Thread by @corbydickerson

Last week bitterly cold air developed over east central Siberia, the likes of which we haven't seen in 40 years this early in December. It was chronicled very well by @ThierryGooseBC on December 6.



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Ever since then the weather prediction models have been showing some crazy, downright silly numbers across Canada & the United States. I'm talking record high Mean Sea Level Pressure, both for December & ALL TIME. Here are those MSLP records from @NWSWPC researchers.

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For those not used to reading these maps, the values shown are the last three digits of the MSLP. For example, in #Missoula the all-time highest surface pressure observed was 1059.9mb on January 1, 1979. The record for December was 1054.5mb on #ChristmasEve, 1983.

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So why is this important? The measure of MSLP is used to understand the strength of weather systems. From lows (hurricanes & mid-latitude cyclones) to highs (summer heat waves, bitterly cold temps in the winter), the measure of MSLP is a key part of the forecast process.

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In this case, extreme high pressure tells us the air is not just polar, but arctic; originating from the coldest parts of the extreme nrn latitudes. The air that was formerly pooled in Siberia has crossed the North Pole 🎅 & now over eastern Alaska, Yukon, & far northern BC.

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Over the course of this week the cold, dense air will gradually seep south across western Canada, eventually spilling into the Northern Plains by Tuesday/Wednesday. But the most extreme, a tidal wave of cold, punches into the Northern Rockies by Thursday morning.

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In fact, that extreme cold tsunami could drive an intense cold front across a significant part of the USA, taking below freezing 🥶 temps to the Gulf Coast & maybe central Florida! Not to mention the possibility of dangerous winter weather through the Midwest & Great Lakes.

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Closer to home, check out the latest GFS model's depiction of MSLP by Thursday morning. Yup, that's 1068mb over central Montana with about 1063mb over #Missoula. If this pans out it would not just break December MSLP records, but all-time records as well.

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So just how cold could it get? Well, that's really hard to forecast. Things like wind & clouds will play key factors. But here's what happened in 1979 & 1983, when the December & all-time MSLP records were set across Montana.

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Some of those values are not just record breaking, but top 10 all-time coldest on record in several places including #Butte, #Potomac, & #Missoula.

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So here's what the latest forecast models are showing. Please remember, these are RAW MODEL values & do not take into account local effects. But these types of extreme values have consistently been in the models for several days.

Also, THESE ARE NOT WIND CHILL VALUES!

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And I'm not even talking about the snowfall expected this week! The first period of snow gets started tonight, becoming heaviest Tuesday PM into Wednesday. Then another round of lighter snow is possible for Christmas! 🎄🎅

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Summary:

👀: An air mass of significant & potentially historic proportions is forecast to descend upon the Lower 48 later this week.

⚠️: Dangerously cold temps are expected across #Montana.

🥶: Temps could be coldest in the last 40 years.

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Please stay tuned to your local @NWS office for the latest on this #Christmas week forecast. Travel could be SIGNIFICANTLY impacted by snow, blowing snow, dangerous wind chill & extreme cold.

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#mtwx
@NWSMissoula @NWSGreatFalls @NWSBillings @NWSGlasgow