037
FOUS30 KWBC 120045
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
845 PM EDT THU SEP 11 2025
DAY 1
VALID 01Z FRI SEP 12 2025 - 12Z FRI SEP 12 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN FLORIDA, THE NORTHERN ROCKIES, AND THE CENTRAL
INTERNATIONAL BORDER WITH CANADA...
..EASTERN FLORIDA
ONSHORE FLOW WITHIN A MOIST AND UNSTABLE AIR MASS BEHIND A FRONT
SAGGING THROUGH THE FLORIDA PENINSULA BRINGS A POSSIBLE THREAT OF
HEAVY RAINFALL DURING THE OVERNIGHT HOURS FROM THE SPACE COAST
SOUTH TO THE GOLD COAST. GIVEN THE EFFECTIVE BULK SHEAR AVAILABLE
REGIONALLY, THERE IS SOME CHANCE THAT SHORT TRAINING BANDS COULD
DEVELOP FROM TIME TO TIME AND POSE AN ISOLATED FLASH FLOODING RISK,
PRIMARILY AFTER 06Z.
..NORTHWEST
AN AREA OF WELL ABOVE AVERAGE MOISTURE ON THE NORTHWESTERN SIDE OF
A NEARLY STATIONARY LONGWAVE TROUGH/CUTOFF LOW OVER THE
INTERMOUNTAIN WEST HAS ACTED AS A FOCUS FOR SHOWER AND
THUNDERSTORM ACTIVITY IN THE TYPICALLY DRY AREAS FROM EASTERN
OREGON NORTHEAST THROUGH MUCH OF MONTANA. BOTH THE 18Z HREF AND 12Z
REFS INDICATE THAT HEAVY RAINFALL WOULD BE POSSIBLE FOR ANOTHER
SEVERAL HOURS. AREAS MOST SENSITIVE TO HEAVY RAINFALL WOULD BE
URBAN, SLOT CANYONS, BURN SCARS, OR ANY DRY WASHES/ARROYOS.
..CENTRAL U.S./CANADIAN BORDER
SHOWERS AND THUNDERSTORMS HAVE SHOWN SOME TRAINING CHARACTER TODAY
IN FAR NORTHERN ND. BOTH THE 12Z HREF AND 18Z REFS INDICATE SOME
CHANCE OF OVERNIGHT ACTIVITY REDEVELOPING AND FESTERING BETWEEN
NORTHEAST MT AND NORTHERN MN, SO KEPT THE MARGINAL RISK IN PLACE.
ROTH
DAY 2
VALID 12Z FRI SEP 12 2025 - 12Z SAT SEP 13 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH FLORIDA AS WELL AS FOR PORTIONS OF SOUTHWESTERN COLORADO INTO
CENTRAL NEW MEXICO...
..SOUTH FLORIDA
THE FRONT ACROSS SOUTH FLORIDA ON FRIDAY WILL MAKE ADDITIONAL
PROGRESS SOUTHBOUND DOWN THE PENINSULA. THE PORTION MOVING DOWN THE
WEST COAST OF THE PENINSULA WILL MAKE BETTER PROGRESS SOUTHWARD
COMPARED TO THE PORTION OF THE FRONT ALONG THE EAST COAST. THIS
SHOULD SUPPORT THE FRONT BECOMING INCREASINGLY ALIGNED SOUTHWEST TO
NORTHEAST. SINCE THE FRONT WILL BE MOVING A BIT MORE ON FRIDAY,
THERE SHOULD BE A BIT BETTER COVERAGE OF STORMS ACROSS SOUTH
FLORIDA, WHERE THE SLIGHT RISK AREA INHERITED REMAINS LARGELY
UNCHANGED. AS THE FRONT PUSHES SOUTH, IT WILL SHOVE THE BULK OF THE
TROPICAL MOISTURE INTO THE FLORIDA STRAITS AND INTO THE BAHAMAS.
THIS SHOULD WORK TO LIMIT STORM COVERAGE ON FUTURE DAYS, AS A MUCH
DRIER AND COOLER AIR MASS CONTINUES TO ENVELOP THE REST OF THE
STATE.
..NEW MEXICO AND COLORADO
21Z UPDATE... THE ENVIRONMENTAL SETUP CONTINUES TO LOOK RATHER
FAVORABLE FOR HEAVY RAINFALL FOR THE HIGHER TERRAIN OF COLORADO AND
NEW MEXICO. THE SLIGHT RISK AREA WAS RESHAPED A BIT ON THE EASTERN
AND SOUTHERN SIDES ACROSS NEW MEXICO TO ACCOUNT FOR THE LATEST WPC
QPF AND MODEL TRENDS. THE MARGINAL RISK REMAINED IN GOOD ORDER.
CAMPBELL
THE POSITIVELY TILTED UPPER LEVEL TROUGH FROM THURSDAY WILL BEGIN
TO PIVOT TO BECOME NEUTRALLY TILTED ON FRIDAY. THIS MEANS THE
SOUTHERN END OF THE TROUGH WILL GET A BOOST TO ADVANCE EASTWARD,
WHILE THE NORTHERN END WILL LARGELY STAY IN PLACE. THE RESULT WILL
BE A MORE SOUTH TO NORTH ORIENTED AXIS OF HEAVY RAINFALL,
STRETCHING FROM THE SOUTHERN END OF NEW MEXICO NORTH ACROSS MUCH OF
WESTERN COLORADO. MOISTURE FROM THE PACIFIC WILL ALSO BE DRAWN
NORTHWARD ALONG THE NEW MEXICO/ARIZONA BORDER. THIS SHOULD EXPAND
THE RAINFALL FOOTPRINT FOR FRIDAY ACROSS MUCH OF THE SOUTHERN
PORTION OF NEW MEXICO AND SOUTHEASTERN ARIZONA. EXPECT FRIDAY TO
THE WETTEST DAY THIS WEEK FOR NEW MEXICO AND COLORADO.
CAMS GUIDANCE SUGGESTS THE PLUME OF MOISTURE STRETCHING NNE FROM
SOUTHEAST ARIZONA THROUGH NORTH-CENTRAL NEW MEXICO AND INTO
COLORADO WILL BE THE FOCUS FOR ADDITIONAL CONVECTION CAPABLE OF
PRODUCING FLASH FLOODING. WITH BETTER SOUTHWARD EXTENT, THE STORMS
WILL BE ABLE TO TAP INTO SOME DEEPER MOISTURE AND INSTABILITY FROM
THE PACIFIC ON FRIDAY. THIS WILL MAKE THUNDERSTORM COVERAGE FRIDAY
GREATER THAN IN PREVIOUS DAYS. SINCE THE WHOLE PLUME WILL ALSO BE
LIFTING NORTH AND EAST, EXPECT A MUCH LARGER AREA OF RAIN TO IMPACT
THE AREA FRIDAY. EMBEDDED THUNDERSTORMS AMONG THE MOSTLY LIGHTER
SHOWER ACTIVITY WILL BE THE FOCI FOR ANY POTENTIAL FLASH FLOODING.
THIS WILL BE PARTICULARLY TRUE IN ANY AREAS WHERE MULTIPLE ROUNDS
OF STORMS PRODUCING HEAVY RAIN OCCUR.
..NORTHWEST INTO NORTHERN HIGH PLAINS
21Z UPDATE... STILL EXPECTING CONVECTION TO FIRE UP ACROSS THE
INTERMOUNTAIN WEST AND TRACK EASTWARD INTO THE HIGH PLAINS DURING
THIS PERIOD. THERE WAS A NOTABLE UPTICK IN QPF FOR WESTERN PORTIONS
OF NORTH DAKOTA, AS SUCH, THE MARGINAL RISK WAS EXPANDED FURTHER
INTO WESTERN NORTH DAKOTA AND NORTHWEST SOUTH DAKOTA TO CAPTURE
THIS TREND.
CAMPBELL
SHORTWAVES AND VORTICITY MAXES ROUNDING THE PERIPHERY OF THE
LONGWAVE TROUGH WILL BE THE FOCI CAUSING ADDITIONAL CONVECTION FROM
NORTHERN IDAHO EAST ACROSS MUCH OF MONTANA. THE EXPECTED RAINFALL
FORECAST HAS DECREASED THE OVERALL RAINFALL IN THIS AREA. HOWEVER,
MOUNTAIN FORCING MAY OFFSET THIS IN SOME AREAS.
WEGMAN
DAY 3
VALID 12Z SAT SEP 13 2025 - 12Z SUN SEP 14 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN NEW
MEXICO THROUGH SOUTHWESTERN KANSAS, AS WELL AS FOR A PORTION OF THE
URBAN CORRIDOR OF SOUTH FLORIDA...
..NEW MEXICO THROUGH KANSAS
21Z UPDATE... THE BEST FOCUS FOR HEAVY RAINFALL AND ASSOCIATED
FLASH FLOODING WILL CONTINUE TO BE ACROSS EASTERN NEW MEXICO,
TEXAS PANHANDLE AND INTO WESTERN KANSAS DURING THIS PERIOD. THE
SLIGHT RISK WAS RESHAPED A LITTLE TO BECOME MORE SSW TO NNE
ORIENTATED, RATHER THAN SW TO NE.
CAMPBELL
AS THE UPPER LEVEL TROUGH OVER THE INTERMOUNTAIN WEST BECOMES MORE
NEGATIVELY TILTED, A PORTION OF THE LOW LEVEL JET OUT AHEAD IT WILL
DRAW DEEPER GULF MOISTURE NORTH INTO THE PANHANDLES AND EASTERN NEW
MEXICO AND INTO SOUTHWESTERN KANSAS. THIS WILL SUPPORT A LINE OF
STORMS THAT WILL DEVELOP OVER CENTRAL NEW MEXICO AND THEN PRESS
EASTWARD INTO THE SLIGHT RISK AREA. ONCE OVER EASTERN NEW MEXICO
AND THE PANHANDLES, THE EASTWARD PROGRESS OF THE STORMS WILL
DIMINISH GREATLY, RESULTING IN MUCH SLOWER-MOVING BUT STILL
ORGANIZED STORMS. THIS WILL BE THE BASIS FOR TRAINING, RESULTING IN
WIDELY SCATTERED INSTANCES OF FLASH FLOODING WHERE THE STORMS ARE
THE MOST PERSISTENT.
..SOUTH FLORIDA
THE FRONT OVER SOUTH FLORIDA WILL FINALLY PUSH EAST AND CLEAR ALL
OF THE DEEPEST TROPICAL MOISTURE WELL OUT INTO THE ATLANTIC AND
BAHAMAS BY SUNDAY MORNING. THUS, EXPECT SATURDAY TO BE A TRANSITION
DAY IN THE AREA...WHERE THERE WILL STILL BE STORMS, PARTICULARLY
ALONG THE URBAN I-95 CORRIDOR FROM MIAMI THROUGH PALM BEACH, BUT
THERE WILL BE LESS COVERAGE OVER FLORIDA AS THE MOISTURE IS PUSHED
EASTWARD OUT INTO THE ATLANTIC. THE NEWLY ISSUED SLIGHT IS A LOW-
CONFIDENCE ONE, AS ONCE THE FRONT CLEARS THE STATE, THAT SHOULD END
THE FLOODING THREAT. HOWEVER, GIVEN THE MANY DAYS OF DAILY
AFTERNOON CONVECTION IN THE I-95 CORRIDOR, SOILS ARE SENSITIVE
ENOUGH THAT EVEN THE REDUCED TIME AND COVERAGE OF STORMS COULD
STILL CAUSE WIDELY SCATTERED INSTANCES OF FLASH FLOODING.
..NORTHERN PLAINS
21Z UPDATE... A MINOR TRIM WAS MADE TO THE SOUTHERN BOUNDS ACROSS
CENTRAL SOUTH DAKOTA AS THE LATEST GUIDANCE SUGGESTS QPF MAY NOT
ADVANCE THAT FAR EAST DURING THIS PERIOD, THUS LOWERING THE THREAT
FOR EXCESSIVE RAINFALL.
CAMPBELL
ACROSS THE NORTHERN PLAINS, AHEAD OF THE NEUTRALLY TILTED TROUGH
THAT IS BECOMING NEGATIVELY TILTED, THE NORTHERN EXTENT OF THE LOW
LEVEL JET AHEAD OF THE TROUGH WILL DRAW DEEPER GULF MOISTURE
WESTWARD ACROSS THE DAKOTAS. THIS SHOULD SUPPORT PERSISTENT
CONVECTION. HOWEVER, THIS AREA OF THE DAKOTAS IS FAIRLY DIFFICULT
TO FLOOD, SO DESPITE THE HIGHER PROBABILITY FOR TRAINING AND HEAVY
RAIN, THE AREA REMAINS IN A HIGHER-END MARGINAL FOR NOW. A SLIGHT
RISK UPGRADE MAY BE NEEDED FOR PORTIONS OF THE AREA AS GUIDANCE
BEGINS TO RESOLVE THIS PART OF THE EVENT ACROSS THE NORTHERN
PLAINS.
WEGMAN
DAY 1 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/94EPOINTS.TXT
DAY 2 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/98EPOINTS.TXT
DAY 3 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/99EPOINTS.TXT
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