181
FOUS30 KWBC 262047
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
447 PM EDT WED MAR 26 2025
DAY 1
VALID 16Z WED MAR 26 2025 - 12Z THU MAR 27 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTH-CENTRAL AND SOUTH TEXAS...
..16Z UPDATE
THE FORECAST REMAINS ON TRACK FOR A CORRIDOR OF VERY HEAVY
RAINFALL TO DEVELOP THIS AFTERNOON ACROSS THE MODERATE RISK AREA IN
SOUTH TEXAS. 12Z SOUNDINGS AT CRP AND BRO DEPICT AN INCREASINGLY
MOIST, UNSTABLE, AND UNCAPPED VERTICAL PROFILE WHICH WILL FOSTER
WIDESPREAD THUNDERSTORM COVERAGE TODAY IN THE PRESENCE OF DIFFLUENT
UPPER-LEVEL FLOW. THE MAIN ADJUSTMENT THIS AFTERNOON INCLUDES A
SLIGHT SOUTHWARD EXPANSION OF THE MODERATE RISK AREA IN SOUTH TEXAS
BASED ON THE 12Z HREF BLENDED MEAN AND NEIGHBORHOOD PROBABILITIES.
ASHERMAN
..PREVIOUS DISCUSSION
MODEL GUIDANCE CONTINUES TO BECOME BETTER CLUSTERED IN TERMS OF
THE TIMING AND AREAL EXTENT OF A HEAVY RAINFALL SIGNAL ACROSS SC-
SOUTH TX LATER THIS AFTERNOON AND OVERNIGHT. INCREASING UPPER LEVEL
DIFLUENCE AND DPVA AHEAD OF A SLOW MOVING UPPER TROUGH WILL
INITIATE ORGANIZED CONVECTION ACROSS ALONG THE RIO GRANDE IN SW TX
LATER THIS AFTERNOON, WHICH WILL BEGIN TO GROW UPSCALE TOWARD THE
EVENING HOURS GIVEN THE UPTICK IN SSE LOW-LEVEL INFLOW (25-40+ KTS
AT 850 MB). THIS AREA HAS BEEN VERY DRY AND COULD USE THE RAIN,
UNFORTUNATELY THIS SETUP LOOKS GOOD ENOUGH TO BE A CASE WHERE IT'S
BENEFICIAL RAIN INITIALLY, BUT THEN QUICKLY OVERWHELMS EVEN THE DRY
SOIL CONDITIONS. MODEL SOUNDINGS BY WED AFTERNOON/EVENING SHOW A
DEEPLY SATURATED PROFILE WITH GOOD DIRECTIONAL SHEAR AND MODERATE
INSTABILITY. THIS SHOULD SUPPORT SOME CONVECTIVE CLUSTERS WITH SOME
WEAK MESOCYCLONES AND RIGHT MOVER MOTIONS ARE VERY SLOW. COMBINE
THAT WITH THE FRONT NEARBY AND ANTICIPATE UPSCALE GROWTH OF SLOW
MOVING CONVECTIVE CLUSTERS THROUGH WED NIGHT WITHIN AN ENVIRONMENT
CAPABLE OF 2-3" PER HOUR RAINFALL. ACTIVITY MAY EVENTUALLY GROW
UPSCALE ENOUGH THAT IT BEGINS PROPAGATING EAST AND POSSIBLY
OFFSHORE, HOWEVER EVEN IF THAT EVENTUALLY TRANSPIRES, THERE WILL
STILL BE A PROLONGED PERIOD WHERE TRAINING CONVECTION IS A
POSSIBILITY.
OVERALL THERE IS ABOVE AVERAGE MODEL AGREEMENT ON THIS OVERALL
EVOLUTION. MOREOVER, THE LATEST (00Z) HREF PROBABILITIES HAVE
BUMPED UP OVER THE PAST FEW RUNS, INCLUDING PROBABILITIES >50% OF
GREATER THAN 5" OF RAIN IN 24HRS (WHICH IS A GOOD PROXY FOR A
MODERATE RISK AREA BASED ON VERIFICATION). THIS IS WHERE THE HREF
HAS 70+ PROBS OF RAINFALL EXCEEDING 5"/24HRS, ALONG WITH 40-50%
PROBS OF >8"/24HRS.
WHILE CONFIDENCE ON THE EVENT HAPPENING IS ABOVE AVERAGE, THERE IS
STILL UNCERTAINTY ON THE EXACT AXIS OF HEAVIEST RAINFALL, WHICH
WILL PROBABLY BE SOMEWHAT NARROW IN NATURE. THE 00Z ECMWF IN FACT
CAME DOWN CONSIDERABLY WITH THE QPF DURING DAY 1, AND AS WITH ITS
PREVIOUS RUN, AGAIN HAS MORE OF A DIFFUSE LOOK WITH MULTIPLE AXES
OF HEAVIER RAIN ON EITHER SIDE OF THE MODERATE RISK AREA. SO AGAIN,
THE MDT RISK IS CENTERED AROUND THE HIGHEST HREF PROBABILITIES,
AND GENERALLY ON THE SOUTHERN GRADIENT OF THE GLOBAL MODEL QPF
SOLUTIONS. IN FACT, CHANGES MADE TO THE PREVIOUS (DAY 2) ERO
INCLUDE PULLING THE SOUTHERN BOUNDARIES OF THE MODERATE, SLIGHT,
AND MARGINAL RISK AREAS A BIT FARTHER SOUTH ACROSS SOUTH TX. FLASH
FLOOD COVERAGE OVER THIS AREA IS EXPECTED TO INCREASE WEDNESDAY
NIGHT INTO THURSDAY MORNING, AND LOCALLY SIGNIFICANT IMPACTS ARE
POSSIBLE.
..PACIFIC NORTHWEST
A MARGINAL RISK CONTINUES ACROSS PORTIONS OF WESTERN OR AND WA.
CONVECTION IS EXPECTED TO DEVELOP OVER PORTIONS OF SOUTHWEST OR BY
21Z WED AND THEN TRACK OFF TO THE NORTH THROUGH THE AFTERNOON AND
EVENING HOURS. AN ANOMALOUS AMOUNTS OF CAPE IS FORECAST,
POTENTIALLY UPWARDS OF 2000 J/KG, AND PWS ARE FORECAST OVER THE
CLIMATOLOGICAL 90TH PERCENTILE. THIS COMBINATION OF CAPE AND PWS
SUPPORTS INTENSE RAINFALL RATES. THESE CELLS SHOULD BE QUICK MOVING
AND THE BIGGER HAZARD IS PROBABLY SEVERE WEATHER (AS HIGHLIGHTED
BY THE SPC SLIGHT RISK), HOWEVER THE INTENSE RAINFALL RATES
PRODUCING OVER 1" IN AN HOUR MAY BE ENOUGH FOR LOCALIZED FLASH
FLOOD CONCERNS. AREAS OF TERRAIN, URBAN AREAS, AND LOW LYING AREAS
WILL BE MOST PRONE TO LOCALIZED FLASH FLOODING DRIVEN BY THESE
SHORT DURATION INTENSE RAINFALL RATES.
HURLEY/CHENARD
DAY 2
VALID 12Z THU MAR 27 2025 - 12Z FRI MAR 28 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTH-CENTRAL AND SOUTH TEXAS AND MUCH OF THE TEXAS GULF
COAST...
..20Z UPDATE
..SOUTH TEXAS
THE PRIMARY CHANGE TO THE DAY 2 ERO INCLUDES A SOUTHWARD EXPANSION
OF THE MODERATE RISK, AND TIGHTENING OF THE MARGINAL AND SLIGHT
RISK AREAS OVER SOUTHEAST TEXAS TO REFLECT A SOUTHEASTWARD SHIFT IN
THE HIGHEST QPF AXIS. WHILE OVERALL CONFIDENCE IN A FLASH FLOODING
EVENT IS HIGH, UNCERTAINTY REMAINS REGARDING THE CONVECTIVE
EVOLUTION BEYOND 12Z TOMORROW, WHICH WILL DEPEND ON OUTFLOW
BOUNDARY PLACEMENT ORIGINATING FROM CONVECTION THIS AFTERNOON AND
TONIGHT.
..MS AND OH VALLEY
MARGINAL FLASH FLOODING ISSUES REMAIN POSSIBLE BEGINNING TOMORROW
MORNING AS SHOWERS AND THUNDERSTORMS PERIODICALLY TRAIN ALONG A
SLOWLY RETURNING WARM FRONT. THE NORTHERN PORTION OF THE INHERITED
MARGINAL RISK AREA WAS TRIMMED TO REFLECT THE CORRIDOR OF HIGHER
HREF NEIGHBORHOOD PROBABILITIES OF 2-3" RAINFALL EXCEEDENCE.
..PACIFIC NORTHWEST
MINOR ADJUSTMENTS OVERALL TO THE OVERNIGHT THINKING REGARDING THE
MARGINAL RISK AREA ASSOCIATED WITH ANOTHER MODEST ATMOSPHERIC
RIVER. RAINFALL TOTALS OF 3-5" THROUGH FRIDAY MORNING MAY SUPPORT
ISOLATED ISSUES OWING TO SATURATED SOILS ACROSS THE REGION.
ASHERMAN
..PREVIOUS DISCUSSION
THE EVENT DESCRIBED IN THE DAY 1 DISCUSSION OVER SOUTH TX WILL
CONTINUE INTO THURSDAY. THERE IS A BIT MORE UNCERTAINTY ON THE
DETAILS BY THIS TIME AS CONVECTIVE EVOLUTION WILL LIKELY BE
IMPACTED BY MESOSCALE INFLUENCES OF THE DAY 1 CONVECTION. ON THE
SYNOPTIC SCALE, INGREDIENTS ARE LIKELY EVEN MORE FAVORABLE FOR
HEAVY RAINFALL ON DAY 3 THEN THEY WERE ON DAY 2. LARGE SCALE
FORCING SHOULD RELOAD AND BE EVEN A BIT STRONGER AS THE MID LEVEL
TROUGH AXIS AND ASSOCIATED SHORTWAVE ENERGY MOVES ACROSS THE AREA
(ENHANCING THE UPPER DIVERGENCE/DPVA).
HOWEVER, THE EXTENT OF THE EXCESSIVE RAINFALL RISK WILL LIKELY BE
AT LEAST PARTIALLY DEPENDENT ON MESOSCALE FACTORS...SUCH AS THE
OUTFLOW LOCATION FORM THE DAY 1 CONVECTION AND ITS IMPACT ON
INSTABILITY. THE EXPECTATION IS THAT THE OUTFLOW BOUNDARY WILL BE
NEAR THE COAST BY THIS TIME, AND SO THAT IS WHERE THE GREATEST
FOCUS FOR TRAINING CONVECTION IS EXPECTED ON THURSDAY, AND THUS
WHERE THE MDT RISK IS IN PLACE. THERE IS SOME CHANCE THIS IS
OFFSHORE BY 12Z THURSDAY, BUT THE CURRENT CONSENSUS SUGGESTS IT MAY
STRUGGLE TO CLEAR THE COAST, LEAVING COASTAL AREAS SUSCEPTIBLE TO
ADDITIONAL HEAVY RAINFALL AND FLASH FLOODING. EVEN IF INSTABILITY
IS ERODED BY THURSDAY, THE INCREASE IN FORCING AND SOUTHERLY FLOW
SHOULD BE ENOUGH TO DESTABILIZE PORTIONS OF THE AREA AGAIN WITH
TIME, AND SO DO THINK AT LEAST SOME FLASH FLOOD RISK EXISTS EVEN IN
A SCENARIO WHERE CONVECTION INITIALLY PROPAGATES OFFSHORE,
ESPECIALLY THU NIGHT WITH THE UPTICK IN S-SSE LOW-LEVEL FLOW.
OVERALL CONFIDENCE IS HIGH IN A FLASH FLOOD EVENT, BUT CONFIDENCE
IN THE DETAILS BY THURSDAY-THU NIGHT REMAINS LOW. IN A WORST CASE
SCENARIO OUTFLOW STALLS NEAR THE COAST ALLOWING FOR REPEAT
CONVECTION AND A SIGNIFICANT FLASH FLOOD EVENT COULD EVOLVE. IN A
BEST CASE SCENARIO CONVECTION PROPAGATES OFFSHORE, BUT
REDEVELOPMENT LATER IN THE DAY STILL RESULTS IN SOME FLASH FLOOD
THREAT, ALBEIT PROBABLY LESS SIGNIFICANT. WHEN COMBINING DAY 2 AND
3 IT SEEMS PROBABLE THAT SOME AREAS RECEIVE 5-10" OF RAIN OVER
SOUTH TX, AND LOCALIZED AMOUNTS OF 10-15" ARE CERTAINLY POSSIBLE.
CONFIDENCE IS INCREASING IN THE COVERAGE OF FLASH FLOODING AND THE
POSSIBILITY OF AT LEAST LOCALIZED SIGNIFICANT IMPACTS, WARRANTING
THE MODERATE RISKS ON BOTH DAYS 2 AND 3.
..MS AND OH VALLEY
A MARGINAL RISK CONTINUES FROM YESTERDAY'S DAY 3 ERO ACROSS PORTIONS
OF NORTHERN MO, SOUTHERN IA, IL AND CENTRAL/SOUTHERN IN. A
STATIONARY FRONT DRAPED ACROSS THIS REGION WILL GRADUALLY LIFT
NORTH AS A WARM FRONT THROUGH THE PERIOD. SOUTHERLY FLOW WILL
RESULT IN AN UPTICK IN INSTABILITY AND MOISTURE, AND AT LEAST
SCATTERED CONVECTION IS EXPECTED THURSDAY INTO THURSDAY NIGHT.
THERE IS AT LEAST SOME CHANCE FOR ORGANIZED CONVECTIVE DEVELOPMENT
NEAR THE FRONT, WHICH COULD BRIEFLY TRAIN FROM WEST TO EAST. GLOBAL
MODEL QPF IS LOCALLY AS HIGH AS 1-3" ALONG THIS CORRIDOR, AND
GIVEN THE EXPECTED CONVECTIVE NATURE OF ACTIVITY, THIS WOULD
SUGGEST ISOLATED AMOUNTS OVER 3" ARE PROBABLE. A LOCALIZED, AND
GENERALLY MINOR, FLASH FLOOD RISK COULD RESULT.
..PACIFIC NORTHWEST
ANOTHER MODEST ATMOSPHERIC RIVER (NARROW RIBBON OF 500+ KG/M/S
IVTS) WILL RESULT IN POCKETS OF 3-5+ INCH 24 HOUR RAINFALL TOTALS
ALONG THE WEST COAST FROM NORTHERN CA NORTHWARD THROUGH OR-WA.
AMONG THE 00Z 3/25 MODELS, THE REGIONAL GEM IS BY FAR THE WETTEST,
WITH ISOLATED TOTALS OF 6-8+ INCHES. DESPITE THE LACK OF
INSTABILITY, THE WET ANTECEDENT SOILS ALONG WITH THE POTENTIAL OF
0.3-0.5 IN/HR RAINFALL RATES COULD LEAD TO LOCALIZED SHORT-TERM
RUNOFF ISSUES, PARTICULARLY ALONG SMALL STREAMS AND OVER BURN
SCARS.
CHENARD/HURLEY
DAY 3
VALID 12Z FRI MAR 28 2025 - 12Z SAT MAR 29 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER TEXAS COAST THROUGH SOUTHWEST AND SOUTH-CENTRAL LOUISIANA...
..20Z UPDATE
ADJUSTED THE NORTHERN EXTENT OF THE SLIGHT RISK A BIT TO REFLECT
THE THEME OF QPF FOCUSING CLOSER TO THE COASTLINE AS SUGGESTED BY
SEVERAL OF THE GLOBAL MODELS, ALTHOUGH SPREAD REMAINS REGARDING THE
PLACEMENT AND MAGNITUDE OF THE HIGHEST QPF AMOUNTS.
ASHERMAN
..PREVIOUS DISCUSSION
SLOW-MOVING MID-UPPER TROUGH WILL PUSH THE AREA OF MOST FAVORABLE
DEEP-LAYER FORCING (UPPER DIVERGENCE/DIFFLUENCE AND DPVA) FARTHER
EAST ALONG THE GULF COAST. COINCIDENT WITH THE FAVORABLE SYNOPTIC-
SCALE FORCING, TPWS BETWEEN 1.75-2.00", ALONG WITH MIXED LAYER
CAPES AROUND 1000-1500 J/KG, WILL RESULT IN A MORE ENHANCED
EXCESSIVE RAINFALL THREAT (I.E. SLIGHT) ACROSS THE UPPER TX COAST
TO SOUTHWEST AND SOUTH-CENTRAL LA. ATTRIBUTING TO A LOWER RISK
(SLIGHT) COMPARED TO THE PREVIOUS DAYS (MODERATE) WILL BE THE
WEAKENING LOW-LEVEL FLOW, WHICH GIVEN THE MORE LIMITED DEEP-LAYER
SHEAR, WOULD MORE LIKELY RESULT IN MORE EFFECTIVE CONVECTIVE
OUTFLOWS AND THUS SHORTER DURATION OF THE MORE INTENSE UPDRAFTS AND
HEAVIER RAINFALL CORES.
HURLEY
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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