November has been a month that has featured a variety of weather conditions for the Tri-Cities, from all-time record heat early in the month, to an extended period of unseasonably cold temperatures later in the month. The month overall finished with temperatures around 0-1° of average for a mean in our region. What about December? Let’s take a look.
Pacific Will Dominate Pattern First Week Of December:
We’re in a pattern currently that’s dominated more by the Pacific. It is a common element of La Niña winters for the Pacific North American (PNA) teleconnection pattern to be negative; favoring deep troughs that buckle the jet stream along Western North America and encourage more of a ridge along areas east of the Rockies, and particularly east of the Mississippi. This ridge tends to favor milder temperatures, as we’ve experienced the last several days since Thanksgiving. Occasionally, some of those troughs in the Western US will traverse eastward and slice through the ridge. It’s not a particularly frigid pattern, nor is it conducive to any wintry precipitation worthy of mention, but a brief period of seasonable cold may occur in transience, and that is what we are seeing now for the first day of the month. Already by the weekend, we are seeing reason to believe temperatures will moderate again. This up and down pattern with warm-ups and brief bland cool spells is something we may see predominantly through the first week of the month as long as the Pacific continues to drive troughs into the West Coast and a ridge over the Eastern US. But what about after that?
Blocking Pattern And What Does It Mean For Our Area?
Something particularly interesting begins to happen over the new few days across other regions of the Northern Hemisphere on the Atlantic side. A blocking pattern develops that may have an important impact on our weather towards the middle of the month. The Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) are both projected to turn negative, and substantially negative at that. When the AO turns negative, high pressure begins to build around the polar regions; causing colder and stormier conditions to favor lower latitudes. When the NAO turns negative, a similar influence to the pattern is noted as high pressure begins to build over areas such as Greenland, and Baffin Island in Canada. This also favors a displacement of colder and stormier conditions to lower latitudes, and tends to lead to more frequency of winter storm systems in the Eastern US, as the jet stream buckles and begins to slow down. These two indices heading into negative territory are often associated with better chances of snow in our region, and it doesn’t take long to put two and two together when you realize significant snowstorms have been less frequent in the last few winters, and have become more anomalous as these indices seldom ever venture into negative territory. Does it feel like we’ve had a lot of “thread the needle” winter weather events in the last few winters? It’s because we have.
When the PNA is negative in tandem with a negative NAO and AO, there is a ridge over the Eastern US; encouraging milder temperatures. However, if the the NAO and AO are negative enough, they will begin to allow troughs to squash that ridge over the Eastern US. If the PNA should relax closer to more neutral levels or even slightly positive, the eastern ridging fades and things get very cold and very stormy. This is something we are watching for towards the middle of the month. The troughs keep barreling into the West Coast, but you’re beginning to get some ridging according to our computer ensemble guidance . This allows these troughs to progress eastward in a fashion that may allow them to travel further south and wrap in colder air as the blocking pattern, courtesy of the negative NAO and AO, begins to take shape. This means that we will need to keep a close eye on individual storm systems in the second and third weeks of the month, and potentially beyond, depending on how long this blocking pattern persists. Some of these may of course be merely rain, but this type of pattern leads us to believe the Tri-Cities region may be at a greater risk of snowfall(s) and/or a legitimate winter storm(s) around the middle of the month. That is by no means a guarantee at this time and may fail to verify, but it is a pattern that needs to be watched carefully. The one possible caveat to the pattern would be if the PNA behaves so stubbornly negative, that ridging lessens the effect of the blocking pattern and therefore lessens the risk of wintry weather.
Preliminary Thoughts On Christmas Into New Years:
Going into the holidays, computer modeling projects we may still be dealing with similar ideas in terms of a potential blocking pattern and the question of how counteracted it is by an eastern ridge. Long range model guidance favors the period leading up to Christmas week and New Years to be cold and stormy, but that is a low confidence forecast right now, and just what computer models indicate.
There’s no way to pinpoint a White Christmas or lack thereof this far in advance, but the odds as it looks now are greater than last year when our weather pattern was not the least bit conducive to snowfall. Normal years lend around a 10% chance or less in our region, but it does happen every once in awhile. The years of 2020 and 2010 are the most recent years to deliver a White Christmas to our region. It just so happens there are a lot of similarities to what we’re looking at for the long range forecast to what happened in 2010, so that is something to keep in mind, but we simply cannot speak definitively on that at this time! Make sure to stay tuned!
What Is The Bottom Line:
• Variability in temperatures and overall conditions is expected in the first week as the Pacific rules the overall weather pattern.
• Second and third weeks of the month may feature blocking pattern with colder and stormier conditions and greater risk of wintry weather as long as the Pacific indices cooperate.
• This may continue into the holiday week, but our forecast confidence is low and very subject to change, depending on longevity of the blocking pattern and other variables.
Stay tuned!
StormTrackerSacher
Assistant Forecaster TWAC2