Very cold airmass over the West, and some snow in unusual places

The much-discussed Arctic front will sweep south from the Gulf of Alaska on Wednesday, heralding the arrival of a very cold airmass aloft. Moisture will be lacking on the southern end of the front, however. The front will elongate a bit on its southern end, with the southernmost segment hanging back as the northern half pivots to the SE. This low pressure area will sink southwards just offshore of OR/CA on Thursday, bringing in even colder air aloft. The most exciting prospect continues to be the chance of snowflakes falling at sea level in NorCal. The moisture content of this system just isn’t very supportive of low elevation snow, even though the temperature profile of the atmosphere is certainly cold enough. Isolated showers are possible in the Bay Area and Central Valley as the coldest and most unstable air moves overhead on Thursday (-7 C at 850 mb over San Francisco). These showers, if they develop, will fall as snow down to 1000 feet in the Bay Area and the Valley, and possibly to sea level. The 00z GFS has an interesting feature that would potentially lead to some rather exciting weather in the Bay Area and southern Sacramento Valley on Friday. For the first time, the model shows a vorticity maximum dropping down the back (west) side of the low as it nears the SF Bay Area, bringing moister air and even colder temps aloft (-8 C at 850 mb). This disturbance, which would be quite uninteresting in most circumstances, could, given the extremely cold air aloft and the instability already present, serve as the trigger for some convective snow showers (not just flurries). These would impact CA from the mid-Mendocino coastline southwards (though showers south of Monterey would be in the form of rain at sea level, w/ snow levels of 1500 feet or so). I will certainly keep everybody posted on that potential! Otherwise, expect places north of Eureka to see the best chances for seeing some of the white stuff at sea level, and some rather significant accums. are possible in WA. Some record lows could be set, especially in the Bay Area as some traditionally cold, sheltered valleys could drop into the mid 10s (15-19 F) Fri/Sat nights. A deep freeze will occur nearly everywhere except within 1/2 mile of the beaches south of Eureka.

Will talk more tomorrow about the potential for…you guessed it…more dry weather and Santa Ana winds, and even the GFS’s prognostications of another day 16 major cold/snow event…

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