223
FXUS21 KWNC 131918
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST NOVEMBER 13 2023
SYNOPSIS: STRONG MID-LEVEL HIGH PRESSURE IS FORECAST TO BUILD ACROSS ALASKA
DURING WEEK-2. THIS FAVORS MID-LEVEL LOW PRESSURE AMPLIFYING DOWNSTREAM OVER
THE WESTERN CONTIGUOUS U.S. (CONUS). FOLLOWING RESIDUAL ENHANCED ONSHORE FLOW
ACROSS THE PACIFIC NORTHWEST AND NORTHERN CALIFORNIA AT THE OUTSET OF THE
PERIOD, MODELS ARE IN GOOD AGREEMENT REGARDING THE ARRIVAL OF ANOMALOUSLY COLD
TEMPERATURES, EXPANDING INTO PARTS OF THE ROCKIES AND GREAT PLAINS BY THE
MIDDLE OF THE PERIOD. SURFACE LOW PRESSURE IS FORECAST TO MOVE THROUGH THE
EASTERN CONUS EARLY IN WEEK-2, WITH ELEVATED POTENTIAL FOR LINGERING EPISODES
OF HIGH WINDS BEHIND THIS SYSTEM STRETCHING FROM THE UPPER MIDWEST AND
NORTHEAST, POSSIBLY PERSISTING INTO THE THANKSGIVING HOLIDAY.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE PACIFIC NORTHWEST AND
NORTHERN CALIFORNIA, TUE, NOV 21.
SLIGHT RISK OF HEAVY SNOW FOR PORTIONS OF THE CASCADES, KLAMATH MOUNTAINS, AND
NORTHERN ROCKIES, TUE, NOV 21.
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE PACIFIC
NORTHWEST, NORTHERN GREAT BASIN, AND THE NORTHERN AND CENTRAL ROCKIES AND
PLAINS, WED-SAT, NOV 22-25.
SLIGHT RISK OF HEAVY PRECIPITATION FOR MUCH OF THE EASTERN THIRD OF THE CONUS,
TUE-WED, NOV 21-22.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF THE UPPER MIDWEST, OHIO VALLEY, GREAT
LAKES, NORTHERN MID-ATLANTIC, AND NORTHEAST, TUE-THU, NOV 21-23.
DETAILED SUMMARY
FOR THURSDAY NOVEMBER 16 - MONDAY NOVEMBER 20:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR TUESDAY NOVEMBER 21 - MONDAY NOVEMBER 27: SURFACE LOW PRESSURE FORECAST OFF
THE COAST OF BRITISH COLUMBIA AT THE END OF WEEK-1 FAVORS ENHANCED ONSHORE FLOW
INTO THE NORTHWESTERN CONUS, POSSIBLY LINGERING INTO THE START OF WEEK-2. THE
ECMWF PROBABILISTIC EXTREMES TOOL (PET) DEPICTS AT LEAST A 20 PERCENT CHANCE OF
3-DAY PRECIPITATION TOTALS EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE AND
1-INCH ACROSS NORTHERN CALIFORNIA EARLY IN THE PERIOD. ADDITIONALLY, ELEVATED
PROBABILITIES (GREATER THAN 20 PERCENT) FOR 3-DAY PRECIPITATION EXCEEDING 1 TO
1.5 INCHES EXTENDS NORTHWARD THROUGH THE PACIFIC NORTHWEST IN BOTH THE GEFS AND
ECMWF PETS, ALTHOUGH PROBABILITIES ARE LOWER BASED ON PERCENTILES. THE
STRONGEST PRECIPITATION SIGNAL IN THE UNCALIBRATED GUIDANCE IS ON DAY-7, AND
LINGERING INTO DAY-8, CORRESPONDING WITH THE SLIGHT RISK OF HEAVY PRECIPITATION
FOR 11/21. SLIGHT RISKS FOR HEAVY SNOW ARE ALSO POSTED ACROSS THE CASCADES,
KLAMATH MOUNTAINS, AND NORTHERN ROCKIES FOR 11/21 WHERE THE GEFS SNOW WATER
EQUIVALENT (SWE) PET INDICATES AT LEAST A 20 PERCENT CHANCE SWE EXCEEDS THE
85TH CLIMATOLOGICAL PERCENTILE AND A HALF-INCH OVER SOME AREAS, AND THE
UNCALIBRATED ECMWF DEPICTS AT LEAST A 20 PERCENT CHANCE DAY-8 SNOWFALL EXCEEDS
4-INCHES.
STRONG RIDGING IS FORECAST TO BUILD ACROSS ALASKA DURING WEEK-2, WHICH SHOULD
ALLOW FOR THE AFOREMENTIONED SURFACE LOW OFF THE COAST OF BRITISH COLUMBIA TO
WEAKEN, AND AN AMPLIFICATION OF TROUGHING DOWNSTREAM OVER THE WESTERN CONUS.
THIS FAVORS A PERIOD OF RELATIVELY DRIER, BUT COLDER WEATHER ACROSS THE
NORTHWESTERN CONUS BEGINNING AROUND DAY-9. THE ECMWF PET DEPICTS A 20 TO 40
PERCENT CHANCE MINIMUM TEMPERATURES FALL BELOW THE 10TH CLIMATOLOGICAL
PERCENTILE AND 0 DEG F SPREADING ACROSS THE NORTHERN ROCKIES AND PLAINS, WITH
SUBFREEZING TEMPERATURES EXTENDING INTO COASTAL AREAS OF THE PACIFIC NORTHWEST.
THE GEFS PET IS NOT AS ROBUST AS THE ECMWF, WITH GENERALLY LOWER PROBABILITIES
FOR TEMPERATURES FALLING BELOW CLIMATOLOGICAL THRESHOLDS, AND LESS COVERAGE OF
SUBZERO TEMPERATURES ACROSS THE NORTHERN TIER. THEREFORE, ONLY A SLIGHT RISK
FOR MUCH BELOW NORMAL TEMPERATURES IS HIGHLIGHTED ACROSS THE NORTHWESTERN
CONUS, EXTENDING INTO PARTS OF THE NORTHERN AND CENTRAL ROCKIES AND PLAINS FOR
NOV 22-25. THE STRENGTH OF THE RIDGE OVER ALASKA WILL BE CLOSELY MONITORED, AS
A TREND TOWARD STRONGER RIDGING WILL LIKELY TELECONNECT WITH COLDER
TEMPERATURES OVER THE WESTERN U.S.
THE 0Z ECMWF, GEFS, AND CANADIAN ENSEMBLES INDICATE THE POTENTIAL FOR SURFACE
LOW PRESSURE TO MOVE THROUGH THE EASTERN CONUS EARLY IN THE PERIOD, IN THE DAYS
LEADING UP TO THANKSGIVING. BOTH THE ECMWF AND GEFS PETS DEPICT 3-DAY
PRECIPITATION TOTALS HAVING AT LEAST A 20 PERCENT CHANCE OF EXCEEDING THE 85TH
CLIMATOLOGICAL PERCENTILE AND 1-INCH ACROSS PARTS OF THE EAST. HOWEVER, THE
DETERMINISTIC GUIDANCE IS INCONSISTENT, WITH THE 0Z GFS DEPICTING A VERY ROBUST
SYSTEM ACROSS THE NORTHEAST AROUND NOV 22, AND A SIGNIFICANT BUT NOT AS
PRONOUNCED SYSTEM IN THE 0Z ECMWF. CONVERSELY, THE 6Z AND 12Z DETERMINISTIC GFS
RUNS DEPICT A WEAKER SYSTEM. A SLIGHT RISK OF HEAVY PRECIPITATION IS
HIGHLIGHTED ACROSS MUCH OF THE EASTERN THIRD OF THE CONUS FROM THE TENNESSEE
VALLEY AND CAROLINAS NORTHWARD THROUGH THE NORTHEAST FOR NOV 21-22, COINCIDING
WITH THE HIGHEST PRECIPITATION TOTALS IN THE UNCALIBRATED 0Z ECMWF, GEFS, AND
CANADIAN ENSEMBLES. DEPENDING ON THE STRENGTH OF THE SYSTEM, EPISODES OF HIGH
WINDS ARE POSSIBLE FROM THE UPPER MIDWEST TO THE NORTHEAST IN ITS WAKE, WITH
THE ECMWF PET INDICATING AT LEAST A 20 PERCENT CHANCE WIND SPEEDS EXCEED THE
85TH PERCENTILE AND 20-MPH. THEREFORE, A SLIGHT RISK OF HIGH WINDS IS POSTED
ACROSS THESE AREAS FROM NOV 21-23, WHICH INCLUDES THE THANKSGIVING HOLIDAY.
THERE ARE NO HAZARDS POSTED ACROSS ALASKA. THE AFOREMENTIONED RIDGING FAVORS
ANOMALOUSLY WARM TEMPERATURES ACROSS MUCH OF THE MAINLAND AND ALEUTIANS, WITH
THE CONSOLIDATED GEFS AND ECMWF REFORECAST TOOL DEPICTING THE HIGHEST MEAN
WEEK-2 TEMPERATURE DEPARTURES ACROSS THE NORTH SLOPE (AT LEAST 12 DEG F).
BELOW-NORMAL PRECIPITATION IS FAVORED FOR SOUTHEAST ALASKA UNDERNEATH INCREASED
OFFSHORE FLOW, WITH SOME ENHANCED PRECIPITATION POSSIBLE ACROSS THE WESTERN
MAINLAND.
FORECASTER: THOMAS COLLOW
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