738
FXUS02 KWBC 221853
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
253 PM EDT MON SEP 22 2025
VALID 12Z THU SEP 25 2025 - 12Z MON SEP 29 2025
..HEAVY RAIN THREAT FOR PORTIONS OF THE EASTERN U.S. LATE WEEK
..OVERVIEW
AN AMPLIFIED TROUGH/CLOSED LOW WILL MOVE SLOWLY FROM THE MIDWEST
ON THURSDAY TO THE EAST BY THE WEEKEND, WITH SOME ENERGY MEANDERING
OVER THE SOUTHEAST. THE ASSOCIATED COLD FRONT WILL FOCUS
POTENTIALLY HEAVY RAINFALL FROM THE GULF COAST/SOUTHEAST INTO THE
APPALACHIANS AND THE MID-ATLANTIC MAINLY THURSDAY AND FRIDAY.
MEANWHILE, PACIFIC MOISTURE AHEAD OF ANOTHER CUTOFF LOW NEAR THE
CALIFORNIA COAST COULD LEAD TO DAILY RAIN CHANCES OVER SOUTHERN
CALIFORNIA INTO THE SOUTHWEST AS IT DRIFTS SOUTHEASTWARD. AN UPPER
LEVEL RIDGE IN BETWEEN THESE TWO SYSTEMS WILL ALLOW FOR ABOVE
NORMAL TEMPERATURES MUCH OF THE PERIOD FROM THE INTERIOR WEST TO
THE NORTHERN PLAINS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE LATEST NUMERICAL GUIDANCE REMAINS IN RELATIVELY GOOD AGREEMENT
WITH THE LARGE SCALE PATTERN THROUGH THE MEDIUM RANGE PERIOD. FOR
DAYS 3 AND 4, THE WPC FORECAST WAS BASED ON A BLEND OF
DETERMINISTIC GUIDANCE, WITH THE 00Z EC SLIGHTLY FAVORED OVER THE
GFS, CMC, AND UKMET. FOR DAY 5, THE UKMET APPEARED TO BE TOO STRONG
WITH THE SOUTHWEST UPPER LOW, AND MUCH TOO SLOW WITH A NORTHERN
STREAM SHORTWAVE NEAR THE U.S./CANADA BORDER... AND THEREFORE WAS
REMOVED. TO COMPENSATE, A SMALL PORTION OF EC/GEFS ENSEMBLE MEANS
WERE INTRODUCED TO THE BLEND. BY DAY 6 AND 7, THE PERCENTAGE OF
ENSEMBLE MEANS WERE INCREASED TO 40 AND 50 PERCENT RESPECTIVELY,
WHILE THE CONTRIBUTION FROM THE GFS WAS REMOVED DUE TO BEING MUCH
TOO FAST AND AGGRESSIVE WITH A POTENTIAL TROPICAL SYSTEM OFF THE
SOUTHEAST U.S. COAST COMPARED TO CONSENSUS. THE WPC FORECAST
OFFERED A FAIR COMPROMISE BETWEEN DETAIL, UNCERTAINTY, AND
CONTINUITY FROM THE PREVIOUS FORECAST.
..WEATHER/HAZARDS HIGHLIGHTS
WIDESPREAD RAIN AND CONVECTION WILL FOCUS ALONG AND AHEAD OF A
COLD FRONT AS IT SLOWLY MOVES INTO THE EAST LATER THIS WEEK.
ANOMALOUS MOISTURE AND AMPLE INSTABILITY WILL BE IN PLACE,
WARRANTING BROAD MARGINAL RISKS ON THE DAYS 4 AND 5 (THURSDAY-
FRIDAY) ERO STRETCHED FROM THE GULF COAST TO THE MID-ATLANTIC. DRY
ANTECEDENT CONDITIONS MAY LIMIT THE OVERALL FLOOD THREAT, BUT THERE
CONTINUES TO BE ENOUGH OF A HEAVY RAINFALL SIGNAL FOR THE MORE
VULNERABLE SOUTHERN APPALACHIANS REGION ON THURSDAY WHERE A SLIGHT
RISK IS HIGHLIGHTED IN THE ERO. A SLIGHT RISK WAS ALSO CONSIDERED
ACROSS CENTRAL NORTH CAROLINA FOR THE DAY 5 FRIDAY ERO, BUT DECIDED
TO HOLD OFF FOR ANOTHER CYCLE AFTER COLLABORATING WITH AFFECTED
WFOS. FUTURE UPGRADES REMAIN POSSIBLE WITHIN THE BROADER MARGINAL
RISKS, AND WILL BE REEVALUATED SHOULD THE GUIDANCE SHOW BETTER
CONSENSUS. RAINFALL MAY LINGER ACROSS THE NORTHEAST AND THE
IMMEDIATE EAST COAST THIS WEEKEND AS WELL.
MEANWHILE, PACIFIC MOISTURE AHEAD OF AN UPPER LOW NEAR CALIFORNIA
WILL RESULT IN INCREASING RAINFALL CHANCES ACROSS MUCH OF THE
SOUTHWEST INTO SOUTHERN CALIFORNIA. MOISTURE ANOMALIES 2 TO 3
STANDARD DEVIATIONS ABOVE NORMAL AND SUFFICIENT INSTABILITY WILL
RESULT IN AT LEAST A LOCALIZED FLASH FLOOD THREAT, PARTICULARLY FOR
VULNERABLE BURN SCAR AREAS. THIS ENTIRE REGION REMAINS HIGHLIGHTED
WITH A MARGINAL RISK FOR BOTH THURSDAY AND FRIDAY'S EROS AND A
MORE TARGETED SLIGHT RISK ALSO REMAINS ON BOTH DAYS ACROSS
SOUTHEAST ARIZONA WHERE QPF AMOUNTS LOOK GREATEST. THIS ACTIVITY
WILL CONTINUE INTO THE WEEKEND AS THE UPPER LOW DROPS SLOWLY SOUTH
AND EASTWARD THROUGH THE REGION.
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. AND MIDWEST UNDER THE LARGE UPPER TROUGH/LOW. AS THE
LOW DRIFTS EASTWARD, TEMPERATURES WILL COOL ACROSS THE EAST AS
WELL.
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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