GFS ensemble cools off
cool crash in pressure
ECM slides down
The ensemble charts above show that we can expect a week of change ahead. At the start of the week fronts will bring modest episodes of rain and swings in temperature to the SE followed by a notable change mid-week as both temperature and pressure fall (see charts above) with the possibility of snow for the SE. By next weekend there is a risk of some proper cold into the start of February. As usual Reigate and the SE will be sheltered from most of the action but the weather will take interesting and notable swings in a predominantly downward direction nonetheless. Models agree on how this transition will happen and it is largely based on the evolution of a LOW starting life off Canada near Newfoundland …. here are charts from the ECM that shows the story of our Canadian LOW and how it is likely to bring another taste of winter to the UK. (Update: worth explaining that the Canada LOW mentioned here is not the same as the Nor’Easter LOW that caused blizzards in NE USA Monday-Tuesday this week: it is a LP preceding it. If you look below at Chart 2 for 28 Jan you can spot the infamous Nor’Easter bashing the NE coast well modelled on this ECM chart from back at the weekend. The US Nor-Easter Blizzard2015 storm is responsible for building the Atlantic BLOCK helping to push Arctic winds our way but it is not travelling to the UK. The NYC storm looks to travel North up the Canadian coast, filling near S Greenland, unable to break through the Atlantic high pressure ridge extending north that it, in part, helped to build. Hope that helps!) In any case, the weather set-up gives Reigate another flaky chance of some snow.
1. The story starts now with the Azores HIGH pressure giving a dry and pleasant weekend for the SE, Saturday has seen brilliant blue skies as a result. The Azores HIGH has been dominating the Atlantic recently and our Canada LOW, deepening rapidly off Newfoundland on Monday, will be forced round the HIGH to the north west to Greenland. This LOW will deepen rapidly because of the great temperature difference between frigid air pouring off Canada and humid sub-tropical air fed up from the south courtesy of the Azores high.
For us in Reigate this period sees a ridge of the Azores high pressure ebb away slightly during Sunday and this will nudge a mild SW flow to raise temperatures temporarily high into the UK and SE overnight into Monday morning. This minor weather episode (marked by the ups and downs in temperature on the 850hPa ensemble chart top of page) will be heralded by increasing cloud on Sunday as fronts bring some patchy rain in on a warm front overnight into Monday. Polar air will follow a cold front later on Monday and a ridge of high pressure will build quickly overnight turning the winds into the NW with a dry chilly night in store for us in Reigate and SE into Tuesday.
2. Later on Tuesday the Canada LOW is forecast to move NW to near southern Greenland where it will feed on a brisk northerly wind of freezing cold air from the Greenland ice cap. This freezing cold air will create a vigorous cold front that will reach the UK early on Wednesday. This active cold front will usher in an unusually cold Polar Maritime NW’ly wind across the country reaching the SE late Wednesday with the possibility of heavier rain or even snow. One to watch carefully.
MetOffice chart cold front
Atlantic COOL POOL
snow showers across the UK
ice sheets to high streets
proper cold front Weds?
A point to note from the charts above is that the central Atlantic is anomalously COLD at the moment and so the brisk NW’ly wind will not warm up as much as usual on its journey over the Atlantic to the UK, increasing the likelihood that it will bring wintry stormy gales to the NW, with some models even showing wintry precipitation for us in the SE too, perhaps reaching us by Thursday, though these are likely to be sparse unless the cold front stalls in which case more significant falls are possible. (Update: GFS 18Z suggests this cold front could be active and bring snow across entire UK in its wake…)
NW airflow 2m temperature anomaly
Check the temperature anomaly chart above which shows how unusually cold this NW’ly wind is going to be. Usually NW winds do not impact SE England with snow unless fronts stall or there is an especially active undercut of unstable cold polar air.
3. Our Canada LOW that started life just off shore from Newfoundland is forecast to cross Iceland mid-week and then slide SE into the North Sea, around the blocking high extending north through the Atlantic to meet rising pressure in Greenland, a good scenario for a COLD Europe! As the LOW transits SE into the North Sea it will bring down Polar and then Arctic northerly winds on its back and snow for the North, NW and NE coasts. Arctic air rarely brings snow to Surrey or Reigate … it usually runs out of moisture and lift on its transit across the cool land and instead the south and SE usually gets azure blue skies with frosty nights. NE and E coasts can get snow showers. Daytime temperatures everywhere may well stay near freezing if this comes off.
It’s too far ahead for any detail but some model runs show Polar LOWS sliding round the edge of the trough on the left exit side of the jetstream (where lows develop rapidly) as the trough moves east . These daughter lows can bring snow to the SE but this is too far off to be certain.
NAO positive is not cool
AO briefly negative
So how long might this cold snap last?
There are indications both for and against a more prolonged cold snap but, on balance, the peak of any Arctic cold looks likely to be shortlived as the high topples east, ebbs south and allows gradually more westerly winds back across the UK. For enduring cold we really need the LOW to move south into Europe and pressure to build to the north bringing in a blocked situation allowing cold easterlies into the UK (beast from the east). However, this scenario looks unlikely because the Azores HIGH is likely to remain relatively dominant. This is shown by the generally positive North Atlantic Oscillation chart above.
North Atlantic Oscillation
The NAO is a forecast measure of the sea level pressure difference between Iceland and the Azores. When the NAO is positive it usually indicates a strong pressure difference with a big HIGH pressure over the Azores and a LOW over Iceland. A positive NAO correlates with a fast zonal westerly jetstream and mild wet winters for the UK. A negative NAO indicates colder winters in which pressure rises to the north (Iceland) and allows easterly or NE winds to bring cold airmasses from the Arctic or more commonly Siberia into the UK.
The Arctic Oscillation (AO) is a related cousin of the NAO. The Arctic Oscillation is a bigger scale comparison of pressure at the Pole compared with that in the mid-latitudes. A positive AO usually correlates with lower than normal pressure over the Poles compared to the higher pressure further south in the Atlantic. This brings mild wet conditions to Europe (zonal westerly winds). A negative AO sees pressure rise higher over the North Pole and this acts like a balloon to push polar air out into mid-latitudes… a cold winter scenario. The AO forecast shows it going negative by early February and this correlates with the cold snap. Unfortunately it looks like the AO will go positive again thereafter but this is too far off to be certain. On the other hand…. !!
ECM forecast pressure anomaly
GFS forecast pressure anomaly
The charts above show a Polar view of the Northern Hemisphere. They show that pressure is forecast to rise over the Pole disturbing the zonal westerly flow of the polar vortex by a displacement of the polar vortex away from the Pole (see below). This might increase the likelihood of a prolonged cold plunge of Arctic air reaching the mid latitudes including the UK This is good for cold weather enthusiasts in Europe!
The other longer-term chart shown above in favour of a cold late winter is a sudden stratospheric warming forecast in early February. A SSW can lead to pressure rising over the Polar regions a few weeks later, disrupting the upper westerly winds, potentially upsetting the jetstream and bringing cold to mid-latitudes. This is well correlated and was significant in bringing a late winter in 2013. So… much of interest at the moment as we enter the final third of winter 2014-2015.
note: for all decision making purposes and forecasts please consult professional agencies: e.g. MetOffice at http://www.metoffice.gov.uk/public/weather/forecast/gcpg7rs0t#?tab=fiveDay&fcTime=1422057600