Is It Going to Get Cold Again This Wintet
Pointers for making the Spring 2022 forecast
We end February with the legacy of an unusually strong polar vortex from winter. Each spring, the polar vortex (PV) slowly weakens as we head towards May, as the polar atmosphere warms up, before it disappears for the summer. Then it returns in the autumn – as the polar atmosphere cools down.
Recent GFS and ECMWF runs have forecast a rapid decrease in zonal winds in the stratosphere. The GFS even shows a brief split in the PV in the next few weeks, before it recovers and strengths again.
These sudden decreases in zonal winds aren't unusual, although it does not indicate a major sudden stratospheric warming. What goes on high up in the stratosphere isn't always mirrored down in the troposphere – where our weather happens. However, recent model forecasts do indeed mirror a similar pattern change in the troposphere, with a weakening of the recently strong tropospheric PV with increased cross polar ridging from northeastern Europe and from the north Pacific side over Alaska, leading to split of the polar vortex in the troposphere, with one vortex over Greenland the other over Siberia. The poleward and upward wave driving of the ridges from the Pacific and European sides of the northern hemisphere will likely be warming the lower stratosphere upwards – leading to a weakening of the zonal winds, putting pressure on the SPV and leading to the brief split. The ridging signal to build to the northeast of the UK, should it eventually form a blocking high, as some models runs and ensembles suggest, will keep the Atlantic storm track further away from the UK to the west and northwest during the first two weeks. Though as the polar vortex recovers as forecast, we may see more unsettled and perhaps even stormy conditions return by/from mid-month, for a time.
The Madden Julian Oscillation (MJO) is perhaps behind some of the tropospheric pattern changes, including buckling of the jet stream and tendency for ridging to the northeast over the next few weeks. Although it is wave of enhanced convection that circum-navigates the global tropics, it does impact the upper patterns at high latitudes of the northern hemisphere, so is a useful tool to use for predictions for 3-4 weeks ahead, but no more than that really. The MJO wave is forecast to move from the maritime continent (phases 4-5) towards the Western Pacific (phases 6-7) over the next 2 weeks on the VP200, though more recent RMM plots have tended to show a weak MJO moving into the 'circle of death'. A move of the MJO wave over the Western Pacific tends increase the chances of blocking in the northern hemisphere in March.
Looking further ahead into April and May, confidence decreases - with forcing on northern hemisphere patterns from the drivers such as the MJO, ENSO and stratosphere becoming far less easy to predict at range. So, the forecast will lean more on analogs of previous years with similar ENSO state, forcing from the tropical Pacific probably one of the main drivers that can be used as a predictive tool at range. So, I have relied on composites of years that were, like this year, a second consecutive year period (2020-21 and 2021-22) with a La Nina and similar with La Nina strength at the start of 2022.
Credit: Oregon Department of Agriculture (ODA)
Spring forecast by month:
March
For the first half of the month, an upper trough looks to extend south over the far North Atlantic in the means and will continue to be a presence close to the west of the UK through the first few weeks of March. However, it looks like its influence will be held back further west than we saw in February, thanks to increasing ridging over eastern and northeastern Europe. Frontal systems will try and push across the UK at times first week, bringing rain at times, but ridging looks to build across the UK over the first weekend before then building northeast over Scandinavia through the second week of March. The high to the northeast perhaps extending far enough west to allow a brief cold easterly flow over the UK, though confidence is 50:50 on this occurring or the Atlantic trough to the west taking control and bringing more unsettled and mild or very mild southerly or southwesterly winds out to mid-month, with frontal systems bringing rain, particularly to the north and west, the southeast drier.
Second half of March, a transition to more unsettled weather from the west, as the upper trough to the west moves further east and brings in a stronger and more zonal jet stream towards NW Europe. So, turning increasingly mobile with low pressure systems bringing unsettled conditions with winds from the west or southwest – so mild, perhaps briefly southerly bringing very mild air, rainfall amounts increasing across all parts too. Towards the end of the month, there is a signal for blocking to develop again. The block may develop to the north over Iceland, perhaps extending towards Scandinavia, with a southerly tracking jet stream bring lows across the south, perhaps with a brief cold spell, with an easterly flow, and a chance of some wintry precipitation, particularly over northern hills.
Overall, the month's temperatures around average to 1C above. Rainfall amounts around average, though with the northwest seeing above average rainfall and the southeast below average.
April
As we head into April, the polar vortex may have significantly weakened by then, the last ECMWF weeklies pointed at significant weakening, while the MJO may have moved through phases 8 and 1 – both of which tend promote a higher likelihood of high latitude blocking. These MJO phases in composites for April shows ridging over the mid-North Atlantic to Greenland and troughing over Scandinavia and Eastern Europe. This would bring cold northerly winds at times. The analog pattern of the 3 years which are the closest matches of La Nina to this stage in the year shows higher heights to the west and southwest over the Atlantic and troughing to the northeast.
April composite following second winter with La Nina
April composite for top 3 analog years with La Nina
So I'm leaning towards a below average first half the month temperature-wise, with cold northerly winds at times and with it showery weather, but generally fairly dry too, with some settled weather encroaching from the west at times. The MJO composites also matches with the April composite of years that are a second consecutive year period with a La Nina too.
As we enter the second half of the month and the MJO fades or moves through phases 2 and 3, then I think any blocking to the west and northwest may break down to allow troughing / low pressure to move into close to the west or northwest bringing a transitions to more unsettled and milder conditions, perhaps warm or very warm briefly late in the month. Temperatures overall for the month likely to be around average, rainfall slightly above average.
May
The MJO loses its usefulness as a predictive tool further away from winter, so by May, and given its too far away to predict where in the cycle it will be, it's not worthwhile considering. The polar vortex would likely have seen it final warming by May, and so we will see its demise until autumn, so will not be driving the weather patterns. So herein there is some difficulties in getting a sense of what might come this month. Recent ENSO forecast from the NOAA Climate Prediction Center is for La Nina to continue through to May, before reaching neutral conditions May to July. Using the second year in a La Nina composite for May, it suggests overall low pressure to the northwest and high pressure to the south and southeast. A warm pattern, perhaps unsettled at times.
Choosing the 3 of the years which are the closest matches of La Nina to this stage in the year has troughing over the Atlantic extending east across the UK, with higher heights to the north and northeast. This would suggest an unsettled westerly pattern.
May composite following second winter with La Nina
May composite for top 3 analog years with La Nina
I'm leaning heavily towards a warm month overall, with winds from the southwest or south for much of the time, but, unsettled at times. So, in contrast to May last year, which was the coldest since 1996, I think this May will be warm at times, perhaps with the odd hot spell later in the month, but there will be spells of unsettled weather, perhaps some thundery spells. Temperatures above average overall perhaps by up to 2C, average to slightly above average rainfall.
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Source: https://www.netweather.tv/weather-forecasts/uk/long-range/seasonal
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