Paul123

I would very much like to know it as well, what threshold is required. By the way, technically spoken it seems more appropriate to me to assign a temperature criterion to any warming event, including a SSW.

The warming you mention is showing more potential every day. Temperaturewise it looks great, but geopotentially it isn't good at all. At 30 hpa there is still no sign of the required zonal wind reversal within the next 10 days. Plenty of problems, indeed, to make it a proper SSW. In the meanwhile the sustained Atlantic warming may very well be going to have some easing effects on the tropospheric jet. Possibly a more southerly track, by the end of the coming week.

Okay, thank you. The point is that given the driving forces of the wave 2, the stratospheric forecasts quite a lot depend on the tropospheric output, more than usually appears to be the case. There is still quite a lot uncentainty in the strat. progs. No way clear, the size of the coming (ongoing) warming.

In order to better understand the mechanism of the stratospheric warming, I have made a sequence of this mornings EC Wave 2 (geopotential) forecasts (+24/48/96/144/198/240): It is obvious that the warming reaches its peak in about one week. Before that I notice a warming in the upper troposphere that lifts into the stratosphere.and consequentily joins with the initial upper stratosphere warming. I assume this is the process of wave breaking, that I often read about on this forum. (The troposphere geopotenial heights are driven by a (shallow) Atlantic Rossby wave.)

From the Judah Cohen blog: 'Possibly the most significant development so far this winter with hemispheric implications is the predicted breakup of the deep polar low in the Barents-Kara seas into two pieces, which will consolidate into the Hudson Bay and East Siberian lows. This projects more favorably onto the climatological wave pattern necessary for active wave driving that results in sudden stratospheric warmings (SSW). These conditions are favorable for increased high latitude blocking, a predominantly negative phase of the AO and an increased frequency of Arctic outbreaks for North America and Eurasia.' http://www.aer.com/science-research/climate-weather/arctic-oscillation The troposphere + 144h: Here we see something of a split. Hopefully it will be enough for a (minor) ssw. The stratosphere charts this morning are somewhat better again. Some significant warming forecasted again, can be totally different in the coming runs.

The split is also forecast in the netweather charts of today. Big change compared to the previous runs (except 18h yesterday). I suppose the change is due to a transistion in the forecast of the troposphere, from a strong PV to a disturbed pattern with a high developing over Scandinavia.

Looks as if this stratospheric warming from your 9 december post is going to affect the tropospere within the foreseeable future, particularly over Northern America. Less vorticity and more blocking in the coming week. (please correct me if I am wrong) [thank you chionominiac for answering my previous question]

I am new on this forum and don't understand much about stratospheric warmings and how they are induced. There are many questions that still remain, after reading many of the posts in this thread over the recent weeks. This is one I would very much like to have answered. The warming at the upper level of the stratospere, say 10 hpa or higher, is it induced by the building of a tropospheric Siberian High, in the coming days? In other words: does the troposphere have a direct impact on the upper stratosphere or is the overlap merely a coincidence?

At his blog http://www.aer.com/science-research/climate-weather/arctic-oscillation he gives his current view on the winter 2014/15. From his summary: 'Confidence in an average negative AO state for the upcoming winter remains high given the robust troposphere stratosphere coupling observed and predicted. The atmospheric response to snow cover so far is mostly consistent with high snow cover which favors increased energy transfer from the troposphere to the stratosphere followed by a weakening of the polar vortex.' He holds firm to his forecast. Judah Cohen goes for a really cold winter for the most of Europe. The reliability of his forecasting for Europe is shown at http://www.vwkweb.nl/index.php?page=Forum&articleid=138&reference=ref222926&threadpage=11, where hindcasts and forecasts from 2002 onwards have been posted (see also 'The Seasonal Forecast Thread', page 32).

This year July Sebastiaan and I contacted Judah for conducting an interview with him, in which he kindly agreed. The 22th of August we received the answers to the questions we submitted. The latter 4 questions have been posed (and answered) this thursday and friday respectively.

Thank you for your thorough in depth forecast. Slightly colder than average (1981-2010!); quite a customary winter, so it seems. Have to say I find the forecast rather disappointing temperaturewise, given the extreme geopotential height anomaly for Dec/Jan/Febr, positioned south of the country.

Judah Cohen still sees opportunities for the triggering of a significant if not major stratospheric warming within foreseeable future. From his blog: 'The latest GFS forecast does call for a new but weak pulse at the end of November and beginning of December. This WAFz needs to be watched closely as continued activity could trigger a significant if not major warming as early as December'. http://www.aer.com/science-research/climate-weather/arctic-oscillation This morning run of netweather is quite promising.

The Judah Cohen winter forecast has been issued at http://www.nsf.gov/news/special_reports/autumnwinter/predicts.jsp A really cold winter for most of Europe and the US. Take into account that the scale of the anomalies is being underestimated by the model. ' Predicted winter surface temperature anomalies for the Northern Hemisphere in Dec-Jan-Feb 2014/2015 in degrees Celsius. The model is forecasting cold for much of the Central and Eastern United States and Northern Eurasia, with warm in Western North America, Southern Europe and North Africa. The model uses October Siberian snow cover, sea level pressure anomalies, predicted El Nino/Southern Oscillation anomalies and observed September Arctic sea ice anomalies. The strongest signal in the model is the October Siberian snow cover, which is the second highest ever observed in the record. This is an indication of an increased probability of a weakened polar vortex or a sudden stratospheric warming and a predominantly negative Arctic Oscillation during the winter.' (Quote: Judah Cohen, AER, Inc.) More on this subject at his recently updated blog: http://www.aer.com/science-research/climate-weather/arctic-oscillation

EC starting to look wintry. High building over Scandinavia; easterlies dominant. It will take some time to get (really) frosty, but it is something to begin with.

I would like to post more about Judah Cohen, his winter forecast and some of my thoughts on the reliability of his (and other) winter forecasts. I am willing to start a new thread, or shall I continue on this one? Any suggestions? This is one of many hindcasts (and forecasts) of the AER, the office Judah Cohen is working for, that I would like to discuss. [16 November 2014] In the coming days I will show you the hindcasts, until 2010. From 2011 onwards it will be a number of real-time forecasts, issued in November. We received these hindcasts and forecasts from Judah Cohen, for an interview we conducted with him. They nicely illustrate the reliability of his winterforecasts. The winter forecasts of Judah Cohen are based on the SAI, the Snow Advance Index, as well as some other indicators. More on this subject is to be found at http://web.mit.edu/jlcohen/www/papers/CohenandJones_GRL11.pdf The hindcast for 2004 obviously is a failure, 2005 is much better, as are 2006, 2007 and 2008. The forecasts clearly show less amplitude compared to the real-time temperatures. That's because 'it is a statistical model and all statistical models by design under-predict the amplitude of the anomalies. That is because the model only captures a fraction of the variance and not 100% of the variance' (quote:Judah Cohen, email). 2009 Was really wrong for Europe; a cold winter overall and a warm forecast, based on a relatively low SAI (little snow-advance). The forecast for the UK was OK. The 1997-2013 SAI [data from Judah Cohen, email August 2014] Raw: [-0.41, -0.12, -0.68, 0.68, -0.07, 0.62, 0.16, -0.55, -0.37, -0.50, -1.44, -0.12, 2.30, -0.50, -0.17, 2.24, -1.07] De-trended: [-0.16, 0.10, -0.50, 0.83, 0.05, 0.72, 0.22, -0.52, -0.37, -0.53, -1.50, -0.21, 2.17, -0.66, -0.36, 2.02, -1.31] Understandably, the hindcast for 2010 is much better. The next four forecasts are real-time forecasts. They were issued after the 'discovery' of the SAI substantially improved the reliabilty of winter forecasting. For the past several years Judah Cohen has been publishing the winter forecast on the National Science Foundation (NSF) website: http://www.nsf.gov/news/special_reports/autumnwinter/predicts.jsp

Anyone interested in a weblog update from Judah Cohen? Visit http://www.aer.com/science-research/climate-weather/arctic-oscillation With Arctic Oscillation Anylysis and forecasts, from the near term to a longer term (30 days) forecast. A discussion about the SAI and its implications for the winter season 2014-2015. Judah anticipates a sudden stratospheric warming within the next two months and a prospensity for a negative AO.