March 27, 2019 – Pacific storm moves through today into Thursday with rain, mountain snow, and possible thunderstorms. Drier weather Friday into the weekend then unsettled weather early next week.
Discussion
Pacific frontal system approaching the CA coast attm will move through today bringing widespread precipitation and gusty wind. Stronger low level pressure gradients ahead of the cold front will result in breezy conditions, with the strongest wind in the Central Valley expected late morning with fropa.
Wind speeds look to remain below advisory criteria. F-Vector local forcing along the front as it moves inland is directed from warm to cold indicating a weakening of the baroclinic zone into the afternoon. Higher QPF amounts are focused over the eastern foothills and mountains, enhanced by orographic lift. Periods of moderate to heavy snow expected in the mountains of Western Plumas and Northern Sierra Nevada today, where a Winter Storm Warning is in effect. Locally heavy amounts of snow are also possible in the Shasta mountains over the next 24 hours, however higher amounts will be above pass levels. S-Band snow level radar at Colfax showing snow level around 6300 feet attm, which is supported by Mesowest data.
Snow levels will trend down to around 4500 to 5500 feet this afternoon as front ushers in colder air. NAM Surface based CAPE values in the post-frontal AMS show a large area of 300-500 J/kg by 18z. CAPE further increases to upwards of 1000 J/kg over eastern portions of the Central Valley this afternoon, extending into the foothills. Bufkit vertical shear profiles from 0-1 km and 0-6 km suggest potential for rotating updrafts with stronger storm development.
Cyclonic upslope flow with embedded waves will keep snow in the mountains of Western Plumas/Northern Sierra Nevada tonight with heavier precip decreasing from north to south overnight. Snow levels lower to around 3500 to 4000 feet by tomorrow morning. Elsewhere, scattered showers are possible overnight with highest POPs over the foothills and mountains.
Additional embedded channeled vort maxes in cyclonic flow will keep showers in the forecast Thursday. Snow levels tomorrow forecast to be around 4000 to 4500 feet. Instability progs showing less CAPE Thursday, but enough to continue to support possible isolated thunderstorms. Precip threat diminishes Thursday night with lingering showers possible Friday, mainly over the northern and eastern mountains. Storm total QPF from today into Friday looks to be around 0.25 to 1.25 in the Central Valley with 1 to 3 inches of liquid equivalent in the foothills and mountains. Stronger convective storm development could lead to localized higher amounts. 1 to 3 feet of snow, possibly higher over summits, expected above 5000 feet.
Subsidence increases over interior NorCal Friday night with dry weather Saturday as building upper level ridging in EPAC approaches. Above normal high temperatures expected on Saturday with highs around 70 in the Central Valley, with mostly upper 40s to mid 60s for the mountains and foothills.
Extended Discussion (Sunday through Wednesday)
Extended period starts out dry under upper leveling ridging and daytime temperatures several degrees above normal. Some overrunning light precipitation possible northern zones as early as Monday morning with upper ridge axis shifting east of the state by Monday afternoon. Models show light warm frontal precipitation over most of the forecast area Monday although areas south of interstate 80 may see very little if any precipitation.
Models vary on timing and strength but frontal passage looks like late Monday or early Tuesday. Still too early to have much confidence but at this time, this looks to be a light to moderate precipitation producer. Showers continue through late Tuesday as upper through shifts through the north state. Parts of the forecast area are forecast to clear out next Wednesday as weak upper ridge rebuilds but threat of light precipitation remains over north and eastern portions of the forecast area.