705
FXUS21 KWNC 132100
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 13 2023
SYNOPSIS: 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. 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.
HAZARDS
MODERATE RISK FOR HEAVY PRECIPITATION FOR COASTAL PORTIONS OF CALIFORNIA SOUTH
OF SAN FRANCISCO, MON-WED, DEC 25-27.
MODERATE RISK FOR HEAVY SNOW FOR THE CENTRAL SIERRA NEVADA MOUNTAINS, MON-WED,
DEC 25-27.
MODERATE RISK FOR HEAVY SNOW FOR THE CENTRAL AND NORTHERN WASATCH MOUNTAINS,
THU-SUN, DEC 21-24.
MODERATE RISK FOR HEAVY SNOW FOR THE CENTRAL AND NORTHERN FRONT RANGE IN
COLORADO, THU-SUN, DEC 21-24.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR PORTIONS OF SOUTHWEST OREGON AND MUCH
OF CALIFORNIA, THU DEC 21, AND SUN-WED DEC 24-27.
SLIGHT RISK FOR HEAVY SNOW FOR THE SIERRA NEVADAS AND COASTAL MOUNTAINS OF
NORTHERN CALIFORNIA AND SOUTHERN OREGON, THU DEC 21, AND SUN-WED DEC 24-27.
SLIGHT RISK FOR HIGH WINDS FOR COASTAL PORTIONS OF CALIFORNIA AND SOUTHERN
OREGON, THU DEC 21, AND SUN-WED DEC 24-27.
SLIGHT RISK FOR HEAVY SNOW FOR THE CENTRAL ROCKIES AND WASATCH MOUNTAINS AS
WELL AS PORTIONS OF WESTERN AND SOUTHERN WYOMING, THU-SUN, DEC 21-24.
SLIGHT RISK FOR HIGH WINDS 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, THU-MON, DEC. 21-25.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR COASTAL PORTIONS OF ALASKA FROM PRINCE
WILLIAMS SOUND SOUTH TO INCLUDE ALL OF THE ALASKAN PANHANDLE, THU-MON, DEC.
21-25.
DETAILED SUMMARY
FOR SATURDAY DECEMBER 16 - WEDNESDAY DECEMBER 20:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY DECEMBER 21 - WEDNESDAY DECEMBER 27: THERE IS STILL A LOT OF
SPREAD AMONG ENSEMBLE MODEL SOLUTIONS WITH REGARD TO WHETHER A CUT-OFF LOW
FORMS OR NOT AT THE BEGINNING OF THE FORECAST PERIOD, WITH THE ECMWF FAVORING
THE CUT-OFF AND THE GEFS NOT. THIS LEAVES A LOT OF UNCERTAINTY IN THE POTENTIAL
FOR HAZARDOUS WEATHER CONDITIONS AT THE OUTSET OF WEEK-2. HOWEVER, THE END OF
THE FORECAST PERIOD HAS COME INTO BETTER FOCUS AS ENSEMBLE SOLUTIONS CONVERGE
ON A RELOADING OF THE MID-LEVEL TROUGH OFF THE WEST COAST AND A FRESH WAVE OF
STORMY ONSHORE FLOW FOR CALIFORNIA DURING THE SECOND HALF OF WEEK-2. AS FOR THE
INITIAL WAVE, REGARDLESS OF WHETHER A CUT-OFF LOW FORMS OR NOT, AS THE SYSTEM
MOVES INLAND, MODELS INDICATE THE POTENTIAL FOR HEAVY SNOW IN THE MOUNTAINS OF
THE INTERIOR WEST IN THE DAYS LEADING UP TO CHRISTMAS.
DIFFERENCES IN MODEL SOLUTIONS NOTWITHSTANDING, THERE IS REASONABLE CONFIDENCE
OF INCREASED CHANCES FOR PERIODS OF HIGH WINDS AND HEAVY PRECIPITATION FOR
SOUTHWESTERN OREGON AND MUCH OF CALIFORNIA DURING THE FORECAST PERIOD,
PARTICULARLY THE LATER HALF. 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 CALIFORNIA AND ADJACENT OREGON. THE ECMWF CONCENTRATES THESE
PROBABILITIES IN CENTRAL AND SOUTHERN CALIFORNIA AND IS MUCH MORE BULLISH THAN
THE GEFS WITH THE CHANCES FOR HEAVY PRECIPITATION. THE INITIAL WAVE WILL BE
DRAWING TO A CLOSE BY THE BEGINNING OF WEEK-2 WITH MOST OF THE ACTIVITY NOW IN
THE WEEK-1 TIME FRAME, AFTER WHICH BOTH MODELS INDICATE A LULL IN STORMINESS
FOR DEC 22-23. AS THE MID-LEVEL TROUGH RELOADS AND ONSHORE FLOW INCREASES,
PROBABILITIES FOR HEAVY PRECIPITATION AGAIN INCREASE. THE ECMWF PET INDICATES
OVER A 60% PROBABILITY OF 3-DAY PRECIPITATION EXCEEDING THE 85TH PERCENTILE AND
AT LEAST 1 INCH FOR CENTRAL AND SOUTHERN CALIFORNIA THROUGHOUT THE FORECAST
PERIOD. THE GEFS PET KEEPS THE PROBABILITIES MUCH LOWER BUT DOES APPROACH 40%
FOR DAYS 12-14. THEREFORE, A MODERATE RISK FOR HEAVY PRECIPITATION IS POSTED
FOR COASTAL PORTIONS OF CALIFORNIA SOUTH OF SAN FRANCISCO, WHERE THE PETS HAVE
THE CLOSEST AGREEMENT. THE PETS FROM THE GEFS AND ECMWF ALSO INDICATE THE
POTENTIAL FOR HIGH WINDS ALONG THE WEST COAST SIMILAR TO WHAT IS SEEN WITH
HEAVY PRECIPITATION. BOTH MODELS INDICATE AT LEAST A 20% PROBABILITY OF WIND
SPEEDS EXCEEDING THE 85TH PERCENTILE AND 25 MPH FOR THE ENTIRE COAST OF
CALIFORNIA AND THE SOUTHERN COAST OF OREGON FOR DAYS 8 AND 11-14.
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. THE SIERRA NEVADA MOUNTAINS
AS WELL AS COASTAL RANGES OF NORTHERN CALIFORNIA AND SOUTHERN OREGON HAVE A
SLIGHT RISK OF HEAVY SNOW POSTED FOR DEC 21 AND 24-27 AND THE CENTRAL SIERRA
NEVADAS HAVE A MODERATE RISK FOR HEAVY SNOW POSTED FOR DEC 25-27, FOLLOWING THE
FORECAST REASONING OUTLINED ABOVE FOR HEAVY PRECIPITATION. FURTHER INLAND, AS
THE INITIAL WAVE OF ENERGY MOVES INLAND AT THE BEGINNING OF WEEK-2, MODELS
DEPICT WIDESPREAD SNOW IN THE HIGHER ELEVATIONS. THIS IS WELL-INDICATED BY THE
GEFS SNOW WATER EQUIVALENT (SWE) PET, WHICH HAS WIDESPREAD PROBABILITIES ABOVE
20% FOR EXCEEDING THE 85TH PERCENTILE AND AT LEAST A HALF INCH, WITH 3-DAY SWE
OVER AN INCH POSSIBLE FOR THE HIGHER ELEVATIONS. THEREFORE A SLIGHT RISK FOR
HEAVY SNOW IS POSTED FOR THE CENTRAL ROCKIES AND WASATCH MOUNTAINS AS WELL AS
PORTIONS OF WESTERN AND SOUTHERN WYOMING FOR DEC 21-24, AND A MODERATE RISK FOR
THE CENTRAL AND NORTHERN WASATCH AND THE CENTRAL AND NORTHERN FRONT RANGE OF
COLORADO FOR THE SAME DATES.
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 21-25 FOR THE AREAS NOTED ABOVE.
FORECASTER: DANNY BARANDIARAN
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