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May 15, 2021 – Cooling trend continues today, with slightly warmer temperatures returning early next week. Scattered showers and thunderstorms are possible this weekend, mainly over the mountains, though a few storms may migrate into the lower elevations later this afternoon and this evening.

Discussion

Satellite imagery indicates the closed mid/upper low is centered over the Sacramento Valley to the northwest of Sacramento. Radar indicates a few showers continuing north of the low early this morning, mainly over Shasta and northern Tehama Counties. The approach of the low has helped to initiate robust onshore flow with gusts in the 30s mph through the west Delta, and gusts of 15-25 mph extending inland into the Sacramento and Stockton areas. Stratus has also developed well inland with IR difference imagery indicating a large patch covering much of the Sacramento region, and another to the northwest of Yuba City.

The main weather concern over the weekend will be the continuing threat for deep convection as the low moves slowly to the east. Most shower and thunderstorm activity today is expected over the mountains between Mt Lassen and Lake Tahoe, with isolated storms developing over the Coast Range. Northeast steering flow may bring some of this activity down to the edge of the Sacramento Valley this evening. Brief downpours and gusty outflow winds, occasional lightning and small hail will be possible with stronger convection.

Shower and thunderstorm chances will be more limited to the Sierra Nevada and foothills on Sunday as the upper low exits toward the Great Basin/Desert Southwest, then dry weather returns Monday/ Tuesday as short-wave ridging moves in. Temperatures will rebound a bit beginning Sunday as onshore flow wanes and gradual synoptic warming returns as the low exits.

Extended discussion (Wednesday through Saturday)

Confidence is high in a much cooler weather pattern over NorCal during the EFP, albeit the timing and details will likely vary in the near term. Max temps are forecast to run some 5 to 15 degrees below normal during the period, with Thu perhaps the coolest day. 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, details on location and timing to be determined. 500 mbs EOF patterns suggest that early in the EFP the main difference among ensemble members is the speed/timing of the trof, then transitioning to more of a difference in magnitude of the trof during the latter portion of the EFP. And in fact, this is what we see in the evening deterministic models.

Appears the GFS is cooler and wetter than the ECMWF at this point. But regardless, a showery weather regime is expected over Norcal especially over the higher terrain due to topographic lift and forcing. Snow showers will likely occur over the higher elevations of the Sierra as well, mostly likely Thu/Fri. (It will be interesting to see if the CPC precip forecast turns the B (below normal precip) to a N (near normal) or just a light green A (above normal) in the coming days.) With the trof forecast to move Ewd across the Great Basin on Sat drier NW flow prevails over NorCal ending the precip chances along with a moderation in temps.