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May 17, 2019 – Mountain showers diminishing today. Rain and mountain snow returning all areas on Saturday afternoon with breezy south winds. Sierra Cascade travel impacts likely late Saturday through Sunday. Thunderstorms possible Sunday afternoon. Shower threat continuing through most of next week.
Upper low over the Great Basin with high pressure building off the coast. Sierra Cascade range still under cyclonic flow, however, and with aid of orographics, Sierra Cascade range is still seeing some scattered light showers. Should see decreasing showers this afternoon as high pressure builds over the west coast.
Warming airmass and increased sunshine will bring warming today but daytime highs will remain well below normal. Most of the forecast area should be dry tonight but GFS and NAM still showing a shortwave clipping the northwest corner of the state tonight so northern Shasta county may see a few light showers.
Upper ridge shifts quickly eastward early Saturday allowing the next Pacific frontal band to move into the north state Saturday afternoon. Precipitation is forecast to spread into the Sierra by the afternoon.
Snow levels below pass levels will mean another period of mountain travel impacts and will issue winter storm watch Saturday afternoon through Sunday night. Most areas will see breezy south winds as well.
Main cold front shifts to the southeast of the area by Sunday morning with main upper low forecast to trek through NorCal during the day on Sunday. Cold air aloft associated with the low will destabilized the airmass enough for isolated thunderstorms.
Showers over the Sierra will continue through the day so winter storm impacts will continue through Sunday night. Should see decreasing showers on Monday with a slight warming as a flat high pressure ridge slides quickly across the state.
Extended Discussion (Tuesday through Friday)
Another cold, upper trough looks to impact NorCal Tuesday with precipitation chances lingering through the end of the period. Models are still a bit off with the timing of this mid-week system with the ECMWF lagging behind the GFS.
The best chances for Valley rain look to be Tuesday and Tuesday night, although amounts look light at this time. Snow levels will be near pass level during this time, although impacts appear minimal with only a few inches of snow expected.
Small chances for snow showers in the Sierra-Cascades linger through Friday. Details will continue to be worked out with future forecasts. Temperatures are on the rise next week with Valley highs Tuesday in the upper 60s, rising to the low 80s by Friday afternoon.