March 8, 2021 – Cool and unsettled weather through mid-week with periods of showers and thunderstorms, and mountain and upper foothill snow. Dry and milder weather expected to return by the end of the week.
Satellite imagery shows clouds increasing across the region as the initial frontal system works its way thru NorCal. Radar is picking up some very light returns over the northern mountains, likely some virga or very light precipitation. With the increase in clouds and wind, current temperatures are considerably milder (5-10 degrees in most areas) compared to 24 hours ago.
Shower chances will increase across the region by this afternoon as the next vort swinging around the base of the closed low off the PacNW coast moves in. At this point it appears the best chance for showers will be generally to the north of I-80 with a slight chance of thunderstorm across the north half of the Sacramento Valley.
More significant precipitation possible for the Central Valley and northern Sierra Tuesday and Wednesday as the closed low drops south to near the Bay Area.
Pattern is classic late winter/early spring heavy snow producer for the northern Sierra with a prolonged period of southwest flow along with steep mid-level lapse rates generating significant snow despite rather meager overall moisture (TPW around 1/2 inch).
Thunderstorms will also be possible each day with small hail expected to be the primary threat, though shear parameters indicate potential for some longer lived storms should sufficient instability materialize.
Winter storm watch has been posted for the far southern Cascade Range and northern Sierra for Tuesday afternoon into Wednesday evening. Several feet of snow will be possible with accumulating snow possible down into the upper foothill elevations leading to significant travel impacts.
Drier weather forecast to return Thursday as the low digs south into SoCal and western Arizona.
Extended discussion (Friday through Monday)
Upper ridging builds inland over interior NorCal Friday into Saturday. Dry weather expected during this period with above normal high temperatures. Ridge axis moves through early Sunday as Pacific frontal system approaches the CWA.
More progressive GFS introduces some precip north of a KUKI-KRBL-KAAT line Sunday afternoon. Short wave trough moves through Sunday night with a threat of showers, mainly over the mountains and northern Sacramento Valley.
Drier northwesterly flow aloft sets up over the forecast area Monday.