I know we aren’t done with November yet, but I wanted to build on the winter forecast.  Now I know there is a lot of talk going around on the weather boards about the models picking up on a very cold pattern for the eastern two-thirds of the United States going into December.  Even the long range models have switched the cold.  However, anyone who’s been keeping an eye on the developments of the stratosphere should not be surprised by this development of the model guidance.  

To recap what everyone is seeing now.  There is strong indication on the GFS Ensembles, GFS, Canadian, and ECWMF Ensembles of a strong -EPO, +PNA, -NAO pattern forming for early December, which will likely lock into a sustained pattern for much of the rest of December.  

The models are showing this out come because of what is happening at the stratosphere.  It was my hypothesis that the stratosphere will average near to slightly above normal with the potential for some warming episodes.  I did not and will not in this discussion forecast when and if major warming episodes will develop, I just don’t have the ability of that skill (yet).  However, I can say that the trend continues to support that the stratosphere is near average to slightly above average for November and going towards December.  Further, look at the location of the coldest temperatures being recorded in the stratosphere.  The coldest temperatures at 30 MB all occur over Asia, exactly opposite of last year, btw.  What does this tell us?

From research I’ve done and my own hypothesis, this stratospheric set up will support an environment favorable for a ridge over central/eastern Asia, which will lead to a negative EPO trough, which supports a +PNA pattern, and thus a trough over eastern North America.  The near normal stratospheric temperatures also lends support for favorable conditions of a negative NAO over time with building heights over Greenland.  The GFS ensembles has been showing this development in the long range for weeks, but has been too aggressive and frankly the stratospheric temperatures were just not in position to support a true negative NAO, but rather a north Atlantic ridge.  Now we are entering that period of development, just in time for early December.  So really, we shouldn’t be surprised by what the models are showing here.

My thoughts for December is the following.  The cold source over northern Canada is building with high temperatures in the -30′s F, so there is no doubt that cold air has been developing thanks to the strongly positive AO in October and the latter half of November.  The pattern will begin to develop next week with the developments in the Pacific leading to an amplification of the northern branch of the Polar Jet Stream.  I think the cold air will first impact the Plains and then the rest of the east for the first week of December.  I wouldn’t be surprised if the forecast area returns to near to slightly above normal for the first few days of December.  Then the bottom drops out for the next three weeks of December.  The pattern locks in, the cold arctic air remains on this side of the hemisphere, which will produce below normal temperatures over the forecast area.  

As far as the potential for storms.  I don’t buy the intensity of the NAO in the long range.  For the past several weeks we’ve seen long range guidance produce too strong of a NAO on either side of the scale.  This aspect of the forecast will be key in determining if the forecast area will have a cold/dry December or a cold/stormy December.  History tells us that a weak negative NAO is the best environment for winter storms for the forecast area.  The weak negative NAO does not suppress the storm track, but keeps cold air locked in over the coastal plain due to a strong high pressure system over southeastern Canada.  The strong negative NAO though suppresses the pattern, which snow lovers likely would not enjoy unless you are in the Great Lakes.  So the state of the NAO, once again, will be a key driver in the type of cold pattern we will see.  However, have no doubt, December will be cold.