The past 48 hours have been cold and damp over NorCal. Snow did fall to very low elevations in the northern Sacramento Valley–less than 1 inch accumulated at the 300-500 foot elevation near Redding and non-sticking snow did fall to the valley floor (50-200 feet). Up to 2 or 3 inches of snow did manage to fall above 1000 feet in parts of the North Bay, which did close highways 101 and 29 in Mendocino and Napa counties. Yesterday’s high of 40 degrees in downtown San Rafael was the coldest in a long time, and the temperature has not risen above 45 degrees in over 48 hours here. The drippy and drizzly weather will decrease overnight and mostly end by Sunday afternoon. The Monday through Wednesday period looks dry and fairly warm (especially compared to the temperatures as of late) for the entire state as the jet streak shifts north and an incredibly strong 1056 mb high develops over the Great basin (that may actually be a record).
Beginning late Wednesday, however, a major pattern change will begin to take places across the Eastern Pacific. The East Asian jet will begin to strengthen and extend as far east as the West Coast, dropping progressively farther south as the week goes on. By Thursday, the jet will be over CA and a strong system will begin to move into NorCal. The first storm in a series such as this one usually weakens more than the models indicate as much energy goes towards knocking down the prevailing high pressure area and moistening the atmosphere, so don’t expect anything historic from this one. That said, however, it could still be the strongest of the season thus far, with heavy rainfall over the entire state and strong winds ahead of the cold front. Expect 1.5-3.5 inches in NorCal with 35-45 mph winds (gusts as high as 60 mph in favored locations) with similar windspeeds and rainfall of 0.5-3 inches in SoCal. The second (and possibly third) system will capable of producing even heavier rainfall and stronger winds, however, and the first will move in late on Friday and the second some time later in the weekend. Each system has the potential to bring 2-5 inches of rainfall to favored parts of NorCal along with wind gusts in excess of 70 mph (to locations that have not recieved winds of this magnitude in at least several years). One of these systems is likely to bring similar rainfall and winds to SoCal, but it unclear at this time which one will be aimed more towards the south (the third and final system may take a more northerly track). This does appear to be a rather intense but brief event, taking place within the 6-10 day time frame and ending rapidly after that. This enhanced jet may be related to a surge in the MJO signal currently over the Western Pacific, and if this continues to propagate across the ocean the very wet period could last longer than currently progged. Stay tuned. This sort of event has the potential to help quite a bit in terms of precip defecits, but may actually hurt the water content of the snowpack if snow levels are too high. At the moment, however, it should be a mostly beneficial event (outside of some flooding or wind damage that may occur).