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FXUS66 KSTO 272134 AFDSTO

Area Forecast Discussion National Weather Service Sacramento CA 134 PM PST Mon Nov 27 2023


Synopsis
Dry weather with mild days and cool nights continues early this week. Cold morning lows are expected through Tuesday morning with areas of valley frost. Rain and mountain snow chances return Wednesday continuing through Sunday.

Discussion
Looking outside our office window, skies are clear around the Sacramento Metro Area and similar conditions are being observed across Northern California via GOES-West satellite. Temperatures this afternoon are gradually warming to their highs of upper 50s to mid 60s in the Valley & foothills. High temps across the mountains range from localized mid 30s to mid 50s. We'll see a general repeat of these high temperatures tomorrow followed by a cooling trend lasting through Saturday associated with a low pressure system.

The present synoptic pattern over our area consists of a blocking pattern created by an encroaching low pressure system of the CA coast and a high pressure ridge over the Pacific Northwest. This set-up is providing a brief window of dry weather through Tuesday.
Near to below freezing morning Valley lows are expected Tuesday morning; slightly more rural locations have a 40-75% chance of lows 32 degrees or lower while more urban areas only have a 15-35% chance.

By Wednesday warmer Valley lows (approx. 35 to 43 deg F)
are expected as warmer air wrapped around the ridge enters the area as the synoptic feature weakens and moves east. This will allow the trough to move over our area bringing a 15-35% chance of 0.25" to the foothills and Sierra Wednesday into Thursday afternoon. A few sprinkles to a few hundredths in the Valley will be possible, but they will have little to no impact other than localized slick roads. Snow levels are anticipated to be around 5000 feet which, with the passing moisture, has a 25-50% chance of dropping 1.00" or more of snow along the Sierra crest during the aforementioned period.

This trough is anticipated to phase with a descending longwave trough late Thursday which will deepen heading into Friday. This secondary storm system will increase the potential for Valley precipitation as well as several inches of mountain snow by Friday lasting through Saturday.

.EXTENDED DISCUSSION (Friday THROUGH Monday)...
Disturbances dropping southeast within cold large scale trough over the Western States Friday and Saturday will bring cooler air and precipitation chances over NorCal. Highest chances for precipitation will be in the northern Sacramento Valley (40-60% probability) and the mountains (60-80% probability through Sunday.
Currently the chance of QPF totals greater than or equal to 0.25 inches is 25-70% in these areas. Snowfall along the Sierra and southern Cascades is also possible, with the NBM projecting a 50-70% chance of accumulations of 2 inches or more by Sunday at pass level. The remainder of the Valley and foothills will see probabilities of 15-40% for precipitation with a 15-35% probability of reaching or surpassing 0.25 inches of precipitation. Ridge building over the area Sunday into early next week will bring warmer and drier conditions.

AVIATION
VFR conditions are expected over interior NorCal for the next 24. Surface winds will generally be below 12 kts.

STO WATCHES/WARNINGS/ADVISORIES
Freeze Warning from 2 AM to 9 AM PST Tuesday for Carquinez Strait and Delta-Central Sacramento Valley-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-Northern San Joaquin Valley-Southern Sacramento Valley.

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