Our many weeks of very dry and warm weather across most of CA are rapidly coming to an end. A strong spring storm is rapidly approaching the coast of California this evening. Current indications are that light warm advection-type rains could begin as early as Sunday in the far north and spread south towards the Bay Area by Sunday night. The main (and rather intense) cold front will move inland on Monday…early afternoon in the north and late evening in the south (and overnight in SoCal). The front…from Santa Barbara northward, will be accompanied by a 3-6 hour period of moderate to heavy rain and possibly some embedded thunderstorms. Winds will be quite gusty…locally in excess of 40 mph, esp. near convective elements along the back edge of the cold front. A very cold (850 mb temps below -3 in the north; down to -1 C in the south) and unstable airmass will move in behind the cold front, and as the core of the low moves inland over NorCal numerous showers and scattered thunderstorms will develop late Monday afternoon into Monday night. Yesterday it appeared that the coldest air aloft and best instability from a dynamic standpoint would coincide with the strong late-March sun angle Tuesday afternoon and give rise to a signficant severe thunderstorm outbreak in NorCal. The system will probably move through faster than expected previously and therefore dynamic support for moist deep convection will be waning on Tuesday across the Central Valley. However…isolated severe thunderstorms with large hail and very heavy rain are still possible…and more numerous (albeit weaker) storms containing heavy showers of small hail will probably occur on Monday night. Snow levels will plummet by late Monday…falling to 1500 feet in the far north to 2000-3000 feet in the Bay Area and as low as 2500-3500 feet in the Los Angeles-area valleys. Some late-season dustings of snow will likely occur on the higher Bay Area peaks and locally in some valley locations in SoCal. Total rainfall from this system will actually be fairly respectable for nearly April–up to 2.5-3 inches in the far north…1-2.5 inches in the Bay Area…0.5-1.5 inches in the Central Valley…and mostly 0.25-0.75 inches in SoCal (except locally higher orographic totals or in places with downpours from isolated thunderstorms). It will also be much colder this week…highs in some parts of NorCal will struggle to reach 50, compared to widepread highs 78-85 on Friday…and blustery NWly cold advective winds will make it seem even colder. This won’t solve our water problems, but it will add a nice foot or two to the Sierra snowpack and help to green up the hills a bit across the rest of the state.
