Light rain and mountain snow on this first day of Spring

March 20, 2019 – Periods of light rain and mountain snow mainly today, Friday and then early next week. Enough snowfall possible for travel delays over the Sierra during each round of precipitation but light rain over the lower elevations will only create minimal impacts.

Discussion

Negatively tilted upper level trough moving into NorCal with shortwave rotating northward ahead of this trough bringing light precipitation across much of the forecast area. Blue Canyon ASOS showing a switch over to light snow with temperature at one degree Celsius so snow levels a little above 5000 feet look okay.

Cloud cover and cooler air with the low will bring down daytime highs today back down to a little below normal for this time of year. Stability progs indicate enough instability associated with the low this afternoon for a threat of thunderstorms over most of the valley and foothills.

Upper trough axis shifts inland over night tonight taking most of the lingering shower threat to the Sierra while showers end at the lower elevations. Upper trough axis moves in to the Great Basin on Thursday but cyclonic flow over the state will keep a threat of showers going over the mountains. May see a brief break in precipitation over the CWA Thursday night as small shortwave ridge slides quickly over the state. Models bring light precipitation into NorCal as early as Friday morning with the frontal band pushing through the north state Friday afternoon and evening. Precipitation amounts are again expected to remain on the light side but there could be enough snowfall over the higher elevations to bring travel impacts to the Sierra Cascade range. Showers will continue going into the weekend as the main upper low moves onshore.

Extended Discussion (Sunday through Wednesday)

Little change to the extended forecast package this morning. Weak high pressure is forecast to build over California on Sunday allowing for warm and dry conditions over Northern California. This will likely be short lived though as a broad low pressure system advances towards the west coast.

Model and ensemble guidance continue to hint at cooler and wetter weather for the week, with rain returning to CWA as early as Monday morning. The low remains quasi-stationary off the coast, which could lead to wet weather through midweek.