The upcoming weather pattern will be rather conducive for some very chilly nighttime temperatures. In fact, our forecast temperatures will expose the region to the coldest readings since the third week of January. So the question is what's delivering this cold air, how long will it last, and how cold can it get?
Let's first examine what got us here. On Tuesday night, the deep trough which delivered widespread rains, and high mountain snows to much of the region shifted east of the Continental Divide and was quickly replaced by cold northwest flow which originated from the Yukon.
|500 mb map for late Tuesday night. Green=moisture, white=dry|
|Time Lapse of dewpoints between Tuesday night 11/19 and Wednesday Morning 11/20.|
Below is a map of the estimated snow depth over the region as of this afternoon. Note there weren't many valleys which had snow on the ground, most of it was found over the mountains.
|Estimated or modeled snow depth map as of Wednesday afternoon 11/20|
So right now we think the coldest night of the week will be tonight/Thursday morning, but why? This airmass really isn't going anywhere. One reason is the air mass transition was really just a glancing blow. Take a look below at the upper level jet stream chart for Thursday afternoon. For us to see a prolonged cold snap, ideally we should be on the cold side of the jet. This would reinforce the surge of cold air from the north or northwest. But in this case the jet will remain to our north and east keeping the coldest air east of the Continental Divide. What that means is each day we should slowly modify or heat up this polar airmass. Without a fresh snowpack in place, that should happen quite readily, the question is how quick?
|Upper level jet stream (blue & purple shading shows fast wind speeds and location of jet)|
Below are maps of the 850 mb or (4500' above sea level) temperatures for Thursday and Saturday.
|Thursday 850 mb temperatures|
|Saturday 850 mb temperatures|