802  
FXUS21 KWNC 091909  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EDT MAY 09 2024  
 
SYNOPSIS: EARLY IN WEEK-2, A TRANSIENT AREA OF FORECAST MID-LEVEL LOW PRESSURE  
SUSTAINS THE RISK OF HEAVY PRECIPITATION AND POSSIBLE FLOODING FOR MANY PARTS  
OF THE SOUTHEASTERN CONTIGUOUS U.S. (CONUS) AND MID-ATLANTIC. UPSTREAM,  
BUILDING MID-LEVEL HIGH PRESSURE OVER THE LOWER FOUR CORNERS IS EXPECTED TO  
BRING UNSEASONABLY WARM TEMPERATURES, RENEWING THE RISK OF EXCESSIVE HEAT  
CONDITIONS FOR PARTS OF THE RIO GRANDE VALLEY. COMBINED WITH DRIER THAN NORMAL  
CONDITIONS AND STRENGTHENING SURFACE PRESSURE, THERE IS AN INCREASED RISK OF  
HIGH WINDS WHICH MAY ELEVATE WILDFIRE POTENTIAL OVER THE SOUTHWESTERN CONUS.  
OVER ALASKA, COOLER THAN NORMAL CONDITIONS ARE FORECAST AS SNOWMELT AND RIVER  
ICE BREAKUP SEASON IS UNDERWAY.  
 
HAZARDS  
 
SLIGHT RISK FOR HEAVY PRECIPITATION FOR PORTIONS OF THE LOWER MISSISSIPPI AND  
TENNESSEE VALLEYS, SOUTHEAST, APPALACHIANS, AND MID-ATLANTIC, FRI-SAT, MAY  
17-18.  
 
SLIGHT RISK FOR EXCESSIVE HEAT FOR PARTS OF THE SOUTHERN PLAINS, SAT-TUE, MAY  
18-21.  
 
SLIGHT RISK OF HIGH WINDS FOR PARTS OF THE FOUR CORNERS AND WESTERN HIGH  
PLAINS, FRI-TUE, MAY 17-21.  
 
FLOODING POSSIBLE FOR PARTS OF THE SOUTHERN PLAINS, LOWER MISSISSIPPI VALLEY  
AND SOUTHEAST.  
 
DETAILED SUMMARY  
 
FOR SUNDAY MAY 12 - THURSDAY MAY 16:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
FOR FRIDAY MAY 17 - THURSDAY MAY 23: SINCE YESTERDAY, THE WEEK-2 HAZARDS  
PERSPECTIVE MOSTLY REMAINS ON TRACK, THOUGH THERE ARE A FEW NEW DEVELOPMENTS  
AND DISCREPANCIES IN THE LATEST MODEL GUIDANCE ADDING TO MORE UNCERTAINTY IN  
THE UPDATED OUTLOOK. LATE IN WEEK-1, BOTH THE GEFS AND ECMWF ENSEMBLES  
CONTINUE TO FEATURE ANOMALOUS 500-HPA RIDGING EXTENDING FROM THE EASTERN  
PACIFIC INTO THE WESTERN CONUS, WITH A FAIRLY LARGE COVERAGE OF ANOMALOUS  
TROUGHING EAST OF THE ROCKIES. WITHIN THIS NEGATIVE HEIGHT ANOMALY COVERAGE,  
TWO SHORTWAVE DISTURBANCES HAVE BECOME MORE EVIDENT IN THE GEFS AND ECMWF  
ENSEMBLES; ONE SITUATED OVER THE TENNESSEE VALLEY, AND A SECONDARY VORTICITY  
MAXIMUM ASSOCIATED WITH THE POSSIBLE REMNANTS OF A CUTOFF LOW UPSTREAM OVER  
NORTHERN MEXICO. IN THE WAKE OF A FRONTAL SYSTEM THAT REMAINS FAVORED TO BRING  
ENHANCED PRECIPITATION OVER THE SOUTHEAST AND MID-ATLANTIC TOWARDS THE MIDDLE  
OF NEXT WEEK, THIS SECONDARY FEATURE IS EXPECTED TO RENEW THE RISK OF HEAVY  
PRECIPITATION OVER PARTS OF THE SOUTHERN AND EASTERN CONUS HEADING INTO WEEK-2.  
THE RETURN OF POTENTIALLY HEAVY PRECIPITATION IS SUPPORTED IN BOTH THE  
UNCALIBRATED AND CALIBRATED PROBABILISTIC EXTREME TOOL (PET) GUIDANCE, THUS THE  
CORRESPONDING SLIGHT RISK AREA IS CONTINUED OVER SOUTHEASTERN CONUS, AND  
EXTENDED THROUGH MAY 18. THIS AREA IS EXPANDED NORTHWARD FURTHER INTO THE  
MID-ATLANTIC IN THE UPDATED OUTLOOK WHERE THE UNCALIBRATED ECMWF DEPICTS  
GREATER THAN A 20% CHANCE FOR 3-DAY PRECIPITATION AMOUNTS EXCEEDING AN INCH.  
 
A POSSIBLE FLOODING HAZARD IS ALSO ADDED IN THE OUTLOOK EXTENDING FROM EASTERN  
TEXAS AND LOUISIANA (WHERE SOME AREAS HAVE REGISTERED MORE THAN 10 INCHES OF  
RAINFALL IN THE PAST TWO WEEKS), EASTWARD INTO PARTS OF WESTERN GEORGIA AND THE  
FLORIDA PANHANDLE WHERE HEAVY PRECIPITATION IS FAVORED DURING WEEK-1. ANY  
ADDITIONAL PRECIPITATION DURING WEEK-2 MAY TRIGGER FLASH FLOODING AS WELL AS  
RIVER FLOODING WITHIN THE HIGHLIGHTED REGION, EVEN IN AREAS WHERE PRECIPITATION  
DEFICITS ARE CURRENTLY REGISTERED.  
 
AS THE SECONDARY SHORTWAVE WEAKENS AND LIFTS OUT, BOTH THE GEFS AND ECMWF  
FAVORINCREASING MID-LEVEL HEIGHTS OVER THE INTERIOR WEST AND GREAT PLAINS, WITH  
A MAXIMUM HEIGHT CENTER AMPLIFYING OVER THE LOWER FOUR CORNERS AND NORTHERN  
MEXICO. HOWEVER, THE ENSEMBLES ARE DIVIDED IN REGARDS TO THE PATTERN EVOLUTION  
BOTH UPSTREAM AND DOWNSTREAM. WHILE THE GEFS SHOWS A SHARPER TREND TOWARDS  
DEVELOPING A TROUGH OVER THE EASTERN PACIFIC AND WEST COAST, THE ECMWF IS  
VIRTUALLY ABSENT OF THIS MID-LEVEL FEATURE AND MAINTAINS A WIDESPREAD COVERAGE  
OF POSITIVE HEIGHT DEPARTURES WHICH SPREAD NORTHWARD ACROSS THE WEST COAST AND  
INTO SOUTHWESTERN CANADA. AS A RESULT OF THE STRONGER RIDGING OVER WESTERN  
NORTH AMERICA, THE ECMWF SHOWS ANOMALOUS TROUGHING REDEVELOPING OVER THE  
MIDWEST BY THE MIDDLE OF THE PERIOD, TRANSLATING INTO PREDOMINANTLY COOLER  
CONDITIONS THAN THE GEFS OVER MANY PARTS OF THE CENTRAL AND EASTERN CONUS LATER  
IN WEEK-2. NOTABLY, THE ECMWF PET AND UNCALIBRATED TOOLS DEPICT THE POSSIBLE  
RETURN OF HEAVY PRECIPITATION OVER THE CENTRAL PLAINS AND MIDDLE MISSISSIPPI  
VALLEY TIED TO THE DEEPENING TROUGH ALOFT. NO CORRESPONDING HAZARDS ARE POSTED  
GIVEN NO SUPPORT IN THE GEFS PET, AND SIGNIFICANT DIFFERENCES AMONG THE MODELS  
AT THIS LEAD, BUT THIS TROUGHING POTENTIAL WILL CONTINUE TO BE MONITORED MOVING  
FORWARD.  
 
DESPITE THESE DIFFERENCES IN THE EVOLUTION OF THE HEIGHT PATTERN, HOWEVER, THE  
POTENTIAL FOR EXCESSIVE HEAT STILL REMAINS A CONCERN ACROSS THE SOUTHWESTERN  
AND SOUTH-CENTRAL CONUS GIVEN MODEL CONSENSUS OF THE AMPLIFYING RIDGE CENTER  
OVER THE LOWER LATITUDES OF NORTH AMERICA. THERE IS FAIR AGREEMENT IN GEFS AND  
ECMWF PETS INDICATING 20-30% CHANCES FOR MAXIMUM TEMPERATURES EXCEEDING THE  
85TH PERCENTILE AND 100 DEGREES F ALONG THE RIO GRANDE DURING THE MIDDLE OF  
WEEK-2 , THEREFORE A SLIGHT RISK FOR EXCESSIVE HEAT REMAINS POSTED FOR THE  
REGION (MAY 18-21). SIMILAR HEAT SIGNALS ARE ALSO EVIDENT FURTHER WEST ACROSS  
THE SONORAN DESERT OF CALIFORNIA AND SOUTHERN ARIZONA, THOUGH MAXIMUM  
TEMPERATURES ARE NOT EXPECTED TO EXCEED HAZARD CRITERIA OVER THIS PART OF THE  
COUNTRY FOR THIS TIME OF YEAR. THE EXCESSIVE HEAT POTENTIAL WILL CONTINUE TO BE  
CLOSELY MONITORED OVER THE DESERT SOUTHWEST, LOWER FOUR CORNERS, AND SOUTHERN  
PLAINS AS THESE REGIONS WERE PLAGUED WITH EXCESSIVE HEAT CONDITIONS FOR MUCH OF  
THE WARM SEASON LAST YEAR. WITH WARMER AND DRIER THAN NORMAL CONDITIONS  
FAVORED, A SLIGHT RISK OF HIGH WINDS IS ALSO POSTED FOR PARTS OF THE DESERT  
SOUTHWEST, SOUTHERN ROCKIES, AND SOUTHERN PLAINS (MAY 17-21). THIS IS SUPPORTED  
IN THE GEFS AND ECMWF PETS AND RAW ENSEMBLES DEPICTING AN ANOMALOUS SURFACE  
HIGH NEAR THE FRONT RANGE OF COLORADO WHICH COULD INDUCE EPISODES OF HIGH WINDS  
AND ELEVATE THE WILDFIRE RISK.  
 
 
 
NO HAZARDS ARE ISSUED FOR ALASKA. SNOWMELT SEASON IS UNDERWAY AND FROZEN RIVERS  
ARE BEGINNING TO BREAK UP, LEADING TO THE POTENTIAL FOR RIVER FLOODING RELATED  
TO ICE-JAMS. CURRENTLY, THERE ARE NO INDICATIONS OF IMPENDING MAJOR RIVER  
BREAK-UPS OR SERIOUS THREAT OF ICE-JAMMING SO NO FLOODING-RELATED HAZARDS ARE  
POSTED AT THIS TIME. CAUTION WILL CONTINUE TO BE EXERCISED AS RIVER BREAK-UP  
CAN BE UNPREDICTABLE AND LOCAL CONDITIONS CAN CHANGE QUICKLY.  
 
FORECASTER: NICK NOVELLA  
 
 
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