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

6 responses to Winter is nigh?

  1. 

    A superb article-put you on my favourites page.
    Thought the ‘r’ in rgs stood for Reading and you had a mole in the ECMWF!

Trackbacks and Pingbacks:

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