232
FXUS02 KWBC 211937
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
337 PM 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
CLOSED AND CUTOFF MID-LEVEL LOW PRESSURE SYSTEM THAT IS FORECAST TO
MOVE VERY SLOWLY EASTWARD ACROSS THE MIDWEST AND OHIO VALLEY
THURSDAY THROUGH 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
THE LATEST NUMERICAL GUIDANCE REMAINS IN RELATIVELY GOOD AGREEMENT
WITH THEIR DEPICTION OF THE LARGE SCALE PATTERN THROUGH THE MEDIUM
RANGE PERIOD. MAJOR FEATURES INCLUDE A LARGE, SLOW-MOVING CLOSED
UPPER LOW OVER THE CENTRAL/EASTERN U.S., A RIDGE OVER THE GREAT
PLAINS, A CUTOFF LOW OVER THE SOUTHWEST, AND VARYING DEGREES OF
TROUGHING WITH FAST-MOVING SHORTWAVES WITHIN AN ACTIVE PACIFIC JET
ACROSS THE EASTERN PACIFIC AND PACIFIC NORTHWEST. FOR DAYS 3 AND 4,
A GENERAL MODEL BLEND CONSISTING OF THE 00Z/06Z DETERMINISTIC RUNS
OF THE EC, CMC, UKMET, AND GFS OFFERED A DECENT STARTING POINT TO
THE FORECAST, WITH A LARGE DEGREE OF CONTINUITY MAINTAINED AND MOST
MAJOR FEATURES REPRESENTED WELL. BY DAY 5, THE 00Z UKMET AND 00Z
CMC BEGAN TO DIVERGE FROM CONSENSUS, WITH SOLUTIONS A BIT TOO FAST
AND AMPLIFIED WITH SHORTWAVE TROUGHING IN THE EASTERN PACIFIC AND
PACIFIC NORTHWEST COMPARED TO THE EC/GFS. THIS RESULTED IN
SIGNIFICANT DOWNSTREAM DISCREPANCIES HEADING INTO DAYS 6 AND 7.
THUS FOR THE LATTER HALF OF THE PERIOD, THE COMPOSITE BLEND WAS
ANCHORED BY 60 PERCENT DETERMINISTIC RUNS OF THE 00Z EC AND 06Z GFS
(50/50 SPLIT) WITH THE REMAINING 40 PERCENT DRIVEN BY THE 00Z EC
AND 06Z GEFS ENSEMBLE MEANS (50/50 SPLIT). THIS BLEND SEEMED TO
MAINTAIN A FAIR AMOUNT OF CONTINUITY WHILE ALSO SMOOTHING OUT SOME
OF THE FINER TEMPORAL AND SPATIAL DIFFERENCES.
..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 REMAINS HIGHLIGHTED FOR THIS REGION WITH ONLY MINOR NORTH AND
EASTWARD ADJUSTMENTS 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 REMAINS IN PLACE 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 GROWING QPF SIGNAL
PROMPTED A NORTHWARD EXTENSION INTO PENNSYLVANIA, SOUTHERN NEW YORK
AND NW NJ WITH THE AFTERNOON ISSUANCE, AND IT WOULD NOT BE OUT OF
THE QUESTION FOR A SLIGHT TO BE INCLUDED IN FUTURE PACKAGES DESPITE
DRY ANTECEDENT CONDITIONS. HEAVY RAIN CONCERNS MAY SPREAD FARTHER
EAST THROUGH THE NORTHEAST ALONG AND AHEAD OF A COLD FRONT FRIDAY
INTO THE WEEKEND.
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 REMAIN IN PLACE
FOR PARTS OF CALIFORNIA AND NEVADA ON DAYS 4 AND 5 FOR ISOLATED
FLOODING POTENTIAL WITH BURN SCARS MOST VULNERABLE. THERE ARE STILL
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.
MILLER/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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