895
FXUS66 KSTO 181211
AFDSTO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
411 AM PST MON NOV 18 2024
SYNOPSIS
COLD VALLEY TEMPERATURES TONIGHT INTO TUESDAY MORNING, WITH LOW
ELEVATION FROST POSSIBLE. PRECIPITATION IMPACTS RAPIDLY RETURN TO
THE REGION BY MIDDAY TUESDAY, WITH PERIODS OF MODERATE TO HEAVY
PRECIPITATION CONTINUING INTO THIS WEEKEND.
*FREEZE WARNING: 10 PM TONIGHT THROUGH 10 AM TUESDAY MORNING FOR
ELEVATIONS BELOW 2000 FEET.
*COLD WEATHER ADVISORY: 10 PM MONDAY NIGHT THROUGH 10 AM TUESDAY
MORNING FOR ELEVATIONS ABOVE 6000 FEET ALONG
THE SIERRA.
*WINTER STORM WATCH: 4 PM TUESDAY THROUGH 10 PM THURSDAY FOR
ELEVATIONS ABOVE 3500 FEET ALONG THE SIERRA/
SOUTHERN CASCADES, COASTAL RANGE, AND SHASTA
COUNTY MOUNTAINS.
DISCUSSION
SCATTERED SHOWERS HAVE MOSTLY EXITED TO THE EAST, OVER THE SIERRA
EARLY THIS MORNING. TEMPERATURES ARE QUITE CHILLY THIS MORNING,
WITH VALLEY TEMPERATURES IN THE 40S WHILE AIR TEMPERATURES IN THE
MOUNTAINS/FOOTHILLS SIT IN THE 20S AND 30S. LOW TEMPERATURES THIS
MORNING ARE EXPECTED TO DIP INTO THE MID TO UPPER 30S IN THE
VALLEY; 20S TO 30S IN THE FOOTHILLS; AND INTO THE TEENS TO LOW 20S
IN THE MOUNTAINS.
COLD MORNING TEMPERATURES CONTINUE TO BE IN THE FORECAST FOR
TOMORROW MORNING WHEN THERE IS A 20 TO 40% CHANCE OF TEMPERATURES
LESS THAN 32 DEG F IN THE VALLEY. AS A RESULT, THERE IS
INCREASING CONFIDENCE IN MORE WIDESPREAD FROST IMPACTS AT LOW
ELEVATIONS ACROSS INTERIOR NORCAL. A FREEZE WARNING HAS BEEN
ISSUED FOR LOCATIONS BELOW 2000 FEET IN ELEVATION BEGINNING
TONIGHT AT 10 PM UNTIL 10 AM TUESDAY. ADDITIONALLY, COLD
TEMPERATURES 3 TO 15 DEGREES ARE EXPECTED ALONG THE SIERRA AT
ELEVATIONS ABOVE 6000 FEET AS WELL.
A BRIEF REPRIEVE FROM PRECIPITATION IMPACTS IS EXPECTED THROUGH
TUESDAY MORNING. WITH BREEZY NORTHERLY WINDS USHERING IN COOLER
AIR AS THE SYSTEM EXITS THE REGION AND CLEARING SKIES EXPECTED BY
THE EVENING/OVERNIGHT HOURS.
A SEPARATE TROUGH IS THEN EXPECTED TO RAPIDLY DEEPEN AS IT DIGS
SOUTHWESTWARD FROM THE GULF OF ALASKA, INTRODUCING SEVERAL PERIODS
OF PRECIPITATION IMPACTS TO THE REGION FROM MIDDAY TUESDAY INTO
NEXT WEEKEND. SHOWERS ARE EXPECTED TO SPREAD EASTWARD INTO
INTERIOR NORCAL BY TUESDAY AFTERNOON, WITH MORE APPRECIABLE
PRECIPITATION IMPACTS, INCLUDING PERIODS OF MODERATE TO HEAVY SNOW
ABOVE 3500 FEET, GENERALLY FROM INTERSTATE 80 NORTHWARD. SNOW
LEVELS WILL INITIALLY SIT AS LOW AS 3000 FEET IN ELEVATIONS IN FAR
NORTH SHASTA COUNTY, WITH SNOW LEVELS ELSEWHERE BETWEEN 3500 TO
4500. THESE ARE EXPECTED TO INCREASE TO 4500 TO 5500 FEET BY
WEDNESDAY AFTERNOON. THE NATIONAL BLEND OF MODELS (NBM) IS
INDICATING A 40 TO 90% CHANCE OF 2.00" OR MORE OF RAIN NORTH OF
I-80, WITH PROBABILITIES HIGHEST OVER THE NORTHERN SACRAMENTO
VALLEY, MOUNTAINS, AND FOOTHILLS. ADDITIONALLY, THE NBM INDICATES
60 TO 90% CHANCE OF 12" OF SNOW OR MORE AT LOCATIONS ABOVE 3500
FEET AND NORTH OF I-80. SNOW SHOWERS WILL GRADUALLY TAPER OFF
OVERNIGHT WEDNESDAY INTO THURSDAY AS SNOW LEVELS INCREASE. AN
ACTIVE WEATHER PATTERN IS EXPECTED TO CONTINUE INTO THE WEEKEND.
EXTENDED DISCUSSION (FRIDAY THROUGH MONDAY)
LATEST ENSEMBLE GUIDANCE AND CLUSTER ANALYSIS INDICATE AN ACTIVE
WEATHER PATTERN CONTINUING OVER THE EXTENDED FORECAST PERIOD AS AN
UPPER LEVEL TROUGH SET UP OFF THE COAST OF THE PACIFIC NORTHWEST
BRINGS ALONG MULTIPLE ROUNDS OF PRECIPITATION THROUGH THE END OF
THE WEEK AND INTO THE WEEKEND. ATMOSPHERIC RIVER MOISTURE WILL
ALLOW FOR THE POTENTIAL FOR MODERATE TO HEAVY PRECIPITATION AT
TIMES, AND THE GFS ENSEMBLE PROBABILITY OF INTEGRATED WATER VAPOR
TRANSPORT (IVT) GREATER THAN 250 KG/(MS) IS PROJECTED TO BE AROUND
80-100% FOR THE SECOND HALF OF THE WEEK. THIS AMPLE MOISTURE IS
REFLECTED IN THE NATIONAL BLEND OF MODELS (NBM) AS WELL, WHICH
CURRENTLY SUGGESTS A 35-65% PROBABILITY OF RAINFALL AMOUNTS
GREATER THAN 1 INCH FOR AREAS SOUTH OF INTERSTATE 80, AND A 70-90%
PROBABILITY FOR AREAS NORTH OF INTERSTATE 80 OVER 48 HOURS FROM
THURSDAY THROUGH FRIDAY NIGHT. SNOW LEVELS EARLY THURSDAY MORNING
WILL BE AROUND 6000 FEET, AND THEN RISE QUICKLY TO AROUND
7000-8000 FEET THROUGH THE DAY AS WARMER AIR MOVES INTO THE AREA.
AS THE WEATHER SYSTEM MOVES INLAND LATER THIS WEEK (SOME
UNCERTAINTY STILL WITH TIMING), AN ADDITIONAL ROUND OF WIDESPREAD
PRECIPITATION IMPACTS IS ANTICIPATED ACROSS INTERIOR NORCAL BY
THE WEEKEND. PROBABILITIES FOR 1 INCH OF PRECIPITATION OR MORE
FROM LATE FRIDAY NIGHT THROUGH SUNDAY ARE AROUND 60-90%. THERE IS
ALSO A 10-20% PROBABILITY OF THUNDERSTORMS OVER THE
FRIDAY/SATURDAY TIMEFRAME THAT WILL BE DEPENDENT ON THE TIMING OF
THE SYSTEM MOVING ONSHORE. PERIODS OF GUSTY SOUTHERLY WINDS ARE
ALSO EXPECTED AT TIMES OVER THE EXTENDED FORECAST PERIOD.
WITH THE ACTIVE WEATHER IN THE FORECAST THIS 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
PRIMARILY VFR CONDITIONS ACROSS INTERIOR NORTHERN CALIFORNIA OVER
THE NEXT 24 HRS. POSSIBLE MVFR IN THE MOUNTAINS DUE TO SHOWERS
AND LOW CEILINGS THROUGH ABOUT 15Z. BREEZY SOUTHWEST WINDS WITH
GUSTS TO AROUND 30 TO 40 KTS OVER THE SIERRA THROUGH 12Z, AND
NORTHWEST GUSTS AT 15 TO 20 KTS IN THE DELTA AND NORTHERN SAN
JOAQUIN VALLEY UNTIL 00Z. ELSEWHERE, SURFACE WINDS GENERALLY AT OR
BELOW 12 KTS.
STO WATCHES/WARNINGS/ADVISORIES
FREEZE WARNING FROM 10 PM THIS EVENING TO 10 AM PST TUESDAY FOR
CARQUINEZ STRAIT AND DELTA-CENTRAL SACRAMENTO VALLEY-MOTHERLODE-
MOUNTAINS SOUTHWESTERN SHASTA COUNTY TO WESTERN COLUSA COUNTY-
NORTHEAST FOOTHILLS/SACRAMENTO VALLEY-NORTHERN SACRAMENTO VALLEY-
NORTHERN SAN JOAQUIN VALLEY-SHASTA LAKE AREA / NORTHERN SHASTA
COUNTY-SOUTHERN SACRAMENTO VALLEY.
WINTER STORM WATCH FROM TUESDAY AFTERNOON THROUGH WEDNESDAY
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.
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