Wet and stormy NorCal; a little wet SoCal

The advertised upcoming wet pattern is shaping up a bit differently than previously forecast. A powerful 946 mb low (see animated GIF below) is moving slowly towards southern British Columbia, dragging a very long cold front with copious entrained tropical and subtropical air behind it. This front is will bring heavy rain and strong winds to the West Coast from about Eureka northward for the next 24 hours, but the main energy will be directed north into Canada. The front will maintain some of its strength on its southern flank as it is dragged slowly south across NorCal later on Monday. As this occurs, a PW plume of 1.5-2 inches will undergo strong isentropic lifting as it nears the coast, bringing increasingly heavy warm advection rains to NorCal from the Bay Area northwards. Rain intensity will increase during the day on Monday, possibly becoming heavy at times. The major change is timing–the moist plume is now forecast to sink very slowly south along the coast–as the main rain event now appears to be slated for Tues/Wed. A relatively deep and surface low–near 986 mb, acc. GFS–will develop and approach the NorCal coast on Tuesday, acting on extremely high PW values (near 2 inches extending back to near the Phillippines). The cold frontal lift will add to the preexisting lift associated with the warm air advection and generate some truly impressive rainfall rates from near Monterey northward on Tues./Wed. This surface low is a new development; it wasn’t indicated by the models yesterday. This could add some stronger gradient winds into the mix–perhaps 40-50 mph–which will make the week all the more stormy. With such a dynamic and moist system, would expect at least some potential for thunderstorms, and as an unstable airmass moves overhead after the strong cold front, will likely see a chance for additional convection. The big story will almost certainly be the rainfall. Sunday night through Wednesday evening totals for far NorCal could approach 10 inches in favored locations, with 3-6 inches in most places. In the Bay Area, it appears that widespread amounts of 2-5 inches will occur, though substaintially higher totals could occur in orographically favored areas. SoCal could see an inch in favored locations, but again, don’t expect any sort of washout from these upcoming storms south of about Monterey. Rains of this magnitude could create some hydrological problems in NorCal, so stay tuned.

The longer term may be interesting, as well. A mostly dry period is expected from days 5-9, though some light rains coudl fall in NorCal. Starting around days 10/11, however, the GFS is beginning to forecast that the East Asian jet will “break on through to the other side.” So to speak. This would bring a much more active pattern to all of CA, potentially allowing major storms to traverse the entire West Coast. This scenario is supported by the current phase of the MJO, and the recent upswing in equitorial East Pacific tropical convection. It appears that there is finally some surface reflection of the ENSO warm event (EL Nino) that is currently underway. That is, thunderstorms are finally firing near the Equator, injecting lots of moisture into the atmosphere and transfering kinetic energy northwards towards the poles as they dissipate, giving new life to the jet stream over the Pacific. We’ll see what happens, but count good ole’ El Nino out just yet…

Merry Christmas and happy holidays…. 

(Click to animate!)