224  
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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