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May 19, 2021 – Next weather system approaches today bringing cooler temperatures. Showers along with a few afternoon thunderstorms expected Thursday into the weekend, mainly over the mountains. The higher elevations will see some late season snowfall accumulations. Dry and milder weather returns early next week.

Discussion

A patch of subtropical high clouds is moving across central California early this morning, and a band of high clouds are beginning to spread into far northern California as the next upper trough approaches. Otherwise, clear skies cover the region. The surface pressure gradient is generally trending toward stronger onshore flow, and local gusts of 15-25 mph are extending inland into the northern San Joaquin Valley and southern half of the Sacramento Valley. Residual northerly gradient is keeping winds lighter across the north end of the Sacramento Valley.

Dry weather will continue today, though temperatures will be around 5-10 degrees cooler from a combination of increased onshore flow and synoptic cooling associated with the incoming trough.

Anomalously cold closed mid/upper low drops south into NorCal tonight and Thursday, and then hangs out to our east over the Great Basin on Friday and Saturday. The result will be an increasing chance of showers over mainly the mountains beginning tonight across the north, then spreading south through the northern Sierra on Thursday. Shower chances will continue Friday and Saturday. A few thunderstorms will also be possible, mainly in the afternoons and early evenings.

Several inches of snow will be possible above 3500-4000 feet in the northern mountains, and above 5000 feet across the northern Sierra (SREF and GEFS plumes indicate potential for 1-2 inches of accumulation at KBLU).

Recent warm temperatures will likely limit accumulations on road surfaces, but there will be a chance for some travel impacts near the higher northern Sierra passes later Thursday and Thursday evening if stronger convective showers develop.

The winter weather advisory has been extended further south to include the crest of the northern Sierra thru Thursday evening.

Breezy onshore winds today will give way to gusty north winds on Thursday and Friday as we transition to the backside of the low. The winds combined with low RH may result in elevated fire weather conditions across the lower elevations.

Extended discussion (Sunday through Wednesday)

Ensembles and cluster analysis continue to indicate that the upper low will exit the region on Sunday. This will result in dry conditions and warmer temperatures Sunday into early next week. Highs will trend closer to normal on Sunday, then above normal by early next week with Valley highs reaching into the upper 80s. Ensembles and clusters hint at another shortwave trough dropping into the region around Tuesday, which may bring some light mountain showers. Have gone with NBM POPs for now.