Many of you, unless you are lucky enough to have sufficient elevation, have probably not seen a flake of snow so far this winter. Or if you have, been disappointed that it has struggled to settle for long enough to warrant getting the sledge out. Many areas of lowland Britain, certainly south of Scotland, are still waiting for their first flake.
So why has this winter been so snowless, so far? And what are the prospects as we head into the last month of meteorogical winter?
To answer the first question, you may probably be aware now that the persistent deep pool of cold air over eastern Canada, a segment of the Polar Vortex, has brought unusually persistent and brutal cold to eastern Canada and northern/northeastern states of the US. This very cold air colliding with warm moist air rising north from Florida and the Caribbean produces a steep thermal gradient, creating a strong and unusually persistent westerly jet stream across the North Atlantic , spawning depression after depression – its destination northwest Europe. This has meant that our wind has predominantly blown from the southwest – a mild and rather moist direction, hence the relentless rain over the last few months. So it really has been a bad winter thus far for cold synoptic patterns to develop over our part of the Northern Hemisphere, even most of the milder winter sof the 2000s and 1990s contained cold snaps that delivered snow to most parts, but not this one so far.
The persistence of the deep wedge of cold air over E Canada and Greenland driving a strong jet and succession of lows towards the UK is all linked to wider picture of slow-moving or stagnant weather patterns across the Northern Hemisphere this winter, particularly apparent by the extremes of weather being experienced across North America and NW Europe so far. A stubborn ridge of high pressure affecting America’s west coast has brought increasingly extreme drought conditions to SW USA, with the winter rains that normally bring most of California’s annual rainfall failing this winter, due to the jet stream being much further north than usual. Then downstream we have seen stubborn troughing in the jet stream over the eastern North America, courtesy of chunks of the polar vortex continually dropping south, keeping the jet stream and very cold air south here. Then the downstream impact of this deep cold air creating a steep temp gradient over the NW Atlantic driving a persistently strong jet towards the UK. Now this month we have the addition of blocking high pressure to the east, so the even though the zonality (west to east movement of depressions) has stopped, the UK still remains at the end of a strong Atlantic jet. So lows pile in and stall against the eastern block over the UK, This morning’s model output runs, like their predecessors, continue the ominously unsettled and wet theme beyond the cold snap developing tomorrow and through Thursday. These wave ridge wave patterns have been and continue to be very slow to change so will unfortunately may continue the extremes of rather different kinds of weather over N America and western Europe.
So in answer to the second question, are there any signs of change? Well, often it is best to look for these changes upstream over N America and even the North Pacific. Looking at some of the teleconnection patterns (large scale weather patterns and circulations around the world) and Stratosphere forecasts that long range forecasters like to use, there appears little impetus for the status quo to change anytime soon. Least not in the next few weeks, which will take us up to mid-February. The Monday update of the US Climate Prediction Center's Madden–Julian oscillation (MJO) Forecast, an intra seasonal tropical circulation which can affect large scale circulations and weather patterns upstream over North America, shows the MJO to stay weak and incoherent, with little eastward propagation from most model guidance over next 1-2 weeks, so not likely to have huge ramifications on weather patterns for now. The Pacific-North American (PNA) teleconnection pattern, which is an indication of the large-scale weather patterns over the North Pacific and North America, has positive and negative phases, and has been in +ve phase recently, meaning persistent ridging over the American west and troughing down stream over Ern N America. The PNA looks to head into -ve territory, so indication of a break down of the NE Pacific ridge.
But closer to home, the North Atlantic Oscillation (NAO) - which in the positive phase indicates lower pressure over Iceland and higher pressure over Portugal (mild and unsettled for UK), looks like remaining +ve, thanks to deep cold air remaining over eastern Canada and Greenland. Often it’s worth a look at the Arctic Oscillation (AO) too, a negative phase indicates higher pressure and blocking towards the arctic which means the jet stream and colder air sometimes moves further south over Europe as the cold air over the arctic is forced south by high latitude blocking. But looking at the forecast pressure and height anomalies over the Arctic, the
AO looks to bee headed to +ve territory too, so the cold air remaining up here, at least as far as NW Europe is concerned.
With regards to the stratosphere, warming of the stratosphere at higher latitudes can have large impacts on weather patterns in the northern hemisphere in winter. If a warming in the stratosphere propagates down to the troposphere, where our weather is, it can lead to blocking and cold conditions for the UK. Currently there is no strong signal for any strat warming having any impact on northern hemisphere weather patterns. So no positive signals for an end to the stalemate between the Atlantic and eastern block, with the UK kept out of reach of sustained cold affecting eastern Europe and Russia. The continuing strong flat jet coming across the Atlantic showing run after run of the models is testament to the lack of impetus for change.
The cold snap developing tomorrow may bring some snow flakes for some in lowland Britain later tomorrow and Thursday, as the cold easterly develops off a cold eastern Europe. Risk of snow more particularly for higher ground on Wednesday and lower levels along the east coast of Britain on Thursday where we could see a few flurries, but many probably will be left waiting for the next opportunity.
We can't rule out colder and perhaps wintry conditions later in February and indeed early March, when we can still see lying snow and sub-zero temperatures. Indeed blocking is more likely to occur in late February and early March when the Atlantic has been so persistent during December and January. So, perhaps still too early to throw the towel in for snowy prospects, if you include early Spring.
