2009

It may be hard to believe after all the spring-like weather as of late, but a sudden and rather dramatic (if short-lived) change in the prevailing weather pattern is about to occur. Spring officially begins at 4:44 AM tomorrow, but the first full day of spring (Saturday) will be a great deal more winter-like than it has been weeks. A potent trough is currently forming over the Gulf of Alaska, and this system will plunge south to a position just off the NorCal coast by late Saturday. Temperatures aloft will approach -6 C at 850 mb in far NorCal and perhaps even -5 C over the Bay Area–which is enough for snow down to 2000 feet or lower, even at this late date. Given the lateness of the season, and the subsequent steeper sun angle, I would expect that insolation alone could drive some significant surface-based convection during this event, particularly on Sunday. Additionally, this system will be rather strong dynamicaly, so there will be additional dynamic forcing for instability. Therefore, I expect at least isolated thunderstorms across essentially the entire state on Sunday, with pockets of enhanced/scattered coverage. Hail could be an issue with some storms given the very cold air at 850 mb and subsequent low melting levels, and isolated severe cells are very possible, though an organized outbreak is unlikely. Precipitation totals across NorCal will generally be up to an inch, and up to a couple tenths in SoCal. Rainfall will be somewhat variable but not necessarily terrain-driven, so I do expet some wide variations in precip totals. Much colder temperatures and much stronger winds will follow the main cold front, with 40+ mph winds possible in NorCal and 60+mph winds possible in favored parts of SoCal. Skies will clear by Monday, and a significant warming trend back to normal to above normal temperatures will take place. It could become quite warm once again by late in the week–but that depends on the flow pattern in the Eastern Pacific. Stay tuned…after Sunday, I don’t expect any rain for a while…