2009

A series of very moist storm systems brought very impressive May rainfall totals to the North Coast since Friday. Many rivers in far NorCal approached flood stage–even the Merced River in Yosemite Valley reached monitor stage. As much as 8 inches of rain fall in the Shasta County mountains, and with the subtropical nature of the source air mass snow levels were well above 8000 feet. There were also several periods of rather spectacular convective activity in the far northern Sacramento Valley between Corning and Redding. One more system…the weakest of them all…will move in tomorrow afternoon, bringing a few showers potentially as far south as Sacramento. The rainfall gradient was very sharp, though: places south of Sacramento and San Francisco saw very little, if any precipitation out of any of these storms.

It will be uncharacteristically warm and muggy once again in the Central Valley tomorrow as highs approach 80 degrees and dewpoints hover around 60 degrees. Warmer temperatures–well into the 80s and even near 90–will be common statewide by later in the week, though the lingering subtropical atmoshperic moisture will long since have scoured out. The models–particularly the GFS–bring a very cold closed low into NorCal from the N/NW on days 6/7.  The GFS portends a system that would have the potential to produce a very impressive round of afternoon thunderstorms about as far south as the Bay Area; other models are weaker and farther to the north with the low (though they have been trending towards the GFS). Cutoff lows are notoriously unpredictable, and the models have performed particularly badly with them this season. Stay tuned, as there is at least a small potential for interesting weather next week.

Update 5/6: I just received the following photograph, taken near Corning, CA during one of the recent convective outbreaks over the far northern Sacramento Valley. There certainly appears to be a mesocyclonic structure apparent, perhaps even a lowering wall cloud with a so-called “beaver’s tail” cloud on the right side of the image. Lightning was observed at the time the photograph was taken. These cells had very impressive radar presentation and were severe-warned by the NWS, so the impressive storm structure is not surprising. These cells were also long-lived, especially for California, so there was apparently enough time for a very visible inflow structure to mature. Very cool stuff! Image courtesy of Sarah Strazzo.