459
FOUS30 KWBC 271530
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1130 AM EDT THU MAR 27 2025
DAY 1
VALID 16Z THU MAR 27 2025 - 12Z FRI MAR 28 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTH-CENTRAL TEXAS AND THE MIDDLE/LOWER TEXAS GULF COAST...
..SOUTH TEXAS
COMPLICATED AND MESSY FORECAST GOING THROUGH THE REST OF THE PERIOD
ACROSS PORTIONS OF SOUTH TEXAS THROUGH THE MIDDLE TEXAS GULF COAST
BUT POTENTIAL EXISTS FOR NUMEROUS FLASH FLOODS, SOME OF WHICH COULD
BE LOCALLY SIGNIFICANT. FOR THIS UPDATE, THE MODERATE RISK WAS
ADJUSTED SOUTHWARD BASED ON BOTH CURRENT OBSERVATIONS AND RADAR
TRENDS AS WELL AS THE 12Z GUIDANCE INCLUDING THE HREF THAT SHIFTED
THE AXIS OF HEAVIEST RAINFALL SOUTHWARD. MPD #088 WAS ISSUED JUST
A BIT AGO TO COVER THE ONGOING AND NEAR- TERM FLASH FLOOD THREAT.
HEADING INTO THIS AFTERNOON THROUGH TONIGHT, GUIDANCE SEEMS TO BE
KEYING ON A CONVECTIVE COMPLEX DEVELOPING AND TRACKING
EAST/NORTHEAST THROUGH SOUTH TEXAS. WHILE INSTABILITY IS MODEST, A BETTER
SHEAR ENVIRONMENT AND AN INCREASE IN THE PWS WILL SUPPORT GREATER
STORM ORGANIZATION AND HIGHER RAIN RATES. THERE ARE SOME
SUGGESTIONS FOR REPEATING/TRAINING CLUSTERS, PARTICULARLY IN THE
SOUTHERN EXTENT OF THE UPDATED MODERATE RISK AREA, WHERE LOCALIZED
HIGHER END ADDITIONAL QPF IS POSSIBLE (4-6").
THIS MAY RESULT IN NUMEROUS FLASH FLOODS AND LOCALLY SIGNIFICANT
FLOODING INTO TONIGHT. THE PREVIOUS DISCUSSION FOLLOWS...
PERIODS OF HEAVY RAINFALL OVER SOUTH TX WILL CONTINUE INTO TODAY
AND TONIGHT, AS AMPLE MOISTURE, SHEAR, AND INSTABILITY (WHICH WILL
RELOAD TODAY VIA BOTH DAYTIME HEATING AND DECREASING LAPSE RATES
ALOFT) REMAIN IN PLACE. THAT SAID, SPATIALLY UNCERTAINTY REMAINS
RELATIVELY HIGH FOR A DAY 1 OUTLOOK, AS MESOSCALE FACTORS THIS
MORNING MAY HAVE A LARGE IMPACT ON THE ULTIMATE EVOLUTION OF
CONVECTION (BEST EXPRESSED BY THE LACK OF CONSISTENCY FROM HOURLY
RUNS OF THE HRRR, AS WELL AS THE 00Z HRRR BEING AN OUTLIER FROM
MOST OF THE OTHER CAMS WITH REGARD TO QPF MAXIMA). ON THE SYNOPTIC
SCALE, INGREDIENTS ARE MORE FAVORABLE FOR HEAVY RAINFALL THAN
YESTERDAY, AND LARGE SCALE FORCING SHOULD RELOAD WITH THE MID-
LEVEL TROUGH AXIS AND ASSOCIATED SHORTWAVE ENERGY MOVING ACROSS
THE AREA (ENHANCING THE UPPER DIVERGENCE/DPVA).
HOWEVER, THE EXTENT OF THE EXCESSIVE RAINFALL RISK WILL BE AT
LEAST PARTIALLY DEPENDENT ON MESOSCALE FACTORS, AND OBSERVATIONAL
TRENDS HAVE ALREADY DIVERGED SOMEWHAT SIGNIFICANTLY FROM THE 00Z
HREF CONSENSUS (WITH CONVECTION PROPAGATING FARTHER SOUTH THAN
INDICATED, TO NEAR BROWNSVILLE). DESPITE THESE TRENDS, THE
INHERITED MODERATE RISK REMAINS LARGELY UNCHANGED, AS LOCALIZED
3-5" TOTALS OVER THE PAST 12-24 HOURS HAVE OCCURRED WITHIN THE
INHERITED AREA WHERE ADDITIONAL TRAINING IS EXPECTED. IN FACT, THE
ONE AREA OF EXPANSION OF THE MODERATE RISK WAS TO BRING THE CONTOUR
FARTHER INLAND WHERE FFGS HAVE BEEN DEPRESSED. WHILE THERE IS STILL
SOME CHANCE THAT THE BULK OF CONVECTION IS OFFSHORE BY 12Z,
OBSERVATIONAL AND MODEL TRENDS SUGGESTS IT MAY STRUGGLE TO CLEAR
THE COAST, LEAVING COASTAL AREAS SUSCEPTIBLE TO ADDITIONAL HEAVY
RAINFALL AND FLASH FLOODING.
WHILE A BREAK IN CONVECTIVE ACTIVITY IS SOMEWHAT LIKELY BY LATE
MORNING TO MIDDAY, THERE IS HIGH CONFIDENCE THAT SCATTERED TO
NUMEROUS INSTANCES OF FLASH FLOODING WILL BECOME LIKELY AGAIN BY
THIS AFTERNOON AND EVENING. IN A WORST CASE SCENARIO, THE OUTFLOW
STALLS NEAR THE COAST ALLOWING FOR REPEAT CONVECTION AND A
SIGNIFICANT FLASH FLOOD EVENT COULD EVOLVE (THOUGH AN INCREASE IN
CAMS SUGGESTING THAT THE CONVECTION PROPAGATES FULLY OFFSHORE HAS
REDUCED THIS RISK A BIT). WHEN COMBINING RAINFALL THAT HAS ALREADY
OCCURRED WITH WHAT IS FORECAST, IT SEEMS PROBABLE THAT SOME AREAS
RECEIVE 5-8" OF STORM TOTALS, AND LOCALIZED AMOUNTS OF 8-12" ARE
CERTAINLY POSSIBLE.
..MS AND OH VALLEY
A MARGINAL RISK WAS MAINTAINED (AND SHIFTED A BIT WEST WITH
OBSERVATIONAL AND MODEL TRENDS) ACROSS PORTIONS OF NORTHERN MO,
SOUTHERN IA, IL AND CENTRAL/SOUTHERN IN. WARM-AIR ADVECTION AND
OVERRUNNING NEAR A STATIONARY FRONT DRAPED ACROSS THIS REGION WILL
GRADUALLY LIFT NORTH AS A WARM FRONT THROUGH THE PERIOD. THROUGH
THE REST OF THE PERIOD, ADDITIONAL RAINFALL AMOUNTS UP TO 1"
(LOCALLY UP TO 2") WILL BE POSSIBLE, RESULTING IN A LOCALIZED AND
GENERALLY MINOR FLASH FLOOD RISK.
..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.
DESPITE THE RELATIVE 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.
CHURCHILL/CHENARD/HURLEY/TAYLOR
DAY 2
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...
AN INHERITED SLIGHT RISK AREA WAS MAINTAINED (AND SHIFTED A BIT
WEST BASED ON MODEL TRENDS) AS A SLOW-MOVING MID-UPPER TROUGH WILL
PUSH THE AREA OF MOST FAVORABLE DEEP-LAYER FORCING (UPPER DIVERGENCE
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 CAPE 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 (AS WELL AS GENERALLY DRIER ANTECEDENT
CONDITIONS). CONSIDERABLE UNCERTAINTY STILL EXISTS, AS CONVECTIVE
DETAILS REMAIN UNCLEAR AT THIS POINT EVEN ON DAY 1, BUT CONFIDENCE
IN 2-4" LOCALIZED TOTALS IS INCREASING PER THE LATEST GUIDANCE.
CHURCHILL/HURLEY
DAY 3
VALID 12Z SAT MAR 29 2025 - 12Z SUN MAR 30 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...
A CONTINUATION OF THE DECREASING FLASH FLOOD RISK IS EXPECTED FROM
DAY 2 INTO DAY 3, AS GUIDANCE LARGELY INDICATES LOCALIZED TOTALS
OF ONLY 1-3" (MOSTLY CONCENTRATED NEAR THE CENTRAL GULF COAST
WHERE FFGS ARE RATHER HIGH). THE INHERITED MARGINAL RISK AREA WAS
TRIMMED RATHER SIGNIFICANTLY TO THE NORTH (INTO THE MID-SOUTH), AS
FORCING AND INSTABILITY LOOK RATHER MINIMAL (WITH THE ECMWF BEING
THE WETTEST GLOBAL MODEL ONLY INDICATING LOCALIZED TOTALS OF 1-2").
A LOCALIZED FLASH FLOOD THREAT SHOULD RESULT FOR AREAS CONFINED
CLOSER TO THE GULF COAST (AND MOSTLY LIMITED TO MORE SENSITIVE
URBAN AREAS).
CHURCHILL
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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