Major storm Wed night through Friday
SF Bay Area Skywarn weather spotters should self-activate for this storm starting Wed night.
From Tropical Tidbits:
From MTR AFD:
.LONG TERM... (Tuesday night through next Sunday) Issued at 1250 AM PST Tue Feb 11 2025 Long term forecast thinking remains unchanged. The previous discussion follows for reference: First round of rainfall continues early Wednesday morning for areas south of the Bay, and largely clearing out by mid-to-late morning. Wednesday afternoon actually has the potential to be mostly dry as shortwave ridging moves over the region and essentially cuts off our large scale vertical motion. By Wednesday night, a vigorous shortwave trough rapidly deepens along with the associated surface low approaching the CA/OR border. This more intense lift will aid in advecting the moisture into what would be a more coherent "atmospheric river" type pattern, whereas the first round of rainfall Tuesday/Wednesday is a more localized moisture plume, though not completely detached, and a much weaker degree of upper level support. So, the Tuesday/Wednesday system currently lacks the punch and will serve as a precursor event. Then, there is high confidence that the Wednesday/Thursday system will be the more impactful given the generous model agreement in robust jet support, not only supporting heavy rainfall, but also potentially damaging winds. Let`s juts say that when my SO told me her flight was Thursday morning out of Monterey, I said, "oh...uhh...gonna be bumpy!" Given the current location of the moisture axis and dynamic features, this system will generally favor areas from about San Francisco southward across the Central Coast. Overall good news in terms of annual rainfall since southern areas are a bit behind the curve so far this water year (since Oct 1). An interesting tidbit to include; the Central Coast watershed which runs along the coast roughly from Monterey to Arroyo Grande is about 4.37" below normal for the water year to date. The current forecast for that watershed by the end of the week is about 3-6". There is a slight chance (at least 15%) of rainfall rates resulting in flash flooding conditions area-wide on Thursday. It is likely that there will be more of a focus as the event nears and more information about convective rainfall rates becomes clear from high res guidance. The system does appear to be rather progressive and should move through the region relatively quickly, thus hindering any increase in the probability of flash flooding for now. Guidance keeps this vigorous wave open and embedded in the flow, so there is relatively low stall potential. As far as rainfall totals go, the North Bay will still see respectable totals in the 2-4" range, the greater Bay Area will see a range of 1.5-3" (less in the rainshadowed South Bay, more along the peninsula), 4-6" and locally up to 8" in the Santa Cruz Mtns, 2-3" across the Central Coast counties, and 3-6" along the Big Sur Coast, locally up to 8". A bit about the winds; guidance is showing reasonable agreement in the placement and strength of the associated surface low, ultimately bringing it into the CA coast near Cape Mendocino and a minimum surface pressure of about 985-991 mb. This would likely result in Wind Advisory criteria being met across a wide swath of the region should guidance begin to converge on the lower end of that range. Current best forecast is for gusty winds 40-45 mph along the coast and ridgetops of coastal ranges. There is also a slight risk for thunderstorms Thursday afternoon as the cooler air settles in aloft and instability increases. By Thursday night things should be settling down with some showers lingering into Friday morning. The weather into the weekend looks to clear out with another, much weaker, embedded shortwave trough entering the PNW late Saturday which may bring more rainfall the the North Bay on Sunday. &&