Archives For warm

2016-05-16_10-57-06

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

Very warm conditions have dominated this December, especially in SE England due to a persistent warm SW sub-tropical airmass. Here in Reigate the mean monthly December temperature so far is over 10C, nearly 5C above the longer term average for the month (5.3C). Records for daily maximum and highest minimum temperatures have been tumbling as 850hPa airmass temperatures have risen 5-10C warmer than normal, making it feel more like May or even June than December, especially overnight!  In addition, the air arriving tonight is loaded with Saharan dust.

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Reigate temps December 9-16, long term av = 5.5C

Reigate daily temperatures have been consistently hitting double figures and recent nights have barely dropped below 10C.

It is the warmest start to December for years but will it break the all-time absolute Tmax record of 18.3C?

Oddly enough probably not, because the December absolute Tmax record was set in the lee of mountains in Scotland where, in the right conditions, a special foehn effect can lift temperatures beyond the reach of even the warmest airmass that we are likely to get in the next few days.  This airmass is cloudy too, so the sun is less likely to break through.

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Nevertheless, daily and local records here in the SE are tumbling and the duration of this warm spell is unusual. The cause is a stubborn high pressure over Europe and a trough in the Atlantic feeding a persistent SW airflow from the warm sub-tropics, places like the Azores.

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Azores: nice!

At this time of year the Saharan HIGH pressure is pretty strong and the southern edge of this creates winds that pour across the Sahara Desert from the east, heading across the continent to the Atlantic.

The Bodélé Depression in Chad is very likely to be the source of any dust arriving in the UK over the next few days.  Weather stations nearby have recorded windy conditions almost entirely from the east.

The Bodélé Depression produces more than half of all Saharan dust, partly due to the way super-geostrophic wind circulate around the Saharan high pressure and are funneled through a mountain barrier into the depression, accelerating wind which then lofts prodigious amounts of dust into the air.

Some 700,000 tonnes of dust can be lofted into the air every day in this location.  The result is some extraordinary rock formations due to the eroding effect of the sand blasting the rocks.

Desert sand tends to be lofted into the lower atmosphere, up to the boundary layer at most some 1km-2km or so above the surface but it can reach higher altitudes in some conditions and be transported further afield.  From Chad it is blown by trade winds into the Atlantic.  A lot of this dust continues across to America but, depending on conditions, some of it can be gathered into the mid-latitude circulation and make its way to Europe. Spot the journey of the dust on these sat pics. taken since late November.

Bodélé Depression

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21 November 2015 largely dust free Sahara

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dust storm early Dec

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Desert dust arrives Atlantic Ocean

By Thursday the desert dust is forecast to join more local particulate pollution in a warm sector to bump up pollution levels in the UK. It also coincides with exceptionally mild airmass. A breeze tomorrow should reduce the threat of a “toxic cloud” developing as stated in some media. Nevertheless, it is worth considering that desert dust is an entirely natural and vital part of the atmospheric circulation.

The outlook is for continued warm/mild weather to continue into Christmas, potentially more unsettled at times as the jetstream perks up and takes on a more direct zonal attack across the Atlantic.  The bigger reason for the mild mid-latitude weather is an exceptionally powerful polar vortex that is keeping pressure and temperature very low in the polar stratosphere.

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The lower tropospheric jetstream is subsequently strong and “locks in” the cold to the polar regions.  This situation is summarized by a strongly positive North Atlantic Oscillation: indicated by high pressure over the Azores and low over Iceland.  Until this situation changes the chances of sustained cold for us are slim.  The only hope for sustained cold this winter in a mega-El Nino year is said by experts in long range forecasting, to be a sudden stratospheric warming that will break down the polar vortex.  That can occurs most commonly Jan-Feb. Here’s hoping!

More on the importance of desert dust in the atmosphere can be found in the post here https://rgsweather.com/2015/04/14/dust/

http://blog.metoffice.gov.uk/2015/12/16/saharan-dust/

After a cool unsettled end to May with a strong zonal jetstream, early June weather prospects are getting interesting for Reigate as models build a tantalizing “heat wave” with potential for warm plumes and thunderstorms, quite a contrast to the start of the week!  The warming is just beginning to cook up on the mean temperature anomaly chart and recent GFS model runs shown below with Tmax temperatures in the high 20’s and some approaching 30C by next weekend/ Saturday especially. Here’s a review, rather than a forecast, of some of the synoptic features unfolding early June weather.  The scenario can change a lot by next weekend of course, so stay tuned, especially to twitter and the fantastic UK weather community (both amateur and professional) for updates.

In the short term, the end of May and start of June will continue to be dominated by LOW pressure to the north sweeping active fronts across the UK with attendant rain and wind, especially Sunday and more on Tuesday, though as usual Reigate and SE will be sheltered from the worst of this which will impact the NW mainly.  Tuesday’s Atlantic depression has an unseasonably low forecast central pressure of 972mb (UKMET) and 968mb (GFS) due to a strong jetstream across Scotland of 160mph.  Expect windy, gales in west and coasts, showery and unseasonably cool weather everywhere but especially in the NW during these episodes. Reigate on Tuesday could have gusts exceeding 40mph.  Update Sunday: MetOffice have issued a weather warning for gales countrywide for this event.

968mb would be close to the lowest June central pressure to impact the UK since 2000**, though the MetOffice chart below showing 972mb is probably nearer what will happen in reality.  Highest winds (40mph inland, 50mph coastal) in Reigate and SE are expected Tuesday am as a daughter frontal wave-low whizzes across the Midlands trailing her bigger parent.

**

 

By the end of this week models build a meridional (wiggly) flow in the jetstream as the North Atlantic Oscillation (NAO) turns negative which means pressure over Iceland rises relative to that over the Azores which falls below average. This weakens the pressure gradient and reduces jetstream strength.  The NAO is not a driver of weather but is an indicator of Atlantic patterns that controls incoming weather for the UK and Europe.  A negative NAO often means fewer Atlantic based LOWS with a less aggressive and frequently more meandering polar front jetstream, with more chance of continental weather impacting the UK as pressure builds to the north (in winter this can bring cold weather from the continent).

A negative North Atlantic Oscillation usually indicates a weaker jetstream and one that meanders with high amplitude waves across latitudes, a so called meridional pattern (rather than zonal which blows purposefully west to east across Atlantic along strong pressure gradient between Azores and Iceland: dragging in frequent LOWS).  A meridional jetstream can slow-down and fix weather patterns into place, especially if a HIGH builds to the north as a so called blocking pattern.  Such a pattern looks possible with current model runs, though with steep temperature gradients building over a heating up USA/Canada, a return to an active unsettled Atlantic pattern seems possible later into June which could breakdown any blocking pattern and finish off our balmy continental flow, but that is way off so remains to be seen!

The meandering jet developing from mid week will encourage a warm / hot continental easterly / SE flow for the UK as pressure builds initially to the north east and pressure lowers to the south from Wednesday.  A cut-off LOW to the west of Iberia sandwiched between the Azores HIGH and the HIGH further NE is also a prime ingredient to waft warm unstable Spanish plumes our way as the HIGH pressure drifts east over Scandinavia (see above charts).

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Spanish Plume June 2015

The threat of heat and thunderstorms peaks next weekend, notably on Saturday, with temperatures peaking as high as 30C. The 850hPa chart above shows the warm plume arriving from Spain.  Upper air temperatures exceeding 15C would yield hot daytime temperatures approaching 30C in sunny conditions.  Such warm plumes of continental air, meeting Atlantic air injected from the cool HIGH offshore, could lead to unstable thundery episodes (more on Spanish Plumes here) On the other hand the pressure is quite high in the East so this could suppress convective action here, the detail will be critical.  On the skew-t chart below spot the steep lapse rates, negative lifted index and high CAPE, high dew points (moist air) and precipitable water content below, all lively indicators of an unstable atmosphere.  It’s a long way off though so things can change a lot and frequently do!

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This set-up is the source of excitement over “heat” by the end of next week: a warm continental plume.  Pressure in this scenario would be highest in the north of the UK so the SE could see more unsettled conditions.  Interestingly, the current Madden-Julian Oscillation Phase 1 and 2 (a tropical disturbance pattern used to forecast patterns in medium range) correlates with this emerging pattern, with P1 and P2 often linked to HIGH pressure to north, LOW over Europe and an unsettled S/SE UK.

Despite this, a prolonged heat wave does not seem to be a strong possibility.  Technically a UK “heat wave” is when daily maximum temperatures for more than five consecutive days exceed the average maximum temperature by 5 °C.  For the SE this usually means exceeding 30C daytime Tmax and 16C night time Tmin, whilst forecasts for the end of next week are warm, sustained heat of this nature does not seem likely.  Pressure and 850hPa temperatures rise this week with some models in the high 20’sC Tmax. Both medium and longer range models suggest either a flat-line or fall in both as June progresses.

Models also suggest a weakening of pressure and some play with a thundery breakdown bringing in wildly high CAPE values (convective potential) into June. This suggests a breakdown as pressure falls. Some CAPE values forecast are extreme for the UK and would not be out of place in a Mid-West tornadic supercell! However, often these scenarios fail to materialise as forecast and often the instability and thunderstorms simply brush past the SW of the UK or stay in France, perhaps clipping Kent alone.

Models in the second week of June seem to suggest the HIGH builds out to the NW, regressing from the location over Scandinavia.  This would put the UK on the cooler side of the HIGH with a northerly flow, thus ending any heat spike. This is just one GFS run and at the unreliable end of the model but a possible solution.

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Finally, over the long term average June is rarely “hot” for a prolonged period and this brief hot spell declining into “warm” would seem to match the emerging pattern.  On average, the hottest day of the year falls in June only 25% of the time and June has shown climate trends of becoming rather duller and wetter bucking the trend of overall warming for other months due to climate change.  The frequency of Atlantic westerlies, on average, also picks up in June making sustained “heat” a rarity.  Nevertheless, a brief warm spell is likely as shown by the end of this week as shown by increasingly convincing model trends. Thereafter, the location of the HIGH and jetstream activity will probably control unfolding June weather events.

Update from @wansteadweather supports idea that June may not live up to any hot start, HIGH dominates but the position may feed average to cool flow from N.  Will be interesting to see how this pans out!

http://www.metoffice.gov.uk/learning/learn-about-the-weather/weather-phenomena/heatwave

http://www.metoffice.gov.uk/public/weather/forecast/gcpg7rs0t#?tab=fiveDay&fcTime=1432944000

2015-05-02_06-58-07

analysis chart shows HIGH edging out with moist Atlantic winds ready to pounce

The analysis chart above shows a weakening ridge of HIGH pressure over the UK being edged out north by a slow moving Atlantic LOW to the W/SW.  Reigate is still currently (Saturday am) in cool dull easterly winds generated by the HIGH but a significant switch in wind direction will take place over the next 12 hours into the bank holiday period as a wholly different mild and humid Sub-Tropical Atlantic air mass, with a source region round the Azores, takes hold from the SW.

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weather cross-section

A mild moist S/SW wind drives in from the Atlantic as the LOW edges north east tonight. The first mass of rain is edging onto radar from the SW and is expected to arrive in Reigate by around mid-late pm today.  Most rain is likely for places further north and west but the SE is still likely to pick up plenty of wet weather overnight with low cloud and rain into Sunday morning when it could turn heavy and showery for a while in the early morning as the trough passes directly overhead and pressure continues to fall. Things are expected to clear to brighter conditions later in the afternoon as pressure rises and winds turn more westerly. Cloud cover will hopefully break and cloud height will lift during the afternoon becoming more cumuliform.

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trough and fronts migrate north, showers follow

If the sun comes out then there could be a low risk of an odd heavy shower Sunday afternoon, possibly thundery, but these are more likely further north of our area where more unstable air makes progress across the Midlands and East Anglia.

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During Sunday winds will be occasionally blustery with moderate convective gusts possible, especially on hills and nearer the coast, and make the mild temperatures Tmax 16C feel considerably cooler. Temperatures overnight could hold up to a balmy 12-13C.

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Overnight Saturday-Sunday rain could linger as showers through the morning

Winds turn from SW to more southerly through Monday and pressure should up-tick slightly giving a mostly dry and warm day and less windy as things stand currently.  Troughs could progress east during Monday and build cloud and produce some showers.  More importantly there is a looming threat of something special for later Monday-Tuesday night.

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As the northern block (high over Greenland) holds on, the Atlantic LOW just west of Ireland will usher in a mild and moist S/SW flow of air from the continent.  An unstable LOW brewing in the topical Atlantic today (Saturday) is forecast to sweep up and intensify from Biscay later Monday and into Tuesday and this might bring heavy rain and winds to the south and SE and a possible thundery episode later Monday but more likely overnight into early Tuesday for SE.  The jetstream is dipping well south and is forecast to perk up and approach the UK from an unusually southerly direction by Tuesday.  If this happens the jetstream could deepen this low considerably, as modelled by some charts (latest UKMet shows 980mb).

Depending on the evolution we could find ourselves in the unstable left exit region of a jet where divergence aloft enhances convective action and creates heavy rain.  Warm air from the south will also contain more moisture.  A dry slot at mid-levels might also enhance instability (rising dry air cools more quickly increasing lapse rates and CAPE, enhancing lift).  High dew points near the surface temperature also encourage condensation and indicate extremely moist warm surface flows.

So all these ingredients stirred up could be a good recipe for some briefly moderate-severe weather in our region especially some briefly torrential rain, though totals are unlikely to amount to more than 10mm.  Gusty winds and gales near coasts could also accompany this system.  Latest metoffice chart shows pressure dipping to 980mb in the North Sea which is significantly LOW pressure for the time of year.

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coastal gales and convective gusts inland

However these episodes have a habit of tracking across Holland and merely clip Kent with thundery showers and miss us entirely.  Models also generally exaggerate these early on and then things flatten out nearer the time considerably.  Nevertheless, it is worth watching this develop as our first potential “warm plume” of the year.  If we take a direct hit the SE could have some heavy rain.

The GEFS summary below clearly shows the two main rainfall spikes tonight and Monday night.

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GEFS 850hPa temperatures and rainfall London

Later mid-week the LOW is expected to drift east across the UK bringing in a more westerly pattern so unsettled showery weather is likely for a while. Thereafter, a rise in pressure from a developing Euro high pressure may then take place from the south and settle things down for us in the SE, though this might only make faltering progress.

Hope you enjoyed this post. Comments are always welcome. Links to websites used to create our blog posts can be found on our links page.

2015-05-02_07-05-01

faltering pressure rise later next week

The end of October has continued to be warmer than average for the season and Friday 31 October could be the warmest Halloween on record (current record around 19C). Hi Res models show the highest temps most likely somewhere inland away from cooler seas and an incoming cold front from the west.  If the sun comes out, which is most likely in the SE or East Anglia and under such a dry southerly air flow, temperatures could rise to over 20C, or even 23C in places.  Whilst this unseasonably freaky warmth cannot really be described as a heatwave (there are technical requirements to classify as a heatwave) the headlines might be correct in suggesting a possible record breaking temperature maximum on Friday.

slide into normal

slide into normal

Thereafter, November temps take a tumble nearer seasonal average through next week and at times even below average. So whilst we can enjoy the treat of near summer like temps tomorrow by next week it will be some 10C cooler as Autumnal temperatures and more rain take a hold, though nothing extraordinary.  Compare this with the first winter storm warning of the year on the East Coast USA where temps are set to freeze and the first substantial snow in the mountains is likely to occur this weekend.

Along with temperature, pressure is set to fall through the weekend and into early next week as a depression hovers NW of the UK over Iceland and brings Reigate some breezy SW winds on fronts with rain reaching us in the SE on occasions over the weekend and into the early part of next week.  A cold front later on Sunday looks especially likely to be the one to usher in the distinctly cooler polar air overnight into Monday as a trough pushes out the high pressure which moves east taking any remaining mild air into the continent.

During the first part of next week the LOW moves SE over the UK and brings unsettled cooler and damper conditions to Reigate.  This LOW is due to then move further to the south / SE over the continent and deepen to become a feature called a “cut-off” low.  This is modelled to move over the continent to the Alps and N Italy where especially heavy rain is possible and then to parts of the Mediterranean which could cause unsettled conditions to arrive in the NW Med later in the week.

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There is some uncertainty as to how the end of next week and next weekend plays out for the UK and us in the SE once this cut-off feature moves off.  It seems likely that another depression with fronts could sweep across the N of the UK and bring more unsettled conditions towards the end of next week with fronts and rain for the NW.  But there are also signs of higher pressure building from the south again.  Either way, temperatures are going to be more averagely Autumnal and November-like, unlike the warm temperature tricks October has played.

A quick reminder that, unless November and December are markedly cooler (and the jury is out on this) then 2014 is STILL on target to become one of the warmest years on record for the UK and certainly one of the warmest.  Every month has been well above average temperature (see below, CET means Central England Temperature) except of course August which was the coldest for 21 years!  Despite this hiccup October looks to be warmer by over 1.5C and ending on a corker.  Nevertheless, perhaps celebration of such temperatures should be tempered by remembering that this kind of anomalous heat is exactly the kind climate change expected by IPCC predictions and, although anomalous warmth is pleasant for humans, it is just as stressful to wildlife and the environment as severe conditions such as storms and cold snaps.

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Finally, it’s worth noting recent press headlines regarding a possible COLD winter.  Such scare-mongering headlines in the Express and Star and similar papers are usually based on just one or two seasonal forecasts from a few meteorologists who might be described as on the fringes of mainstream weather forecasting.

Whilst RGSweather does not  write seasonal forecasts, it is worth sharing that some of the wider expert and reputable weather community, including both professionals and amateur, is pretty animated about the possibility of a colder than average 2014-2015 winter (that is of course Dec, Jan and Feb).  This is despite several of the standard weather models indicating a rather warm winter at this stage (including our own UK Met Office).  There is a lot of discussion about it.

Making a winter forecast is a complicated process because it is based on many interacting factors in the atmosphere, the cryosphere and the oceans which are combined together by expert forecasters to assess likely winter conditions.

Winter forecast indicators include factors such as the extent and build up of early Autumn snow cover over Siberia/Russia (encouraging HIGH pressure and cold easterlies), Arctic Ice cover (low ice cover warms Poles, increasing pressure, pushing cold air out to mid latitudes), the North Atlantic Oscillation and Arctic Oscillation (based on development of Azores High and Icelandic LOW) and associated amplification of the jetstream, how strong it is and how wiggly (meridional) or straight (zonal) it is, (a weak jetstream with -ve NAO allows polar air to leak out into mid latitudes), the strength of the polar vortex in the stratosphere as indicated by the an equatorial lower stratospheric wind pattern called the Quasi-biennial oscillation (QBO).  A westerly phased QBO is correlated with possible increase in likelihood of Sudden Stratospheric Warming episodes that have been linked to downwellling of cold air into mid latitudes some weeks after a SSW).  Phew, you can see there are loads of tricky indicators to watch.

This year, the October Pattern Index (OPI) is gaining attention as a possibly reliable predictor of winter outcomes.  Basically, the OPI is a clever measure of upper atmospheric conditions in October and particularly how amplified (or wiggly) the jetstream is looking.  Scientists have found that atmospheric conditions in October correlate well with winter outcomes.  A negative OPI, for example,  correlates amazingly well with a negative NAO (north atlantic oscillation) occurring later in the winter.  A negative NAO indicates a weak jetstream, itself possibly caused by low Arctic Sea Ice and high Siberian snow cover.  A negative NAO can, given the right synoptic pressure patterns (placing of HIGH and LOW pressure), be conducive to incursions of COLD air from the Poles reaching mid latitudes like the UK.  All this is experimental but extremely interesting and rather important given the rapid climate change going on in the Arctic.

Update!

Records were smashed today… warmest end to October on record at 24C

 

Reigate weather for next week to 10 days is overall set to calm down somewhat and, for us in the south, be generally drier and warmer than average for the time of year, most of the time. Temperatures are unseasonably warm over the next few days with night times barely falling below teens and daytime reaching 21C.  This is due to the southerly winds bringing warm air from Spain.

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A slack static cold front divides the really warm air over the SE from the cooler air to the NW. This cold front will bring cloud and rain at times across the SE, especially to the south coast, as it drifts SE over the weekend.  Winds on the coast will also be more noticeable over the weekend with 20mph+ possible, but staying mild over the weekend. The temps are likely to slip very slightly as the weak cold front edges SE on Sunday bringing some rain through the SE and more wind to the south coast.

Unfortunately, there is a hiccup to this generally benign warm weather. A gradual rise in pressure is set to be sharply upset temporarily by the remains of Hurricane Gonzalo, now battering Bermuda as a Cat 3 storm and due to arrive UK early next week late Mon /Tues.

Gonzalo hiccup

Gonzalo hiccup

Gonzalo is modelled to arrive early/mid next week.  Forecasts suggest Gonzalo will merge with the persistent Atlantic low near iceland and ride the jetstream across the Atlantic arriving late Monday as a 980mb low (not especially low) , bringing gales initially to the west and NW coasts and then gales to through the North Sea later the same day.  Some heavy rain is likely but the SE and Reigate looks currently likely to miss the worst.

After its passage across the north of the UK, the centre of Gonzalo appears to move SE down into the North Sea and behind a potentially vigorous cold front usher in significantly cooler NW winds of some potency, albeit briefly.  These appear to peg down temps a while, especially on NE coasts. Thereafter, indications are for a gradual improvement in the south as high pressure builds over the continent, albeit with some fronts reaching across to the south at times before a HIGH pressure seems to be suggested for the last week in October.

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If this comes off a dry and settled spell can be expected for half term.  The location of the HIGH starts in the south where mild conditions can be expected but the anticyclone could slip north and topple east.  if this happens then temps will fall as a cool easterly flow are brought in from a cooler continent creating a more foggy autumnal feel.  The charts below show the possible scenario for the last week in October and very start of November with things cooling off.

So in summary the weather for half term is generally settled, warmer and drier than average for the time of year but with a significant hiccup as Gonzalo arrive early next week bringing a plunge of cool polar maritime air down across the UK and the north sea behind this sharp active system.  Thereafter, a gradual improvement to the last week of October when an anticyclone is set to build across the UK bringing autumnal foggy end to October as temperatures drop to November.  Note that extra-tropical storms upset models so keep an eye on any changes to this forecast.

forecasters: SAC and Chris M

 

 

 

5,000 runners and thousands of spectators are expected to enjoy a fantastic day for the Run Reigate Half Marathon on Sunday 21 Sept in Priory Park. It is a long way ahead to forecast with any accuracy but the weather at the moment for Sunday 21 Sept suggest Reigate is maintaining warm temperatures (Tmax 22-25C), light winds with sunny spells but with possible scattered showers.  This week is warming up and could even reach a heat peak by Thurs/Friday of up to 25C but Sunday currently looks to cool slightly.

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Next Sunday is a long way ahead of course and things can change a lot so updates will be necessary.  In particular, Atlantic frontal systems are nearby but a re-building high pressure to the NE looks to keep them at bay for next weekend.  Do check back and follow twitter @RGSweather for updates. Also please note this is a local amateur educational weather station site so of course consult professional weather providers such as the UK Met Office for important decision-making purposes 😉 .  Here are synoptic details to explain more about what is likely to happen this week in the lead up to Run Reigate.

The current blocking omega high pressure that has brought such warm and settled weather over the last two weeks or so takes a bit of a nudge from the cut-off LOW near Iberia to the SW this week.  The chart above left shows pressure falling throughout this week to Friday and then steadying out but with a hint of a rise in pressure for Sunday.  Falling pressure in warm conditions means scattered showers become more likely. By mid-week the Fax met office chart below shows a warm easterly / SE flow across the South from a warm continent as a shallow trough extends across N France and the Channel with weak fronts to the south bringing a risk of showers that could reach S UK at times, though no drama is expected with a slack pressure pattern and the jetstream hundreds of miles to the north of the UK, well out of the way.

slack easterly flow mid-week

slack easterly flow mid-week

The charts below show the same LOW to the west of Portugal/Spain niggling away at the HIGH and moving a little closer to UK SW shores.  This is not indicative of a full breakdown, however, as the HIGH looks to build back next weekend, albeit to the NE and probably briefly before a possible collapse east thereafter as fronts and depressions eventually break through from the Atlantic, but that is all uncertain.  Next weekend could be Summer’s last stand!

As said, during mid-week the HIGH is forecast to edge further away to the NE causing pressure to fall in the SW and South, but not enough to allow a complete breakdown.  Lower pressure will increase the threat of showers that could push into the SW and south of the UK, especially Thurs/Fri, though details are sketchy on how far they reach east, especially in light winds and rainfall totals look small and patchy.  Reigate could stay dry the whole time in this scenario.  On the other hand, any showers that do perk up could potentially be thundery and quite heavy (apologies if this appears to be weather obfuscation!) . The most noticeable effect of the niggling Biscay located LOW is to build a slack warm flow through the week from the SE / S / E bringing in potentially warm / very warm temperatures from a hot continent for the time of year.  These could even reach mid to high 20’s at end of next week according to current charts, though this is the max likely and the weekend looks to cool down somewhat which is good news for runners.  France is set to hit 30C in places later this week if this pattern comes off.

Despite the scary looking rainfall charts above, the HIGH pressure should keep fronts at bay at least on current model runs for next weekend.  Note the rain spikes in the black chart above show that MEAN rainfall values (the white line at the bottom) are generally much less than 5mm which is hardly very much rain at all and certainly no wash out.  Also, take note of the overall conditions for the forthcoming week in the summary ensemble charts below: warmer than average temperatures and drier than usual, showing that any showers are likely to be scattered and not add up to significant totals.  The high CAPE values (convective available potential energy) give an indication of potential instability which could trigger sharp showers or thunderstorms but these are often over-done this far out and settle down much lower nearer the time.  Needs watching tho.

SO… for Run Reigate this is a good outlook for the time of year.  It is especially good as a breakdown in the weather has been repeatedly threatened for sometime but is still being effectively held off by the blocking High which models currently suggest will build back for a while next weekend.  Here’s hoping they stay on target with this idea!  Good luck runners… RGS will be out assisting, running and spectating.

 

Quick update #2 02/09/2014

Several models (GFS, GEM, ECM) are getting wobbly with the high pressure this weekend and bringing in a trough or upper low pressure down the North Sea over the weekend or up the English Channel early next week.  Either way such scenarios will bring less settled weather to the SE this weekend and early next week: cloudy, some showers, but probably light mostly.  This is not a collapse of HP (high pressure) because pressure remains generally high and the jetstream well to the north of the UK.  Most models also suggest a building back of pressure next week, probably further to the north leaving the south vulnerable to unsettled conditions at times.  Best of the weather by Week 2 of September is likely to be Scotland. Point to note is that UKMet models do not see such an unsettled weekend, so things not certain… but seems to hint at vulnerable nature of the anticylcone.  HIGH building back stronger than ever second week sept.  

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High pressure… getting the wobbles

 

 

Quick update #1 31/08/2014

More agreement amongst models as to firmness of the High pressure lasting into second week, to around 9/10 Sept. ECM latest run shows nice firm blob of HIGH NW of UK.  Only snag with this is IF the subglacial Bardarbunga caldera in Iceland decides to erupt it would be when upper air flow is northerly, direct to UK and Europe.  However, this is all speculative and of course the most unlucky scenario. Check this website for volcano updates http://www.ruv.is/ and the Icelandic Met Office http://en.vedur.is/earthquakes-and-volcanism/earthquakes/

Forecast:

Anticyclonic conditions are building over the UK in the coming week or so courtesy of ex-hurricane Cristobal which is tracking NE across Iceland this weekend. The ex-hurricane has morphed into a deep low pressure that is dragging warm tropical air behind it into the Mid-Atlantic thanks to the jetstream and this is helping to build high pressure across the Atlantic and over Scandinavia.

Ex-hurricane Cristobal leaves trail of construction

Ex-hurricane Cristobal leaves trail of construction

 

Whilst the black and white UKMet synoptic chart (below left) shows surface pressure, the colourful chart (below right) shows important UPPER air flow as well as surface pressure on a so-called “500hPa” chart.  Upper air charts often represent conditions at at 500hPa (hectopascal) and are commonly used by meteorologists to get a better idea of how the atmospheric conditions will develop over longer periods of time.  This is because upper air is less disturbed by surface features such as oceans and mountains and by the influence of day and night that can upset surface charts and make them awkward to interpret predictions over longer time periods.  Upper air charts most commonly show temperature at 500hPa, or 5km altitude, which is about “half way” up through the troposphere, well away from surface upsets.  Upper air temperatures usually correlate with surface pressure, but not always.  See below to compare 2 charts, one surface and one upper air, for the same time period.

Warm upper air, usually of tropical origin, is shown on the 500hPa charts in orange and red colours.  Cool polar air at height is shown in green and blue. Where the colours are tightly packed together it shows a steep temperature gradient and this is often associated with the jetstream.  Warmer air is less dense and, rather like fluffing-up a thick duvet,  takes up more “space” in the atmospheric column compared to the the thin “blanket” of cold Polar air.  As warm air increases the 500mb “height”, fluffing up the atmospheric duvet, more pressure is exerted on the surface which increases the air pressure.  Cloud formation is inhibited as air sinks, warms and dries out.  The HIGH being built early next week, initially over the Atlantic, is formed by a warm upper flow of Tropical air pumped into the Azores high by an upper SW jet following on the heels of ex-hurricane Cristobal.

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So HIGH pressure will bring settled weather across much of the UK, including here in Reigate and the SE, for about a week with some warm temperatures and light winds and generally pleasant weather with some sunshine.   However, not all models show a uniformly “clean” high i.e. an anticyclone that is free from cloud and blocking all incursions of cool or moist air.  Note the UKMET chart (up) shows some weak fronts wriggling across the HIGH and this could mean cloudy skies for some and even odd showers at times.  Rainfall charts also do not show completely dry weather throughout next week (see below).  The ECMWF model (above right) shows a weak trough / cut off low feature to the west of the UK mid-end of next week and eventually forming a low feature in Biscay.  If this scenario comes off it could temporarily spoil the HIGH and possibly send showers into the South as it drags in warmer humid air from a southerly direction later next week.  So the exact duration and nature of the high is still a little uncertain so check back and also check UKMO of course for updates.

 

 

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By later in the second week of September and beyond an active Atlantic with the possibility of more extra-tropical cyclones will probably start to breakdown any remaining high pressure. The chart above shows the chances of tropical cyclone formation (TCFP) in the Atlantic and, whilst % figures are low, this still represents a fair chance of cyclones forming and ultimately entering the mid-latitudes later in September.  So this might not be a durable blocking high or one that lasts.   Nevertheless, it is good enough to keep active Atlantic weather systems at bay for a week.

The positioning of the HIGH is also critical to how “warm” we will get in the UK.  This time of year the continent is still warm so high pressure over Scandinavia to the NE or East of the UK brings in some warm continental air on an easterly flow.  This is how the GFS and UKMO sees things developing initially  with a  high positioned over Scandinavia and Europe through mid-week.  This flow could yield temps by Thursday in the mid-20’s, up to 25c is possible for SE.  With sea surfaces at their warmest this time of year it could be a good time to head to the beach.  By next weekend most models (above) see some movement of the HIGH to the west and NW of the UK where it could introduce a cooler Atlantic flow from Iceland.  So peak temps are probably reached before the end of this week.

Further ahead several models suggest a breakdown mid-September to a more unsettled regime.  The CFS temperature chart shows a rather sudden decline Mid-Sept and then again in October.  Whilst  this is not surprising for the time of year, such steps in the CFS can indicate frontal systems and depressions as different air masses arrive across the UK.  Warmer than average Atlantic sea surface temperatures, in some places nearly 5c above normal, is likely to encourage the formation of active depressions as is usual for Autumn.

 

Update: BIG cool off likely from mid-week as high slips west and draws down COOL Northerlies / NE polar winds for E UK: Tmax drop from 19c to 10c by mid week! Matches Northern blocking suggested by NAO (see text below) Likely to bring first snow cover for continental Europe. ( check @RGSweather)

cool off mid week

The movement of this high has changed in latest models – update coming later:

Rainfall, or lack of it, is the theme for next week in / update: cool off is the theme now for Eastern UK, SE and in Reigate. Warm upper air flowing from the South and SW is building a blocking HIGH pressure which will sit to the South of the UK and then migrate WEST keeping rainfall and LOW pressure way out west in the Atlantic for much of the coming week. Tmax temps during the day could top 20c and night time temps could even stay in double figures.  Given these high nighttime figures and low dew points it is unlikely that fog will be much of problem with low humidity in the SE, unless the cloud clears at night and ground temps fall away. Is this an Indian Summer? Read here. http://www.bbc.co.uk/news/magazine-15127159

The NAO (north atlantic oscillation) is a measure of pressure difference between the Azores and Iceland.  If the pressure difference is high then it yields a positive NAO and this usually accelerates westerly winds and the jetstream and brings fast moving wet and warm (winter) Atlantic weather to the UK.  Negative NAO usually indicates a weak pressure difference, weak westerly winds and a blocking pattern with higher pressure than usual and slow blocked weather which sticks around for ages.  The weaker westerlies in mid-winter can allow cold polar air to flow out of the Arctic depending on the location of the HIGH. Slow moving conditions can mean extreme weather if a HIGH causes dry weather for weeks (drought) or a static slow moving LOW accumulates a lot of rainfall.  So negative NAO is not necessarily “good” weather.  For us, at present, we have a very negative NAO, one of the most negative NAO’s of this year but the ridge is bringing in a warm flow from the continent.

The longer range CFS chart shows a major gear change down to cooler conditions later in October. To be expected of course, but it looks rather a harsh step function in the weather.  It’s long range so things can change.
Enjoy the warm dry weather and Autumn sunshine but think of the USA today where Winter storm Atlas is bringing blizzards to S Dakota and Wyoming, Tropical Storm Karen is making landfall this weekend over Louisiana and Mississippi and, in between, there are tornado warnings in Iowa, Kansas and Nebraska. Update: severe tornadoes breaking out near Sioux City, NE Nebraska. Thoughts to people there. Stay safe.

Last Day of Summer 2013… Reigate and SE England enjoyed cloudless blue skies throughout the last day of 2013 meteorological summer today. Reigate reached Tmax 22ºC, was exceptionally sunny with no cloud all day. A light NW breeze meant shadows felt a little on the cool side, but this is just being picky. A very dry NW airmass aloft circling round the Azores High kept the sky cloud free. Relative humidity at 700hPa was 10% and at the surface Reigate had RH less than 40%.  

Is this the end of warm weather, no not at all!  Even warmer conditions are expected to build across the South through the coming week with Tmax perhaps reaching 28ºC by mid-week.  Thereafter, a cold front attached to a LOW could bring a fairly rapidly conclusion to our warmest weather with maybe some heavy showers and thundery rain before the end of the week.  Nevertheless, Autumn is typically characterised by fluctuating waves of alternating warm Tropical air battling it out with cool Polar air, so we can expect periodic flows of warm air at times through early Autumn.  The first big Autumn storm has yet to appear on the horizon, they are stuck across the Atlantic giving E Canada a soaking!  Our time will come, though 🙂

Meteorological (weather) Summer differs from astronomical Summer in that the weather seasons are simply broken into three month chunks: Summer is June, July and August; Autumn is September, October and November; Winter is December, January and February and Spring is March, April and May.

More on this here http://www.ncdc.noaa.gov/news/meteorological-versus-astronomical-summer%E2%80%94what%E2%80%99s-difference