‘SSW’ rumours and what it means for UK

Over the past few days there has been a lot of speculation on the possibility of a sudden stratospheric warming event happening. These events can contribute to unseasonably cold snaps in winter though exactly where their effects happen on the globe is impossible to forecast at small scale resolution. ssw

In short it looks like the eastern US is most likely to see any severe cold weather from this episode – what seems to happen a lot in recent winters in the UK.

Far from being an expert on this there was a very good posting on the usenet forum uk.sci.weather by Stephen Davenport.

“As far as the stratospheric warming is concerned, we’ve been keeping track of this for the last 1-2 weeks. Models have recently been consistent on this being a strong warming (10hPa 60-90 deg N zonal temperatures rising above 240K – see http://acdb-ext.gsfc.nasa.gov/Data_services/met/metdata/annual/merra/t60_90n_10_2015_merra.pdf).

“A temporary wind reversal is likely in the upper stratosphere; for example, see ECMWF at http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/, and have a look at the off-the-chart EPV (although not so much poleward). However, I do not see this as a major SSW (by definition) and everything points to a recovering but weaker circumpolar vortex mid-month onwards.

“Meanwhile, though, the vortex is being shunted off the pole by warming and big height rises firstly over Siberia then the Bering Sea, with exaggerated deformation which will see it extend far south across eastern North America and introduce cross-polar flow (which you can see here: http://www.tropicaltidbits.com/analysis/models/?model=gfs&region=nhem&pkg=Tz10&runtime=2016020412&fh=132&xpos=0&ypos=255).

“At 500hPa a very deep trough digs southwards east of the Mississippi as far as the Gulf of Mexico. GFS shows a -3 to -4 sigma anomaly over the Southeast with ECMWF similar but a little less deep: http://www.tropicaltidbits.com/analysis/models/?model=gfs&region=eus&pkg=z500a_sd&runtime=2016020412&fh=120&xpos=0&ypos=155

“That opens the door to Arctic air pouring far southwards, and there’ll be snow chances at least as far south as the Tennessee Valley. Incidentally there is a risk of another Nor’easter around Feb 9th-10th.

“Downstream a mid-Atlantic ridge could build rather strongly from the subtropical high towards Greenland with slowed zonal flow but most likely surface low pressure developments eastern Atlantic / near the British Isles.

“If the long wave pattern shifts a little eastwards then the increased meridionality *could* see LP progression pull in temporary subsequent N-NW flows for the UK – a more likely route to short-lived cold shots than via any high latitude blocking in the medium range.

“I think that elsewhere people have got hung up on the stratospheric warming and, as so often, drawn excitable conclusions. Cold impacts are more likely for the eastern U.S. than Europe with a displaced rather than a split vortex; and the circumpolar vortex was so strong first half of winter it was always going to take a lot to break it down fully. And sure enough, as noted there should be a recovery after this “attack”. You can see the vortex distortion and latterly the beginnings of recovery in this rather nice animation of 10hPa potential vorticity:


“And here are some 3D representations:


“GFS has occasionally pulled out a split vortex at various levels during the past week but doesn’t now.

“That’s not to say that a possible further warming (early final warming?) wouldn’t bring colder chances from the end of February into March.

“I was surprised to read recently that a well-known online forecaster had predicted significantly cold/blocked conditions for Europe in January via a major SSW. A bit premature, that.”


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