Archives For December 2015

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Storm Frank 30/12/2015

Storm Frank brought gales and heavy rain and more flooding to the NW of Britain and especially for parts of Ireland.  It was perhaps fortunate that the storm did not cross directly over the UK but swept north towards Iceland and the Pole. The northward track was due to a developing blocking high of 1047mb over Scandinavia which directed the jetstream and attendant depression north.

The “cause” of Storm Frank was an increasing temperature gradient over the Atlantic which increased the power of the jetstream.

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Polar and Tropical airmass collision

Polar air pouring into the Atlantic from a cold Arctic Canada and Greenland met Tropical air moving up from the Gulf.  This contrast was increased by a cooler Mid-Atlantic and a warm Gulf Stream creating a large pressure difference.

The ingredients for a perfect storm were created as the mother of a jetstream embraced her baby Frank in the Mid-Atlantic.  Infant Frank lay underneath the left-exit of the jetstream and this caused surface pressure to “bomb” extremely rapidly, more than 24mb in 24 hours, creating an unusually low central pressure of 931mb on 30/12/2015.  This process is known as bomb-cyclogenesis and occurred with St Jude storm.

Storm Frank is not a record breaking storm in terms of low pressure. The lowest pressure recorded for a North Atlantic cyclone was 913mb in the Braer Storm in January 1993.  This storm skirted closer to NW Scotland.

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Sub-930mb storms in the Atlantic are not unprecedented, especially in winter.  Several storms have fallen below 930mb in the last 200 years, although only two have been recorded below 920mb.

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However, Storm Frank is unusual because, with assistance from the high pressure over Scandinavia, it has squeezed unusually warm air far north into the Arctic causing a “heat wave” over the North Pole.

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pressure LOW over Atlantic, blocking over Scandinavia

This process is called warm air advection and it has heated parts of the Pole to between 20 or even 30C above “normal”.

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Airmass temperature anomalies

Svalbard, usually around -14C in December, has risen to +8.1C and people in Longyearbyen are revelling in summer clothes outside in the Polar night.

Even the North Pole, usually about -30C in late December, is expected to “melt” for a time even rising above freezing for a short period.

This is probably only the second time this has happened on record.  It is especially remarkable considering, of course, that there is 24 hour darkness and the sun is not set to rise until March 21!  Any warmth is therefore entirely due to warm air advecting north on the back of Storm Frank.

With the HIGH over Scandinavia pushing cold easterlies into SE Europe Athens and Istanbul are expected to be colder than Longyearbyen in Svalbard. Infact it is snowing in Athens and Istanbul while it is now raining in Longyearbyen on Spitsbergen in the High Arctic.

Unfortunately the weather is set to continue stormy with more depressions arriving through New Year.  This time they might even impact the South of UK more with gales and heavy rain expected here too.  The temperature is set to cool to average so the anticipated cold lurking out east is not expected to arrive soon. Northern blocking is expected but this does not seem to be leading to any cold break-outs imminently. Nevertheless, the weather is telling some extraordinary stories at the moment.

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http://www.telegraph.co.uk/news/weather/12075282/North-Pole-temperatures-spike-above-freezing-as-Storm-Frank-sends-warm-air-north.html

http://mashable.com/2015/12/28/freak-atlantic-storm-uk-frank/#ZA9mGgejhaqi

https://www.washingtonpost.com/news/capital-weather-gang/wp/2015/12/30/freak-storm-has-pushed-north-pole-to-freezing-point-50-degrees-above-normal/

Winter is nigh?

December 29, 2015 — 6 Comments

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The synoptic situation above shows the remarkable contrast building up around Europe this week.  While much of the UK still sits in a mild SW flow brought in by a powerful SW jetstream, a HIGH pressure building over Scandinavia is set to drag in Siberian easterly winds to Eastern and SE Europe which is due to get much colder.

Whilst the jetstream is forecast to sink a little south of the UK by early January, which is the colder side, it is not certain whether any proper cold will reach the UK just yet. The UK looks increasingly sandwiched between bitterly cold easterly air and cool Atlantic NW air.  A stormy and wet set-up is likely with temperature contrasts like these, but will it snow?

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The ECMWF is certainly keen on something cold as pressure falls across the UK and Europe and allows the chance of LOWS ingesting nearby cold continental air.  This could mean snowy weather for the North, especially on hills, in early January, for example.

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However, for the SE Tmins stay above freezing on the chart below for nearby Gatwick, Surrey, and note the absence of snowfall, at least for now.

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While the ECM shows the potential for cold UK conditions by early January, the GFS maintains a more broadly westerly Atlantic flow with temperatures falling to average.

Both show a cooler regime and more persistent rainfall for us in the SE. A fall in pressure means LOWS will also make more direct passage across the UK during early January.

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The jetstream is shown to move south of the UK over the next few weeks. This will bring colder air across the country and lower pressure.

The terrible flooding in the North and West of England and Wales contrasts with the drier than normal December down here in the SE, running at about 70% of normal rainfall so far this month.  Unfortunately, a significant Atlantic storm, named Frank, is bombing-out right now in the Atlantic and is set to bring gales and more heavy rain to the NW, albeit not due to track directly across the UK.

Storm Frank will stay mostly in the Atlantic and arrive in Iceland on Wednesday night. It is unlikely to impact us much here in the south east other than some blustery and wet weather especially through Wednesday pm/evening as the cold front passes over.

Polar Maritime air behind this front will bring a cooler average feel by Thursday. Further wet and windy weather is likely later in the week and into New Year.  The arrival of this NW/westerly air probably spells an end to the extraordinarily warm long-fetch south westerlies that have made December 2015 by far the warmest on record. Whilst there will be warm sectors passing through the persistent warmth is less likely through January.

The wider atmospheric conditions hold more interesting clues than models as to which way the weather could proceed through January.  A more powerful than usual stratospheric vortex has built around the Pole this December. The extra-powerful vortex has possibly been caused by the excessive heat injected into the global atmosphere by the mega-El Nino: increasing the temperature gradient between mid-latitudes and the Pole and thereby increasing the strength of polar vortex as temperatures in the Polar stratosphere have fallen in the polar night.  Much catastrophic weather has been blamed on the El Nino “bar fire” burning across the Pacific.  Whilst Texas tornadoes and Pacific hurricanes are more likely to be directly linked to ENSO, UK flooding and weather has only tenuous links.  El Nino has now reached a peak but will continue to output through Spring until likely turning neutral and even reaching an opposite cool La Nina state by next winter. (more on El Nino impact on UK weather here )

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Mega El Nino December 2015

The powerful stratospheric vortex has “sealed in” cold air into the Pole throughout December (Globe a below).  However, the vortex has been taking a hammering from perturbations from the troposphere known as vertical waves (Globe b). If sufficiently powerful, these waves can lead to sudden warming in the stratosphere which can distort, split or even destroy the vortex, allowing cold Polar air to “escape” into mid-latitudes (Globe c).

Stratospheric experts differ somewhat in their forecasts but, overall, the feeling is that something is afoot high up which could erupt into a full break up of the vortex by Mid-January, something called a sudden stratospheric warming.  This what a SSW looks like in 3D.

A Sudden Stratospheric Warming could then mean, depending on how surface pressure patterns pan out, that some sustained proper cold arrives a few weeks later around late January into February.  This is due to pressure rising over the Poles as a result of warming in the Stratosphere. The increased pressure over the Poles further strengthens the negative Arctic Oscillation giving greater risk of cold outbreaks across mid-latitudes and snowy weather.  Even without a major warming, the super-powerful zonal westerly winds in the stratosphere appear to be abating (chart below: top arrows) due to the perturbation from the troposphere which is forecast to continue.  Whilst lower down the tropospheric jetstream nudges south of our latitude (lower black arrow) as anomalous or neutral winds expand out from 60N (red circles).  This means an increased possibility of cold leaks from the Pole reaching our latitude above 50N.

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Unfortunately, at the moment, none of this is catastrophic for the stratospheric vortex because, to date, the waves and warming have been insufficient to knock this King Vortex off his perch.  Nevertheless, El Nino years see a greater chance of SSWs and, as the westerly QBO weakens, it seems odds-on for such a full sudden stratospheric warming event.

Meanwhile, action nearer the surface in the troposphere (up to 10km) has been even more interesting with changes taking place that may render any stratospheric influence less relevant at least in the medium term. Notably, the Arctic and North Atlantic Oscillations are both trending negative.  This means pressure is building in the north relative to the mid-latitudes and could allow more polar air to push out into Europe. A cold outbreak is also possible in the USA.

Until now, December has been dominated by a positive Arctic Oscillation and positive North Atlantic Oscillation, hence the mild conditions.  A change to negative AO and NAO is therefore a significant indicator of cooler weather ahead.

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warming Pole negative AO

The chart above shows mean anomaly 2m temperatures for the 5 days 08-13 Jan 2016.  Note the extraordinary warmth over the Pole which helps to build pressure.  Note also the cold pool in the Atlantic, associated with cooler sea surface temperatures residing there.  This will be significant because NW polar maritime winds will be cooler than usual and could bring more snow to the NW and especially upland UK, even if we fail to get any truly bitter easterlies. Spot the cold lurking in the N/east of Europe and Russia waiting to pounce should we get a LOW sliding east through the Channel, for example!  Finally, the MJO is an important influence on winter outcomes worth exploring.

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The MJO or MOJO or Madden Julien Oscillation is a measure of convective activity which starts in the Indian Ocean and migrates east as a series of thunderstorms across to the Pacific and thence around the planet in a 30-60 day cycle. Here is a video about how the MJO impacts Australia, but it also impacts weather elsewhere.

The intensity and state of the MJO connects with global weather and correlates to known pressure patterns in the northern hemisphere. We have just left MJO Phase 5.  The expected mean pressure pattern associated with MJO Phase 5 (in ENSO positive phase) correlated pretty well with the pattern that turned out.  See charts below.

The MJO is now going through a nicely organised phase. This could mean that the correlation with real weather patterns continues into the weeks ahead.  Here below is the expected 500mb pressure pattern for Phase 6, which we are just entering and then Phase 7 and 8 which are due in early January.  Note the strong westerlies of Phase 6, which certainly equates with the current state of affairs, as does the building Scandinavian HIGH. This is not a cool phase for the UK.

Phase 7 sees a HIGH pressure building out from Scandinavia as a possible northern block.  This equates with the emerging negative NAO and potential easterly / Arctic winds winding round the base of the HIGH.  It is an increasingly cool phase for Europe, so matches expectations as we move into early January.  Phase 8 is a high pressure phase in Europe as the LOW moves further into the Mid-Atlantic.  High pressure can be cool dry frosty at the surface depending on the exact location of the high pressure.  This is expected by 13 Jan.

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Finally, Phase 1 and 2 (above) are both cool phases as they build Atlantic / Greenland blocks to the NW and place a trough over the UK dragging in potentially cold NE or NW. These MJO phases could weaken according to MJO forecasts but the signal is due later Jan/early Feb.  The MJO is just one teleconnection in winter weather forecasting it will be one to watch in the coming weeks and most interesting to see how it verifies with prevailing conditions.  Below are some links to explore the MJO yourself.

This is a round-up of the atmospheric situation and not a forecast. In summary, however:

  • models are struggling with all the action, ecm might be preferred as gfs does not take account of much vertical extent into the stratosphere, while ecm does. ecm is showing colder runs generally.
  • stratospheric vortex is taking a hammering and a SSW is predicted for January (AER)
  • QBO westerly regime is weakening somewhat, allowing more potential for cold.
  • MJO entering cool phases upcoming in January (esp Phase 7,1 and 2)
  • AO and NAO going negative which indicate blocking in North.
  • latest ensembles show high latitude blocking across Scandinavia to Greenland.

All the above give more likelihood of colder weather for the Northern Hemisphere, albeit not necessarily for the south of England!

http://www.americanwx.com/raleighwx/MJO/MJO.html

http://www.meteonetwork.it/models/mjo/

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/clivar_wh.shtm

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/

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November 2015 statistics for Reigate, Surrey

  • Temperature average 10C
  • Tmax 16.3C
  • Tmin -2.6C
  • Rainfall 66mm
  • Max gust 40mph
  • Sunshine 69 hours

In the UK November 2015 was the third warmest on record since 1910 and averaged 10C in Reigate with Tmax 16.3C and Tmin -2.6C.  The overall CET (Central England Temp) for November came out at 9.5C, nearly 3C above the long term average.

The month started with a trough in the Atlantic and HIGH pressure to the east bringing a mild wet southerly //SW flow to our area in the south.

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November 2015 mild SW flow

Rainfall was about average for our area at a total of 66mm for the month, though this is significantly drier than the MetOffice SE data shown below which shows the SE being wetter than average as a whole.

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South East rainfall Nov 2015

A number of named MetOffice storms impacted the UK but did not significantly affect the SE.  Ex-Tropical storm Kate swept unnoticed across the South on the 14/15 Nov and brought some blustery weather to the Downs.

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Pressure and wind gusts Reigate November 2015

Temperatures took a brief dip later in the month when a short-lived Arctic plunge brought brief wet snow which fell and settled momentarily in Reigate on the morning of 21 November.

November rain was about average for Reigate, though wetter in the NW.

November was the dullest ever on record especially here in the south.

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Some other pics of Reigate during November

Here is the MetOffice summary and blog records for the month

http://www.metoffice.gov.uk/climate/uk/summaries/2015/november

http://blog.metoffice.gov.uk/2015/11/

Wanstead Meteo

Over the next two weeks speculation on Christmas Day weather will inevitably build. Will Christmas be white or green?

Shoulder of Mutton lake, Wanstead PArk

At this range it is impossible to tell from standard meteorological models though from the 15th, 10 days before the big day and when models can start to be relied upon for at least a general trend, the pieces of the weather jigsaw will start to fall into place.

Latest odds offered by bookies William Hill for a single snowflake at London Heathrow are currently 5-1, slightly shorter than I’d expect at this time of year. With the predominance of the European high I’d expect those odds to start to come down.

In terms of proper snow falling and settling, there has not been a white Christmas in Wanstead for over 30 years. In 2010, we could still see the Christmas card Victorian snow scene in small patches if our gardens…

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