Folks, its hard to believe that we are in the last days of Meteorological Autumn but indeed we are. The calendar flips over to December on Friday and with the start of Meteorological Winter we begin to look at the weather pattern and what could be coming down the road as we get into the last month of 2017 and the holiday season.
We have to admit that current computer model guidance looks very intriguing across a good part of the United States including here in the Western Carolinas. First off all you need to know about the current weather pattern lies in the jet stream flow.
If you look at the image below all of the Continental US is being dominated by a very strong Pacific Jet Stream that is moving from west to east. We are in one of the more impressive zonal flow regimes in recent history. This all started back toward the end of October once the Atlantic Hurricane activity began to slow down. As you can see even with a significant blocking signature across the Northern Atlantic, cold air has been very hard to sustain itself across the US due to the uninterrupted flow from the Pacific going all the way back to Asia.
This zonal jet regime will remain through the first half of next week allowing the country to remain in a relative mild and dry weather pattern. However all signs are heading to a dramatic reversal in the Pacific as we get about 7 days out from now. A more amplified pattern will develop in the Pacific with a trough several hundred miles off the West Coast, that will in response pump ridging in the Western United States. The Pacific jet while converted to this pattern will also split somewhat with one stream following the ridge will the other slides under the ridge…creating a split-flow type setup into the US.
The initial result of this change is a shift to much colder weather as the jet will be diving down from the northwest. Our part of the country will begin to see this change in the 7-10 day range, models already show a stout cold front sliding through around that Day 7-8 range bringing us a chance as stormy weather followed by a significant cool down.
As you can see in the images below the Pacific ridge combined with continued blocking in the Atlantic will work in tandem for this change in the pattern. This is a great example for those who follow Southeast US weather of how the tele-connections out west mean as much if not more than what we have going on in the Atlantic.
Speaking of tele-connections… both the Pacific-North America (PNA) and North Atlantic Oscillation (NAO) once they come in place in the 7-10 Day range could sustain itself for a good while. For a colder pattern you want the PNA to be in a positive phase while the NAO needs to remain neutral to negative…and indeed both of these look to evolve in that fashion.
The +PNA look as if it may reach its climax somewhere in the mid-month timeframe, there are some indications on the model ensembles ridging may connect across the Pole and shift the majority of the available cold air away from Siberia and the Pole into North America. This is the kind of weather pattern that can lead to arctic outbreaks in the month of December across the Eastern half of the US.
By the time we get to the middle part of December with this kind of jet stream pattern all options are on the table for weather possibilities thereafter. Does that southern split in the Pacific jet try to activate a Southern storm track later on in the month? That’s way too early to predict with accuracy but its something that we will have to examine once we get through the initial stages of this pattern change.
Needless to say December looks very interesting across the US as temperatures will begin at or above average the first few days of the month before a quick switch to below normal temperatures. Just for reference, average temperatures for the first week of December in the Piedmont and Foothills are in the mid 50’s during the day and low 30’s at night. As you can see of the ensembles we go significantly below normal by late next week.