May 18, 2019 – Rain and mountain snow return to NorCal today along with gusty southerly winds. Sierra Cascade travel impacts expected late Saturday through late Sunday. Thunderstorms possible Sunday afternoon. Cool and showery weather is expected to continue through most of next week.
Weak ridging will give way as an upper low makes its way towards northern California today, with rain and mountain snow expected today through Monday. Precipitation will begin pushing into the coastal range and northern mountains this morning, overspreading all of the area by the evening hours.
With this onset of precipitation, expect gusty surface winds up to 40-45 mph mainly in the Sacramento Valley through this evening. Have maintained the previously issued Wind Advisory for this potential.
Snow may begin across the Sierra as early as Saturday afternoon near pass level with snow levels dropping to as low as 5000 feet by early Sunday.
Expect accumulations of 6 to 12 inches at pass level, with only minor accumulations down to 5000-5500 feet overnight. With this late season snow, the Winter Storm Watch has been upgraded to a Warning beginning today and continuing through late Sunday night. If traveling over the Sierra, be prepared for winter-like travel.
By Sunday, the main upper low will be overhead of northern California with continued showers, storms, and snow expected. CAPE values may exceed 500 J/kg by late Sunday morning through the evening hours leading to the potential for thunderstorms across NorCal.
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Shear is limited so the main threats look to be gusty winds, accumulating hail, heavy rain, and lightning with any storms.
There may be a brief break in precipitation Monday before another system moves in from Pacific Northwest bringing additional Valley rain and mountain snow to NorCal. Temperatures this day warm slightly into the upper 60s, but stay well below average highs for mid to late May. Similar temperatures will be seen Tuesday along with some chances for storms across portions of the Sacramento Valley.
Extended Discussion (Wednesday through Saturday)
Additional disturbances will drop down from the north later next week continuing the threat for mainly mountain showers and cooler than average temperatures. Models vary in the details, so will continue to lean on the NBM and ensembles for precipitation chances and amounts. Temperatures gradually rise later in the week.