May 22, 2019 – Light Sierra showers next couple of days as an upper low remains over the Great Basin. Isolated Sierra thunderstorms as well. Mainly dry elsewhere next couple of days with near normal temperatures. Showers and isolated thunderstorms Friday through early next week as another low drops out of western Canada.
Discussion
Isolated light showers continue over the southern half of the CWA this morning as an upper trough digs southward through central California. The southern Cascades, northern Sierra and parts of the San Joaquin valley will remain under cyclonic flow today so isolated showers will continue these areas with enough instability indicated to warrant an afternoon thunderstorm threat as well.
A cool airmass combined with mixed sun and clouds today should bring up highs at the lower elevations to a little below normal for this time of year. Upper troughing over the Great Basin will continue the shower and thunderstorm threat over the Sierra Cascade range and foothills on Thursday. The airmass warms slightly so should see a little warm up to near normal for this time of year.
Another upper low dropping southward out of western Canada Thursday night will nudge the Great Basin low eastward bringing a temporary dry period all areas Thursday night and Friday morning. This break will be short lived however with shortwave troughs forecast to pivot out of the Pacific Northwest low through NorCal bringing a threat of showers or thunderstorms by most areas by Friday afternoon.
Daytime highs slip a bit on Friday then much more on Saturday as the upper low continues southward into NorCal. This low will bring showers and isolated thunderstorms to nearly all of NorCal with daytime highs expected to drop to 10 or more degrees below normal.
Extended Discussion (Sunday through Wednesday)
Models continue to differ with progression of upper low progged to dig south near the CA coast and eventually track inland across SoCal in the Sunday to Monday timeframe. GFS/GEFS spaghetti plots showing good consistently among ensembles and thus the preferred model. As a result, unsettled weather expected to continue Sunday into Monday with best chances for showers expected over the foothills and mountains. Model CAPE values look high enough to support a slight chance of thunderstorms Sunday. Channeled vorticity along the Sierra Nevada Tuesday will maintain a slight chance of showers over higher elevations that persists into Wednesday. Below normal temperatures progged into Monday with high temps returning to near normal Tuesday, and above normal Wednesday.