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April 4, 2020 – The first of a pair storm systems arrives into NorCal this morning. Rain, gusty winds, chances for thunderstorms, heavy mountain snow, and travel issues are forecast into early next week. Below average temperatures likely through Tuesday with a gradual warm up thereafter.

Discussion

Precipitation is currently spreading into northwest California this morning, with perhaps some showers and virga hitting the western portion of our CWA. Chances for rain and snow, above 4500 feet, will gradually become more widespread throughout the day. The Winter Storm Warning remains in effect elevations above 4500 feet, with the coastal range starting at 5AM while the Sierra`s warning begins at 11AM. 6 to 12 inches of snow accumulation is expected today, with higher amounts expected for tomorrow as the stronger and more potent wave moves through. Valley and foothill rain for today will range from a couple hundredths down to the northern San Joaquin Valley to nearly an inch up in Redding and some foothill communities. The bulk of this wet weather is expected to occur this morning and into the afternoon, with showers possibly lingering through the evening and overnight hours.

On the heels of the initial shortwave will be a much stronger, colder system set to arrive on tomorrow. 500-mb temperatures are expected to run between -29 and -32C which is fairly anomalous for early April. While the colder plume of air aloft stays just offshore, there will be many impacts across interior Northern California. The mountains can expect a brunt of the more high- impact level issues which includes potential road closures, chain controls, snow-covered roads, and whiteout conditions at times. 2 to 3 inch hourly snow rates are a distinct possibility with tomorrow’s snow totals alone possibly reaching 2 feet. For those who must travel, tomorrow (Sunday) will be a treacherous day on the roads so plan accordingly. Storm snow totals could easily reach 2 to 3 feet, locally up to 4 feet by Monday morning.

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Across the Valley and foothills, Sunday could prove to be a very active day. Simulated radar imagery shows some periods of locally heavy rainfall tracking through the region, especially over the foothill locations. Some ponding on roads can be expected although not anticipating any hydrologic issues given how dry the region has been this winter. Total Valley rainfall is expected to run between 1 to 2 inches, running on the higher side of the range over the northern Sacramento Valley. 2 to 4 inches will be more commonplace into the foothills given the flow getting augmented by the terrain. On the thunderstorm realm, fairly impressive vertical shear profiles set up over the west side of the Valley. Model soundings and hodographs support the threat for a few more organized storms capable of small hail, gusty winds, and perhaps a weak tornado or two. The wild card is the available instability which will be dependent on the amount of clearing which can take place. Aside from the threat for precipitation, breezy southerly winds will dominate the weekend, particularly on Sunday with gusts likely in the 30 to 40 mph range.

On Monday, the upper low will fill some but remains a slow mover as it drifts toward the south. This will keep showers in the forecast with a particular focus over the mountains where a Winter Storm Warning will remain in effect into the morning hours. Snow levels do drop to around 3,000 to 4,000 feet on Monday. Throughout the remainder of the day, instability-driven showers are likely to fire with the activity waning into the evening hours. Given the close proximity of the parent low to the coast, additional thunderstorm chances exist but should be weaker given more limited vertical shear.

Tuesday, conditions will start to dry out with showers potentially lingering over the Sierra. The upper low will be tracking farther south which will eventually lead to the Pacific high nudging its way back in to NorCal. Well see temperatures start return closer to normal for some areas, with the departure from normal being anywhere zero to fifteen below the seasonal average. More seasonal weather is currently expected for Wednesday.

Extended Discussion (Wednesday through Saturday)

Deep closed low forecast to be to our south during the second half of next week keeping a threat of showers across the northern Sierra Nevada, mainly to the south of Lake Tahoe. Drier weather with northerly flow expected across the remainder of the region with gradually milder temperatures. Highs expected to be above average by next weekend as ensembles in decent agreement indicating ridging from the eastern Pacific building into NorCal.