289
FXUS02 KWBC 071921
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
321 PM EDT SUN SEP 7 2025
VALID 12Z WED SEP 10 2025 - 12Z SUN SEP 14 2025
..OVERVIEW
THE OVERALL WEATHER PATTERN DURING THE UPCOMING MEDIUM RANGE
PERIOD BEGINS WITH AMPLIFIED TROUGHS ON BOTH THE EAST AND WEST
COASTS WITH RIDGING OVER THE CENTRAL U.S.. AT LEAST INITIALLY,
THESE WILL BE SLOW TO MOVE OUT BUT ENERGY EJECTING OUT OF THE WEST
SHOULD HELP TO FINALLY DISLODGE THE PATTERN SOME. IN TERMS OF
SENSIBLE WEATHER, WAVY FRONTAL SYSTEMS WILL FOCUS RAIN CHANCES FROM
THE NORTHWEST/WEST TO THE ROCKIES/PLAINS WITH GRADUAL FLOW
TRANSLATION. HEAVY RAIN POTENTIAL REMAINS PRESENT FROM FLORIDA
THROUGH THE COASTAL CAROLINAS AND POSSIBLY SOUTHEAST NEW ENGLAND AS
DEEPENED MOISTURE INTERACTS WITH A WAVY WESTERN ATLANTIC FRONT.
THIS ACTIVITY WILL FOCUS ON THE LEADING EDGE OF A COOL CANADIAN
AIRMASS INSURGENCE FOR THE CENTRAL AND EASTERN U.S..
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE LATEST GUIDANCE SEEMS GENERALLY WELL CLUSTERED THE FIRST HALF
OF THE PERIOD ON THIS OVERALL PATTERN WITH THE EXCEPTION OF THE
CMC. THE CMC SHOWS MORE INTERACTION BETWEEN THE WESTERN U.S. TROUGH
ENERGIES AND AN ADDITIONAL SHORTWAVE DROPPING SOUTH FROM WESTERN
CANADA. THIS LEADS TO THE OVERALL WESTERN U.S. PATTERN SHIFTING
EASTWARD FASTER THAN THE CONSENSUS. THERE IS AGREEMENT THAT ENERGY
FROM THE WESTERN TROUGH WILL EJECT EASTWARD BY THE WEEKEND, BUT FOR
TIMING AND AMPLITUDE, PREFER THE ECMWF AND GFS COUPLED WITH THE
ENSEMBLE MEANS. ANOTHER DEEP TROUGH WILL REACH THE WEST COAST LATE
WEEKEND WITH SOME MINOR TIMING UNCERTAINTIES.
ELSEWHERE, THE EASTERN U.S. TROUGH SHOULD GENERALLY HOLD THROUGH
THE PERIOD BUT BY NEXT WEEKEND SOME MORE SIGNIFICANT UNCERTAINTIES
ARISE ATTRIBUTED TO ENERGY DROPPING SOUTHWARD FROM EASTERN CANADA.
THE ECMWF WAS THE OUTLIER HERE SHOWING A DEEP CLOSED LOW MOVING
INTO THE LOWER GREAT LAKES/NEW ENGLAND REGION NEXT SUNDAY.
GENERALLY FAVORED A BLEND OF THE DETERMINISTIC MODELS DAY 3 AND 4,
REMOVING THE CMC COMPLETELY FROM THE BLEND BY DAY 5. FOR DAYS 6
AND 7, INCREASED WEIGHTING OF THE ENSEMBLE MEANS TO 70 PERCENT TO
ACCOUNT FOR GREATER PATTERN AND DETAIL UNCERTAINTIES. THIS APPROACH
STILL MAINTAINS GOOD CONTINUITY WITH THE PREVIOUS WPC FORECAST.
..WEATHER/HAZARDS HIGHLIGHTS
A PERSISTENT AND WAVY FRONTAL SYSTEM OVER THE GULF AND FLORIDA
ALSO EXTENDS JUST OFF/UP THE EASTERN SEABOARD. THIS PROVIDES A MAIN
FOCUS FOR TROPICAL MOISTURE TO POOL, LEADING TO DAILY ENHANCED AND
DIURNALLY DRIVEN RAINFALL POTENTIAL ASSOCIATED WITH SCATTERED
THUNDERSTORMS. A NEARLY STATIONARY VORTEX OVER THE EASTERN GULF
WHICH MAY EVENTUALLY MERGE WITH THE FRONT OVER THE GULF COULD HELP
TRIGGER AND PUSH SOME ORGANIZED BANDS OF THUNDERSTORMS ALSO ONSHORE
INTO WESTERN FLORIDA PENINSULA INTO MID- LATE THIS WEEK. SATURATED
GROUNDS FROM PRE- CURSOR RAINFALL WARRANT A MARGINAL RISK OF
EXCESSIVE RAINFALL OUTLOOK AREA TO BE KEPT ACROSS THE FLORIDA
PENINSULA FOR WEDNESDAY AND THURSDAY. FARTHER NORTH, DAILY RAINFALL
AMOUNTS FROM THE COASTAL SOUTHEAST/CAROLINAS AND UP THE COAST
OFFERS UNCERTAINTY FOR HOW MUCH CONVECTION OCCURS ONSHORE/OFFSHORE
ON ANY GIVEN DAY. SMALL SHIFTS IN THE WAVY FRONTAL BOUNDARY COULD
PRODUCE SIGNIFICANT DIFFERENCES IN ONSHORE RAINFALL AMOUNTS. THE
AMPLIFIED AND SLOWED PATTERN NATURE MAY LEND FAVOR TO ONSHORE FLOW
TRENDS AND BEHAVIOR.
AN UPPER TROUGH/CLOSED LOW WORKING OVER THE NORTHWEST/WEST WILL
SUPPORT MULTIPLE DAYS WITH POTENTIAL RAINFALLS INTO MID-LATE WEEK,
SHIFTING SLOWLY WITH TIME FROM THE PACIFIC NORTHWEST/NORTHERN
CALIFORNIA TO THE NORTH-CENTRAL GREAT BASIN/INTERMOUNTAIN WEST.
THIS COOL SEASON TYPE OF RAINFALL SHOULD BE MARKED BY MODERATE
RATES THAT ARE GENERALLY NOT ENOUGH TO CAUSE FLOODING CONCERNS. THE
LOWERED HEIGHTS ALOFT WILL YIELD COOLER TEMPERATURES SUPPORTIVE OF
SOME SNOW FOR THE HIGHEST ELEVATIONS. MONSOONAL FLOW GENERALLY
WANES INTO THIS WEEK FOR THE WEST/SOUTHWEST, BUT LINGERING MOISTURE
AND INSTABILITY FUELING SHOWERS REMAIN POSSIBLE. ACTIVITY FOCUS
MAY INCREASE THROUGH LATER WEEK AS SOME RENEWED LOWER LATITUDE
MOISTURE CHANNELS BETWEEN THE TRANSITIONING WESTERN U.S. TROUGH AND
THE CENTRAL U.S. RIDGE. SHOWERS AND STORMS WILL INCREASINGLY
SPREAD FROM THE ROCKIES TO THE NORTH-CENTRAL PLAINS TO MONITOR FOR
POSSIBLE RUNOFF ISSUES LATE WEEK INTO NEXT WEEKEND GIVEN LOCALLY
GROWING INSTABILITY AND BOUNDARY FOCUS/HEIGHT FALLS.
SANTORELLI/SCHICHTEL
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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