994
FXUS02 KWBC 230646
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
246 AM EDT TUE SEP 23 2025
VALID 12Z FRI SEP 26 2025 - 12Z TUE SEP 30 2025
...HEAVY RAIN THREAT FOR PORTIONS OF THE EASTERN U.S. ON FRIDAY,
WITH HEAVY RAINS ALSO LIKELY ACROSS THE SOUTHWEST THROUGH THE
WEEKEND...
..OVERVIEW
AN AMPLIFIED TROUGH WILL BE IN PLACE ACROSS THE EASTERN U.S. BY
FRIDAY, AND THE GUIDANCE SUGGESTS IT WILL SPLIT BETWEEN NORTHERN
STREAM ENERGY THAT QUICKLY EXITS THE NORTHEAST NEXT WEEKEND, AND
SOUTHERN STREAM ENERGY OR A WEAK CLOSED LOW MEANDERING OVER THE
SOUTHEAST. THE ASSOCIATED COLD FRONT WILL FOCUS WIDESPREAD RAINS
FROM THE GULF COAST/SOUTHEAST IN PARTS OF THE MID- ATLANTIC THROUGH
AT LEAST FRIDAY. MEANWHILE, PACIFIC MOISTURE AHEAD OF ANOTHER
CUTOFF LOW NEAR THE CALIFORNIA COAST COULD LEAD TO DAILY RAIN
CHANCES OVER PARTS OF CALIFORNIA INTO MUCH OF 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 AND THE NORTHEAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE LATEST GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT ON THE LARGE
SCALE SET UP THROUGH MUCH OF THE PERIOD, BUT PLENTY OF UNCERTAINTY
IN THE DETAILS. MODELS AND ENSEMBLES AGREE ON A SPLIT WITH THE
EASTERN TROUGH, BUT SOME QUESTION ON HOW STRONG THE ENERGY IS OVER
THE SOUTHEAST. THE GFS WAS MUCH MORE AMPLIFIED WITH A SECONDARY
NORTHERN STREAM SHORTWAVE THROUGH THE GREAT LAKES THIS WEEKEND.
MODELS ARE ALSO VERY UNCERTAIN WITH SOME POSSIBLE SMALL TROPICAL
SYSTEM OFF THE SOUTHEAST COAST EARLY NEXT WEEK. THE ECMWF AND CMC
WERE MOST AGGRESSIVE BRINGING IT INLAND (THOUGH THE NEW 00Z CMC
TONIGHT BACKED OFF ON THIS) AND THE GFS WAS WELL OFF THE COAST AND
CURVED IT AWAY. THE FORECAST TONIGHT OPTED TO STICK CLOSE TO
CONTINUITY WHICH WERE COLLABORATED WITH NHC AND ARE SORT OF IN A
MIDDLE OF THE TWO CAMPS.
OUT WEST, THE GUIDANCE STILL STRUGGLES WITH EXACT PLACEMENT OF THE
UPPER LOW OVER THE SOUTHWEST WHICH HAS BIG IMPLICATIONS FOR
AMOUNTS AND DISTRIBUTION OF QPF ACROSS THE REGION. TROUGHING WILL
AMPLIFY OFF THE NORTHWEST COAST WHICH MAY DRIVE HOW QUICKLY THE
SOUTHWEST LOW GETS PUSHED OUT OF THE REGION LATE IN THE PERIOD. THE
CMC WAS QUICKEST WITH THE SHORTWAVE INTO THE NORTHWEST EARLY NEXT
WEEK, WITH THE GFS SLOWER AND THE ECMWF A NICE MIDDLE GROUND (WITH
THE ENSEMBLE MEANS).
THE WPC FORECAST TONIGHT WAS BASED ON A DETERMINISTIC BLEND FOR
DAYS 3 AND 4, EVENTUALLY SHIFTING TOWARDS AN ECMWF AND ENSEMBLE
MEAN BLEND FOR THE LATTER HALF OF THE PERIOD. OVERALL, MAINTAINED
GOOD CONTINUITY WITH THE PREVIOUS SHIFT FORECAST AS WELL.
..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 A BROAD MARGINAL RISK ON THE DAYS 4/FRIDAY ERO STRETCHED
FROM THE GULF COAST TO THE MID-ATLANTIC. DRY ANTECEDENT CONDITIONS
MAY LIMIT THE OVERALL FLOOD THREAT, BUT FUTURE UPGRADES REMAIN
POSSIBLE WITHIN THE BROADER MARGINAL RISKS, AND WILL BE REEVALUATED
SHOULD THE GUIDANCE SHOW BETTER CONSENSUS. RAINFALL MAY LINGER
ACROSS PARTS OF THE EAST INTO THE WEEKEND, AND A MARGINAL RISK WAS
INCLUDED ON THE DAY 5/SATURDAY ERO FROM EASTERN GEORGIA TO
SOUTHEAST VIRGINIA.
MEANWHILE, PACIFIC MOISTURE AHEAD OF AN UPPER LOW NEAR CALIFORNIA
WILL RESULT IN INCREASING RAINFALL CHANCES ACROSS MUCH OF THE
SOUTHWEST INTO 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 IS HIGHLIGHTED WITH
A MARGINAL RISK FOR BOTH FRIDAY AND SATURDAY EROS WITH A MORE
TARGETED SLIGHT RISK ON BOTH DAYS ACROSS SOUTHEAST ARIZONA TO
SOUTHWEST NEW MEXICO WHERE QPF AMOUNTS LOOK GREATEST. THIS ACTIVITY
WILL CONTINUE INTO AT LEAST SUNDAY AS THE UPPER LOW DROPS SLOWLY
SOUTH AND EASTWARD THROUGH THE REGION AND MAY BEGIN TO TAPER OFF BY
MONDAY.
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 WITH ONLY A GRADUAL EASTWARD SHIFT. TEMPERATURES
ACROSS THE SOUTHWEST WILL BE BELOW NORMAL UNDERNEATH THE LINGERING
UPPER LOW, WITH NEAR NORMAL TEMPERATURES ELSEWHERE ACROSS THE
CONUS.
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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