160
FXUS21 KWNC 122043
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 12 2023
SYNOPSIS: A SERIES OF SURFACE LOW PRESSURE SYSTEMS ARE PREDICTED TO MOVE FROM
THE BERING SEA INTO THE GULF OF ALASKA DURING THE FORECAST PERIOD, RESULTING IN
A STORMY FORECAST WITH THE POTENTIAL FOR HIGH WINDS AND HEAVY PRECIPITATION FOR
MUCH OF THE SOUTHERN COAST OF ALASKA. MID-LEVEL LOW PRESSURE IS FAVORED ACROSS
THE WEST COAST OF THE CONTIGUOUS U.S. (CONUS) THROUGHOUT THE FORECAST PERIOD,
RESULTING IN THE POTENTIAL FOR HEAVY PRECIPITATION AND HIGH WINDS ALONG THE
COAST, AS WELL AS HEAVY SNOW FOR HIGHER ELEVATIONS IN THE INTERIOR WEST.
HAZARDS
SLIGHT RISK FOR HEAVY PRECIPITATION FOR COASTAL PORTIONS OF CALIFORNIA FROM THE
U.S.-MEXICO BORDER TO THE SAN FRANCISCO BAY, WED-THU AND SAT-TUE, DEC. 20-21,
23-26.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR COASTAL PORTIONS OF WASHINGTON, OREGON,
AND CALIFORNIA NORTH OF SAN FRANCISCO BAY, SAT-TUE, DEC. 23-26.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR COASTAL PORTIONS OF ALASKA FROM PRINCE
WILLIAMS SOUND SOUTH TO INCLUDE ALL OF THE ALASKAN PANHANDLE, WED-SUN, DEC.
20-24.
SLIGHT RISK FOR HIGH WIND FOR COASTAL PORTIONS OF CALIFORNIA FROM THE
U.S.-MEXICO BORDER TO THE SAN FRANCISCO BAY, WED-THU AND SAT-TUE, DEC. 20-21,
23-26.
SLIGHT RISK FOR HIGH WIND FOR COASTAL PORTIONS OF WASHINGTON, OREGON, AND
CALIFORNIA NORTH OF SAN FRANCISCO BAY, SAT-TUE, DEC. 23-26.
SLIGHT RISK FOR HIGH WIND FOR COASTAL PORTIONS OF ALASKA FROM NUNIVAK ISLAND
SOUTH AND EAST TO INCLUDE ALL OF THE ALASKAN PENINSULA, KODIAK ISLAND, THE
KENAI PENINSULA, AND THE ALASKA PANHANDLE, WED-SUN, DEC. 20-24.
SLIGHT RISK FOR HEAVY SNOW FOR THE CENTRAL AND SOUTHERN SIERRA NEVADAS, WED-THU
AND SAT-TUE, DEC. 20-21, 23-26.
SLIGHT RISK FOR HEAVY SNOW FOR THE NORTHERN SIERRA NEVADAS, KLAMATH AND CASCADE
RANGES, SAT-TUE, DEC. 23-26.
SLIGHT RISK FOR HEAVY SNOW FOR PORTIONS OF THE CENTRAL AND NORTHERN GREAT
BASIN, AND THE SOUTHERN AND CENTRAL ROCKIES, FRI-TUE, DEC. 22-26.
SLIGHT RISK FOR HEAVY SNOW FOR PORTIONS OF THE NORTHERN ROCKIES, SUN-TUE, DEC.
24-26.
DETAILED SUMMARY
FOR FRIDAY DECEMBER 15 - TUESDAY DECEMBER 19:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY DECEMBER 20 - TUESDAY DECEMBER 26: THE WEEK-2 FORECAST PERIOD IS
GENERALLY CHARACTERIZED BY WIDESPREAD MID-LEVEL TROUGHING FROM THE BERING SEA
DOWN THE WEST COAST OF THE CONUS AND RIDGING OVER THE CENTRAL AND EASTERN
CONUS. THERE IS A GREAT DEAL OF UNCERTAINTY REGARDING THE DETAILS OF THE
FORECAST HOWEVER AS THERE ARE NOTABLE DIFFERENCES IN THE SOLUTIONS OFFERED BY
THE GEFS AND ECMWF, WITH VERY DIFFERENT OUTCOMES FOR CALIFORNIA IN PARTICULAR.
THE ECMWF FEATURES A DISTINCT CUT-OFF LOW OFF THE COAST OF CALIFORNIA AT THE
BEGINNING OF WEEK-2, WHILE THE GEFS FAVORS BROAD TROUGHING EXTENDING THE FULL
LENGTH OF THE WEST COAST. DESPITE THESE DIFFERENCES HOWEVER, THERE IS
REASONABLE CONFIDENCE OF INCREASED CHANCES FOR PERIODS OF HIGH WINDS AND HEAVY
PRECIPITATION FOR MUCH OF CALIFORNIA FROM SAN FRANCISCO SOUTH FOR MOST OF
WEEK-2, AND FOR THE REST OF THE WEST COAST FOR THE SECOND HALF OF THE FORECAST
PERIOD. THIS IS SUPPORTED BY THE ECMWF AND GEFS PROBABILISTIC EXTREMES TOOLS
(PETS), WHICH BOTH INDICATE AT LEAST A 20% PROBABILITY OF 3-DAY PRECIPITATION
TOTALS EXCEEDING THE 85TH PERCENTILE AND 1 INCH FOR MUCH OF THE WEST COAST. THE
ECMWF CONCENTRATES THESE PROBABILITIES IN CENTRAL AND SOUTHERN CALIFORNIA AND
IS MUCH MORE BULLISH THAN THE GEFS WITH THE CHANCES FOR HEAVY PRECIPITATION.
REGARDLESS, BOTH MODELS INDICATE A SLIGHT LULL IN STORMINESS ON DEC 10, WHICH
IS REFLECTED IN THE HAZARDS POSTED SOUTH OF SAN FRANCISCO. THE PETS FROM THE
GEFS AND ECMWF ALSO INDICATE THE POTENTIAL FOR HIGH WINDS ALONG THE WEST COAST
IN A SIMILAR PROGRESSION TO WHAT IS SEEN WITH HEAVY PRECIPITATION. BOTH MODELS
INDICATE AT LEAST A 20% PROBABILITY OF WIND SPEED EXCEEDING THE 85TH PERCENTILE
AND 25 MPH SOUTH OF SAN FRANCISCO FOR MOST OF WEEK-2, AND FOR DEC 11-14 NORTH
OF SAN FRANCISCO.
IN ADDITION TO THE POTENTIAL FOR HEAVY PRECIPITATION AND HIGH WINDS ALONG THE
WEST COAST, THERE IS THE POTENTIAL FOR HEAVY SNOW FOR MUCH OF THE MOUNTAINOUS
AREAS OF THE WEST THROUGHOUT THE FORECAST PERIOD. LIKE THE PRECIPITATION AND
WIND HAZARDS DISCUSSED EARLIER, THE HEAVY SNOW HAZARDS POSTED ARE DIVIDED
NORTH-SOUTH WITH REGARD TO THE TIME PERIODS COVERED. FOR THE SIERRA NEVADAS AND
MOUNTAIN RANGES ALONG THE COAST, THE DIVIDING LINE IS JUST SOUTH OF LAKE TAHOE,
WITH A SLIGHT RISK FOR HEAVY SNOW SOUTH OF THIS LINE FOR DEC 20-21 AND 23-26,
AND JUST FOR DEC 23-26 NORTH OF THIS LINE. FOR THE INTERIOR WEST, HIGHER
ELEVATIONS SOUTH OF THE SNAKE RIVER IN IDAHO AND THE WIND RIVER MOUNTAINS IN
WYOMING A SLIGHT RISK FOR HEAVY SNOW IS POSTED FOR DEC 22-26, WHILE THE
BITTERROOTS AND THE MOUNTAINS OF MONTANA ARE HIGHLIGHTED FOR A SLIGHT RISK FOR
HEAVY SNOW DEC 24-26, SUPPORTED BY THE GEFS PET FOR SNOW WATER EQUIVALENT
(SWE). IT SHOULD BE NOTED THAT THE TEMPERATURE OUTLOOK FOR THE CONUS FAVORS
ABOVE-NORMAL TEMPERATURES PRETTY STRONGLY, SO IT IS LIKELY THAT SNOW LEVELS
WILL BE A LITTLE HIGHER THAN USUAL FOR A WINTER STORM DURING THIS TIME OF YEAR.
ENSEMBLE SOLUTIONS FROM THE GEFS, ECMWF, AND CMCE ALL INDICATE A SERIES OF DEEP
LOW PRESSURE SYSTEMS MOVING FROM THE BERING SEA INTO THE GULF OF ALASKA
THROUGHOUT THE WEEK-2 PERIOD, RESULTING IN ELEVATED CHANCES FOR HIGH WIND AND
HEAVY PRECIPITATION. PETS FROM ALL THREE MODELS INDICATE AT LEAST A 20%
PROBABILITY OF WIND SPEEDS EXCEEDING THE 85TH PERCENTILE AND AT LEAST 40 MPH
ALONG THE COAST FROM NUNIVAK ISLAND SOUTH, AND A 20% PROBABILITY OF 3-DAY
PRECIPITATION TOTALS EXCEEDING THE 85TH PERCENTILE ROUGHLY FROM PRINCE WILLIAM
SOUND SOUTH FOR MOST OF THE FORECAST PERIOD. DAILY ANALYSIS FROM THE MODEL
ENSEMBLES SUGGESTS THAT THE STRONGEST WINDS AND PRECIPITATION SHOULD EASE
TOWARDS THE END OF WEEK-2, THEREFORE A SLIGHT RISK FOR HEAVY PRECIPITATION AND
HIGH WINDS IS POSTED FOR DEC 20-24 FOR THE AREAS NOTED ABOVE.
FORECASTER: DANNY BARANDIARAN
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