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FXUS21 KWNC 291901
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST NOVEMBER 29 2023
SYNOPSIS: ENHANCED ONSHORE FLOW IS FAVORED TO LINGER FROM LATE WEEK-1 INTO
EARLY WEEK-2 WHERE LOCALLY HEAVY PRECIPITATION AMOUNTS AND HIGH ELEVATION HEAVY
SNOWFALL ARE POSSIBLE OVER PARTS OF THE WEST COAST. MID-LEVEL LOW PRESSURE
SHIFTING INLAND FROM THE NORTHEASTERN PACIFIC INCREASES THE RISK OF HIGH
ELEVATION HEAVY SNOWFALL AND EPISODES OF HIGH WINDS THROUGH MUCH OF WESTERN
CONTIGUOUS U.S. (CONUS). OVER ALASKA, STRONG MID-LEVEL LOW PRESSURE IS LIKELY
TO MAINTAIN STORMY CONDITIONS OVER THE SOUTH-CENTRAL MAINLAND AND THE SOUTHEAST
EARLY IN WEEK-2.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND NORTHERN
CALIFORNIA, THU, DEC 7.
SLIGHT RISK OF HEAVY SNOW FOR THE HIGHER ELEVATIONS OF THE CASCADES, AND PARTS
OF THE SIERRA NEVADAS, THU, DEC 7.
SLIGHT RISK OF HEAVY SNOW FOR THE HIGHER ELEVATIONS OF THE INTERMOUNTAIN WEST
AND ROCKIES, THU-SUN, DEC 7-10.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF THE WEST INCLUDING THE NORTHERN HIGH
PLAINS, THU-SUN, DEC 7-10.
SLIGHT RISK OF HIGH WINDS FOR PARTS OF SOUTHERN CALIFORNIA, THU, DEC 7.
SLIGHT RISK OF HIGH WINDS FOR THE KENAI PENINSULA OF ALASKA, COASTAL SOUTHERN
ALASKA, AND THE SOUTHEAST, THU-SAT, DEC 7-9.
DETAILED SUMMARY
FOR SATURDAY DECEMBER 02 - WEDNESDAY DECEMBER 06:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY DECEMBER 07 - WEDNESDAY DECEMBER 13: TOWARDS THE MIDDLE TO LATTER
PART OF NEXT WEEK, THERE IS GOOD AGREEMENT AMONG THE DYNAMICAL MODELS FEATURING
500-HPA TROUGHING OVER THE NORTHEASTERN PACIFIC WITH ANOMALOUS RIDGING
OVERSPREADING MUCH OF THE WESTERN AND CENTRAL CONUS. FURTHER DOWNSTREAM, HIGHLY
AMPLIFIED TROUGHING REMAINS FAVORED TO LIFT OUT INTO THE NORTHWESTERN ATLANTIC,
ALLOWING MANY OF THE WEATHER RELATED HAZARDS HIGHLIGHTED IN PREVIOUS OUTLOOKS
OVER THE EASTERN U.S. TO TIME OFF BY THE START OF WEEK-2. ONE EXCEPTION
CONCERNS SOME POTENTIAL FOR MUCH BELOW NORMAL TEMPERATURES TO LINGER OVER THE
SOUTHEASTERN CONUS IN THE WAKE OF A FRONTAL SYSTEM FAVORED LATE IN WEEK-1. BOTH
THE GEFS AND ECMWF PROBABILISTIC EXTREMES TOOLS (PETS) CONTINUE TO SHOW
INCREASED CHANCES FOR MINIMUM TEMPERATURES FALLING BELOW THE 15TH PERCENTILE
OVER PARTS OF THE SOUTHEAST, COASTAL CAROLINAS AND WELL INTO THE FLORIDA
PENINSULA. HOWEVER, MANY OF THESE AREAS ARE CURRENTLY EXPERIENCING THEIR FIRST
FREEZE OF THE SEASON, AND THERE IS LITTLE SUPPORT FOR MINIMUM TEMPERATURES
REACHING HAZARD CRITERIA SO NO CORRESPONDING HAZARD IS ISSUED.
WEST OF THE MISSISSIPPI, THERE IS GOOD CONSENSUS AMONG THE TEMPERATURE TOOLS
FOR UNSEASONABLY WARM CONDITIONS TIED TO THE ANOMALOUS RIDGING ALOFT AND THE
FLOW PATTERN DOMINATED BY PACIFIC AIR. MUCH OF THE ENHANCED ONSHORE FLOW AND
ATMOSPHERIC RIVER (AR) POTENTIAL OVER THE WEST COAST LATE IN WEEK-1 REMAINS
FAVORED TO BE WINDING DOWN BY THE OUTSET OF WEEK-2, WHERE BOTH THE GEFS AND
ECMWF PETS DEPICT LESSER CHANCES FOR PRECIPITATION AMOUNTS EXCEEDING THE 85TH
PERCENTILE COMPARED TO PREVIOUS GUIDANCE. WHILE THE GEFS APPEARS THE LEAST
SUPPORTIVE OF A HEAVY PRECIPITATION RISK, BOTH THE UNCALIBRATED AND CALIBRATED
ECMWF, AS WELL AS THE ECMWF BASED INTEGRATED VAPOR TRANSPORT (IVT) TOOL SUPPORT
THE CONTINUATION OF A SLIGHT RISK OF HEAVY PRECIPITATION FOR DAY 8 (DEC 7). THE
FAVORED CONTINUATION OF ABOVE-NORMAL PRECIPITATION INTO WEEK-2 IS EXPECTED TO
BE BENEFICIAL FOR AREAS EXPERIENCING DROUGHT OVER THE WEST COAST, HOWEVER ANY
LOCALLY HEAVY PRECIPITATION OVER BURN SCARRED AREAS WITHIN THE HIGHLIGHTED
SLIGHT RISK AREA MAY LEAD TO LOCALIZED FLOODING AND/OR DEBRIS FLOWS. WITH
ENHANCED MOISTURE IN PLACE, A SLIGHT RISK OF HEAVY SNOW ALSO REMAINS ISSUED FOR
DAY 8 (DEC 7) FOR THE HIGHER ELEVATIONS OF CASCADES AND SIERRA NEVADAS OF
NORTHERN CALIFORNIA. BASED ON THESE TOOLS, THE SLIGHT RISK COVERAGE FOR BOTH
HEAVY PRECIPITATION AND HEAVY SNOW IS REMOVED OVER PARTS OF WASHINGTON IN THE
UPDATED OUTLOOK WHERE WET SIGNALS HAVE TRENDED DRIER.
RELATIVE TO YESTERDAY, THERE IS BETTER AGREEMENT IN THE ENSEMBLE MEAN HEIGHT
FORECASTS FOR MUCH OF THE ANOMALOUS MID-LEVEL TROUGHING OVER THE NORTHEASTERN
PACIFIC TO SHIFT INLAND AND BRING ANOMALOUS TROPOSPHERIC MOISTURE INTO THE
WESTERN CONUS. A SLIGHT RISK OF HEAVY SNOW REMAINS ISSUED (DEC 7-10) OVER PARTS
OF THE INTERMOUNTAIN WEST AND ROCKIES WHERE THE GEFS BASED SNOW WATER
EQUIVALENT (SWE) PET INDICATES INCREASED CHANCES FOR VALUES EXCEEDING THE 85TH
PERCENTILE. A SLIGHT RISK OF HIGH WINDS ALSO REMAINS POSTED (DEC 7-10) OVER A
BROAD AREA OVER THE WESTERN CONUS AND INCLUDING THE HIGH PLAINS, WHICH IS
SUPPORTED IN THE PETS AND ENSEMBLE MEAN SURFACE PRESSURE ANALYSES. THE COVERAGE
OF BOTH THESE SNOW AND WIND HAZARDS ARE EXPANDED SOUTHWARD BASED ON LOWER
MID-LEVEL MEAN 500-HPA HEIGHTS IN THE GEFS AND ECMWF ENSEMBLES COMPARED TO
YESTERDAY. A SEPARATE SLIGHT RISK OF HIGH WINDS REMAINS POSTED FOR PARTS OF
SOUTHERN CALIFORNIA FOR DEC 7 BEFORE THE APPROACHING TROUGH BEGINS TO BREAK
DOWN MUCH OF SURFACE HIGH PRESSURE CENTERED OVER THE GREAT BASIN AND REDUCE THE
POTENTIAL FOR EPISODES OF SANTA ANA WINDS.
BY DAYS 10 AND 11 (DEC 9-10), THERE ARE GROWING DISPARITIES BETWEEN THE GEFS
AND ECMWF ENSEMBLES IN REGARDS TO THE EVOLUTION OF THE MID-LEVEL TROUGHING OVER
THE WEST. THE ECMWF FAVORS A DEEPER SOLUTION OF THE MEAN TROUGH AXIS, WITH LEE
SURFACE CYCLOGENESIS TO INTRODUCE THE RISK OF HEAVY PRECIPITATION DURING THE
MIDDLE OF WEEK-2. THIS IS REFLECTED IN THE ECMWF PET INDICATING AT LEAST A 20%
CHANCE OF 3-DAY AMOUNTS EXCEEDING THE 85TH PERCENTILE AND 1 INCH OVER PARTS OF
THE PLAINS, MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS, AND THE SOUTHEAST. BY
CONTRAST, THE GEFS FAVORS A SHALLOWER MEAN TROUGH WITH LITTLE TO NO INDICATION
OF HEAVY PRECIPITATION POTENTIAL IN THE TOOLS. DUE TO DISAGREEMENT BETWEEN THE
MODELS AT THIS LEAD AND THE NOVELTY OF THIS DEVELOPMENT IN THE ECMWF, NO
CORRESPONDING HAZARD IS POSTED BUT ANY HEAVY PRECIPITATION POTENTIAL, AS WELL
AS HEAVY SNOWFALL POTENTIAL ON THE BACKSIDE OF ANY SURFACE LOW THAT FORMS, WILL
BE MONITORED IN UPCOMING OUTLOOKS.
TIED TO THE AFOREMENTIONED MID-LEVEL TROUGHING OVER THE NORTHEASTERN PACIFIC,
STRONG SURFACE LOW PRESSURE REMAINS FAVORED OVER THE GULF OF ALASKA WHICH IS
EXPECTED TO PROLONG STORMY CONDITIONS OVER SOUTHEASTERN ALASKA INTO WEEK-2. THE
STRONG MEAN PRESSURE GRADIENTS EVIDENT IN BOTH THE GEFS AND ECMWF, AS WELL AS
PETS SUPPORT A SLIGHT RISK OF HIGH WINDS EXTENDING FROM THE KENAI PENINSULA TO
THE SOUTHEAST FOR DEC 7-9. ALTHOUGH PRECIPITATION AMOUNTS ARE NOT LIKELY TO
EXCEED HAZARD THRESHOLDS BASED ON PET GUIDANCE, THE CONTINUATION OF
ABOVE-NORMAL PRECIPITATION AND HIGH WINDS ARE LIKELY TO WORSEN ALREADY
SATURATED SOILS, AND MAY LEAD TO ADDITIONAL LANDSLIDES OVER PARTS OF THE STATE.
FOR TEMPERATURES, THERE IS BETTER MODEL AGREEMENT IN REGARDS TO STRONGER
RIDGING OVER THE BERING STRAIT WHERE INCREASED NORTHERLY FLOW APPEARS MORE
LIKELY TO BRING NEAR TO BELOW NORMAL TEMPERATURES OVER THE SOUTHWESTERN
PORTIONS OF THE STATE DURING WEEK-2. MINIMUM TEMPERATURES ARE NOT EXPECTED TO
REACH HAZARD CRITERIA, HOWEVER THE ARRIVAL OF COLDER TEMPERATURES FOLLOWING A
VERY WARM NOVEMBER IS LIKELY TO AID IN RIVER FREEZES. A RECENT AERIAL FREEZE-UP
SURVEY FOUND OPEN WATER AND THIN ICE ALONG THE KUSKOKWIM RIVER WHICH MAKES
RIVER TRAVEL HAZARDOUS.
FORECASTER: NICK NOVELLA
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