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FXUS66 KSTO 040930 AFDSTO

Area Forecast Discussion National Weather Service Sacramento CA 230 AM PDT Sat May 4 2024

SYNOPSIS
A weather system moves through the area today bringing showers, mountain snow, isolated thunderstorms, gusty winds and cooler temperatures. Warmer and drier conditions return next week with gusty north to east winds mid-week.

Current radar shows the weather system starting to move into far northwestern California this morning. Precipitation will spread inland through the day today, in the form of Valley and foothill showers, and mountain snow. Latest guidance from WPC shows storm total QPF of around 0.50"-1.25" in the Valley, with 1"-3" in the foothills and mountains. Isolated thunderstorms are also in the forecast for today (15-30% probability). Brief heavy rain, small hail, lightning, gusty winds and brief funnel clouds will be possible with any thunderstorms that develop. Best chances are north of Interstate 80.

For the mountains, snow is expected today, heavy at times. Storm total snow amounts have increased slightly. The National Blend of Models (NBM) advertises a 75-95% probability of 8 inches of snow or more, and 50-90% probability of 12 inches or more. Snow levels have also trended slightly lower, and the Winter Storm Warning area has been expanded to include elevations above 4000 feet. Snow levels will initially be around 4000-5000 feet this afternoon and lower to around 2500-3500 feet by early Sunday morning. Snow rates of 1-2 inches per hour, combined with gusty winds will lead to travel delays, reduced visibilities, snow covered roads and chain controls. People should check the forecast and road conditions before traveling in the mountains this weekend.

Gusty south to southwest winds are also developing ahead of the storm. A Wind Advisory has been issued for portions of the Northern and Central Sacramento Valley, mainly east of Interstate 5, and the Northeast Foothills through 11 AM this morning. The NBM shows a 65-95% probability of 40 mph gusts or greater in this area, and a 55-95% probability of 50 mph gusts in the mountains where the Winter Storm Warning is in place. Much cooler temperatures will also influence the area, with highs around 15 to 20 degrees below the climatological normal for this time of year today, which is also highlighted with a strong signature on the Extreme Forecast Index (EFI). Precipitation tapers off on Sunday, as just a few showers and mountain snow showers linger in the mountains. Temperatures will gradually warm next week, with periods of breezy winds.

.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)...
Ensemble guidance and cluster analysis indicate a dry, warm and windy pattern setting up for mid to late next week. There is still some uncertainty with the exact details such as timing and magnitude of the winds. The Extreme Forecast Index (EFI) is highlighting strong north to east winds in the typical wind prone areas for this direction which would include the western portion of the Valley and in the mountains. The National Blend of Models (NBM) shows a 55 to 85 percent probability of wind gusts greater than 40 mph in those areas on Wednesday. Temperatures gradually warm over the extended forecast period, returning to the 80s in the Valley.

AVIATION
Pacific weather system moves through today with widespread MVFR conditions spreading south in the Central Valley through the morning, then generally VFR after 21Z except MVFR vicinity scattered showers and isolated thunderstorms. In the Sierra, widespread IFR/LIFR conditions develop after 15Z with rain, snow and thunderstorms. Snow levels lowering to 5000-6000 feet this afternoon, and to around 3500 feet tonight. Widespread southerly surface wind gusts 25-35 kts with 35-45 kts over higher terrain.
Wind decreasing in the Central Valley after 00Z Sunday.

STO WATCHES/WARNINGS/ADVISORIES
Wind Advisory until 11 AM PDT this morning for Central Sacramento Valley-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley.

Winter Storm Warning from 11 AM this morning to 8 AM PDT Sunday for West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park.

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