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County Warning Area [CWA]: STO Regional NWS Weather Office: Sacramento, CA 089 FXUS66 KSTO 292043 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 143 PM PDT Tue Oct 29 2024 .SYNOPSIS... Cooler temperatures this week, with breezy north winds today. First notable precipitation event of the Water Year arrives tomorrow, bringing widespread precipitation, mountain snow, breezy winds, and thunderstorm chances through Saturday. Breezy northerly winds develop on Sunday and into Monday. .DISCUSSION... Key Points: -First notable precipitation event of the Water Year will bring TWO WAVES of widespread measurable rainfall and mountain snow accumulations Wednesday PM thru Saturday. -First Wave (Wednesday PM-Thursday), Second Wave (Friday PM- Saturday) -Following this system, north winds increase through the Valley on Sunday & Monday TODAY Clear skies and breezy conditions are being observed across interior Northern California this afternoon. Current observations have northerly winds gusting 15 to 25 mph along the I-5 corridor in the Sacramento Valley. These are expected to weaken throughout the afternoon and evening. Cloud cover will gradually increase overnight and into tomorrow morning ahead of an incoming weather system. LATE WEEK WEATHER SYSTEM (Wed PM-Sat) An impactful weather system arrives tomorrow evening as a low pressure system digs into the region. This will bring TWO waves of precipitation and mountain snow beginning Wednesday PM and continuing through Saturday. The first wave will move through on Wednesday and into Thursday. Ahead of the initial band of precipitation, increased southerly winds are expected to develop with gusts 20-30 mph. Scattered showers will move in from the northwest throughout the later half of the day on Wednesday. Valley locations will see a lull in rainfall Thursday and into early Friday with a few scattered showers, before the second (stronger) wave moves through later in the day Friday continuing into Saturday. This second wave will bring a 15 to 20% chance for thunderstorm development Friday afternoon/evening across the Valley and foothills, along with the potential for debris flow concerns over the Park Fire burn scar. Let`s talk amounts... Recent forecast precipitation amounts have once again trended up, with the heaviest precipitation accompanying the second wave (Friday PM-Saturday). Over the course of three days, NBM probabilistic guidance shows a 60-90% chance for 1.00" inch of rainfall across the Valley and similar chances for 2.00" or more over Shasta County, the mountains, and foothills. Highest chances (80% and up) remain north of I-80. Forecast rainfall totals currently range from around 0.75"-2.00" inches in the Valley, with the higher totals (1.75"-3.25") in the mountains, foothills and Shasta County. Regarding snowfall, the NBM indicates a 65 to 90% chance of 12" or more above 6000 ft, with a 50 to 80% chance of 24" above 7000 ft. Forecast snowfall totals range from 8-16" above 6000 ft, with up to 24" along the highest peaks of the Sierra. Snow levels are currently forecast to be 5000-6500 feet throughout the duration of this multi-day event. If you have travel plans through the Sierra later this week, it is strongly encouraged to check quickmap.dot.ca.gov to check road conditions before heading out. && .EXTENDED DISCUSSION (Saturday THROUGH Tuesday)... EPAC upper ridging builds into NorCal Sunday into early next week resulting in dry weather with warming temperatures. Gusty north to east expected Sunday into early Tuesday. NBM probabilities show a 40-65% chance of north wind gusts greater than 30 MPH on Sunday, 25-50% on Monday along Interstate 5 westward. High temperatures expected to be near to slightly below normal. Expected rainfall and increased moisture will negate any critical fire thresholds. && .AVIATION... VFR conditions are mainly over interior Northern California for the next 24 hours except a weather system is brining MVFR/IFR conditions after 12Z Wednesday. Northwestern/northern surface winds are expected to be up to 20 knots with local gusts up to 25 knots until 23Z Tuesday for western part of Valley/foothills otherwise surface winds are mainly below 12 knots. && .STO WATCHES/WARNINGS/ADVISORIES... None. && $$
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