The long-advertised pattern change is still on track to take place on Friday for California and the rest of the West Coast. A storm system will move into a split flow regime west of CA, allowing the southernmost part of the system to cut off and possibly form a closed low as it approaches NorCal. Surface pressure gradients will not be overly impressive with this event, but some gusty winds of 40-45 mph could occur in the usually favored areas (hills and coast). Rain could be heavy for a time, mainly near frontal passage, for a grand total of .75 to 1.50 inches in the wetter areas. A second storm will follow close on the first storm’s heels. Initially, the GFS had indicated that the second system would be substantially stronger than the first and would not split nearly as much. Then the GFS depicted a fairly weak, badly splitting front reaching the coast of CA with little fanfare. Now, in the 18z run, the GFS has a fairly potent low ploughing right into the Bay Area, with possible wind gusts in excess of 50 mph, some heavy-ish rains on the order of 1-2.5 inches, and a decent possibility of convective activity (thunderstorms). We’ll see which scenario pans out. Models have a hard time with split-flow conditions, and as always, the devil’s in the details. If the forecast for the next few days seems murky, the extended is really a product of the fabled numerical model “fantasyland.” With so much uncertainty with regard initial conditions, next week’s weather is anyone’s guess. It does appear that a fairly active pattern will prevail along the West Coast, but the latest runs of the GFS mave the core of the 150 kt jet up on OR or WA. This would leave NorCal on the southern tail of any storm systems. The jet could shift back south in future runs just as easily as it shifted north in the last few. In any case, it will be interesting to see what develops. The current moderate El Nino in the Eastern Pacific is really not making its presence known in North America; the prevailing synoptic setup has remained rather stagnant and un-El Nino-like thus far. This could change, but probably not until later in the winter season–late January or February. Stay tuned…
