0800 14 NOV 2011
If it wasn’t clear last week in the weather models… it certainly is clear now: there is little chance of a turn towards a significantly colder pattern over any portion of the CONUS before December 1. Again that in itself is not that unusual per se. But obviously as we move towards Thanksgiving and the last days of VOV there will be a big emphasis on whether or not we are going to see a significant large scale pattern change. Last December that is exactly what happened… but back in 2007-8 the pacific jet never let up and all of the DEC cold in the eastern CONUS forecasts busted horribly.
In looking at the “overall” pattern I do see some changes of current pattern. But with respect to affecting sensible weather conditions in the Conus…. there is not much to get excited about if you like cold weather. It is important that as a meteorologist we communicate in a clear and effective manner. So while I can talk about changes occurring in the overall pattern at the high latitudes which over the long term could be more positive towards getting a colder pattern… the fact is that for most people this will sound like a bunch of esoteric gobbledygook because their forecast shows more mild temps. From the average person point of view there appears to be no significant change in the overall pattern.
If we compare the current hemispheric 500 MB map as of 0z NOV 14 to the one from NOV 7 we do indeed see significant changes. The PV has split which is what most of the models were e forecasting and we now have one of the PV centers moving into far northwest portions of North America (the north slop of eastern Alaska/ Yukon). The other PV has retrograted and is now moving towards the north side of central Siberia on the arctic circle. And we also seeing the widespread introduction of arctic air across all of Central and Northern Canada which is something we did not see last week or the week before. This is leading to increase in snow cover across Canada as well.
Another key aspect to the overall pattern is the powerful European or Scandinavian Ridge. Last week most of the model data show this Ridge retrograting into Iceland or Greenland but clearly that is not happening on any the model data over the weekend or this morning and that’s one of the reasons why the pattern change at the high latitudes is not working its way down to the mid latitudes.
Part of the problem is that the new polar vortex over Northwest North America splits into two sections. The Eastern portions slides to Eastern Canada by the end of the week and moving towards Baffin island and the Davis straits by Day 7. This movement initially causes a moderate trough to develop over the eastern CONUS but once this feature moves into Eastern Canada were labrador in actually cuases the NAO to flip back to Neutral or Positive and it also kills any chance of the Scandinavian ridge retrograting to Iceland.
This is reflected in the the Teleconnections Indicies… http://raleighwx.americanwx.com/models/00zlinegraphs.html
However the western portion of the North American PV…. continues to move from the north slope of Alaska southeast into the gulf Alaska and along the Alaskan panhandle by day 3. This feature slides down towards Vancouver Island and the Pacific Northwest by day 7. It is a very intense vortex and if this was the middle of December there is no doubt that this feature bring a major snowstorm to locations such as Vancouver and Seattle. It still might give an how impressive this feature is. A piece of this energy breaks off and develops a significant surface Low that tracks of the western high plains this weekend with the arctic air pressing south across sea was Canada border into Montana and the Dakotas. This will probably be the first serious winter storm for the Upper Plains of the season — there was a moderate one last week .
By day 10 the operational European and the European ensemble show a dual center pattern with one PV over central Siberia and the other one located over northwest Canada –perhaps in the Yukon. The PV over the Yukon is clearly was stronger on most of the model data with its value below 500 DM . The Teleconnections from this intense vortex over far Northwest North America of course supports a strong southeast U.S. to ridge so all of the Model data here apears to be very consistent and meteorologically sound.
In addition if you take a look at the 500 MB height lines over the north Pacific you will see that the extremely strong central north Pacific positive height anomaly — the Ridge at 588dm– is located surprisingly close to the 498 dm very strong and negative height anomaly over the Yukon. This of course is causing the Pacific jet to become squeezed causing ncreased velocities to a extremely powerful 180 knots on the 0z GFS at 192 hrs. And having the jet streak slam into the West coast / br Columbia ensures the mean trough stays on the West coast and the the cold air building Canada cannot possiblly drop south of the US Canada Border .
Clearly the impediment in terms of getting the pattern to turn significantly colder is the extremely intense polar vortex over the Yukon. This fat pig has got to MOVE or slide southeast into north central Canada. As long as that feature is in that part of the western hemisphere the pattern is absolutely no chance of changing. This position for a PV has long been know as the Kiss of death if one is forecasting any sort of sustained colder outbreak were pattern developing over the eastern half of North America. Of course there have been plenty of winters where the vortex up there in northwest Canada in November has moved to the southeast which ends up altering the entire pattern. But as long as that feature is there and the 180 knot long jet streak …. the central and eastern CONUS stays Mild.