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April 27, 2016

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2016-04-27_17-13-24

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Reigate Grammar School, UK. Local weather station and forecasts for education. Reporting on local and global weather and climate. RMetS education committee. Town VP2 updates website every 10mins, wind every 5secs. CoCoRaHS manual rain gauge. NEW Hartswood sportsground Vantage Vue AWS reports every 15 mins. Data to Weatherlink Met Office + Weather Underground.

RGS Status: Reigate data : all good;  Hartswood status: rain gauge stopped working; checking asap:-/

badge_2015Reigate Grammar School awarded MetOffice and Royal Meteorology Society MetMark Award for excellence in weather teaching and promotion of weather understanding and climate awareness. Read some of our best weather club events that helped win the award: HAB launch; BBC school report; River Mole and Gatwick flood reports and St Jude storm post as reported on Radio4 and published student authors in Weather magazine.

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Reigate June 2016 weather statistics

  • T average 16C
  • Tmax 25.6C
  • Tmin 8.5C
  • Total rainfall 100mm
  • Max gust 28mph
  • total sunshine 114 hours

June 2016 was wet in Surrey with over 246% of normal rainfall.  SE England as a whole received 113mm of rain, 208% of long term average.  At 118.4mm, Surrey was the wettest county in the UK regarding anomalously high precipitation. Despite this, both 1971 and 2012 were wetter months.

The rain was delivered in a number of distinct heavy showers and convective thunderstorm events, notably on 23 June when parts of South London were especially badly flooded in flash flooding during torrential rainfall of more than 50mm in one storm.

http://www.huffingtonpost.co.uk/entry/uk-weather-flood-warnings-as-torrential-rain-falls-on-london_uk_576b9225e4b0b1f1704feabb?7liv7jj9ylkz3q5mi&

The heaviest rainfall was characteristically patchy so not every part of Surrey experienced the same soaking, some parts were a lot wetter than others. For example, while Caterham received over 40mm in one thunderstorm, Reigate received only 10mm from the same storm.  In total, Reigate received 100mm of rain in June which compares with 15mm for June 2015 and 30mm for 2014.

“Surrey with 118.4mm of rain endured the worst rainfall of any county, relative to its average. The June average rainfall for Surrey between 1981 and 2010 is just 50.7mm. This month’s total was 246% of normal.” MetOffice

A thunderstorm on 23 June brought an amber warning and local flooding and lightning damage to some places in the SE. Below are some news reports from the time.

A shelf cloud was spotted over Reigate, one of the first observed over the town.

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Horsham experienced intense lightning and one bolt struck a conifer at 2am which was destroyed in a spectacular explosion.  Fortunately no one was hurt, but cars and property was damaged.

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convective June 2016 Priory Park

June started well but pressure fell from 10 June and the month remained mostly unsettled thereafter.

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Due to the unsettled conditions and rainfall and associated cloud, June was less sunny than usual, with only 114 hours of sunshine recorded in Reigate. June 2015 had 192 hours of sunshine in comparison.

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June was just about 1C above the long term 1981-2010 UK average at 13.9C.  Reigate recorded a monthly average of 16C which is considerably higher than the UK average being located in the warmer SE.

Globally June 2016 was the warmest June since records began in 1880 and was the 14th month in a row to beat global long term average temperature. The charts below show NOAA, NASA and UAH global temp anomaly maps for June. All agree that June was hot.

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With one month after another breaking heat records, 2016 is easily on target to be the hottest year on record despite a waning El Nino.

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This continues the inexorable rise in global temperature in recent years.

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Reigate May 2016 summary statistics

  • Tmax 27.4C
  • Tmin 0.3C
  • Tav 13.8C (UK 11.3C)
  • total rainfall 42mm (town) 45mm (Hartswood)
  • max gust 36mph
  • average wind direction NNE
  • sunshine 181.7 hours (May 2015 161 hours)

Whilst there were fortunately no severe weather events in Reigate and few across the UK in May, the weather we experienced more widely could be linked tenuously to climate change.  Of course, caution is required with such speculative statements but attribution studies on the May floods in Paris, not so far away, have concluded that they were made 90% more likely due to climate change. The same stalled low pressure system delivered our easterly winds so we were influenced, albeit on the edges, by the same blocked weather pattern.

People attempting to climb Snowdon in North Wales in May were lucky to experience sunnier-than-usual conditions for much of the month (south wales had more thunderstorms which reduced the sunshine totals there).  Meanwhile, in Surrey, we experienced occasionally warm conditions with an unusual mean monthly wind direction from the NNE.

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Reigate pressure rose hesitantly mid-month

Pressure fell across the UK to start May but then rose mid-month, especially to the North, bringing a relatively unusual easterly flow into Reigate and the south.  Whilst there were few severe weather events during the month, this post briefly explores some of the wider factors that may have contributed to this Easterly flow and the possibility of it being linked to climate change.

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Whilst mostly dry for the UK as a whole, occasional showers, some thundery, brought Reigate rainfall totals to just above average at around 42-45mm as recorded from our two weather stations respectively in and out of town.  SE England as a whole recorded rainfall at 111% of normal rainfall, mostly falling in thundery showers, more common in SE wind regimes.

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With relatively dry Easterly winds, sunshine totals for the UK were accordingly above average given the relatively high pressure overall.  Reigate experienced 182 hours of sunshine in total for the month.

Unusually, the sunniest places in the UK were in the North and West given the easterly winds bringing occasionally cloudier conditions off the North Sea to the south and east.  May 24 shows a typical scenario with the higher pressure to the North dragging in E/NE winds across the southern part of the UK with cloud across eastern areas and clearer conditions to the west.

Some great sunny days were recorded in the mountains of Wales, Cumbria and Scotland!

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Sunny Snowdonia with kelvin-helmholtz type wave clouds trying to form over Ogwen Valley

The higher than average rainfall patches shown below in the south were associated with showers on occasionally unstable warm and humid SE winds.  Reigate reached a Tmax of over 27C in this warm flow.

The wider pressure pattern across the northern hemisphere was characterized by anomalously high heights over the Arctic and LOWER than normal pressure in mid-latitudes including Europe. This situation is called “northern blocking” and in winter could cause cold conditions in mid-latitudes.  In Spring, as the continent rapidly warms up in stronger sunshine, easterly winds can be warm or even hot for the UK.

northern blocking over Pole

northern blocking over Pole

Northern Hemisphere pressure patterns are measured by the Arctic Oscillation which, as can be seen below, remained unusually negative through much of April and May showing high pressure persisting over the Arctic relative to low pressure in the mid-latitudes.  This pressure pattern turned winds from the usual westerlies into easterlies in the UK and Europe.

negative arctic oscillation Spring 2016

negative arctic oscillation Spring 2016

The causes of this reversal of the usual mid-latitude zonal westerly wind set-up have been linked to low sea ice extent in the Arctic, especially the Kara and Arctic Gateway seas. Warmer influxes of air into the Arctic builds air pressure which then links to higher chances of Easterly winds in mid-latitudes.

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low Arctic sea ice cover March 2016

The very low sea ice extent this year was brought about by much warmer-than-usual conditions during the Polar winter, where monthly average temperatures in the Arctic (>60N) were at times 3.5C or more above average during the cold season of 2015-16. This Arctic amplification is widely accepted as being caused by human induced climate change.

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It turns out that Spring Arctic sea ice extent is some of the lowest recorded in the 38 year satellite series.

 

So, unusual sunshine in North Wales, a warm NNE mean wind direction in Reigate and cloudy conditions on the east coast can be linked to the above tele-connecting weather patterns which, in turn, can be linked to climate change in the far flung Arctic.

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Meanwhile, the strong 2015-16 El Niño declined rapidly through May and ENSO conditions were neutral by early June. Models suggest the chance of La Niña (cool Pacific) conditions by Autumn 2016 are as high as 60%.  Some forecasters bring La Nina through the summer.  La Nina, and the warmer SSTs of the tropical Atlantic, are associated with more frequent hurricanes in the Atlantic basin.  In turn, high hurricane accumulated energy transfered to the North Pole during such seasons can build Polar heights in Northern Hemisphere winters, warming the Arctic and further melting sea ice.  Whilst this is just outrageous long term amateur speculation, it is nevertheless interesting to ponder the potential for feedbacks to accelerate further climate change in the near future.

The turning down of the vast heat engine of the El Nino might be linked to the slightly lower May global average temperature, though confirmation from expert sources has not verified this as yet.

 

Local data for May and all months stretching back to 2012 can be found on our data page here

http://www.metoffice.gov.uk/climate/uk/summaries/2016/may

 

Dust!

May 22, 2016 — 3 Comments

Saharan dust reaching the UK gets in the news quite regularly, usually unfavourably in connection with pollution events. Desert dust is one of several types of minute particles, called aerosols, that are emitted into the atmosphere including salt, carbon and volcanic ash. Human made aerosols, such as CFCs and sulfate aerosols from the burning of fossil fuels, are infamous for destroying the ozone layer and causing climate change but 90% of atmospheric aerosols have natural origins and they all contribute in major ways to the global weather machine. The aerosols constantly floating around in the air that we breathe are made up of a complex mix of particles clumping together in an invisible soup that we are unaware of most of the time.  On occasions dust concentration becomes “thick” enough to become visible and reveals itself as haze. Here is some information about desert dust to help get to grips with this impressive weather phenomenon and hopefully clear the air of those mysterious arid particles!  This post concentrates mostly on Saharan desert dust but volcanic dust is mentioned as a comparison and is worthy of a separate post at a later date.

Where does it come from?

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Saharan dust blown across the Atlantic reaches South America

Dust is naturally lifted into the atmosphere from deserts and is an important component of global weather and climate processes and nutrient transport to ecosystems.  Global mineral / desert dust emissions into the atmosphere are estimated to be up to 1500-1800 Tg/year (teragrams) per year and emerge from numerous arid and semi-arid source regions.  For comparison, the average global annual volcanic output of ash from average scale small eruptions has been estimated as an average of only 20 Tg/yr (20 million tonnes per year).  Less frequent, larger eruptions, inject much more ash into the atmosphere. The Icelandic volcano, Eyjafjallajökull, erupted 250 million tons of volcanic ash during the eruption in 2011. This was still small compared with the largest mega eruptions which blast huge volumes of volcanic dust higher into the atmosphere. Desert dust is usually swept up by winds only as high as the planetary boundary layer 2-4km (PBL), whereas volcanic ash can be injected into the high troposphere or even the stratosphere where it encircles the Earth quickly. Nevertheless, in average conditions desert dust usually dwarfs volcanic ash in the atmosphere, unless there is a colossal eruption. (1 Tg = 1 million tonnes ) and albeit desert dust resides at lower altitudes.

global dust source regions

global dust source regions

The biggest global source of atmospheric dust is the Sahara Desert, a huge area of sand dunes, stone and gravel plateaus, dry valleys and salt plains creating nearly 5 million sq km of potential dust producing terrain.  Within the Sahara Desert the Bodele Depression in Chad is thought to contribute half of all Saharan dust.

How does dust get into the air?

Dust is lifted by strong surface winds produced at different scales, from small local convective processes such as dust devils to meso-scale convective systems such as large thunderstorms through to regional scale frontal depressions. Importantly, rainfall in arid areas contributes to available dust by causing flash floods that wash fine debris into river and lake beds. These rivers and lakes then dry out and provide an important source of desert dust when the wind blows. A good example is the Bodélé Depression in Chad, which is part of the dried out Lake Chad.  This area has dust storms on average of 100 days per year and can loft 700,000 tonnes of dust into the atmosphere every day.

http://en.wikipedia.org/wiki/Bod%C3%A9l%C3%A9_Depression

Meso-scale convective weather systems in deserts can cause strong cold downdrafts of out-flowing evaporatively cooled air descending from cumulonimbus storm clouds that can entrain particles and lift them vertically into powerful upward thermals. Sandstorms known locally as haboobs are created in this way and appear as frightening “Hollywood”style dust fronts in Africa, Australia, China, the USA and recently in the film Interstellar.

haboob dust storm formation

haboob dust storm formation

Dust can also be lifted from the surface by powerful winds covering a large area associated with troughs and fronts sweeping across, or near to, desert regions.  One such wind is called the Sirocco which occurs in eastward tracking Mediterranean lows where the warm sector produces strong southerly winds which can bring dusty conditions into Europe especially in Spring and Autumn.

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A significant sirocco event occurred 23 March 2016.  The event shows up well on a synoptic chart and satellite photo.

Various other synoptic scale meteorological scenarios bringing European / UK dust events are discussed below.  Once elevated, coarse dust (sand) falls out nearest to the origin but fine dust (clay), less than 0.002mm in diameter, can be lifted high into the troposphere, up to 10km, where it can remain aloft for weeks and be driven thousands of miles across oceans by jetstreams.  Saharan dust routinely travels to the Caribbean in the summer on an easterly jetstream.  Dust is eventually deposited in light winds, usually in anticyclonic high pressure systems, or is washed out in rainfall.  In this way some 40 million tons of dust is transported from the Sahara and deposited in the Amazon rainforest every year.

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desert dust entrainment and transport

There are broadly two types of dust storm.

  1. Dust plumes have a streaky linear point pattern of dust emerging from a point source and spreading into a cone.
  2. Dust fronts are walls of dust rising on an extensive, frequently curved path.

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Desertification of environments in China and Africa seem likely to be increasing the area of global dust producing regions and potentially making the planet more dusty. However, it is not certain whether global atmospheric dustiness will increase or decrease due to expected climate change in source regions like North Africa.  The world has certainly been more dusty in the past. It is understood that during past glacial periods (last glacial maximum 18,000 years before present) water was locked up in glaciers creating drier conditions particularly in periglacial mid-latitudes.  In Europe, China (Yellow River) and the US (Idaho, Washington, Iowa and Mississippi), huge areas of wind-born dust deposited thick aeolian sediments, one of which extends across the North European plain which now forms very fertile soil called loess.  Loess has become some of the most productive agricultural terrain in the world.  “Dust to dust” seems more apposite than ever considering our reliance on natural dust transport for our food.

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loess hills, fertile farmland courtesy of dust!

What are the impacts of dust on climate and environment?

Over long time scales dustiness increases during cold climatic periods (glacials or ice ages). Evidence for this comes from ice cores in Antarctica and Greenland shown below.

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Reasons for increased dustiness during cold glacial periods includes:

  • Increase desertification (less rainfall generally in cold periods as more water locked up as ice)
  • Increased land area (due to falling sea levels, so more dust sources available)
  • Increased winds

Over shorter time scales, dust plays various complex and sometimes contradictory roles in atmospheric processes, including modification of solar energy receipt, temperature, cloud formation and influencing rainfall.  Dust also has impacts on ecosystems and human activities which can be beneficial or detrimental and even hazardous. So, what can atmospheric dust do exactly?

DUST…

  • absorbs and scatters incoming sunshine causing surface cooling
  • increases cloud condensation nuclei enhancing rainfall or…
  • increases cloud condensation nuclei enhancing condensation of small droplets which stay aloft so reducing rainfall
  • causes “blood / red / mud rain” events creating dirty cars and windows
  • neutralizes acid rain: dominant minerals in dust are usually >pH7 and include acid neutralizing carbonates
  • imports important beneficial minerals and nutrients such as nitrates, phosphates, iron, calcium, silicates etc to ecosystems like the Amazon rainforest. 200 million tons of fertilizing dust is transported from Africa to the Amazon each year of which about 40 million tons is deposited directly into the forest ecosystem: this is possibly the main nutrient source for the forest. Marine ecosystems also benefit from dust inputs e.g. stimulating growth of phytoplankton and subsequent food chain.
  • imports pernicious alien spores and soil fungus to coral reefs potentially causing coral death events
  • reduces Atlantic hurricane formation: enhanced dust from the Saharan Air Layer (SAL) over the Atlantic during the hurricane season has been correlated with reduced numbers of hurricanes, possibly due to the dust reducing sunshine which suppresses Atlantic sea surface temperatures in the hurricane development zone.  The EUMETSAT satellite image below shows a dust veil (pink) killing off convection cells (brown and green) as it moves across the Atlantic towards the Caribbean.  The Saharan Air Layer is a hot, dry and dusty stream of upper air emanating from West Africa, especially during summer.  The SAL could also inhibit convection, and hurricane formation, by creating an inversion preventing updrafts necessary to kick-start tropical storms.

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  • Dust also impacts human activities and health.  Severe dust storms impact activities requiring good visibility such as air travel and some sports. It can also carry organisms such as spores, fungus, bacteria and viruses which could introduce disease far away from the origin of the dust.  Serious cardiovascular and respiratory problems might also be aggravated by fine airborne dust. http://rt.com/news/smog-britain-sahara-pollution-981/

So, dust is clearly a critical part of the weather machine and can bring both benefits and problems.  The next section attempts to explain how desert dust can get all the way to Britain from the Sahara.

What weather patterns typically bring dust to the UK and Europe?

2015-04-11_09-36-51chart1

In Winter, the subtropical Saharan HIGH pressure is strong with winds wanting to spill away in all directions, potentially carrying dust.  However, with a more southerly jetstream and visits by low pressure systems, the Mediterranean is often unsettled and wet during winter. Despite the Sahara being dusty in winter, dust events extending to Europe in winter tend to be restricted because particles are washed-out by winter rainfall before it gets very far north.

2015-04-11_09-36-51chart3

The transitional seasons of Spring and Autumn can produce the most significant dust episodes in Europe. The desert heats up and dries out creating ideal conditions for dust to be elevated by strong winds.  Low pressure can still dip south on meridional jetstreams and create Genoa low pressure which typically increases wind speeds across North Africa.  A cooler Mediterranean Sea surface temperature means that less convection occurs and creates less wash out opportunities as any dust travels north. Therefore, springtime is potentially more dusty for Europe given the right conditions.

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In Summer the Mediterranean  HIGH pressure develops as a semi-permanent feature.  This inhibits transport of dust from the Sahara.  Nevertheless, occasional heat lows over Iberia or cut-off lows can create the right southerly wind on a Spanish Plume to bring dust as far as the UK.

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Saharan dust moves north associated with Spanish Plume

Dust events in Europe vary in scale and can occur at any time of year but it seems usually and most effectively in transition seasons, especially Spring. In 1901 an historic dust event created the first recorded “blood rain” across Europe.  In this single dust event, well documented, some 50,000 tonnes of dust was deposited across Europe (this would have required a 250km long convoy of 2500 20-tonnes lorries to transport). It has been estimated that dust build up across Europe is 4-5mm per century.

Some case studies of European dust events

Here are some examples of past European dust events showing the synoptic evolution of how dust gets to Europe. Note that meso-scale convective systems (MCS) typically producing dust storms in the Sahara are sub-synoptic and sometimes the dust lofting event barely shows up on these charts.  Nevertheless, the synoptic patterns transporting the dust into populated parts of Europe are well illustrated in these examples.

European Dust March 2014

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European Dust April 2011

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So what is the future for dust?

There is no certainty on the impact of climate change on the future of dustiness in the atmosphere.  There have been press articles suggest there is increasing Saharan dust emission due to population increase, intensive farming and land degradation in North Africa.

“There has been a dramatic increase in some aspects of dust flux [emissions], which have doubled over the last 50 years. Population pressure alone is likely to exacerbate the problem and if current trends continue the amount could double again over the next 50 years,” said Dr Bryant, a Reader in Dryland Processes at the University of Sheffield.

Nevertheless, the impact of these activities is not certain and others suggest dust emissions are not increasing. For example, despite human desertification and degradation of semi-arid environments causing increased potential source areas of dust, it appears that the most significant dust source globally, the Sahara  desert, has not in fact been perturbed by human activities since the major dust sources are mostly in uninhabited areas and in true-deserts.

The IPCC predict that North Africa will get drier and therefore presumably more dusty.  However, models suggest that specific dust source regions could become wetter.  There are significant uncertainties over African dust and climate change and there seems to be no clear correlation over recent decades between measurable climate change and dust load in the atmosphere. Models cannot agree on rainfall changes in North Africa.

Here are some links for further information on dust…

aerosol http://www.nasa.gov/centers/langley/news/factsheets/Aerosols.html

excellent detail:  http://www.goes-r.gov/users/comet/EUMETSAT/at_dust/print.htm#page_1.3.0

Greek forecasting dust: http://forecast.uoa.gr/dustindx.php?domain=med

Barcelona dust forecast centre http://dust.aemet.es/forecast

satellite dust over Western Europe: http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/DUST/WESTERNEUROPE/index.htm

cross-sections of dust across Europe: http://charadmexp.gr/instruments/16/

ecosystem impacts http://gallery.usgs.gov/videos/223#.VS1CZfnF_To

http://earthobservatory.nasa.gov/Features/Aerosols/

http://www.independent.co.uk/environment/saharan-dust-six-things-you-didnt-know-about-red-dust-storms-9232879.html

more info: https://books.google.co.uk/books?id=RyYyWQrn-D8C&pg=PA288&lpg=PA288&dq=dust+events+europe&source=bl&ots=EAi_LZA045&sig=7lvbivQoXeOXESgsqRaI8ERn27w&hl=en&sa=X&ei=h78rVf-2AeWQ7Abqx4GgDg&ved=0CDgQ6AEwAw#v=onepage&q=dust%20events%20europe&f=false

http://www.hindawi.com/journals/isrn/2013/245076/

http://www.climatenewsnetwork.net/desert-dust-feeds-deep-ocean-life/

https://news.cnrs.fr/articles/dust-blowing-in-the-winds-of-climate-change?utm_content=buffercc37b&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

notes

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How can there possibly be a link between a modestly cool month in Reigate and the earliest start to the melt-season in Greenland, the devastating wild fires in Canada and the seventh hottest-ever global month in succession?

April summary weather statistics for Reigate

  • Average Temp 8.2C
  • Tmax 17.7C
  • Tmin 0.1C
  • precipitation 43.4mm (local Reigate) SE PPT 55mm
  • sunshine 140.4 hours
  • Max wind gust 30mph
  • average wind bearing 199 degrees

Reigate, like the UK as a whole, had a cooler than average April at 8.2C. The town even experienced some unusual snow showers on 26 April in a cool northerly air flow.

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The cool month for the UK is in stark contrast to the bulk of the planet which experienced a much much warmer month than average, at over 1.1C warmer than any previously measured April.

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Astonishingly, this is the seventh month straight that has brought record breaking global temperature anomalies.  This continuing succession of warm months globally should be of concern to everyone.  More on this below.

Back to the UK… The Central England Temperature came out at 7.5C,  0.4C below average, and the UK mean was even lower at 6.5C, 0.9C below the long term average.

Rainfall was about average in Reigate with around 40mm of rainfall.  The MetOffice SE figure came out at 55mm.

April was sunnier than usual with a total of 140 hours of sunshine.

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This continues the trend of drier and sunnier Aprils in the UK in recent years.

The first half of April was unsettled with most of the rain falling associated with low pressure systems and fronts. The second half of April saw an unusual cool period as northern blocking over the Arctic sent cool northerly winds south with attendant sunshine and showers.

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Globally April was the warmest ever April on record.  An anomaly of 1.1C sent the Paris target of keeping global temperatures below 1.5C into grave doubt as this is the 7th month in succession to yield much higher temperatures than ever. This is now being dubbed a “Climate Emergency” because of the sudden and rapid increase in global temperature to levels not expected to occur so soon.

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The UK / NW Europe was about the only part of the planet, with NE Canada, to record below average temperatures.

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The cool spot over the UK  was due to northern blocking (high pressure) over the Arctic. As pressure rose over the Arctic, cold air pushed out into mid-latitudes.

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It is a matter of chance where high pressure and low pressure set up that determines where cold polar air penetrates in these northern blocking scenarios.  This time the pattern sent the cold air to the UK and N Europe.  The Northern Hemisphere as a whole saw anomalously low snow cover as a result of incredibly high temperatures elsewhere.

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Arctic Amplification, where the northern latitudes experience highest rates of warming, is well documented and of increasing concern to climate change.  It is acting as both a response and a further driving force behind rapid climate change.

Temperatures rocketed over the Arctic this cold season with temperature departures over 3C widely across the Polar regions.  The Greenland ice sheet experienced one of the earliest starts to the ice melt season on record.

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Arctic Sea cover also recorded another record low maximum winter extent.

“On March 24, Arctic sea ice extent peaked at 5.607 million square miles (14.52 million square kilometers), a new record low winter maximum extent in the satellite record that started in 1979. It is slightly smaller than the previous record low maximum extent of 5.612 million square miles (14.54 million square kilometers) that occurred last year. The 13 smallest maximum extents on the satellite record have happened in the last 13 years.” NASA 

This is both a response and a further catastrophe for climate change.  As snow and ice melt in the Polar regions there are connections with further warming as darker sea and land surfaces heat up more readily.

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This Polar warming itself is connected with a weaker jetstream as latitudinal temperature gradients in the atmosphere decline.  It is temperature gradient, especially in Mid-Latitudes, that generates the driving force behind the jetstream.  A weaker jetstream is said to cause more blocked atmospheric conditions as it meanders with greater amplitude in a meridional pattern that locks in swoops of northerly and southerly winds. More extreme weather is caused as these pressure patterns persist for longer.  Sweeps of warmer air penetrate into the Arctic, melting more ice over Greenland and, for mid-latitudes, cooler dry Polar air leaks out causing damaging late frosts and wild fires.

So, whilst it seems tenuous to connect these far-off events to our own rather benignly cool April, it is still important to think globally when considering how our own weather links to increasingly extreme weather elsewhere.

http://www.theguardian.com/environment/2016/may/16/april-third-month-in-row-to-break-global-temperature-records?CMP=share_btn_tw

http://www.bbc.co.uk/news/science-environment-36212145

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Today we put up a new weather station at our sports ground at Hartswood outside Reigate. This location will complement our established town weather station located at Reigate Grammar School. Hartswood is an out of town location with more exposure from all wind directions. It is already recording different conditions to the town (see links below).

The new weather station is a robust self-contained Davis Vantage Vue automatic weather station (AWS).  This model was chosen for its ability to cope with exposed sites and it has a reputation for being relatively maintenance free for longer periods.  It is commonly put on masts on rooves, as we have done here.

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This AWS is unusual because it uses the new Vantage Connect system.  The Connect system uses the mobile phone network to transmit data at 15 minute intervals to the Weatherlink website where it is pushed onwards to other websites, such as Weather Underground.  There is also a handy local live read out of weather on a console in the Tea Hut window.

The Vantage Vue weather station is simple to set up being a single housed unit. Attaching the anemometer and wind vane involves tightening screws with a tiny allen key. The Connect System is also easy to set up. Insert and connect batteries, start both systems up and they will endeavour to discover each other with little intervention.

The console unit also discovers the AWS and starts displaying data almost immediately with little user input.

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Once the systems are working and data is uploading reliably to the internet then assembly and fitting onto the roof is the next step.  An aerial expert was employed for this bit.

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Orientation to the South is important for both the Vantage Vue and Connect systems. Not only do they both use solar panels to maintain battery power (used at night of course) the Vantage Vue also requires a southerly orientation to ensure that wind direction readings are accurately recorded by the wind vane.  This is all explained in the manuals.

 

Roof top sites for AWS are popular but they have pros and cons.  Whilst wind readings benefit greatly from a clear wind run at height (so long as the mast exceeds a metre or so above the roof line to avoid eddies and turbulence), the accuracy of rain recordings can sometimes suffer with greater wind speed rendering totals somewhat less reliable than traditional ground based rain gauges (although ground based AWS often do not entirely satisfy strict meteorological conditions for rain gauge placement either).  Roof locations benefit from better security and connectivity.  Overall, with single-unit compact weather stations a roof top location is a good compromise and the most effective use of this technology.  Our Vantage Pro 2 AWS in town allowed us to divide the rain gauge and temperature sensor units on the ground from the anemometer on the roof, a better solution.

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“Live” weather data from Hartswood can now be viewed on the internet in these locations:

www.wunderground.com

weatherlink.com

Weatherlink summary

It is hoped the data will prove to be useful for checking the weather conditions before matches for staff, students, players and spectators preparing for their match or visit.

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Eventually a ground frost sensor can be added to issue alarms when ground temperatures fall to near zero. This will save some guess-work and early visits to check if pitches are frozen or not. Data will also be useful for students doing weather studies in urban micro-climate and the data can also be used by computing and maths students amongst many other applications.

RGSweather will also be able to compare data between town and edge of town locations.

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Reigate weather summary statistics for March 2016:

  • Tmax 14.8C
  • Tmin -2.8C
  • Taverage 6.3C
  • Total rainfall 80mm (66.4 aws)
  • Max gust 51mph
  • Total sunshine 118.6 hours

March in Reigate started and finished unsettled with Storm Jake (no impact locally) on 2 March and Storm Katie (moderate impact, see previous post) on 27-28 March.  High pressure located to the north of the UK mid-month kept things more settled and mostly dry here.

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Here’s a time-lapse video from Reigate of showers and a defined cold front passing through associated with Storm Jake on 2 March.  Note the distinct drop in temperature as the cold front arrives (windows fog up) and then the change (veer) in wind direction as the front passes and skies clear.  Spot the wind shear too during the cold front passage.

March was sunnier than average for Reigate with 118.6 hours of sunshine.

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UK-wise the month was sunnier but wetter in the south, mostly due to heavy rain associated with a deep low on 9 March and Katie later in the month.

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deep LOW brought most of March rain to the South

March was also slightly below average temperature in the south.  The CET came out 0.1C above average for March at 5.8C.

Air pressure was lower across the south of Europe than the north during March, hence the drier conditions to the north.

The Atlantic cool blob sea surface temperature anomaly continued to develop during March and this may have had some influence on moderating temperatures regionally.

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Globally, March was the warmest ever recorded March by a considerable margin at over 1.29C above the long term average for the month.

 

Whilst Europe was about average temperature, there were notably extreme temperatures across the Tropics, in particular Indonesia, parts of India and Australia, as well as tropical North and South America and parts of Africa.

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This continues the trend of warming shown in the graph below. Note the correlation of anomalously warm years with El Nino events.

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The current record breaking 2015-16 El Nino is fading fast and is forecast to be replaced by cooler than average Pacific equatorial sea surface temperatures known as a La Nina by the autumn.  This might have implications for a more active Atlantic hurricane season.

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Met Office March summary

refs

http://www.metoffice.gov.uk/climate/uk/summaries/2016/march

 

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Currently Reigate and the SE is enjoying a settled sunny spell under the continental flow of a HIGH pressure sat directly over the UK.

Light surface easterly/SE winds and unbroken sunny skies under high pressure have brought temperatures up to 17C locally today.

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The 500mb chart above shows the upper southerly / SE flow bringing recently warm conditions in the daytime.  Unfortunately things change by the end of the week as the upper flow swings round to arrive from the north over the weekend and push temperatures well below average for the time of year.

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The jetstream will be arriving direct from Greenland by Sunday, as HIGH pressure builds over the Atlantic and falls to the east of the UK.

Airmass temperatures in this northerly polar flow (measured at 850hPa) could even be low enough for showery wintry precipitation.

Cold late Springs are not unheard of in the SE, 1981 was a cold Spring with snow, for example. Recent Springs over the last 25 years have been milder than the long term, so this event is still “normal” weather.

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The HIGH is set to regress NW into the Atlantic and merge with a high pressure over Greenland.  Pressure over the Poles is rising as well, shown by the swing to negative Arctic Oscillation and related NAO.

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The result of higher pressure over the Polar regions and low pressure in mid latitudes is to push cold Arctic air south.  If this was January it would be a truly cold period coming up.

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With LOW pressure over the North Sea the resulting funnel of cold northerly winds will dig south across the UK and into Europe especially from Sunday into next week.  Sunday is the London marathon and, whilst showers are forecast coming down the UK during the day, it looks a mostly dry but cool start for runners.

Most of the wintry action looks set for Benelux but here the Tmax during the daytime are likely to be around 10C, but fall to near freezing at night, with frosts under any clear skies.  Occasional wintry showers are possible, especially Monday. Note the Tmax temperatures on Monday daytime shown below.

Cool for the time of year for sure, as shown by the contrasting anomaly charts below between this week and next.

During this cold spell next week fresh falls of snow are expected across mountain regions of Europe, especially Norway and the Alps and the highlands of Scotland and Northern England.

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At lower elevations snow might fall at times during this cold spell, however, strong April sunshine (as strong as August) will quickly raise temperatures between showers so that, even with any wintry precipitation, ice and frost will melt rapidly away.  Icy roads overnight are also unlikely at lower elevations because soil and surface temperatures have risen well above freezing.  Nevertheless, gardeners should be aware of the chance of frost and 10cm soil temperatures will fall to near 5C over the course of next week.

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How long will this last?  Charts show the cool spell lasting for much of next week but with some amelioration throughout likely.  Stronger sunny periods will make it feel satisfactory out of any wind but night time on east coasts in heavy wintry showers, for example, will feel distinctly chilly!

Where’s the heat gone?  It’s worth pointing out the very high temperature anomalies over N Africa and Greenland at the moment.  The highest and earliest surface melt across the Greenland ice cap is likely to continue next week in the upstream flow around the Atlantic block. The build up of heat in N Africa (shown below) could, in the right conditions, bring continental heat to the UK later and, perhaps some decent thunderstorms, but this is all just speculation.

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Whilst Storm Katie was not a record-breaker by any means she did provide some evidence of stingjet winds in the wrap-around feature that showed up especially on IR satellite photos and rainfall radar in the later stages of her track across the SE and UK.

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Here is some analysis from local Reigate Surrey records of these winds to explore this feature more. Stingjets can be the most damaging winds, in this case they were not especially strong.  Professionals, like Matt Hugo (NorthWstWx services) and Simon Lee (MMetReading), saw hints of stingjets during the passage of the storm but what evidence of stingjet winds is there “on the ground” and how can amateur observers watch out for these potent weather features in future?

Katie’s rapid cyclogenesis prior to landfall over the UK was a precursor to the formation of stingjet winds.

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Stingjets are associated with vigorous bomb-depressions developed in a process called rapid cyclogenesis: in RaCy depressions central pressure falls very rapidly and slows the horizontal surface speed of the cyclone.  A stingjet is a narrow band of gusty winds that descend from high altitude in the latter stages of RaCy depressions.  They arrive from some 3-4km above the ground and are associated with descending stratospheric air into the low core.  Ironically stingjets are associated with weakening fronts and aging cyclones.  This descent of upper air pushes the jetstream lower and this can deliver extremely gusty conditions to the surface in a narrow band that is shown by a cloud hook and matching rainfall pattern.

Watervapour satellite loop from 00hrs to 8am showing dry descending stratospheric air forming a dark slot – dry intrusion – that wraps into the developing low core: a sure signal of rapid intensification.

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The hooked rainfall signature below matched a marked increase in the strength of wind and the modest strongest gusts of the whole event of 52mph in Reigate. In exposed places like Redhill aerodrome this exceeded 60mph and over the North Downs at Kenley 68mph was recorded with 70mph in places. 2016-03-28_07-32-34 Arriving from high altitude, stingjet winds are unsurprisingly cold with low dew points. So you’d expect a dip in temperature at the surface.  Parts of Wales and Shropshire had snow in this airstream wrapping round the back of the low as it moved into the North Sea. Stingjets match the mature stage of the Shapiro-Keyser cyclogenesis model.  In all respects, they are aptly named as the “sting in the tale”.

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metoffice stingjet info

Storm Katie had some generally strong winds (for the SE of the UK!) associated with the southern edge of this cyclone where the tightest pressure gradients developed as pressure fell across the SE: in the narrow warm sector especially.  The lowest central pressure was 971mb.  Our Wight-Wash Oscillation reached 16 or 17mb at one stage: the difference between the pressure over the Isle of Wight and the Wash.

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Most significant storms to impact SE England have similar NE tracks through the Bristol Channel and exiting through the Wash. This was certainly the case with October 1987, St Jude 2013 and Storm Katie March 2016.

A complex warm sector and bent-back wrapped occlusion make it tricky to identify the normal Norwegian model of warm-cold frontal passage.

It is more likely that Katie, like other RaCY depressions, developed according to the Shapiro-Keyser model of rapid cyclogenesis which involves a break away cold front fracturing away from the depression core: T-bone.

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The wrapped rainfall radar and cloud hook above both suggest a stingjet feature associated with this kind of development.

Now, onto evidence stingjets… the passage of the strongest gusts shown below do not match the passage of a “normal” cold front but suggest some other process was at work to deliver the strongest gusts.  The strongest winds occurred 3 hours after the passage of the front that started to deliver the expected colder polar air behind the wrapped occlusion. This can be seen from the chart of Reigate wind speeds and temperature below. 2016-03-28_22-25-38 The chart shows the persistent warm sector gales proceeding from midnight on 28 March through to about 5am.  These gales do not exceed 80kmh but they modestly peak just before the passage of the front, an expected pattern.  Fronts then pass through Reigate at about 5am and temperatures fall as expected, as do gusts.  However, from 7am gust strength sharply increase, this matches the timing of the sting jet cloud feature on satellite photos.  This increase in wind speed had no front associated with it and therefore suggests evidence of a stingjet process: arriving out of the blue!

Note the temporary drop of temperature to a minimum and recovery after the departure of strongest winds.  This again suggests these winds are not frontal in origin but are part of the stingjet process.  Are they associated with a sting jet of descending high altitude air originating some 3-4km in the troposphere?  I’d like to think this is a sting jet signature but will need confirmation from official sources to pin this down. 2016-03-29_07-29-56 For comparison shown above is a “normal” cold front passage from the previous weekend when a cold front squall line of some note passed through.  Note there is no dip to minimum temperature associated with the maximum gusts and temperatures remain cold after the passage of the front because of the insurgence of cold polar air.   This is more typical behaviour when it comes to frontal passage.

 

 

Other charts and references support this idea, but some not unfortunately not quite with same timings.

http://www.metoffice.gov.uk/media/pdf/2/p/Sting_Jet_Flyer.PDF

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Before we get to Katie, it’s worth mentioning recent and current weather for Easter Sunday: the first convective day of 2016.  A cold pool of polar upper air followed the impressive cold front yesterday which developed a very long squall line.

Thundery conditions followed the cold front in an unstable Polar airmass with notable CAPE (convective available energy) and negative 2 Lifted Index for the time of year – both indicators of potential thundery heavy shower activity.  The limiting factor on multicell thunderstorms locally was lack of wind shear and an anticyclonic jetstream, both subduing anything truly impressive or sustained. Nevertheless, some hail and thunderstorm activity is rife across the UK today in this unstable polar maritime airmass.

Storm Katie is riding a powerful jetstream and deepening rapidly over the Atlantic through today.  She is some 600miles away to the SW but approaching and deepening rapidly as she undergoes rapid cyclogenesis (RaCy) falling from 999mb to 975mb:  24mb in 24 hours, qualifying as a “bomb depression”.

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Katie emerged out of Canada earlier this week as cold continental air met humid sub-tropical maritime air.  She engaged with an active jetstream to be launched across the Atlantic.  Storms tend not to move fast and deepen rapidly at the same time, so her rapid progress across the Atlantic will now slow as she now expends more energy lowering pressure on her approach to the UK.

The evolution and track of this storm is similar to St Jude October 2013, but Katie is forecast to be less powerful, due in part to the slacker pressure rise on her departure into the North Sea.** (update: in the event Katie pipped StJude, see below!)

Katie also has some similarities to the October 1987 but again is significantly less powerful than that rare 200 year event.

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The water vapour sat pics below show the evolution of Katie and cirrus cloud associated with the jetstream.

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Baroclinic leaf in the Atlantic, jetsream cirrus to the UK

 

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dark slot shows rapid intensification stage as strat air descends into LOW, showers over UK

Storm Katie is forecast to deepen through the SW approaches to below 980mb on an expected track overnight Sunday-Monday from Biscay into the Bristol Channel through to exit around the Wash.

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The pressure gradient between the Isle of Wight and the Wash could be as much as 12mb… the highest of our Wight-Wash Oscillations recorded, and greater than the 10mb WWO recorded in StJude, if this comes off.

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However, the pressure rise behind Katie is expected to be less rapid and sustained than St Jude, so this critically reduces the potential for highest gusts, sting jets aside.

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The highest wind speeds for Storm Katie will be south of the low core in the occluding warm sector in the small hours of Monday morning.

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Max gusts of over 80mph are possible in the Channel, 70mph along Channel coasts and 60mph further inland.  For Reigate this means a brief episode of gusts possibly exceeding 50mph and more over exposed hills.  This could wake people up momentarily as gusts roar through trees and round houses.

Rainfall will be significant too and some hi res models put down >40mm over parts of the North Downs, showing the significance of small hills in generating orographic rainfall.

Rainfall more widely is more likely to be around 20-30mm, enough for local flooding. The MetOffice have issued a yellow warning for Storm Katie.  Check the weather impact matrix showing moderate likelihood of moderate impact (low yellow).

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Update: Amber warning issued for Storm Katie.

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The shipping forecast is less equivocal and shows storm warnings right around the UK associated with Katie and other low pressure systems.  Storm Force 10 is possible in the Channel for a while. http://bbc.in/1RtwvyS

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Furthermore, evolution so far matching StJude and the Wight-Wash pressure gradient forecast seems to be matching or beating St Jude.

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Below is a Meteosat picture (courtesy Dundee sat receiving station) showing Katie moving into the North Sea, deepening again slightly to 972mb,

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Meteosat visible photo of Storm Katie moving into North Sea on Monday 28 March

Post-Katie we enter a broadly zonal westerly flow but turning cooler as slack winds swing northerly through the week as LOW pressure moves into North Sea/Scandinavia. Unsettled and showery activity continues through the week but with pressure slowly rising from the west by Thursday. No further significant storms are expected for the time being. Into the start of April, pressure over the Atlantic is expected to rise and progress over the south of the UK into Europe to give a settled period for the start of April.

RGSweather is off to East Iceland, so expect occasional tweets and updates from cooler climes.

Storm Katie: In the Event:

various meteorological impacts show Katie exceed StJude in power but both fall well short of October 1987 Great Storm.

  • Max wind gusts Needles 105mph (St Jude 99mph)
  • Reigate max wind gust 51mph, widely 50-60mph and 70mph on North Downs (Kenley) Gatwick 59mph
  • Redhill aerodrome recorded 70mph gust at 04:50
  • Lowest pressure: 971mb (St Jude 2013 976mb; Oct’87 958mb)
  • Reigate lowest pressure 976mb (St Jude 980mb; Oct ’87 est
  • Reigate rainfall 20mm
  • Wight-Wash Oscillation 14mb (St Jude 12mb, Oct ’87 storm 20mb!)
  • Rapid rise in River Mole catchment – no significant flooding but overtopped bankfull stage.

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Note, compared with the Oct ’87 storm Katie and St Jude are found wanting.  The Wight-Wash Oscillation for the 200 year storm of October 1987 was a whopping 20mb! See the chart below for the exit of the Oct 87 storm through the Wash on 16 October 1987.

Finally, which of these big SE storms are most powerful? Play SE storms TRUMPS to find out..

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http://www.bbc.co.uk/news/in-pictures-35910151

http://www.bbc.co.uk/news/uk-24708614

http://www.chichester.co.uk/news/update-more-than-600-incident-in-sussex-as-storm-katie-batters-the-south-1-7299324?utm_medium=twitter&utm_source=dlvr.it

http://www.telegraph.co.uk/news/weather/12205613/Storm-Katie.html?frame=3601967

http://www.bbc.co.uk/news/uk-england-35909651

http://www.dailymail.co.uk/news/article-3512119/Storm-Katie-batters-Britain-Gales-100mph-dozens-flights-cancelled-UK-wakes-Bank-Holiday-washout.html

http://www.theguardian.com/uk-news/2016/mar/28/storm-katie-flight-diversions-wind-rain-batter-southern-england-and-wales

http://www.mirror.co.uk/news/uk-news/see-full-carnage-storm-katie-7644830

local coverage

http://www.dorkingandleatherheadadvertiser.co.uk/Storm-Katie-Trees-power-Surrey-storm-hits-PHOTOS/story-29006658-detail/story.html