262
FXUS66 KSTO 201157
AFDSTO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
357 AM PST WED NOV 20 2024
SYNOPSIS
A STRONG ATMOSPHERIC RIVER STORM BRINGS PERIODS OF MODERATE TO HEAVY
RAIN AND MOUNTAIN SNOW, ISOLATED THUNDERSTORMS, AND GUSTY SOUTHERLY
WINDS TO INTERIOR NORCAL THROUGH THE END OF THE WEEK AND INTO THE
WEEKEND.
*WINTER STORM WARNING: THROUGH 10 PM TODAY FOR ELEVATIONS ABOVE
3500 FEET ALONG THE SIERRA/SOUTHERN CASCADES INCLUDING INTERSTATE
80, THE COASTAL RANGE, AND ABOVE 3000 FEET IN THE SHASTA COUNTY
MOUNTAINS.
*FLOOD WATCH: THROUGH 4 AM SATURDAY FOR VALLEY/FOOTHILLS
LOCATIONS GENERALLY NORTH INTERSTATE 80 AND BELOW 3000 FEET.
*WIND ADVISORY: THROUGH 10 PM TODAY FOR THE NORTHERN/CENTRAL
SACRAMENTO VALLEY AND NORTHEAST FOOTHILLS.
DISCUSSION
CURRENT RADAR SHOWS PRECIPITATION CONTINUING TO MOVE INLAND ACROSS
THE COASTAL RANGE AND NORTHERNMOST PORTIONS OF THE SACRAMENTO
VALLEY EARLY THIS MORNING [AS OF 300 AM]. OVER THE LAST 12 HOURS,
PRECIPITATION AMOUNTS RANGE FROM 0.15 TO 0.75 INCHES OVER THE
COASTAL RANGE AND SHASTA COUNTY MOUNTAINS. FURTHER INTO THE
NORTHERN SACRAMENTO VALLEY AND SIERRA/SOUTHERN CASCADES,
ACCUMULATIONS ARE FAR LESS AT AROUND 0.05 TO 0.10 INCHES.
SHOWER COVERAGE WILL EXPAND THROUGHOUT THE DAY TODAY, WITH
HIGHEST IMPACTS EXPECTED NORTH OF US 50.
PERIODS OF MODERATE TO HEAVY RAINFALL ARE EXPECTED THROUGHOUT THE
DAY WHICH MAY RESULT IN PONDING OF WATER ON ROADS, HAZARDOUS
TRAVEL CONDITIONS, AND LONGER THAN NORMAL COMMUTES. ADDITIONALLY,
SOUTHERLY WINDS ARE ALREADY GUSTING 20 TO 30 MPH THROUGHOUT THE VALLEY
WITH GUSTS 45 TO 50 MPH OVER THE MOUNTAINS. STRONGEST LOWER
ELEVATION WIND GUSTS 35 TO 45 MPH ARE ANTICIPATED ACROSS THE
NORTHERN/CENTRAL SACRAMENTO VALLEY, WITH GUSTS 25 TO 35 MPH
ELSEWHERE THROUGH THIS EVENING. ALONG THE SIERRA, GUSTS TO 55 MPH
WILL BE POSSIBLE AS WELL. HEAVIEST PRECIPITATION WITH THIS
INITIAL WAVE IS CURRENTLY EXPECTED TO PERIODICALLY FALL THROUGH
FRIDAY MORNING, WITH COMPARATIVELY LIGHTER PRECIPITATION THEN
ANTICIPATED THROUGH SATURDAY MORNING. ALTHOUGH, HIGHEST
THUNDERSTORM POTENTIAL (15% TO 25% ACROSS THE VALLEY/FOOTHILLS) IS
EXPECTED FRIDAY EVENING INTO SATURDAY AS THE TROUGH EVENTUALLY
TRAVELS INLAND, WITH ADDITIONAL THUNDERSTORM CHANCES (10-20%) ON
SUNDAY.
FORECAST PRECIPITATION TOTALS THROUGH SATURDAY MORNING CONTINUE
TO FAVOR ACCUMULATIONS OF 7 TO 13 INCHES AT HIGH ELEVATIONS NORTH
OF US 50, 3 TO 6 INCHES FOR VALLEY/FOOTHILLS LOCATIONS NORTH OF
US 50, AND 1 TO 3 INCHES ELSEWHERE. AS A RESULT, PROBABILITIES OF
EXCEEDING 3" OF PRECIPITATION AROUND 40% TO 70% FOR LOCATIONS FROM
US 50 SOUTHWARD, WITH 70% TO 95% PROBABILITIES FOR LOCATIONS
GENERALLY NORTH OF US 50.
AS THIS TROUGH PROGRESSES AND RETROGRADES FROM WESTWARD FROM THE
PACIFIC NORTHWEST COAST, SNOW LEVELS ARE EXPECTED TO FLUCTUATE
THROUGH THE REMAINDER OF THE WEEK. INITIAL SNOW LEVELS THIS MORNING
ARE EXPECTED TO SIT AROUND 3500 TO 4500 FEET, BEFORE RISING TO
4500 TO 5500 FEET BY WEDNESDAY AFTERNOON. SNOW LEVELS IN SHASTA
COUNTY WILL STILL BE AROUND 4000 TO 5000 FEET THIS AFTERNOON. SNOW
LEVELS THEN RISE TO 7000 TO 8000 FEET THURSDAY INTO FRIDAY.
LOWEST SNOW LEVELS DOWN TO 3000 FEET ARE EXPECTED TO REMAIN
CONFINED TO THE SHASTA COUNTY MOUNTAINS. FORECAST SNOWFALL
ACCUMULATIONS OF 10 TO 20 INCHES WILL BE POSSIBLE THROUGH SATURDAY
MORNING AT ELEVATIONS ABOVE 4500 FEET, WITH ACCUMULATIONS 5 TO 10
INCHES DOWN TO AROUND 3500 FEET ALONG THE SIERRA AND DOWN TO
AROUND 3000 FEET ACROSS THE SHASTA COUNTY MOUNTAINS. LOCALLY
HIGHER TOTALS OF 2 TO 4 FEET WILL BE POSSIBLE OVER PEAKS AS WELL.
RESULTANT PROBABILITIES OF SNOWFALL EXCEEDING 1 FOOT ARE AROUND
30% TO 70% ABOVE 4500', WITH 30% TO 60% PROBABILITIES OF EXCEEDING
6 INCHES OF SNOWFALL BETWEEN 3000 AND 4500 FEET.
EXTENDED DISCUSSION (SUNDAY THROUGH WEDNESDAY)
ENSEMBLE GUIDANCE AND CLUSTER ANALYSIS INDICATE UPPER LEVEL
TROUGHING AS THE PRIMARY SYNOPTIC INFLUENCE OVER MUCH OF THE
EXTENDED FORECAST PERIOD, BRINGING CONTINUED PRECIPITATION CHANCES
AND PERIODS OF BREEZY WINDS TO THE REGION THROUGH MID NEXT WEEK.
THE OVERALL PROGRESSION OF THE TROUGH LOOKS TO BE WEAKENING AND
MOVING INLAND EARLY NEXT WEEK. THERE IS STILL SOME UNCERTAINTY IN
EXACT DETAILS, HOWEVER WIDESPREAD PRECIPITATION RAIN AND MOUNTAIN
SNOW CHANCES CONTINUE TO BE IN THE FORECAST THROUGH TUESDAY, WITH
SOME LINGERING MOUNTAIN SHOWER CHANCES ON WEDNESDAY. THE NATIONAL
BLEND OF MODELS (NBM) CURRENTLY SUGGESTS A 30-60% PROBABILITY OF
PRECIPITATION AMOUNTS OF 1 INCH OR GREATER OVER THE SUNDAY THROUGH
TUESDAY TIMEFRAME IN THE VALLEY, WITH A 55-85% PROBABILITY IN THE
FOOTHILLS AND MOUNTAINS.
SNOW LEVELS LOOK TO GENERALLY BE AROUND 4000-5000 FEET, WITH JUST
SOME MINOR FLUCTUATIONS WITH THIS ROUND. THE NBM OFFERS A 55-85%
PROBABILITY OF SNOW AMOUNTS OF 12 INCHES OR GREATER, MAINLY ABOVE
5000 FEET OVER THE SAME SUNDAY THROUGH TUESDAY TIMEFRAME.
ISOLATED THUNDERSTORMS WILL ALSO BE POSSIBLE ON SUNDAY, WITH
GENERALLY A 10-15% PROBABILITY OF THUNDERSTORM DEVELOPMENT. WITH
THE ACTIVE WEATHER IN THE FORECAST THIS WEEK AND INTO NEXT WEEK,
BE SURE TO STAY UP TO DATE WITH THE LATEST FORECAST AT
WEATHER.GOV/STO AND CHECK CURRENT ROAD CONDITIONS AT
QUICKMAP.DOT.CA.GOV BEFORE TRAVELING!
AVIATION
GENERAL VFR CONDITIONS EXPECTED FOR MOST INTERIOR NORCAL
LOCATIONS THROUGH 12Z, THEN PERIODS OF MVFR/IFR CONDITIONS
EXPECTED WITH LIFR IN THE MOUNTAINS AS A STORM SYSTEM BRINGS
MODERATE TO HEAVY PRECIPITATION TO THE REGION, SPREADING INLAND
FROM THE NORTHWEST TO THE SOUTHEAST THROUGH THE DAY. GUSTY
SOUTHERLY WINDS EXPECTED, WITH GUSTS 20 TO 35 KTS FOR THE DELTA,
VALLEY, AND FOOTHILLS, AND GUSTS OF 30 TO 50 KTS ALONG THE SIERRA.
SOME LLWS ALSO POSSIBLE AT RDD AND RBL.
STO WATCHES/WARNINGS/ADVISORIES
WINTER STORM WARNING UNTIL 10 PM PST THIS EVENING FOR MOUNTAINS
SOUTHWESTERN SHASTA COUNTY TO WESTERN COLUSA COUNTY-SHASTA LAKE
AREA / NORTHERN SHASTA COUNTY-WEST SLOPE NORTHERN SIERRA NEVADA-
WESTERN PLUMAS COUNTY/LASSEN PARK.
FLOOD WATCH THROUGH LATE FRIDAY NIGHT FOR CENTRAL SACRAMENTO
VALLEY-MOUNTAINS SOUTHWESTERN SHASTA COUNTY TO WESTERN COLUSA
COUNTY-NORTHEAST FOOTHILLS/SACRAMENTO VALLEY-NORTHERN SACRAMENTO
VALLEY-SHASTA LAKE AREA / NORTHERN SHASTA COUNTY.
WIND ADVISORY UNTIL 10 PM PST THIS EVENING FOR CENTRAL
SACRAMENTO VALLEY-NORTHEAST FOOTHILLS/SACRAMENTO VALLEY-NORTHERN
SACRAMENTO VALLEY.
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