After an exceptionally and record-breaking dry period in February, March, and most of April, a pattern shift may bring some significant spring precipitation tomorrow and Wednesday in NorCal. That retrogressive low pressure pattern that has brought the cold and windy conditions over the past week (and was originally progged to bring some precipitation) is continuing to evolve, and the large gyre is developing a piece of energy and sending it directly towards NorCal. This system will be fairly decent for late April, bringing a period of soaking to locally heavy rain to many areas north of San Jose. Given the time of year and the decent dynamics involved (inc. several vorticity maxima), isolated thunder will be possible both along the cold front and in the post-frontal airmass in NorCal. 850 mb temps drop to slightly below zero–not nearly as low as yesterday, when snow fell for almost an hour at sea level in Arcata, CA (North Coast–right on the ocean!). As much as an inch of rain could fall in some favored locations, with most Valley and Bay Area locations seeing 0.15-0.45 inches (considering the dryness lately, we’ll take what we can get). Snow levels will be fairly low in the Sierras for the time of year–locally down to 3000 feet or so. There doesn’t look to be a terrible profusion of cold air cumulus behind this system, but some scattered showers and possible isolated thunder (esp. Sac. Valley) do appear possible on Wednesday. SoCal will not see any precip out of this system except perhaps some drizzle/light showers on the Central Coast. After Wednesday, skies clear and temps warm above normal once again. The GFS, however, has been adamant in developing a very deep and persistent trough off the West Coast by day 6, eventually bringing cold and showery weather to CA for a good portion of the 7-16 day period. Stay tuned–but it looks like there will be at least the potential for some beneficial late-season showers over the next week or so…
