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FXUS21 KWNC 201910
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 20 2023
SYNOPSIS: MID-LEVEL LOW PRESSURE OVER THE NORTH PACIFIC AT THE END OF WEEK-1 IS
PREDICTED TO WEAKEN THROUGHOUT WEEK-2. HEAVY PRECIPITATION (HIGH ELEVATION
SNOW) AND HIGH WINDS ACROSS THE WEST COAST AND INTERIOR CALIFORNIA IS PREDICTED
TO LINGER UNTIL THE MIDDLE OF THE PERIOD. HEAVY PRECIPITATION IS POSSIBLE
ACROSS THE NORTHEAST AT THE ONSET OF THE PERIOD AS SURFACE LOW PRESSURE OVER
THE EASTERN CONTIGUOUS U.S. (CONUS) AT THE END OF WEEK-1 TRACKS TOWARDS THE
EAST COAST BY THE BEGINNING OF WEEK-2.
HAZARDS
MODERATE RISK FOR HEAVY PRECIPITATION FOR PARTS OF NORTHERN AND CENTRAL
CALIFORNIA, THU-SAT, DEC 28-30.
MODERATE RISK FOR HEAVY SNOW FOR THE CENTRAL AND NORTHERN SIERRA NEVADA,
SOUTHERN CASCADES, AND COASTAL RANGES OF NORTHERN CALIFORNIA, THU-SAT, DEC
28-30.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR MUCH OF THE WEST COAST AND INTERIOR
CALIFORNIA, THU-MON, DEC 28-JAN 1.
SLIGHT RISK FOR HEAVY SNOW FOR THE SIERRA NEVADA, SOUTHERN CASCADES, AND
COASTAL RANGES OF NORTHERN CALIFORNIA, THU-MON, DEC 28-JAN 1.
SLIGHT RISK FOR HIGH WINDS FOR MUCH OF THE WEST COAST, INCLUDING THE CALIFORNIA
VALLEYS, SIERRA NEVADA, AND SOUTHERN CASCADES, THU-MON, DEC 28-JAN 1.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR PARTS OF THE NORTHEAST, THU, DEC 28.
DETAILED SUMMARY
FOR SATURDAY DECEMBER 23 - WEDNESDAY DECEMBER 27:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY DECEMBER 28 - WEDNESDAY JANUARY 03: THERE IS RELATIVELY GOOD
AGREEMENT AMONG MULTIPLE MODEL ENSEMBLE MEANS INDICATING AMPLIFIED MID-LEVEL
TROUGHING OVER THE NORTH PACIFIC PEAKING BY THE END OF WEEK-1 AND GRADUALLY
WEAKENING THROUGHOUT WEEK-2. INCREASED CHANCES FOR HEAVY PRECIPITATION AND HIGH
WINDS ACROSS THE WEST COAST ARE PREDICTED TO LINGER UNTIL THE MIDDLE OF THE
PERIOD.
PROBABILITIES FOR EXCEEDING THE 85TH PERCENTILE HAVE DECREASED IN THE ECMWF AND
CANADIAN PROBABILISTIC EXTREMES TOOLS (PETS), WITH THE GEFS INDICATING A SLIGHT
INCREASE TO >30% CHANCES FOR A SCOPED AREA ACROSS COASTAL CENTRAL CALIFORNIA. A
MODERATE RISK (>40% CHANCE) OF HEAVY PRECIPITATION IS CONTINUED ACROSS MUCH OF
CALIFORNIA, DEC 28-30, WITH THE NORTHERN EXTENT REMOVED IN TODAY'S OUTLOOK. THE
SLIGHT RISK (>20% CHANCE) OF HEAVY PRECIPITATION ACROSS THE PACIFIC NORTHWEST
AND CALIFORNIA IS SCALED BACK SLIGHTLY IN TIME AND SPACE COMPARED TO
YESTERDAY'S OUTLOOK, FOR DEC 28-JAN 1 AND THE SOUTHERN EXTENT DECREASED. THE
GEFS SNOW WATER EQUIVALENT (SWE) PET NO LONGER SHOWS PROBABILITIES REACHING 20%
FOR 3-DAY SWE EXCEEDING THE 85TH PERCENTILE. HOWEVER DUE TO THE GIVEN
ANTICIPATED CONTINUED ENHANCED MOIST FLOW TO THE REGION AND SUPPORT FROM
DETERMINISTIC MODELS AND UNCALIBRATED ECMWF, SLIGHT AND MODERATE RISKS FOR
HEAVY SNOW ARE CONTINUED ACROSS HIGHER ELEVATIONS OF THE SIERRA NEVADA,
SOUTHERN CASCADES, AND COASTAL RANGES OF NORTHERN CALIFORNIA UNTIL THE MIDDLE
OF THE PERIOD. PET WIND SIGNALS HAVE DECREASED SOMEWHAT COMPARED TO YESTERDAY,
THUS THE MODERATE RISK FOR HIGH WINDS IS REMOVED IN TODAY'S OUTLOOK, AND THE
SLIGHT RISK REMAINS FOR DEC 28-30. MULTIPLE MODEL PETS SHOW AT LEAST A 20%
CHANCE OF WIND SPEEDS EXCEEDING THE 85TH PERCENTILE AND 40 MPH NEAR THE COAST
OF THE PACIFIC NORTHWEST AND NORTHERN CALIFORNIA (>25 MPH FURTHER SOUTH).
A SURFACE LOW PRESSURE SYSTEM OVER THE EASTERN CONUS AT THE END OF WEEK-1 IS
PREDICTED TO REACH THE EAST COAST BY THE START OF WEEK-2, ALTHOUGH THERE ARE
MODEL DIFFERENCES REGARDING THE SPECIFICS OF THE EVOLUTION OF THIS STORM.
MULTIPLE MODELS GENERALLY FAVOR MUCH OF THE HEAVIEST PRECIPITATION REMAINING
OFFSHORE BY DAY 8, ALTHOUGH THERE COULD BE LINGERING HEAVY PRECIPITATION ACROSS
THE NORTHEAST. THE GEFS PET SHOWS A SMALL REGION OVER MAINE WITH AT LEAST A 20%
CHANCE OF 3-DAY PRECIPITATION EXCEEDING THE 85TH PERCENTILE, WITH THE ECMWF PET
KEEPING THESE THRESHOLDS OFFSHORE. DETERMINISTIC MODEL GUIDANCE AND ENSEMBLE
MEANS STILL HINT AT THE POSSIBILITY OF REMNANT HEAVY PRECIPITATION LOCALLY
ACROSS THE NORTHEAST, THUS A SLIGHT RISK FOR HEAVY PRECIPITATION IS POSTED FOR
THIS REGION FOR DEC 28. THE ADVANCED HYDROLOGIC PREDICTION SERVICE (AHPS)
INDICATES SOME AREAS ACROSS THE NORTHEAST CURRENTLY EXPERIENCING MINOR TO MAJOR
FLOODING. EXPECTED ENHANCED PRECIPITATION DURING WEEK-2 COULD EXACERBATE
CURRENT FLOOD CONDITIONS. HOWEVER, NO SPECIFIC REGION CAN BE IDENTIFIED WITH AN
ENHANCED RISK DUE TO THE UNCERTAINTIES IN THE EVOLUTION OF THIS STORM SYSTEM.
MID-LEVEL TROUGHING IS PREDICTED OVER THE BERING SEA AND WESTERN ALASKA WITH
RIDGING EASTWARD. SURFACE LOW PRESSURE IS FORECAST OVER THE ALEUTIANS,
SOUTHWESTERN ALASKA, AND THE GULF OF ALASKA. HOWEVER, MODEL GUIDANCE AND TOOLS
DO NOT INDICATE ANY ASSOCIATED PRECIPITATION OR WINDS REACHING HAZARDS CRITERIA
AT THIS TIME, WITH MUCH OF THE ANOMALOUSLY WET AND WINDY WEATHER FURTHER SOUTH
OF THE STATE. STORMY WEATHER IN THE GULF OF ALASKA MAY HOWEVER BRING PERIODS OF
SIGNIFICANT WIND SPEEDS OVER MARITIME AREAS THROUGHOUT WEEK-2.
FORECASTER: MELISSA OU
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