746
FXUS21 KWNC 041911
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 04 2023
SYNOPSIS: MID-LEVEL LOW PRESSURE IS FORECAST TO LIFT OUT OF THE EASTERN
CONTIGUOUS U.S. (CONUS) AT THE START OF WEEK-2. WHILE MOST OF THE ASSOCIATED
PRECIPITATION IS FORECAST TO OCCUR DURING THE LATE WEEK-1 TIMEFRAME, SOME
LINGERING RAINFALL MAY PERSIST INTO THE START OF WEEK-2 ACROSS THE FLORIDA
PENINSULA. ON THE BACKSIDE OF THE MID-LEVEL LOW, HIGH WINDS ARE POSSIBLE ACROSS
PORTIONS OF THE GREAT LAKES AND NORTHEAST EARLY IN THE PERIOD, WITH AN ELEVATED
POTENTIAL FOR LAKE EFFECT SNOW OVER SOME AREAS. LATER IN WEEK-2, MID-LEVEL HIGH
PRESSURE IS FORECAST TO BRING MORE TRANQUIL AND WARM WEATHER BY MID-DECEMBER
STANDARDS TO THE CENTRAL AND EASTERN CONUS. MODELS DEPICT DIFFERING PATTERN
EVOLUTIONS ACROSS WESTERN NORTH AMERICA RESULTING IN A LESS CONFIDENT FORECAST,
WITH THE BEST AGREEMENT OVER SOUTHEASTERN ALASKA WHERE STRONG ONSHORE FLOW AND
RESULTANT EPISODES OF HEAVY PRECIPITATION AND HIGH WINDS REMAIN FAVORED.
HAZARDS
SLIGHT RISK OF HEAVY SNOW DOWNWIND OF LAKES SUPERIOR, MICHIGAN, ERIE, AND
ONTARIO, TUE-WED, DEC 12-13.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF THE GREAT LAKES AND NORTHEAST,
TUE-WED, DEC 12-13.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF SOUTHEAST ALASKA, TUE-FRI,
DEC 12-15.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF SOUTHEAST ALASKA, TUE-FRI, DEC 12-15.
DETAILED SUMMARY
FOR THURSDAY DECEMBER 07 - MONDAY DECEMBER 11:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR TUESDAY DECEMBER 12 - MONDAY DECEMBER 18: LATE IN WEEK-1, TROUGHING IS
FORECAST ACROSS THE EAST, WITH AN ASSOCIATED AREA OF SURFACE LOW PRESSURE AND
FRONTAL SYSTEM PREDICTED TO BRING AREAS OF HEAVY PRECIPITATION TO MUCH OF THE
EASTERN SEABOARD. HOWEVER, BY THE START OF WEEK-2, THIS SYSTEM IS LIKELY TO BE
EXITING THE U.S., WITH MANY RELATED IMPACTS TIMING OFF. THE ECMWF PROBABILISTIC
EXTREMES TOOL (PET) DEPICTS ELEVATED PROBABILITIES (AT LEAST 40 PERCENT) FOR
3-DAY PRECIPITATION TOTALS GREATER THAN THE 85TH CLIMATOLOGICAL PERCENTILE
ACROSS SOUTH FLORIDA PERSISTING INTO EARLY WEEK-2, AS THE FRONT IS SLOWER TO
DEPART THE REGION. HOWEVER, PRECIPITATION CLIMATOLOGIES DECREASE ACROSS THE
REGION DURING DECEMBER, AND PROBABILITIES FOR 3-DAY PRECIPITATION TOTALS
EXCEEDING 1-INCH ARE LESS THAN 20 PERCENT IN THE GEFS PET. THEREFORE, NO
RELATED HEAVY PRECIPITATION HAZARD IS POSTED ACROSS FLORIDA, ALTHOUGH LINGERING
ROUNDS OF HEAVY PRECIPITATION CANNOT BE RULED OUT, ESPECIALLY IF THE FRONT IS
SLOW TO MOVE OUT OF FLORIDA AS DEPICTED IN THE ECMWF GUIDANCE.
ON THE BACKSIDE OF THE DEPARTING SYSTEM, SURFACE HIGH PRESSURE IS FORECAST TO
BUILD ACROSS THE EASTERN CONUS. A TIGHT PRESSURE GRADIENT FAVORS A SLIGHT RISK
FOR HIGH WINDS ACROSS PORTIONS OF THE GREAT LAKES AND NORTHEAST, DEC 12-13.
AREAS OF LAKE EFFECT SNOW ARE POSSIBLE AS WELL, ALTHOUGH MODELS SLIGHTLY DIFFER
IN THE ORIENTATION OF THE SURFACE WIND FIELD. THE ECMWF ENSEMBLE INDICATES MORE
OF A NORTHERLY COMPONENT TO THE FLOW WHICH WOULD BE MORE FAVORABLE FOR SNOW.
HOWEVER, THE GEFS IS QUICKER TO BUILD THE SURFACE HIGH TO THE EAST, WHICH
RESULTS IN MORE SOUTHERLY SURFACE FLOW. THIS COULD LEAD TO RELATIVELY WARMER
DAYTIME SURFACE TEMPERATURES, LIMITING THE POTENTIAL FOR SIGNIFICANT SNOWFALL.
DUE TO THE POTENTIAL FOR A SOMEWHAT FAVORABLE SET-UP, ALONG WITH ABOVE-NORMAL
LAKE TEMPERATURES, A SLIGHT RISK OF HEAVY SNOW IS POSTED DOWNWIND OF LAKES
SUPERIOR, MICHIGAN, ERIE, AND ONTARIO, DEC 12-13. WHILE A PERIOD OF
BELOW-NORMAL TEMPERATURES IS POSSIBLE OVER THE SOUTHEAST EARLY IN WEEK-2,
IMPACTS TO VEGETATION ARE FORECAST TO BE MINIMAL, EVEN IF SOME AREAS HAVE
TEMPERATURES NEAR OR BELOW 40 DEG F, PRECLUDING A RELATED HAZARD.
THERE IS LESS CONFIDENCE IN THE WEEK-2 FORECAST ACROSS THE WEST DUE TO
DIFFERENCES BETWEEN THE MODELS AT THE OUTSET OF THE PERIOD. WHILE THE ECMWF
ENSEMBLE DEPICTS MORE RIDGING ALONG THE WEST COAST EARLY IN THE PERIOD, THE
GEFS INDICATES A MORE ZONAL FLOW PATTERN. NEITHER THE GEFS OR ECMWF PET DEPICT
PROBABILITIES ABOVE 20 PERCENT FOR 3-DAY PRECIPITATION EXCEEDING THE 85TH
CLIMATOLOGICAL PERCENTILE ACROSS THE WEST DURING THE FIRST PART OF THE PERIOD,
AND INTEGRATED VAPOR TRANSPORT IS FAVORED TO BE LOW. HOWEVER, BY THE MIDDLE OF
THE PERIOD, BOTH MODELS DEPICT MORE TROUGHING DEVELOPING NEAR THE WEST COAST,
AND A CORRESPONDING UPTICK IN PROBABILITIES IN THE PETS, WITH THE ECMWF PET
EXTENDING 20 PERCENT OR GREATER PROBABILITIES INTO PARTS OF SOUTHERN
CALIFORNIA. NO RELATED HAZARDS ARE POSTED IN TODAY’S OUTLOOK, BUT TRENDS WILL
CONTINUE TO BE MONITORED FOR POSSIBLE INCLUSION OF HEAVY PRECIPITATION AND/OR
SNOW HAZARDS ACROSS PARTS OF THE WEST LATER IN THE PERIOD.
SURFACE LOW PRESSURE FORECAST OVER THE GULF OF ALASKA FAVORS INCREASED ONSHORE
FLOW OVER SOUTHEASTERN ALASKA FOR THE FIRST HALF OF THE PERIOD, WITH BOTH THE
GEFS AND ECMWF PETS INDICATING AT LEAST A 20 PERCENT CHANCE 3-DAY PRECIPITATION
TOTALS EXCEED THE 85TH CLIMATOLOGICAL PERCENTILE AND 2-INCHES ACROSS THE
REGION, ALONG WITH POTENTIAL FOR WIND SPEEDS EXCEEDING 40-MPH. THEREFORE,
SLIGHT RISKS OF HEAVY PRECIPITATION AND HIGH WINDS ARE POSTED ACROSS SOUTHEAST
ALASKA FOR DEC 12-15. RIDGING FORECAST ACROSS THE BERING SEA AND INCREASED
NORTHERLY FLOW OVER ALASKA FAVORS BELOW-NORMAL TEMPERATURES OVER MUCH OF THE
STATE DURING WEEK-2, WITH THE LARGEST NEGATIVE TEMPERATURE ANOMALIES PREDICTED
OVER THE WESTERN MAINLAND. ALTHOUGH MINIMUM TEMPERATURES ARE NOT EXPECTED TO
REACH HAZARDS CRITERIA, THE ARRIVAL OF COLDER TEMPERATURES FOLLOWING A VERY
WARM NOVEMBER IS LIKELY TO AID IN RIVER FREEZES.
FORECASTER: THOMAS COLLOW
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