At this time of year increasingly thoughts draw to what may lay ahead in the upcoming winter months, will there be snow? With seemingly helpless task in finding direction in the atmospheric ‘chaos’ which never stands still, ebbs and flows in global circulation is which drives what weather we see. In this winter forecast blog post I’ll try make sense of multitude of teleconnections these list from: the ENSO state (El Nino and La Nina); the QBO (Quasi Biennial Oscillation); Stratospheric Developments: Solar Activity and more. Some of these drivers are more dominant to one another whereas some drivers can work destructively with another accessing their weight is a very difficult task but I'll give it a go.
Forecast summary:
A turbulent winter season with a highly meridional jet stream strong chance of significant cold and snow but also much milder/unsettled conditions.
Risk of significant disruption to Polar Vortex from mid Dec with ‘fallout’ seen in January possibly precipitating in much colder conditions. I am however expecting first cold spell later December but more intense cold spell in January. It is very unlikely to be continuous from December. I’d expect pattern to relax with westerlies taking hold and then for pattern to reamplify. As of now zonal winds at 10hPa are stronger than normal indicating a stronger than average stratospheric PV. Compelling signs into December for a strong weakening of zonal winds. Thus, continuing the disconnect with troposphere the lower atmosphere where weather happens, a strong PV often propagates vertically into troposphere; greatly increasing, risk of a stronger jet stream and milder conditions therefore these potential developments, make colder, blocked weather patterns more likely during the first half of winter. Late winter period in contrast to winter 2017-2018 expecting it to be relatively mild and this goes against grain of others winter forecasts and many seasonal models.
Temperatures; for winter 2018-2019 statistically unremarkable while tempted to go for an overall colder winter I do have hesitation with some variables such as Atlantic SSTs and W-QBO. However, do not let overall picture deceive you, these can hide extremes of significant cold in which I’d say is likely to feature and similarly much milder, wetter conditions.
December– near normal to touch above; very mild start offset by drier, possibly much colder second half possibly with the first snowfall esp. E courtesy of easterly winds.
January– coldest month of the winter, perhaps well below average long overdue.
February– near normal, cold weather is likely but possibly early spring like conditions.
Rainfall; likely to be above average across England and Wales this is needed following a very dry summer; owing to a southerly tracking jet stream, drier conditions possible for Scotland nearest to blocking highs and displaced storm track. Although I expect certainty won’t be immune to multiple bouts of wet and very windy weather.
Forecast drivers:
QBO
First of what is it? The QBO is in essence the directional flow of upper winds on fringe of atmosphere above equator which flow either westerly or easterly we pretty much know from November where QBO will be for winter making it a very useful winter forecast indicator of being constant. An easterly phase increases chance of cold weather patterns and increased chance of disruption to PV you may have heard about SSW (Sudden Stratospheric Warming) in the run up to ‘Beast From The East’ 2018, while westerly vice versa, strong jet & increased winter rainfall. Currently we are in descending negative phase (westerly) and through winter 2018-2019 this will only ‘strengthen’ – this could possibly mean a backloaded winter is unlikely contrary to last winter 2017-2018 which also featured an E-QBO.
ENSO state
This winter we face a weak El Nino a climate cycle in Pacific Ocean which covers 30% of Earth’s surface. As a result, its sea surface temperature (SST) pattern configuration, has a great influence on weather and climate around the world. During La Nina conditions we see colder than normal SSTs dominate equatorial parts of Pacific whereas in El Nino conditions which are being seen this winter vice versa. The ensemble guidance above are indicating a weak to moderate El Nino with magnitude peaking at roughly +1 to +1.5 there are indications El Nino will weaken even further towards spring, some word around Twitter is that El Nino has already peaked. It’s quite possible it will not be particularly influential this winter not being overly ‘strong’. In winter 2010-2011 in which saw the coldest December since 1890 we had a record strength La Nina historically this favours a cold start to winter with milder conditions seen late winter. Some argue it was this which led to that winter burning out. Presumably on the face of it the ENSO state this year favours a backloaded winter, although I am doubtful it will be strong enough to have much of an imprint on atmospheric circulation in UK and Europe this winter.
What is El Nino? This Met Office video is a good introduction.
https://youtu.be/WPA-KpldDVc
Solar Activity
There is emerging and recognised evidence that low solar activity years tend to correlate with more frequent high latitude blocking events and a more meridional jet stream for reasons not well understood. Thus, increasing the chance of cold blocked weather patterns. Currently we are at end of weakest solar cycle 24 and we are approaching the next solar minimum – the least active part of a solar cycle. What is very interesting the last time we had an inactive sun with El Nino conditions was in winter of 2009-2010 - a cold and snowy winter in UK, I do see this winter having some similarities. Low solar activity this winter and winters following 18/19 are going to be an exciting time for theory to be well tested in digital age.
Northern Hemisphere snowpack
Research studies suggest that a healthy snowpack in Northern Hemisphere during the autumn can be a valuable tool to predict subsequent winter with a feedback mechanism in HLB and cold and snow in Eurasia and U.S. Dr Judah Cohen theory looks particularly at region below 60N during month of October being the critical month, October 2018 featured a very slow build in snow cover however towards the end of the month the snowpack exploded. Therefore, giving mixed signals, currently Northern Hemispheric snowpack is above average, this is good for winter prospects in UK. A deeper, extensive snowpack results in formation of colder and denser air-masses through albedo, fresh snow cover can reflect as much as 90% of incoming solar radiation. Already I have noticed unusually low 850hPa temperatures given the time of year in NH. And as such the cold air ‘reservoir’ this winter may be colder than normal increasing risk of UK seeing significant cold and snow.
Conclusion
Unfortunately due to time constraints I have not been able to really scratch surface but I tried, a winter of discontent for 'coldies' I absolutely do not see, you may have got an impression from above I'm optimistic and indeed I am. With midwinter statistically coldest time of year my main area of interest, December should not be dismissed too for wintry weather. I hope you liked this winter forecast my first official one following a successful summer forecast, in experience these forecasts either go spectacularly wrong or right, no in between - will review later. Cheers :-)
Dan
A very detailed blog. So talented. You really know your stuff. Is it still a case of wait and see re Christma snow?