711
FOUS30 KWBC 270049
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
849 PM EDT WED MAR 26 2025
DAY 1
VALID 01Z THU MAR 27 2025 - 12Z THU MAR 27 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTH-CENTRAL AND SOUTH TEXAS...
ONLY MINOR ADJUSTMENT NEEDED TO THE PREVIOUS OUTLOOK ACROSS
PORTIONS OF SOUTH CENTRAL AND DEEP SOUTH TEXAS BASED ON SHORT TERM
TRENDS IN SATELLITE AND RADAR IMAGERY. 18Z SOUNDINGS AT CRP AND
BRO DEPICTED AN INCREASINGLY MOIST, UNSTABLE, AND UNCAPPED
VERTICAL PROFILE WHICH SHOULD CONTINUE TO FOSTER INCREASING
COVERAGE OF SHOWERS AND THUNDERSTORMS CAPABLE OF PRODUCING
TORRENTIAL DOWNPOURS LATER TONIGHT/EARLY THURSDAY MORNING.
MESO-ANALYSIS SHOWED DIFFLUENT UPPER- LEVEL FLOW, DPVA AND AMPLE
MOISTURE TRANSPORT INTO THE SOUTHERN END OF THE MODERATE RISK AREA
ALREADY UNDERWAY. A DEEPLY SATURATED PROFILE WITH GOOD DIRECTIONAL
SHEAR...MODERATE INSTABILITY WITH THE FRONT NEARBY SHOULD RESULT
UPSCALE GROWTH OF SLOW MOVING CONVECTIVE CLUSTERS THROUGHOUT THE
UPCOMING NIGHT. THE 18Z HREF NEIGHBORHOOD PROBABILITIES OF 3
INCHES OF RAINFALL IN AN HOUR PEAKING IN THE 15 TO 20 PERCENT RANGE
MAINLY IN PARTS OF JIM HOGG AND DUVAL COUNTIES...WITH AN AXIS OF
THE PROBABILITY OF RAINFALL EXCEEDING 5 INCHES DURING THE PERIOD
FROM 27/00Z THROUGH 27/12Z ALIGNED SOUTHWEST TO NORTHEAST ACROSS
THE SOUTHERN PARTS OF THE MODERATE RISK AREA. THESE TOTALS SEEM TO
BE WITHIN REACH GIVEN THE 1.9 INCH PRECIPITABLE WATER AT
BROWNSVILLE AND 1.7 INCH PRECIPITABLE WATER VALUE AT CORPUS CHRISTI
FROM THEIR 27/00Z SOUNDINGS...ESPECIALLY IF/WHERE CONVECTIVE
TRAINING OCCURS.
..PACIFIC NORTHWEST
A MARGINAL RISK REMAINED IN PLACE ACROSS PORTIONS OF WESTERN OREGON
AND WESTERN WASHINGTON. CONVECTION IS EXPECTED TO DEVELOP OVER
PORTIONS THIS EVENING AND THEN TRACK OFF TO THE NORTH. 00Z
SOUNDINGS FROM SALEM AND MEDFORD OR BOTH SHOWED ROUGHLY 600 TO 800
J PER KG OF CAPE AND PRECIPITABLE WATER VALUES RUNNING BETWEEN 0.75
AND 1.00 INCHES WHICH SHOULD SUPPORT LOCALLY HEAVY INTENSE
RAINFALL RATES. THE BIG LIMITING FACTOR SHOULD BE QUICK MOVING
CELLS WHICH SHOULD LIMIT ACCUMULATIONS. ANY RAINFALL RATES
PRODUCING OVER 1" IN AN HOUR...HOWEVER...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. AS A
RESULT...NO CHANGES WERE MADE TO THE ON-GOING MARGINAL RISK AREA AT
THIS POINT.
BANN
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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