Very cold and stormy weather over California

After an impressively long quiescent weather period over the entire state of California, a much more active pattern has returned to the Eastern Pacific. A significant cold front moved overhead yesterday into last night over NorCal and is continuing to progress southward into to SoCal this morning. More impressive than sensible weather associated with the front itself, however, is the very cold air that is filtering in behind it. 1000-500 mb thicknesses are expected to drop into the mid 520s by late today, with 850 mb temperatures less than -3 C in parts of Northern California. Snow levels will continue to plummet from nearly 7000 feet ahead of the cold front to less than 1500 feet in far Northern California. Local inland valleys east of Eureka and possibly near Redding may see some snowfall down to 500-1000 feet tomorrow and Friday. These cold temperatures aloft, in combination with strong surface heating in temporary post-frontal clearing, is rapidly destabilizing the atmosphere over NorCal this morning. Thunderstorms are likely to develop by around noon, possibly continuing well into the evening as several vorticity maxima move overhead and cyclonic curvature of the jet remains favorable. Given the relatively large amounts of instability (500 J/kg is a lot by California standards!) and some local wind shear aided by terrain-forced convergence in the Central Valley, there may be some isolated severe thunderstorms this afternoon. Most storms should be capable of small hail, and we could see some hazardous accumulations on roadways where intense precip sets up.

Visible satellite view of NorCal (NWS)

Tomorrow and Friday may be interesting days once again as a new system begins to develop right off the CA coast and move rapidly inland. The models have been struggling with the development and progression of this storm, but the GFS finally seems to be coming into agreement with the ECMWF that a fairly potent “cool” front will develop along a positive vorticity axis and  plume of strong upward vertical motion. As this feature moves south over the state in a fairly unstable airmass, convection may develop, and it may be rather intense if recent NAM precip schemes are to be believed. The highest potential for heavy precipitation, gusty winds, and possible strong thunderstorms will be from about the Monterey Bay Area southward to the Mexican border, though the northern extent is less certain. In any case, I would expect late tomorrow and Friday to be active weather days across most of the state.

NAM output for late tomorrow (NCEP)

The cold trough and associated low will slowly fill and begin to move off to the east this weekend, but before high pressure is able to reestablish itself a very strong impulse will drop down the backside of the highly amplified meridional trough and bring a renewede shot at some very cold and unsettled weather statewide. The ECMWF and GFS have been hinting that this next system, slated for Sunday-Tuesday of next week, has the potential to be quite impressive over CA. The 12z GFS, for example, brings a modified Arctic front over the region late Sunday into Monday, bringing some heavy precipitation, gusty winds, and possible convection. The front will be moving into an airmass that is already quite cold, however, with 850 mb temps still below 0 C in NorCal.

GFS output for early next week. Note location of 522 dm line... (NCEP)

Behind the front, thicknesses drop below 525 dm and 850 mb temps drop below -6 C in far NorCal. Now that this scenario is only 6 days out, it’s worth keeping an eye on the potential for a very low snow event early next week. If the 12z GFS is somewhere in the ballpark, snow would be possible in parts of the Sacramento Valley next week. It’s still too early to get excited, but this pattern bears watching. Stay tuned.


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