690  
FXUS02 KWBC 210727  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
327 AM EDT SUN SEP 21 2025  
 
VALID 12Z WED SEP 24 2025 - 12Z SUN SEP 28 2025  
 
...HEAVY RAIN THREAT FOR PORTIONS OF THE EAST-CENTRAL TO EASTERN  
U.S. LATER THIS WEEK...  
   
..OVERVIEW
 
 
THE MAIN SYSTEM OF NOTE DURING THE MEDIUM RANGE PERIOD WILL BE A  
SHORTWAVE THROUGH THE CENTRAL U.S. WHICH LOOKS TO CLOSE OFF INTO A  
CUTOFF LOW AND MOVE VERY SLOWLY EASTWARD ACROSS THE MIDWEST AND  
OHIO VALLEY THURSDAY-SUNDAY. THIS WILL FOCUS POTENTIALLY HEAVY  
RAINFALL NEAR A RESULTANT SURFACE LOW PRESSURE/FRONTAL SYSTEM INTO  
THE OHIO VALLEY/MID-SOUTH, THE SOUTHERN/CENTRAL APPALACHIANS, AND  
EVENTUALLY THE EAST. MEANWHILE, ANOTHER CUTOFF LOW NEAR THE  
CALIFORNIA COAST COULD LEAD TO A ROUND OF RAIN OVER SOUTHERN  
CALIFORNIA INTO THE SOUTHWEST AS IT LIFTS NORTH AND THEN EAST,  
WHILE AN UPPER RIDGE AHEAD OF THIS LOW OVER THE SOUTHWEST COULD  
LEAD TO A PERIOD OF TRIPLE DIGIT HEAT INTO MID-WEEK.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
AN OMEGA BLOCK PATTERN SHOULD BE IN PLACE OVER THE CONUS LATER  
THIS WEEK, CONSISTING OF AN UPPER LOW NEAR/OVER CALIFORNIA, A LARGE  
CUTOFF LOW OVER THE EAST-CENTRAL U.S., AND AN AMPLIFIED RIDGE IN  
BETWEEN. GUIDANCE CONTINUES TO SHOW GOOD OVERALL AGREEMENT ON THIS  
PATTERN, BUT PLENTY OF UNCERTAINTY IN THE DETAILS WHICH WILL HAVE  
IMPLICATIONS OF FOR EXACT PLACEMENT OF HEAVY RAINFALL, ESPECIALLY  
ACROSS THE EASTERN THIRD OF THE COUNTRY. THE ONLY THING NOTABLE  
WITH THE EASTERN LOW IS THE UKMET WAS A BIT SOUTH AND WEST OF THE  
BETTER CONSENSUS. IN CALIFORNIA, THERE REMAINS SOME DIFFERENCES IN  
PLACEMENT OF THE LOW, BUT ALL THE GUIDANCE SHOWS THIS LOW GRADUALLY  
MOVING INLAND, WEAKENING, AND DROPPING SOUTHEASTWARD. THERE IS  
GREATER UNCERTAINTY LATE PERIOD WITH TROUGHING AS IT MOVES  
TOWARDS/INTO THE NORTHWEST. THE ECMWF AT THIS POINT IS MOST  
AGGRESSIVE WITH BRINGING SHORTWAVE ENERGY INLAND NEXT WEEKEND.  
 
THE WPC FORECAST TONIGHT USED A BLEND OF THE GFS, ECMWF, AND CMC  
FOR THE FIRST HALF OF THE PERIOD WITH RELATIVELY GOOD AGREEMENT.  
INCREASED WEIGHTING OF THE ENSEMBLE MEANS BY NEXT WEEKEND TO  
ACCOUNT FOR INCREASING UNCERTAINTY, BUT OVERALL MAINTAINED GOOD  
AGREEMENT WITH THE PREVIOUS WPC FORECAST.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
WIDESPREAD RAIN AND CONVECTION WILL FOCUS NEAR THE UPPER LOW AND  
ASSOCIATED FRONTAL BOUNDARIES AS IT BECOMES MORE DEFINED INTO MID-  
WEEK. ON WEDNESDAY, THE HEAVIEST RAIN LOOKS TO FOCUS ACROSS PARTS  
OF THE TENNESSEE VALLEY INTO THE LOWER OHIO VALLEY AND A SLIGHT  
RISK IS HIGHLIGHTED FOR THIS REGION ON THE ERO. A BROADER MARGINAL  
RISK SURROUNDS THIS AREA TO THE CENTRAL GULF COAST AND BACK INTO  
THE MID-MISSISSIPPI VALLEY AND EASTERN PARTS OF THE CENTRAL PLAINS.  
THIS ACTIVITY WILL MOVE SLOWLY EASTWARD WITH TIME WITH A LIKELY  
FOCUS ON THURSDAY ACROSS THE SOUTHERN AND CENTRAL APPALACHIANS.  
ANOTHER SLIGHT RISK WAS INCLUDED ON THE DAY 5 ERO FOR THIS REGION  
SURROUNDED BY A BROAD MARGINAL RISK FROM THE CENTRAL GULF COAST  
INTO THE OHIO VALLEY AND MID-ATLANTIC. A COLD FRONT MOVING INTO THE  
NORTHEAST MAY BRING A PERIOD OF MODERATE TO HEAVY RAIN TO THAT  
REGION LATER IN THE WEEK OR NEXT WEEKEND AS WELL.  
 
MEANWHILE, AN UPPER LOW LINGERING NEAR CALIFORNIA SHOULD RESULT IN  
SOME RAINFALL OVER THE STATE AND INTO THE SOUTHWEST AS AMPLE  
MOISTURE ADVECTS AHEAD OF IT. PRECIPITABLE WATER ANOMALIES COULD BE  
3-4 STANDARD DEVIATIONS ABOVE NORMAL INTO CALIFORNIA, THOUGH  
INSTABILITY IS NOT AS IMPRESSIVE. MARGINAL RISKS ARE IN PLACE FOR  
PARTS OF CALIFORNIA AND NEVADA ON DAYS 4 AND 5 FOR ISOLATED  
FLOODING POTENTIAL WITH BURN SCARS MOST VULNERABLE. THERE ARE SOME  
TIMING DIFFERENCES WITH THE UPPER LOW AND THUS PRECIPITATION  
TIMING, SO CONTINUE TO MONITOR FORECASTS FOR REFINED DETAILS.  
MOISTURE WILL INCREASE ACROSS THE SOUTHWEST THROUGH THE PERIOD AS  
THE UPPER LOW DRAWS PACIFIC MOISTURE INTO THE REGION. MARGINAL  
RISKS FOR THE DAYS 4 AND 5 ERO IN THE SOUTHWEST REGION REMAIN ON  
TRACK RIGHT NOW. LIGHTER PRECIPITATION WILL ALSO SPREAD INTO PARTS  
OF THE INTERMOUNTAIN WEST AS THE ENERGY MOVES NORTH AND EAST. THE  
NORTHWEST SHOULD GENERALLY BE DRY UNTIL THE WEEKEND AHEAD OF A  
POSSIBLE UPPER TROUGH.  
 
A RIDGE AXIS WILL BUILD FROM THE FOUR CORNERS STATES INTO THE WEST  
WITH THE MOST ANOMALOUSLY WARM TEMPERATURES EXPECTED FROM THE  
NORTHWEST INTO THE NORTHERN ROCKIES AND PLAINS WHICH MAY HOLD MUCH  
OF THE PERIOD. THE DESERT SOUTHWEST SHOULD ALSO SEE TEMPERATURES  
REACHING OR EXCEEDING 100F, MAINLY INTO THURSDAY BEFORE THE RIDGE  
SHIFTS EAST. EXPECT NEAR TO BELOW NORMAL TEMPERATURES FOR THE  
CENTRAL U.S. UNDER THE LARGE UPPER LOW. AS THE LOW DRIFTS EASTWARD  
AS THE WEEK PROGRESSES, TEMPERATURES WILL COOL A BIT FROM ABOVE  
AVERAGE ACROSS THE EAST TO NEAR AVERAGE.  
 
SANTORELLI  
 
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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