170  
FXUS02 KWBC 210659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
259 AM EDT WED MAY 21 2025  
 
VALID 12Z SAT MAY 24 2025 - 12Z WED MAY 28 2025  
 
 
...MULTI-DAY HEAVY RAIN THREAT IN THE OZARKS AND VICINITY THIS  
WEEKEND...  
 
   
..OVERVIEW  
 
AS THE MEDIUM RANGE PERIOD BEGINS SATURDAY, A SHORTWAVE ATOP THE  
ROCKIES WILL UNDERCUT A BROADER OMEGA-TYPE PATTERN CONSISTING OF A  
NORTHEAST PACIFIC UPPER LOW, A CENTRAL CANADA AND U.S. RIDGE, AND  
TROUGHING OVER THE EAST. THIS SHORTWAVE AND A FRONTAL SYSTEM WILL  
PROVIDE SUPPORT FOR HEAVY RAIN AND THUNDERSTORMS IN A MOIST AND  
UNSTABLE ENVIRONMENT IN PARTS OF THE CENTRAL U.S. THIS WEEKEND.  
FLASH FLOODING IS POSSIBLE IN THE OZARKS AND VICINITY. THE  
SHORTWAVE IS FORECAST TO MERGE WITH THE WESTERN SIDE OF THE TROUGH  
IN THE EAST AND CAUSE IT TO DIG INTO THE EAST-CENTRAL U.S. EARLY  
NEXT WEEK, WHICH SHOULD PRESS THE FRONT SOUTH AND EAST, SHIFTING  
PRECIPITATION ACROSS THE SOUTHERN TIER INTO THE APPALACHIANS AND  
EVENTUALLY THE EASTERN SEABOARD. THE ROUNDS OF TROUGHING ALOFT WILL  
CAUSE COOLER THAN NORMAL TEMPERATURES IN PARTS OF THE CENTRAL AND  
EASTERN U.S., WHILE UPPER RIDGING BUILDING IN THE INTERMOUNTAIN  
WEST SHOULD ALLOW FOR WARMER THAN AVERAGE TEMPERATURES THERE.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT  
 
MODEL GUIDANCE STARTS OUT IN GOOD AGREEMENT WITH A WAVY UPPER  
PATTERN THIS WEEKEND. THE FIRST MODEL TO DIVERGE WITH THE PATTERN  
WAS THE 12Z UKMET, WHICH CUT OFF ENERGY IN THE SOUTHWEST UNDERNEATH  
THE RIDGE BY EARLY NEXT WEEK AND THUS WAS OUT OF PHASE WITH THE  
OTHER GUIDANCE. THERE IS SOME MODERATE SPREAD WITH THE WAY THE  
ROCKIES SHORTWAVE EMERGES AND JOINS WITH THE ELONGATED EASTERN  
TROUGH EARLY NEXT WEEK, CAUSING THE TROUGH TO DIG SOUTHWEST. AROUND  
MONDAY, THE NEWER 00Z MODELS HAVE THE TROUGH REACHING FARTHER  
SOUTHWEST THAN THE 12/18Z GUIDANCE CYCLE THAT WAS AVAILABLE FOR  
FORECAST GENERATION, THOUGH WITH THE 00Z CMC PERHAPS ON THE EXTREME  
WESTERN SIDE. THE TROUGH POSITION WILL BE MEANINGFUL FOR THE  
FRONTAL AND QPF POSITION EARLY NEXT WEEK, SO FORECAST CHANGES MAY  
BE NEEDED.  
 
MODELS ARE IN GOOD AGREEMENT THAT UPPER RIDGING WILL SET UP ATOP  
THE INTERIOR WEST BEHIND THIS TROUGH, FORMING PART OF A  
REDEVELOPING OMEGA BLOCK ALONG WITH EASTERN PACIFIC TROUGHING. THE  
EASTERN PACIFIC TROUGH SHOWS A FAIR AMOUNT OF MODEL SPREAD THOUGH.  
GUIDANCE VARIES WITH HOW MUCH ENERGY MAY PINCH OFF ON THE SOUTHERN  
SIDE AND EVEN FORM A SOUTHERN STREAM CLOSED LOW MOVING INTO  
CALIFORNIA AROUND TUESDAY OR SO. THE 18Z GFS AND 12Z CMC INDICATED  
THIS UPPER LOW FORMING, AS DO THEIR NEW 00Z RUNS. THE 12Z ECMWF DID  
NOT SHOW IT BUT THE 00Z RUN SHOWS HINTS OF STRONGER ENERGY EVEN IF  
IT MAY NOT QUITE CLOSE. 12Z AI MODELS DID NOT FAVOR A CLOSED LOW,  
SO LEANED AGAINST IT FOR THIS FORECAST BY LESSENING THE GFS/CMC  
PROPORTION IN THE FORECAST BLEND, BUT WILL CONTINUE TO MONITOR THIS  
POTENTIAL.  
 
THE WPC FORECAST WAS BASED ON A BLEND OF THE 12/18Z GFS, 12Z  
ECMWF, AND 12Z CMC EARLY IN THE PERIOD. INCLUDED THE ENSEMBLE MEANS  
IN THE BLEND LATER ON, REACHING HALF THE BLEND DAY 6 AND A BIT  
MORE DAY 7 AS SPREAD INCREASED.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS  
 
A HEAVY RAINFALL PATTERN WILL BE IN PLACE BY SATURDAY AS AMPLE  
GULF MOISTURE AND INSTABILITY STREAM NORTH DUE TO RETURN FLOW FROM  
SURFACE HIGH PRESSURE OVER THE SOUTHEAST AND IN RESPONSE TO A  
SHORTWAVE/RIGHT ENTRANCE REGION OF THE UPPER JET COMING INTO THE  
NORTH- CENTRAL ROCKIES AND HIGH PLAINS. RAIN AND STORMS ARE LIKELY  
TO OVERLAP WITH FRIDAY'S (DAY 3 ERO PERIOD) CONVECTION WHERE THERE  
WILL BE WETTER ANTECEDENT CONDITIONS. A SLIGHT RISK REMAINS  
CENTERED OVER THE OZARKS (WHICH CAN BE SENSITIVE TO FLASH FLOODING  
ANYWAY) IN THE DAY 4/SATURDAY ERO. CONVECTION ONGOING FROM SATURDAY  
NIGHT COULD CONTINUE EAST INTO SUNDAY ACROSS THE MID-MISSISSIPPI  
VALLEY, WITH UNCERTAINTY IN IF/WHERE IT COULD PERSIST INTO THE  
TENNESSEE OR OHIO VALLEYS. THEN ADDITIONAL CONVECTION IS FORECAST  
TO FORM FARTHER SOUTHWEST IN THE SOUTHERN PLAINS/LOWER MISSISSIPPI  
VALLEY AS WELL ON SUNDAY. THUS PLAN TO SHOW A FAIRLY LARGE SLIGHT  
RISK FROM NORTHEASTERN TEXAS ACROSS PARTS OF THE LOWER AND MIDDLE  
MISSISSIPPI VALLEY FOR THE DAY 5/SUNDAY ERO TO COVER BOTH AREAS.  
INTO THE WORKWEEK, THE FRONT WILL PRESS SOUTH FARTHER INTO THE  
SOUTHERN TIER AND EAST INTO THE APPALACHIANS AND EVENTUALLY THE  
EASTERN SEABOARD. AREAS OF HEAVY RAINFALL ARE POSSIBLE AND THE  
DETAILS WILL CONTINUE TO GET REFINED WITH TIME.  
 
PRECIPITATION, IN THE FORM OF RAIN EXCEPT FOR SNOW IN THE HIGHEST  
PEAKS, IS POSSIBLE ACROSS THE NORTH-CENTRAL ROCKIES AND PLAINS INTO  
SATURDAY UNDERNEATH UPPER TROUGHING AND AS FRONTAL SYSTEMS MOVE  
THROUGH. THE DETAILS ARE UNCERTAIN BASED ON THE MODEL SPREAD WITH  
THE SUPPORTING ENERGIES. BUT THE SHORTWAVE PROVIDING DYNAMICAL  
SUPPORT, ABOVE NORMAL PWS, AND THE CUSP OF THE INSTABILITY GRADIENT  
COULD PROVIDE SOME HEAVY RAIN AND A LOW END FLASH FLOOD RISK IN  
THE NORTH-CENTRAL HIGH PLAINS. A MARGINAL RISK IS IN PLACE THERE  
FOR THE DAY 4/SATURDAY ERO.  
 
ELSEWHERE, LIGHT SHOWERS COULD CONTINUE ACROSS THE NORTHEAST BUT  
GRADUALLY LESSEN IN MAGNITUDE THROUGH THE WEEKEND AS THE UPPER AND  
SURFACE LOWS ARE SLOW TO PULL AWAY THERE. SUMMERTIME SCATTERED  
STORMS ARE LIKELY ACROSS THE FLORIDA PENINSULA THROUGH THE PERIOD.  
THE NORTHWEST MAY SEE RENEWED PRECIPITATION CHANCES EARLY NEXT  
WEEK.  
 
WARMER THAN NORMAL TEMPERATURES WILL BE IN PLACE ACROSS THE  
SOUTHERN PLAINS ON SATURDAY UNDERNEATH A CENTRAL U.S. RIDGE.  
TEMPERATURES WELL INTO THE 90S TO AROUND 100 WILL SUPPORT AREAS OF  
MAJOR HEATRISK IN PARTS OF TEXAS AND LOUISIANA THIS WEEKEND. THEN  
AS THE MEAN UPPER RIDGE REBUILDS FARTHER WEST, THE GREAT BASIN AND  
NORTHERN/CENTRAL ROCKIES CAN EXPECT TEMPERATURES GENERALLY AROUND  
10 TO 15 DEGREES ABOVE AVERAGE. MEANWHILE, COOLER THAN NORMAL  
TEMPERATURES BY 5 TO 15 DEGREES WILL TAKE HOLD OVER THE NORTH-  
CENTRAL TO EASTERN U.S. INTO THE WEEKEND UNDERNEATH UPPER  
TROUGHING. THE TROUGH WILL ALLOW FOR TEMPERATURES TO COOL IN THE  
SOUTHERN TIER DURING THE WORKWEEK COMPARED TO THE HOT WEEKEND, BUT  
OVERALL TEMPERATURES SHOULD GRADUALLY TREND CLOSER TO NORMAL AS  
NEXT WEEK PROGRESSES.  
 
TATE  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION FORECAST (QPF), EXCESSIVE RAINFALL  
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT  
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW  
 
 
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