May 17, 2021 – Other than a few late day showers and thunderstorms over the Sierra south of Lake Tahoe, dry weather with slightly warmer temperatures is expected early this week. Unsettled weather along with cooler temperatures returns after mid-week.


Yesterday’s deep convection has died out and skies are clear other than an area of stratus developing across the southern Sacramento and northern San Joaquin Valleys as a strong Delta Breeze continues. Current temperatures range from the upper 30s in the colder mountain valleys, to the 50s to mid 60s across the Central Valley.

The weekend low continues moving east across northern Arizona and short-wave ridging is moving over NorCal. Enough moisture and instability lingers for a few late day showers and thunderstorms over the Sierra south of Lake Tahoe, otherwise dry weather is expected into mid-week.

Another trough is forecast to move into the region on Thursday bringing another cool down along with a chance of showers and thunderstorms, mainly over the mountains.

Extended discussion (Friday through Monday)

Confidence remains high in a much cooler weather pattern over NorCal at least during the first half of the EFP as indicated in the 5H Cluster Analysis. Max temps are forecast to run some 5 to 15 degrees below normal Fri and Sat with the higher Sierra elevations 20 degrees or more below normal. Temps will moderate to near normal on Sun, then a little above normal by Mon.

A Gulf of AK (GOA) trof is forecast to deepen along the W Coast early in the EFP period, eventually forming a closed low over CA/NV by Fri, details of which are to be determined. Most of the ensemble members maintain the deep low over NV on Sat as a highly amplified blocking pattern develops over the USA with a strong high over the Gulf Coastal states blocking the Ewd progression of the NV low.

Deterministic models suggest that multiple vort maxes will rotate around the closed low resulting in some retrogression of the synoptic system due to the downstream blocking pattern. This will likely keep some showery wx over the Sierra/Nrn Mtns not captured by the NBM pops this evening.

The cold nature of this low will cause some snow showers over the higher elevations of the Sierra as well. On Sun and Mon, energy lifting Nwd from the base of the low center will lift the system Nwd allowing for moderation in temps and a more stable wx pattern.