030  
FXUS02 KWBC 171945  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
345 PM EDT WED SEP 17 2025  
 
VALID 12Z SAT SEP 20 2025 - 12Z WED SEP 24 2025  
   
..DESERT SOUTHWEST HEAT WAVE FIRST HALF OF NEXT WEEK  
 
   
..OVERVIEW  
 
THE UPPER AIR PATTERN THIS WEEKEND BEGINS WITH A DEEP LOW LIFTING  
NORTH OVER THE LABRADOR SEA WITH A DESCENDANT TROUGH THAT ALSO  
LIFTS NORTH FROM THE NORTHEAST, A FILLING/WEAKENING LOW OVER THE  
UPPER MIDWEST, AND A POTENT TROUGH REACHING THE BRITISH COLUMBIA  
COAST BY SATURDAY NIGHT WHICH BREAKS OFF INTO A CUTOFF LOW OFF THE  
CALIFORNIA COAST. THAT POTENT TROUGH THEN LIKELY DEVELOPS INTO A  
LARGE CUTOFF LOW OVER THE CENTRAL U.S. TUESDAY INTO WEDNESDAY AND  
BECOMES THE NOTABLE WEATHER FEATURE BY THE MIDDLE OF NEXT WEEK.  
MEANWHILE, STRONG RIDGING OVER THE INTERMOUNTAIN WEST LIKELY BRINGS  
A HEAT WAVE TO THE DESERT SOUTHWEST EARLY TO MID NEXT WEEK, WITH  
RECORD CHALLENGING TEMPERATURES POSSIBLE. SOME LOCALLY HEAVY RAIN  
MAY LINGER THIS WEEKEND OVER THE UPPER MIDWEST WITH SOME  
PROGRESSION TO DEVELOPMENT OVER THE CENTRAL/SOUTHERN PLAINS. COOL  
AIR DAMMING WILL SPREAD DOWN THE EASTERN SEABOARD THROUGH THE  
CAROLINA PIEDMONT SATURDAY AND LINGER POSSIBLY INTO TUESDAY.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT  
 
MOST GUIDANCE CONTINUES TO HAVE A SOLID HANDLE ON THE EVOLUTION OF  
THE LARGE SCALE PATTERN THROUGH THROUGH THE MEDIUM RANGE PERIOD. A  
LARGE UPPER TROUGH WILL BE LIFTING OUT OF EASTERN CANADA WHILE  
TROUGHING PUSHING INTO WESTERN CANADA WILL HELP DEPOSIT A CUTOFF  
LOW OFF THE COAST OF CALIFORNIA. IN BETWEEN, SEVERAL SLOWER-MOVING  
SHORTWAVES WILL INITIALLY ROTATE AROUND A COMMON AXIS OVER THE  
CENTRAL AND NORTHERN PLAINS/UPPER MIDWEST, WHILE INCREASINGLY  
AMPLIFIED RIDGING BUILDS OVER THE INTERMOUNTAIN WEST. WHILE THE  
SPATIAL AND TEMPORAL DETAILS OF INDIVIDUAL WAVES STILL NEEDS SOME  
FINE TUNING OVER THE NEXT FEW DAYS, CONFIDENCE IN THE OVERALL  
PATTERN IS FAIRLY GOOD. IN ADDITION TO THE PLAYERS MENTIONED ABOVE,  
A SIZABLE AMOUNT OF GUIDANCE, INCLUDING THE 00Z/12Z RUNS OF THE  
ECMWF AND THE 00Z/06Z/12Z RUNS OF THE GFS, IS HIGHLIGHTING THE  
DEVELOPMENT OF A LARGE CUTOFF LOW SOMEWHERE OVER THE CENTRAL U.S.  
DOWNSTREAM OF THE AMPLIFYING RIDGE BY DAYS 6 AND 7. CONFIDENCE IN  
THIS DEVELOPMENT IS STRENGTHENED BY THE SUPPORT OF THE MACHINE  
LEARNING GUIDANCE, PARTICULARLY THE LATEST RUNS OF THE EC-AIFS. THE  
CMC ON THE OTHER HAND HAS BEEN MUCH TOO PROGRESSIVE THUS FAR AND  
IS A CLEAR OUTLIER COMPARED TO OTHER GUIDANCE SOURCES. AS A RESULT,  
THE WPC FORECAST UTILIZED A BLEND OF THE DETERMINISTIC RUNS OF THE  
ECMWF, GFS, UKMET, AND CMC WITH 20 TO 40 PERCENT EPS/GEFS ENSEMBLE  
MEANS THROUGH DAY 5 BEFORE TRANSITIONING TO 40 PERCENT EC/GFS AND  
60 PERCENT EPS/GEFS ENSEMBLE MEANS FOR DAYS 6 AND 7, COMPLETELY  
PHASING OUT ANY CONTRIBUTION FROM THE CMC.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS  
 
STALLED LOW PRESSURE OVER THE UPPER MIDWEST ON SATURDAY CONTINUES  
TO PROVIDE A FRONTAL FOCUS FOR GULF-SOURCED MOISTURE STREAMING UP  
THE PLAINS. A DAY 4 MARGINAL RISK ERO REMAINS IN PLACE WITH MINOR  
ADJUSTMENTS MADE WITH THE AFTERNOON PACKAGE.  
 
FARTHER SOUTH, CONVERGENCE OF THIS GULF MOISTURE ALLOWS FOR SOME  
SLOWER MOVING CLUSTERS OF SHOWERS AND THUNDERSTORMS OVER PORTIONS  
OF THE SOUTHERN PLAINS/MID-SOUTH, SOME OF WHICH COULD HAVE HEAVY  
RAINFALL RATES. DAY 4 AND 5 MARGINALS REMAIN IN PLACE, BUT WERE  
EXPANDED RADIALLY IN ALL DIRECTIONS BOTH DAYS TO INCLUDE A LARGER  
QPF FOOTPRINT. MOISTURE IS NOT THAT ANOMALOUS, BUT PW ON THE ORDER  
OF 1.5" COMBINED WITH ADEQUATE INSTABILITY AND FORCING IS  
SUFFICIENT FOR EXCESSIVE RAINFALL CONCERNS WITH SLOW MOVING  
ACTIVITY/FLOW PARALLEL TO ANY BOUNDARIES.  
 
AN UPPER RIDGE WILL ALLOW HEAT TO PERSIST OVER THE MIDWEST AND  
SOUTHEAST THIS WEEKEND BEFORE SHIFTING TO THE INTERIOR NORTHEAST  
FOR EARLY NEXT WEEK WITH MAX TEMP ANOMALIES OF 5-15 DEGREES ABOVE  
NORMAL. A STRONG SURFACE HIGH CROSSING THE NORTHEAST SATURDAY WILL  
SEND A DECENT COLD FRONT DOWN MUCH OF THE EASTERN SEABOARD WITH  
COOL AIR DAMMING LINGERING EAST OF THE APPALACHIANS AND LIKELY  
THROUGH THE CAROLINA PIEDMONT AT LEAST THROUGH MONDAY AND POSSIBLY  
TUESDAY. ONSHORE FLOW, SCATTERED CLOUDS, AND POSSIBLE SHOWERS WILL  
RESULT IN BELOW NORMAL TEMPERATURES FOR MUCH OF THE NORTHEAST AND  
MID-ATLANTIC, PARTICULARLY FOR COASTAL LOCATIONS.  
 
AMPLIFYING RIDGING OVER THE INTERMOUNTAIN WEST WILL LEAD TO  
INCREASINGLY HOTTER WEATHER AND A POSSIBLE HEAT WAVE ACROSS THE  
DESERT SOUTHWEST EARLY TO MID NEXT WEEK. THIS HEAT MAY CHALLENGE  
HIGH TEMPERATURE RECORDS WITH HIGHS SOARING INTO THE MID TO UPPER  
100S FOR SOME LOCATIONS. DOWNSTREAM OF THE RIDGE, A SERIES OF  
SHORTWAVES WILL LIKELY COMBINE TO FORM A DEEP, CUTOFF MID-LEVEL LOW  
PRESSURE SYSTEM ACROSS THE CENTRAL U.S. BY TUESDAY AND WEDNESDAY.  
THIS WILL LIKELY RESULT IN FAIRLY WIDESPREAD UNSETTLED WEATHER WITH  
TEMPERATURES TRENDING NEAR TO BELOW NORMAL.  
 
MILLER/JACKSON  
 
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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