Cool weather continues with unsettled weather bringing periods of rain and mountain snow into the weekend. Warmer with dry weather expected Sunday. An unsettled weather pattern returns next week with the potential for more rain and mountain snow.
Discussion
We are going to remain in an active and unsettle weather pattern the next few days with periods of rain and mountain snow. A short wave trough pushing into far southern OR is showing up nicely on water vapor. This trough is pushing a cold front into NorCal stretching from the Lassen Park area southwest through the Ukiah area. We are seeing light Valley showers ahead of this front with widespread rain and snow over the higher elevations. PWATs ahead of the front are 0.50 to 0.90″ and we are seeing hourly rain rates generally 0.10 to 0.35″ over the higher elevations and less than 0.05″ in the lower elevations.
Expect these rain rates to continue for the next few hours but things will start to let up quickly after 8 pm this evening. Snow levels are generally around 5000-5500 feet and we should see them come up about 500 feet this evening. Mountain snow will continue to bring mountains travel issues into the evening.
The HREF and other CAM models such as the HRRR are building in instability over the northern Sacramento Valley and Shasta County. Currently Meso-analysis is starting to show 100 mb of MUCAPE over this region and CAPE has been slow to build in due to cloud cover and this should limit the thunderstorm threat but there is about a 15% chance or so to see a few thunderstorms. CAPE in the area looks to linger into the later evening but we should see the thunderstorm threat end by midnight.
Soundings do show some low level veering but 0-1 km shear is pretty low and that should limit any funnel potential with small hail and gusty winds the main threat with any thunderstorms. PWATs even behind the front are quiet high, ~0.50″ but somewhat fast storm motion should limit heavy rain but brief downpours can be expected.
Things will dry out overnight and we will see a mainly dry start to Friday. The dry conditions will be short lived though as another short wave trough digs over the Pacific and moves over NorCal Friday night into Saturday.
This will be a quick moving system but is able to pick up decent amount of moisture again with PWATs 0.50-0.90″. With the further south track of this trough we will see higher snow levels in the 5500-6000 foot range over the Sierra and 4500-5500 feet over the Coastal Range and Shasta County Mountains.
With the trough axis moving just to our south the highest QPF can be expected from about I-80 north. QPF totals are looking to be 0.90-2.25″ over the mountains with locally higher amounts over the Lassen Park area. Valley totals 0.15-0.75″ and 0.75-1-75″ in the foothills.
Snow totals are looking to be 6-12″ above 5500 feet with locally higher amounts over Lessen Park. We do see some CAPE build in Saturday afternoon mainly over the northern Sierra Foothills and the Sierra north of I-80.
Forcing will be rather weak by the afternoon and the thunderstorm threat is looking low but not zero. An isolated cell or two will be possible. The environment on Saturday will be very similar to today and similar threats can be expected with any thunderstorms that end up developing.
Our active and unsettled weather will take a break on Easter Sunday as weak ridging builds in and we will see temperatures warm to near average.
Extended Discussion (Monday through Thursday)
Unsettled weather is going to return for the extended forecast as we will see a few troughs push over the region. The first one will dig out of the Gulf of Alaska later Monday into Tuesday. There is some spread in the ensembles mainly with how deep the trough digs but also some with timing. The northern mountains will see the best chances for showers.
Wednesday should start out dry but another trough will bring rain and snow chances but uncertainty is quite high with this trough in the ensembles. Highs will generally be near seasonal averages.