526
FXUS02 KWBC 200712
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
312 AM 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
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 MAY CLOSE OFF INTO A CUTOFF LOW AND LINGER
OVER THE MIDWEST FOR A DAY OR TWO LATER IN THE WEEK. THIS WOULD
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 APPALACHIANS. MEANWHILE, ANOTHER
CUTOFF LOW OFF THE CALIFORNIA COAST COULD LEAD TO A ROUND OF RAIN
OVER SOUTHERN CALIFORNIA INTO THE DESERT SOUTHWEST AS IT LIFTS
NORTH, AND A POSSIBLE UPPER RIDGE 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 OVER CALIFORNIA, A LARGE CUTOFF LOW OVER
THE CENTRAL U.S., AND AN AMPLIFIED RIDGE IN BETWEEN. THE MAIN
FEATURE OF INTEREST AND FOCUS FOR POSSIBLY IMPACTFUL WEATHER WILL
BE THE LOW OVER THE CENTRAL U.S.. THERE REMAINS SOME UNCERTAINTY IN
THE DETAILS AND PLACEMENT, BUT MUCH OF THE GUIDANCE NOW SUPPORTS A
CUTOFF LOW OVER THE MID-MISSSIPPI VALLEY TO MIDWEST WHICH MAY
LINGER FOR A FEW DAYS BEFORE SLOWLY SHIFTING EAST NEXT WEEKEND. THE
WPC FORECAST FAVORED THE GFS AND ECMWF FOR THIS SYSTEM, WITH THE
ECMWF BEING THE MOST CONSISTENT IN SHOWING THIS FEATURE FOR SEVERAL
DAYS NOW.
OUT WEST, THE GUIDANCE STRUGGLES STILL WITH PLACEMENT OF THE UPPER
LOW NEAR CALIFORNIA AND HOW IT MAY GET ABSORBED INTO A NORTHERN
STREAM SHORTWAVE MID WEEK. ANOTHER SHORTWAVE TOWARDS THE NORTHWEST
ON FRIDAY SHOWS EVEN GREATER UNCERTAINTY WITH THE GFS SHOWING A
MUCH MORE AMPLIFIED TROUGH IN THE NORTHEAST PACIFIC WHICH THE ECMWF
AND CMC ARE MORE TRANSIENT WITH A WEAKER SHORTWAVE.
THE WPC FORECAST FOR TONIGHT USED A BLEND OF THE DETERMINISTIC
GUIDANCE FOR DAYS 4 AND 5 WITH SUFFICIENT AGREEMENT. INCREASED
WEIGHTING OF THE ENSEMBLE MEANS WITH THE GFS AND ECMWF LATE PERIOD
TO ACCOUNT FOR GREATER UNCERTAINTY. OVERALL, MAINTAINED GOOD
CONTINUITY WITH THE PREVIOUS SHIFT FOR DAYS 3-6.
..WEATHER/HAZARDS HIGHLIGHTS
WIDESPREAD RAIN WILL FOCUS NEAR THE UPPER LOW AND ASSOCIATED
FRONTAL BOUNDARIES AS IT BECOMES MORE DEFINED BY TUESDAY. GUIDANCE
IS SHOWING MORE AGREEMENT ON AN AREA OF POTENTIALLY HEAVY RAINFALL
FROM THE CENTRAL PLAINS INTO THE MID-MISSISSIPPI VALLEY ON TUESDAY
WHICH IS HIGHLIGHTED WITH A BROAD MARGINAL RISK AREA ON THE DAY 4
ERO. THERE SEEMS TO BE SUFFICIENT CONSENSUS ON A MORE FOCUSED AREA
ACROSS SOUTHERN MISSOURI AND A SLIGHT RISK WAS ADDED TO THE OUTLOOK
TONIGHT FOR THAT AREA. THE HEAVY RAINFALL SHIFTS JUST SLIGHTLY
EASTWARD ON WEDNESDAY, WITH THE ERO MARGINAL RISK OVER PARTS OF THE
TENNESSEE VALLEY INTO THE LOWER OHIO VALLEY. THERE IS SOME RECENT
AGREEMENT ON HEAVY RAIN FOCUSING ALONG THE COLD FRONT FROM WEST-
CENTRAL TENNESSEE INTO SOUTHERN KENTUCKY, BUT DRY ANTECEDENT
CONDITIONS AND SOME LINGERING UNCERTAINTY PRECLUDES A SLIGHT RISK
THERE YET. FARTHER EAST, RAIN IS BECOMING MORE LIKELY IN THE
NORTHEAST ESPECIALLY TUESDAY AHEAD OF THE EASTERN PART OF THE
FRONTAL SYSTEM, BUT RAIN AMOUNTS AND PLACEMENT ARE UNCERTAIN THERE
AS WELL. THE RAIN ACTIVITY WITH THE PRIMARY LOW WILL MOVE SLOWLY
EASTWARD WITH TIME AFTER MID WEEK INTO PARTS OF THE EAST, WITH A
LIKELY HEAVY RAINFALL FOCUS IN THE SOUTHERN APPALACHIANS.
ELSEWHERE, AN UPPER LOW NEAR CALIFORNIA EARLY IN THE WEEK SHOULD
RESULT IN SOME RAINFALL OVER THE STATE AND POSSIBLY INTO THE
SOUTHWEST. CURRENTLY THE RAINFALL DOES NOT APPEAR TO BE AS HEAVY AS
THE RECENT EVENT, BUT WILL CONTINUE TO MONITOR IF THE ABOVE
AVERAGE MOISTURE AND INSTABILITY IN SOUTHERN CALIFORNIA WOULD
REQUIRE A MARGINAL RISK IN THE ERO PARTICULARLY TUESDAY. LIGHTER
PRECIPITATION IS FORECAST TO SPREAD INTO PARTS OF THE INTERMOUNTAIN
WEST AS THE ENERGY MOVES NORTH AND EAST. THE NORTHWEST SHOULD
GENERALLY BE DRY UNTIL POSSIBLY LATE WEEK AHEAD OF A POSSIBLE UPPER
TROUGH. MEANWHILE, THE FLORIDA PENINSULA CAN EXPECT MAINLY
DIURNALLY DRIVEN SHOWERS AND STORMS.
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 EARLY IN THE WEEK.
ELSEWHERE, EXPECT NEAR TO BELOW NORMAL TEMPERATURES FROM THE
CENTRAL U.S. EASTWARD ASSOCIATED WITH THE LARGE UPPER LOW.
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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