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FXUS21 KWNC 221841
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT APRIL 22 2021
SYNOPSIS: IN GENERAL, HAZARD RISKS OVER THE U.S. SEEM MORE MARGINAL TODAY, WITH
DISPARITIES IN THE MODEL GUIDANCE AND TOOLS ADDING TO UNCERTAINTY IN THE WEEK-2
OUTLOOK. A COLD FRONT MOVING ACROSS THE EASTERN SEABOARD IS LIKELY TO BRING
ENHANCED PRECIPITATION ACROSS PORTIONS OF THE SOUTHEAST AND MID-ATLANTIC BY THE
START OF WEEK-2. DEVELOPING MID-LEVEL LOW PRESSURE FAVORS POSSIBLE HEAVY
PRECIPITATION OVER PARTS OF THE PLAINS AND THE MISSISSIPPI VALLEY DURING THE
MIDDLE OF THE OUTLOOK PERIOD.
HAZARDS
NO HAZARDS ARE DEPICTED IN TODAY'S OUTLOOK AS ODDS ARE NOT ANTICIPATED TO REACH
PRE-DEFINED CRITERIA DURING THE FORECAST PERIOD AT THIS TIME.
DETAILED SUMMARY
FOR SUNDAY APRIL 25 - THURSDAY APRIL 29:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY APRIL 30 - THURSDAY MAY 06: DURING EARLY WEEK-2, BOTH THE 0Z GEFS
AND 0Z ECMWF ENSEMBLE MEAN 500-HPA HEIGHT ANOMALIES CONTINUE TO FEATURE A
SHORTWAVE TROUGH TRAVERSING THE EASTERN CONUS. COMPARED TO PREVIOUS ENSEMBLE
RUNS, THE ECMWF IS CONSIDERABLY WEAKER WITH THIS MID-LEVEL FEATURE, AS IT IS
PREDICTED TO DEAMPLIFY OVER THE SOUTHEASTERN U.S BY EARLY MAY. FARTHER WEST,
BOTH MODELS CONTINUE TO FEATURE AN AMPLIFYING MID-LEVEL RIDGE OVER THE EAST
PACIFIC ALLOWING FOR THE DEVELOPMENT OF DOWNSTREAM ANOMALOUS TROUGHING OVER
PORTIONS OF THE WESTERN AND CENTRAL CONUS LATER IN THE PERIOD. THE GEFS
CONTINUES TO BE STRONGER THAN THE ECMWF WITH THIS DEVELOPING MID-LEVEL FEATURE,
WHILE FAVORING AN ELEVATED POTENTIAL FOR ANOMALOUS COLD RETURNING OVER PORTIONS
OF THE ROCKIES AND THE PLAINS, AS WELL AS POSSIBLE HEAVY PRECIPITATION MAINLY
OVER THE MISSISSIPPI VALLEY TIED TO SURFACE LOW DEVELOPMENT BY THE MIDDLE OF
THE OUTLOOK PERIOD. HOWEVER, THE ECMWF GUIDANCE HAS BECOME LESS SUPPORTIVE OF
THIS POTENTIAL, WHICH INTRODUCES ADDED UNCERTAINTY IN TODAY'S HAZARD OUTLOOK.
SURFACE LOW PRESSURE REMAINS FORECAST TO TRACK ACROSS THE GREAT LAKES WITH A
LARGE COLD FRONT EXTENDING INTO THE GULF OF MEXICO BY THE END OF WEEK-1. BASED
ON THE ENSEMBLE MEAN PRECIPITATION TOTALS FROM THE GEFS AND ECMWF, THE HIGHEST
PRECIPITATION AMOUNTS ARE EXPECTED OVER THE MISSISSIPPI VALLEY PRIOR TO THE
START OF WEEK-2, ALTHOUGH THERE IS SOME SUPPORT IN THE ECMWF REFORECAST TOOL
SUGGESTING SOME HEAVY PRECIPITATION MAY PERSIST INTO THE START OF THE OUTLOOK
PERIOD. HOWEVER, GIVEN THE LOW PROBABILITIES FOR PRECIPITATION AMOUNTS
EXCEEDING AN INCH ON DAY 8 (APR 30) IN THE 0Z ECMWF ENSEMBLE, AND LITTLE SIGNAL
IN THE GEFS REFORECAST TOOL, THE SLIGHT RISK FOR HEAVY PRECIPITATION IS REMOVED
FROM THE OUTLOOK. AS THIS FRONTAL SYSTEM TRACKS ACROSS THE EASTERN SEABOARD,
BELOW NORMAL TEMPERATURES ARE LIKELY TO PREVAIL FROM THE SOUTHERN PLAINS TO THE
GREAT LAKES EARLY IN THE PERIOD, WITH THE POTENTIAL FOR MINIMUM TEMPERATURES
FALLING BELOW 40 DEGREES F OVER PORTIONS OF THE MIDWEST WHICH MAY LEAD TO FROST
OVER SENSITIVE VEGETATION IN THE REGION. FARTHER SOUTH, INCREASED WIND SPEEDS
ARE ALSO POSSIBLE, AS REFORECAST TOOLS MAINTAIN ELEVATED PROBABILITIES FOR
SUSTAINED WINDS EXCEEDING THE 85TH PERCENTILE OVER THE SOUTHEAST. HOWEVER, WIND
SPEEDS ARE NOT EXPECTED TO EXCEED HAZARD THRESHOLDS AND NO WIND-RELATED HAZARDS
ARE ISSUED.
BY DAY 10 (MAY 2), THE 0Z AND 6Z GEFS CONTINUE TO FEATURE A BROAD AREA OF
SURFACE LOW PRESSURE OVER THE SOUTHERN HIGH PLAINS TIED TO THE AFOREMENTIONED
TROUGHING ALOFT. SINCE YESTERDAY, THE STRENGTH OF THIS MEAN LOW PRESSURE AREA
IN THE GEFS HAS LESSENED AND ENSEMBLE MEAN PRECIPITATION AMOUNTS OVER THE
SOUTH-CENTRAL CONUS HAVE ALSO DECREASED. WHILE THE GEFS REFORECAST TOOL
MAINTAINS AT LEAST A 20% CHANCE THAT 3-DAY ACCUMULATIONS EXCEED THE 85TH
PERCENTILE AND 1 INCH OVER THE MIDDLE AND LOWER MISSISSIPPI VALLEY, THIS SIGNAL
IS NOW ABSENT IN THE ECMWF REFORECAST TOOL WHERE THERE IS ALSO A SIGNIFICANT
REDUCTION IN PROBABILITIES FOR PRECIPITATION EXCEEDING AN INCH OVER THE REGION
IN THE RAW ECMWF ENSEMBLE DURING THE MIDDLE OF THE PERIOD. GIVEN THE DRIER
TREND AND DISAGREEMENT IN THE REFORECAST TOOLS, THERE IS INSUFFICIENT
CONFIDENCE THAT PRECIPITATION AMOUNTS WILL EXCEED HAZARD THRESHOLDS, AND THE
SLIGHT RISK AREA FOR HEAVY PRECIPITATION IS OMITTED FROM THE OUTLOOK. IT IS
IMPORTANT TO NOTE THAT ANY ABOVE-NORMAL PRECIPITATION THAT OCCURS OVER THE
LOWER MISSISSIPPI VALLEY DURING WEEK-2 IS LIKELY TO WORSEN ANOMALOUSLY WET
ANTECEDENT CONDITIONS AND MAY TRIGGER LOCALIZED FLOODING IN THE REGION.
ACROSS WESTERN NORTH AMERICA, ANOMALOUS MID-LEVEL RIDGING FORECAST IS EXPECTED
TO PROMOTE ABOVE-NORMAL TEMPERATURES AND BELOW NORMAL PRECIPITATION OVER MUCH
OF THE WEST COAST AND ALASKA EARLY IN THE PERIOD. THERE IS AGREEMENT IN THE
GEFS AND ECMWF REFORECAST TOOLS INDICATING ELEVATED PROBABILITIES OF MAXIMUM
TEMPERATURES EXCEEDING THE 85TH PERCENTILE FROM THE CENTRAL CALIFORNIA VALLEY
TO THE DESERT SOUTHWEST, HOWEVER THESE TEMPERATURES ARE NOT EXPECTED TO SURPASS
HAZARDOUS THRESHOLDS AS TEMPERATURES EXCEEDING 95 DEGREES F ARE LIKELY TO BE
CONFINED TO THE SONORAN DESERT. IN ALASKA, THE ANOMALOUSLY WARM CONDITIONS
FAVORED BY THE GEFS ARE LIKELY TO TRANSLATE INTO MAXIMUM TEMPERATURES CLIMBING
UPWARDS OF 40 AND 50 DEGREES F OVER PORTIONS OF THE CENTRAL AND SOUTHERN
MAINLAND DURING WEEK-2. SUCH TEMPERATURES ARE LIKELY TO ACCELERATE ICE AND SNOW
MELT DURING RIVER ICE BREAK UP SEASON HEADING INTO EARLY MAY.
FORECASTER: NICK NOVELLA
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