March 15, 2020 – Cold weather system will bring widespread rain and mountain snow this weekend into next week with mountain travel impacts and low snow levels. Below average temperatures continue through next week with unsettled weather at times.
Discussion
A deep, cold core upper level low will gradually shift southward along the Northern California coast through Monday. This storm system will bring 1 to 3 feet of additional snow across the Sierra Cascade Range, with up to 5 feet feet over higher peaks. A Winter Storm Warning continues through 5 PM PDT Monday for that area. Snow will continue into Tuesday as the low shifts southward off the Southern California coast, with gradually diminishing precipitation intensity across Northern California. Gusty winds will continue to combine with heavy snow to produce low visibility in the mountains, which has been reported down to 30 feet at times in the evening. Snowfall at Kingvale was estimated by Caltrans to total 25 inches at midnight. Several trees were reported to have fallen on I-80 from strong winds. Numerous spinouts and accidents have closed I-80 east and west bound, between Colfax and the Nevada state line, as well as Highway 20. This closure is currently expected to continue through 6 am this morning. Numerous other mountain roads have chain closures. Mountain travel is strongly discouraged today and tonight due to dangerous driving conditions!
Moisture and lift will continue to be less favorable over the northern Coastal Range, with lighter snowfall amounts expected. The Winter Storm Warning for the mountains of southwesternShasta County to western Colusa County has been downgraded to a Winter Weather Advisory, through 11 pm this evening. Additional snowfall there is expected to be 2 to 6 inches, with 10 inches over higher peaks.
As the upper low pushes closer, stronger ascent with the trough combined with enhanced upslope flow will continue high precipitation rates through Sunday. Hourly snowfall rates should easily reach 2 to 3 inches, potentially higher at times within heftier bands. Snow levels are likely to hover between 3,000 and 3,500 feet, locally lowering during periods of heavy snow. By late Sunday into Monday, snow levels will lower further possibly bringing 4 to 8 inch accumulations into areas between 2,500 to 3,000 feet. These should be slightly lower up into the Shasta County mountains and southern Cascades, nearing 1,500 to 2,000 feet. When it is all said and done, several feet of snow are forecast across sections of the Sierra which includes the major mountain passes. Totals of up to 7 feet of snow are forecast for higher peaks south of Highway 50. A foot or more is possible over the mountains of northern Shasta County. This could impact travelers going along I-5 as snow levels drop to around and just below 2000 feet.
Across the Valley, a decent convective pattern emerges today as as the upper low further increases the vertical wind shear. Instability is the typical wildcard when it comes to convection. If enough clearing can occur, a few semi-organized thunderstorms are possible, particularly during the afternoon hours. Overall, instability showers will be the primary focus with brief downpours and accumulating small hail. The activity should gradually taper beyond sunset given the diminished instability. Overall rainfall totals will likely sit in the 0.50 to 2 inch range, while doubling into foothill locations.
With the mentioned system being a relatively slow mover, its influence will keep scattered showers in the forecast through Tuesday. Scattered thunderstorms are possible Monday afternoon over the foothills. Shower coverage will decrease the days after as the upper low further exits the region. The overall pattern will remain cool and unsettled with high temperatures generally running around 10 to 15 degrees below climatology.
Extended discussion (Thursday through Sunday)
Model differences exist with progression and strength of upper low Thursday leading to lowered forecast confidence. Ensembles look more like ECMWF-Hi Res model which is the solution forecast leans to attm. Upper low therefore expected to fill and shift east into the Great Basin Friday. At same time, another large cold closed upper low digs slowly near 130 W into Sunday. This will continue unsettled weather over Interior NorCal through the extended forecast period.