This weekend’s weather: Mild, highs in the 60s to near 70, perhaps a few showers (mostly sprinkles) but no widespread significant rain likely until Monday. Should be a good weekend for outdoor activities. Normal temperatures this time of year run mid 40s-near 50 highs and mid 20s-low 30s lows. Being early December, and with what may be coming down the pike, enjoy it!
When the North Atlantic Oscillation and Arctic Oscillation start tanking in tandem, I pay attention. Model forecasts on each show precipitous drops in the next several days. Almost inevitably, this means a colder weather pattern setting up for the Eastern U.S., as the negative phases of the NAO and AO signal high pressure building over Greenland and the North Pole, respectively, forcing Arctic air southward and buckling the jet stream southward into the Eastern U.S. Whether this means occasional shots of cold, a seasonably cold long-term pattern or something even more long-lasting and extreme depends on some details that can’t entirely be foreseen yet, such as exact placement of the high pressure systems causing the northern latitude “blocking.” A third oscillation we often watch in winter, the Pacific-North America pattern or PNA, is showing no signs of budging from its slighly negative phase, which works somewhat against colder weather parking over us with continued low-pressure in the Pacific Northwest and westerly to west-southwesterly upper-level flow across the U.S. Still, if the forecasts for the NAO and AO are correct in their deep nosedives, that level of high pressure blocking in the far northern latitudes will be likely to overcome the relatively weak PNA- pattern and allow more true shots of Arctic air farther south. December 2010 was a month with the same combination — NAO-, AO- and PNA- — that ended up as Roanoke’s fifth coldest December since records began in 1912 and produced a series of small snow events.
The first manifestation of this changing pattern in the northern latitudes will be pushes of Arctic air sliding southward into the central U.S., creating boundaries with the mild air that has dominated since the start of the month, leading to a series of low-pressure systems forming and moving northeastward. Expect these systems about every 3-5 days, starting with Monday and continuing perhaps to Christmas or beyond. The first will bring some rain on Monday (1/2 to 1 inch possible, per latest HPC map), then windy cold and some mountain snow showers by Tuesday. Each one will bring a little more cold farther southward and eastward, with mild weather in between — at least at first. If some of the longer-range models are right, cold air may be dispersed across the nation enough about 2 weeks down the road (Climate Prediction Center holding on to warmer-than-normal lean for most of next 2 weeks) that systems after that may have a chance to do something besides just rain on us. But we have several steps in the process to go before we get to that point, and it can still derail. I expect Monday’s “snow meter” will have pretty low numbers for next week.