420
FXUS02 KWBC 081952
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
352 PM EDT MON SEP 8 2025
VALID 12Z THU SEP 11 2025 - 12Z MON SEP 15 2025
..OVERVIEW
THE UPPER FLOW PATTERN FOR THIS MEDIUM RANGE PERIOD BEGINS WITH
AMPLIFIED MEAN TROUGHS SETTLED OVER BOTH THE EASTERN AND THE
WESTERN LOWER 48 TO SANDWICH A WARMING CENTRAL U.S. RIDGE. THESE
WILL BE SLOW TO MOVE OUT INITIALLY, BUT ENERGY EJECTING OUT OF THE
WEST SHOULD HELP TO FINALLY DISLODGE THE PATTERN DURING THIS
PERIOD. IN TERMS OF SENSIBLE WEATHER, WAVY FRONTAL SYSTEMS WILL
FOCUS RAIN CHANCES FROM THE NORTHWEST/WEST TO THE ROCKIES/PLAINS
WITH GRADUAL FLOW TRANSLATION AND ALSO DOWNSTREAM THROUGH THE GREAT
LAKES REGION WITH FRONTAL PASSAGE AS NORTHERN STREAM ENERGY DIGS
TO THE LEE OF THE RIDGE. HEAVY RAIN POTENTIAL WILL PERSIST FROM
FLORIDA PERHAPS THROUGH THE COASTAL CAROLINAS AS DEEPENED MOISTURE
INTERACTS WITH A WAVY WESTERN ATLANTIC FRONT. THIS ACTIVITY WILL
FOCUS ON THE LEADING EDGE OF A COOL CANADIAN AIRMASS OVER THE
EASTERN SEABOARD.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODELS AND ENSEMBLES SEEM GENERALLY WELL CLUSTERED THROUGH THE
MEDIUM RANGE, AS UPPER TROUGHING PIVOTS THROUGH THE WEST THROUGH
THE END OF THE WEEK AND ANOTHER TROUGH MOVES INTO THE WEST COAST
EARLY NEXT WEEK. UPPER RIDGING IS FORECAST AHEAD OF IT, ALONG WITH
A NARROW TROUGH MOVING THROUGH THE EAST THROUGH LATE WEEK. A RECENT
MODEL TREND IS FOR A SMALL CLOSED LOW TO DIVE SOUTH FROM THE
HUDSON BAY AND BE LOCATED SOMEWHERE OVER THE GREAT LAKES BY
SUNDAY/MONDAY OF NEXT WEEK, THOUGH WITH SOME PLACEMENT SPREAD. THIS
WOULD INCREASE QPF/POPS COMPARED TO MODEL BLENDS CONTAINING OLDER
MODEL RUNS LIKE THE 13Z NATIONAL BLEND OF MODELS.
THE WPC FORECAST USED A MULTI-MODEL DETERMINISTIC BLEND FAVORING
THE 06Z GFS AND 00Z ECMWF EARLY IN THE PERIOD. INCREASED THE
PROPORTION OF ENSEMBLE MEANS AS THE PERIOD PROGRESSED, WITH THE
MEANS REACHING HALF THE BLEND DAY 6 AND MORE DAY 7 GIVEN INCREASING
UNCERTAINTY IN THE DETAILS OF THE PATTERN.
..WEATHER/HAZARDS HIGHLIGHTS
IT OVERALL REMAINS THE CASE THAT A PERSISTENT AND WAVY FRONTAL
SYSTEM OVER THE GULF AND FLORIDA ALSO EXTENDS JUST OFF/UP THE
EASTERN SEABOARD. THIS WILL PROVIDE A MAIN FOCUS FOR TROPICAL
MOISTURE TO POOL, LEADING TO DAILY ENHANCED AND DIURNALLY DRIVEN
RAINFALL POTENTIAL ASSOCIATED WITH SCATTERED THUNDERSTORMS.
SATURATED GROUNDS FROM PRECURSOR RAINFALL WARRANTS MARGINAL RISK OF
EXCESSIVE RAINFALL OUTLOOK (ERO) THREAT AREAS FOR THE
CENTRAL/SOUTHERN FLORIDA PENINSULA INTO THURSDAY AND FRIDAY.
FARTHER NORTH, RAINS FOR THE COASTAL SOUTHEAST/CAROLINAS STILL
OFFER UNCERTAINTY FOR HOW MUCH CONVECTION OCCURS ONSHORE/OFFSHORE
ON ANY GIVEN DAY. FOR THE MOST PART, RAIN LOOKS TO REMAIN OFFSHORE,
BUT SMALL SHIFTS IN THE WAVY FRONTAL BOUNDARY MAY PRODUCE
SIGNIFICANT DIFFERENCES IN ONSHORE RAINFALL AMOUNTS.
FOR THE WEST, A SERIES OF UPPER TROUGHS/CLOSED LOWS SET TO WORK
OVER THE NORTHWEST/WEST WILL SUPPORT LOCALLY ENHANCED RAINFALL INTO
LATER THIS WEEK AND AGAIN INTO EARLY NEXT WEEK. RAINFALL AREAS
WITH EACH SYSTEM MAY TRANSLATE SLOWLY FROM THE PACIFIC
NORTHWEST/NORTHERN CALIFORNIA TO THE GREAT BASIN/INTERMOUNTAIN WEST
AND THEN TOWARD THE PLAINS. TERRAIN AND FRONTAL LIFT MAY FOCUS
SOME RUNOFF THREAT OVER THE NORTHERN ROCKIES/HIGH PLAINS AND ALSO
SOUTHWEST COLORADO AND VICINITY WHERE DEEPER MOISTURE MAY CHANNEL
BETWEEN EJECTING WESTERN U.S. TROUGHING AND THE CENTRAL U.S. RIDGE.
MARGINAL RISK ERO THREAT AREAS ARE IN PLACE THURSDAY/FRIDAY WHERE
MODEST INSTABILITY WILL COMBINE WITH SHORTWAVE ENERGIES PROVIDING
LIFT AND PW ANOMALIES OF +3 STANDARD DEVIATIONS ESPECIALLY IN THE
NORTHERN TIER. ALSO, THIS PATTERN MAY YIELD COOLER TEMPERATURES
SUPPORTIVE OF SOME SNOW FOR THE HIGHEST ELEVATIONS. GRADUAL FLOW
AND FRONTAL TRANSLATION DOWNSTREAM SHOULD ALSO SUPPORT RAINFALL
THROUGH THE GREAT LAKES REGION AND VICINITY WITH FRONTAL PASSAGE AS
NORTHERN STREAM ENERGY DIGS TO THE LEE OF THE MAIN RIDGE.
COOLER THAN NORMAL TEMPERATURES ESPECIALLY FOR HIGHS ARE LIKELY
ACROSS THE WESTERN U.S. UNDERNEATH PERIODS OF UPPER TROUGHING. NEAR
TO BELOW AVERAGE TEMPERATURES ARE ALSO FORECAST FOR THE EAST. BUT
UNDERNEATH THE UPPER RIDGE IN BETWEEN THESE FEATURES, EXPECT ABOVE
AVERAGE TEMPERATURES FOR THE PLAINS TO MISSISSIPPI VALLEY.
TEMPERATURE ANOMALIES OF +10 TO +20 DEGREES WILL LEAD TO HIGHS IN
THE 90S AS FAR NORTH AS SOUTH DAKOTA AND IOWA AT TIMES.
TATE/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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