675  
FXUS02 KWBC 201959  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
359 PM EDT SAT SEP 20 2025  
 
VALID 12Z TUE SEP 23 2025 - 12Z SAT SEP 27 2025  
 
...HEAVY RAIN THREAT FOR PORTIONS OF THE CENTRAL U.S. TO  
SOUTHERN/CENTRAL APPALACHIANS NEXT WEEK...  
   
..OVERVIEW  
 
THE MAIN SYSTEM OF NOTE DURING THE MEDIUM RANGE PERIOD WILL BE A  
DEEPENING SHORTWAVE INTO THE CENTRAL U.S. ON TUESDAY WITH  
INCREASING CONFIDENCE THE ENERGY WILL CLOSE OFF INTO A CUTOFF LOW  
AND MOVE RATHER SLOWLY EASTWARD ACROSS THE MIDWEST AND OHIO VALLEY  
FOR THE LATTER HALF OF THE WEEK. THIS SHOULD FOCUS RAIN THAT COULD  
BE HEAVY NEAR A RESULTANT SURFACE LOW PRESSURE/FRONTAL SYSTEM IN  
THE LOWER/MID-MISSISSIPPI VALLEY, OHIO VALLEY/MID-SOUTH, AND  
SOUTHERN/CENTRAL APPALACHIANS. MEANWHILE, ANOTHER CUTOFF LOW OFF  
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 EARLY-MID WEEK.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT  
 
AN OMEGA BLOCK PATTERN LOOKS TO SET UP OVER THE CONUS NEXT WEEK,  
CONSISTING OF AN UPPER LOW NEAR/OVER CALIFORNIA, A LARGE CUTOFF LOW  
OVER THE CENTRAL U.S. MOVING EAST, AND AN AMPLIFIED RIDGE IN  
BETWEEN. RECENT MODEL AGREEMENT IS BETTER THAN A DAY OR TWO AGO  
WITH THE EVOLUTION OF THE UPPER LOW FORMING NEAR THE CENTRAL  
ROCKIES/HIGH PLAINS BY TUESDAY AND TRACKING EAST SLOWLY THROUGH THE  
MID-MISSISSIPPI VALLEY AND REACHING THE OHIO VALLEY BY LATE WEEK.  
THE ASSOCIATED SURFACE LOW ALSO SHOWS REASONABLE CONSENSUS. A  
MULTI-MODEL BLEND WITH SOME ENSEMBLE INCLUSION WORKED WELL FOR  
THESE FEATURES AND MAINTAINED GOOD CONTINUITY WITH THE PREVIOUS  
FORECAST. BUT THERE IS STILL SOME MODEL SPREAD WITH THE TIMING OF A  
NORTHERN STREAM TROUGH MOVING ACROSS THE NORTHEAST, AFFECTING A  
COLD FRONTAL POSITION AHEAD OF IT AND PRECIPITATION CHANCES. THE  
00/06Z ENSEMBLE MEANS, ESPECIALLY THE CMC/ECENS, AND THE 00Z CMC  
TENDED TO BE FARTHER WEST THAN THE GFS AND ECMWF RUNS THAT WERE  
EAST. NOW WITH THE 12Z RUNS, THE ECMWF HAS SLOWED DOWN BUT THE CMC  
HAS SPED UP. SO WILL CONTINUE TO WAIT FOR BETTER CONSENSUS IN THAT  
REGION.  
 
THERE IS GENERAL AGREEMENT FOR A SMALLER SOUTHERN STREAM UPPER LOW  
TO DRIFT NORTHWARD FROM THE EASTERN PACIFIC INTO CALIFORNIA AND  
THEN MOVE EAST THROUGH THE PERIOD. SOME TIMING AND PLACEMENT  
DIFFERENCES ARE SEEN, BUT TYPICAL FOR THE MEDIUM RANGE PERIOD. IN  
THE NORTHERN STREAM, MODEL AGREEMENT HAS INCREASED WITH SHOWING LOW  
AMPLITUDE FLOW AS A WEAK SHORTWAVE MOVES THROUGH BRITISH COLUMBIA  
AROUND MIDWEEK, AND A NORTHEASTERN PACIFIC TROUGH PERHAPS EDGING  
JUST INTO WASHINGTON LATE WEEK. THIS LEADS TO THE SOUTHERN STREAM  
LOW POTENTIALLY REMAINING A SEPARATE FEATURE IN THE INTERMOUNTAIN  
WEST EVEN INTO LATE WEEK. THE WEST COULD ALSO BE HANDLED WITH THE  
WPC FORECAST APPROACH OF A MULTI-MODEL BLEND EARLY IN THE PERIOD  
WITH INCREASING PROPORTIONS OF THE ENSEMBLE MEANS TO HALF BY THE  
END OF THE PERIOD.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS  
 
WIDESPREAD RAIN WILL FOCUS NEAR THE UPPER LOW AND ASSOCIATED  
FRONTAL BOUNDARIES AS IT BECOMES MORE DEFINED BY TUESDAY. GUIDANCE  
CONTINUES TO SHOW INCREASING AGREEMENT ON AN AREA OF HEAVY RAINFALL  
FOR EASTERN PARTS OF THE SOUTH-CENTRAL PLAINS AND THE LOWER/MID-  
MISSISSIPPI VALLEY. SHOW A SLIGHT RISK FOR THESE AREAS IN THE DAY  
4/TUESDAY ERO WITH A BROADER MARGINAL RISK FOR SURROUNDING AREAS.  
THIS IS CONSIDERED A HIGHER-END SLIGHT RISK ATOP PARTS OF THE  
OZARKS, WHERE HEAVY RAIN RATES ARE FORECAST TO TRAIN OVER THIS  
TYPICALLY FLOOD-PRONE AREA. BY WEDNESDAY, THE SLOW MOVEMENT OF THE  
UPPER/SURFACE LOW EASTWARD SHOULD CARRY THE HEAVIEST RAIN CHANCES  
EAST WITH IT, INTO PARTS OF THE TENNESSEE VALLEY INTO THE LOWER  
OHIO VALLEY. UNCERTAINTY INCREASES WITH THE DETAILS BY THEN, BUT  
THIS AREA SEEMS TO HAVE ENOUGH CONSENSUS FOR A SLIGHT RISK ON DAY  
5/WEDNESDAY, THOUGH ITS PLACEMENT MAY MOVE IN FUTURE ISSUANCES.  
THERE IS INCREASING POTENTIAL FOR LOCALIZED HEAVY RAIN IN UNSTABLE  
AIR AHEAD OF THE LOW'S COLD FRONT, SO EXTENDED THE MARGINAL RISK TO  
THE GULF COAST, DESPITE SOME MODEL SPREAD IN PLACEMENT. THERE IS  
QUITE A BIT OF UNCERTAINTY IN THE NORTHERN SIDE OF THE FRONTAL  
SYSTEM THAT COULD ALLOW FOR MORE RAINFALL IN THE GREAT LAKES AND/OR  
NORTHEAST TUESDAY/WEDNESDAY. THE RAIN ACTIVITY WITH THE PRIMARY  
LOW WILL MOVE SLOWLY EASTWARD WITH TIME AFTER MIDWEEK INTO PARTS OF  
THE EAST, WITH A LIKELY HEAVY RAINFALL FOCUS IN THE SOUTH-CENTRAL  
APPALACHIANS. FARTHER SOUTH, SOME DIURNALLY DRIVEN SHOWERS AND  
STORMS ARE POSSIBLE ACROSS THE FLORIDA PENINSULA THROUGHOUT THE  
PERIOD.  
 
MEANWHILE, AN UPPER LOW LINGERING NEAR CALIFORNIA EARLY-MIDWEEK  
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 APPEARS WEAKER IN THIS EVENT  
COMPARED TO THE RECENT RAIN EVENT. WILL SHOW MARGINAL RISKS FOR  
PARTS OF CALIFORNIA AND NEVADA ON DAYS 4 AND 5 FOR ISOLATED  
FLOODING POTENTIAL WITH THE MUCH ABOVE NORMAL MOISTURE. BURN SCARS  
WILL BE PARTICULARLY VULNERABLE TO FLOODING. THERE ARE SOME TIMING  
DIFFERENCES WITH THE UPPER LOW AND THUS PRECIPITATION TIMING, SO  
CONTINUE TO MONITOR FORECASTS FOR REFINED DETAILS. LIGHTER  
PRECIPITATION IS FORECAST TO SPREAD INTO PARTS OF THE INTERMOUNTAIN  
WEST AS THE ENERGY MOVES NORTH AND EAST. THE PACIFIC MOISTURE  
COULD ALLOW FOR CONVECTION ACROSS THE SOUTHWEST AS WELL, SO  
DELINEATED A MARGINAL RISK FOR PORTIONS OF ARIZONA AND NEW MEXICO  
INTO FAR WEST TEXAS ON DAY 5/WEDNESDAY. THE NORTHWEST SHOULD  
GENERALLY BE DRY UNTIL POSSIBLY LATE WEEK AHEAD OF A POSSIBLE UPPER  
TROUGH.  
 
A RIDGE AXIS WILL BUILD FROM THE FOUR CORNERS STATES INTO THE WEST  
WITH SOME ABOVE NORMAL TEMPERATURES ACROSS NORTHERN CALIFORNIA AND  
THE NORTHERN TIER STATES. THE DESERT SOUTHWEST MAY SEE  
TEMPERATURES REACHING OR EXCEEDING 100F, MAINLY TUESDAY-THURSDAY.  
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.  
 
TATE/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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