March 27, 2020 – Dry and warmer today, then unsettled weather over the weekend with showers and mountain snow showers. Threat of showers persists in the north Monday and Tuesday, otherwise drier and warmer into the middle of next week.
Colder AMS in place in wake of upper trough that moved through. Clear skies overnight allowing for strong radiational cooling and morning temperatures will be in the 30s in the Central Valley. Local areas will be around freezing by sunrise with some patchy frost possible. Otherwise EPAC upper ridging builds inland over NorCal today resulting in drier weather. Clouds associated with approaching low, currently centered near 48N 139, will spread in from the NW today. AMS will be slow to warm and forecast highs today are generally within about 5 degrees difference from Thursday.
Upper ridge axis shifts through interior NorCal tonight as POPs increase over the NW with approaching Pacific storm. NAM/GFS look similar in strength and timing of upper trough but have some differences with QPF. Main theme is diffluent flow aloft producing synoptic scale UVM, thus supporting a threat of showers over interior NorCal Saturday. Best chances for precipitation expected over the mountains. 7H temps and 1000-500mb thicknesses support snow levels around 3000 to 4000 feet Saturday morning, rising to 4000 to 5000 by Saturday evening. Trough shifts inland Saturday night as secondary vort max follows inland Sunday with little change in snow levels. Moisture looks limited with these disturbances as PWS generally are around three-quarters of an inch or less in the Central Valley and below half an inch over the mountains of Western Plumas and the Sierra Nevada. Thus overall weekend QPF looks fairly light with rain totals in the Central Valley generally below half an inch and values approaching an inch in the wetter foothill and mountain locations. Snow totals attm look to be generally light, around 2 to 6 inches over the higher mountain elevations.
Decreasing threat of showers Sunday night as upper troughing shifts into the Great Basin, and EPAC upper ridging begins to rebuild inland. Models keep a threat of showers over the Shasta and Coastal mountains and in the Northern Sacramento Valley Monday into Tuesday as weak embedded speed maxima move through in NWly flow aloft. Otherwise, drier weather expected early next week with daytime temperatures warming 5 to 10 degrees above normal by Tuesday.
Extended Discussion (Tuesday through Friday)
An upper level trough will be pushing into BC and the northern PacNW for the start of the extended period. This will bring shower chances to the higher elevations in the northern parts of the area as a cold front sets up just to our north. There is still quite a bit of uncertainty on how far south this trough digs. The deterministic models continue to dig it further south then the ensemble means and have continue to favor the ensemble means in the forecast. Activity we do see will remain light with just some light QPF amounts. The pattern still looks uncertain on Wednesday but we should see mainly dry conditions. A short wave trough will bring a slight chance for showers again on Thursday mainly in northern areas. Generally high temperatures will be near average.