190
FOUS30 KWBC 281911
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
311 PM EDT FRI MAR 28 2025
DAY 1
VALID 16Z 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 LOUISIANA...
16Z UPDATE: MAINTAINED THE SLIGHT RISK FOR THE DAY 1 ERO UPDATE
WITH MINIMAL ADJUSTMENTS. IT'S STILL RATHER UNCERTAIN HOW
TODAY/TONIGHT WILL PLAY OUT BUT GIVEN THE ENVIRONMENTAL SETUP
(MID/UPPER LEVEL TROUGH APPROACHING AND FAVORABLE MOISTURE
TRANSPORT) AND THE MOST RECENT MODEL GUIDANCE STILL POINTING
TOWARDS SOME SCATTERED HIGHER END QPF, SAW NO REASON TO
DRAMATICALLY CHANGE THE MESSAGING AND ERO RISK LEVEL. FINER DETAILS
AND MESOSCALE INTERACTIONS WILL BE DRIVING HOW THIS WILL EVOLVE,
BUT THE LATEST HREF PROBS POINT TOWARD UPPER TX COAST AND COASTAL
LA AS THE PRIMARY FOCUS AREA FOR REPEATING/TRAINING CONVECTION.
NEIGHBORHOOD PROBS FOR 5" APPROACH 40-50% ALONG THE COASTAL AREAS
WITH A LOW-END (10-15%) SIGNAL FOR 8" ACCUMULATIONS THROUGH 12Z
SATURDAY. OVERALL, THIS LINES UP WITH THE PREVIOUS FORECAST AND
ERO, SO SAW NO REASON TO SIGNIFICANTLY ALTER WHAT WAS INHERITED.
PREVIOUS DISCUSSION FOLLOWS...
AN INHERITED SLIGHT RISK AREA WAS MAINTAINED (AND ADJUSTED) ONCE
AGAIN FOR THIS UPDATE, 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. HOWEVER, CONSIDERABLY
UNCERTAINTY REMAINS (ESPECIALLY FOR AN INITIAL DAY 1 OUTLOOK) AS
MUCH IS DEPENDENT ON THE EVOLUTION OF CONVECTION THIS MORNING, AS
CONVECTION CONTINUES TO CONSOLIDATE FARTHER SOUTH THAN VIRTUALLY
ALL OF THE GUIDANCE INDICATED AT THIS TIME YESTERDAY (RESULTING IN
THE HEAVIEST RAINFALL TOTALS OF 6"+ OVER THE PAST 24-48 HOURS
OCCURRING OVER FAR SOUTH TX). SHOULD A SIMILAR ERROR OCCUR TODAY,
A SLIGHT RISK MAY WELL NOT VERIFY (AS THE CONSOLIDATING MCS AT THE
TIME OF WRITING OVER FAR SOUTH TX WILL PROPAGATE OFFSHORE AND MAY
PREVENT CRITICALLY NECESSARY MOISTURE FLUX TRANSPORT NORTHWARD INTO
THE GULF COAST, AS BEST DEPICTED BY THE 00Z HRRR). IN ADDITION,
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).
HOWEVER, SHOULD MORE INTENSE CONVECTION INITIATE AND SUSTAIN
ITSELF FARTHER NORTH (AS MUCH OF THE REST OF THE 00Z HREF
INDICATES), THEN LOCALIZED TOTALS OF 3-6"+ TOTALS WILL BE POSSIBLE
FROM SOUTHEAST TX AND THE UPPER TX COAST THROUGH MUCH OF
SOUTHEASTERN LA (PER 00Z HREF PMM QPF AND ASSOCIATED 40-KM
NEIGHBORHOOD PROBABILITIES FOR 3" AND 5" EXCEEDANCE OF ~30-60% AND
~20-40%, RESPECTIVELY). METEOROLOGICAL FACTORS SUPPORTING THIS
POTENTIAL INCLUDE FAVORABLE SYNOPTIC-SCALE FORCING (PARTICULARLY
THE CONTINUED IMPRESSIVE DIVERGENCE ALOFT), PRECIPITABLE WATER
VALUES BETWEEN 1.75-2.00", AND CAPE BUILDING TO 1000-1500 J/KG.
THESE SAME FACTORS MAY STILL FUEL INTENSE CONVECTION OFFSHORE WHICH
ULTIMATELY FAIL TO MATERIALIZE FURTHER INLAND, THUS THE SLIGHT
RISK IS CONSIDERED TO BE CONDITIONAL. ON THAT NOTE, THE MARGINAL
RISK WAS ALSO EXPANDED SOUTHWESTWARD TO INCLUDE SOUTH TX, GIVEN THE
ONGOING FLOODING AND LOW-END POTENTIAL FOR CONVECTIVE
REDEVELOPMENT ONCE AGAIN WITH DAYTIME HEATING (THOUGH THIS
CONVECTION SHOULD BE MUCH LESS INTENSE WITH THE BULK OF THE FORCING
SHIFTING EASTWARD).
CHURCHILL/TAYLOR
DAY 2
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 1 INTO DAY 2, AS ENSEMBLE GUIDANCE INDICATES THE POTENTIAL FOR
LOCALIZED TOTALS OF 1-3" (MOSTLY CONCENTRATED NEAR THE GULF COAST
WHERE FFGS ARE RATHER HIGH). THE LIMITED INHERITED MARGINAL RISK
AREA WAS MAINTAINED FOR THIS UPDATE, AS FORCING AND INSTABILITY
CONTINUE TO LOOK RATHER MINIMAL (AND MOST DETERMINISTIC GUIDANCE AT
THIS POINT INDICATES LOCALIZED TOTALS OF LESS THAN 1" FOR THE MOST
PART, INCLUDING THE NEW WPC FORECAST, BUT DECIDED TO MAINTAIN THE
MARGINAL. ANY LOCALIZED FLASH FLOOD THREAT SHOULD MOSTLY BE
CONFINED CLOSER TO THE GULF COAST (AND LIMITED TO MORE SENSITIVE
URBAN AREAS).
CHURCHILL/TAYLOR
DAY 3
VALID 12Z SUN MAR 30 2025 - 12Z MON MAR 31 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS...
20Z UPDATE:
A BROAD AREA OF CONVECTION IS EXPECTED SUNDAY INTO SUNDAY NIGHT
WITH A PAIR OF SHORTWAVES MOVING INTO THE CENTRAL U.S. DURING THE
PERIOD. THE FORCING FOR ASCENT AND ASSOCIATED MOISTURE TRANSPORT
SUGGESTS SEVERAL AREAS OF HEAVIER PRECIPITATION FROM THE LOWER MS
VALLEY NORTHWARD INTO THE OHIO VALLEY. THE OVERLAP OF THE BEST
INSTABILITY AND FORCING RIGHT NOW POINTS TOWARD PORTIONS OF THE TN
VALLEY INTO THE MID SOUTH REGION AND THIS COINCIDES WHERE THE
ENSEMBLES SHOW THE GREATEST POTENTIAL FOR SOME HIGHER RAINFALL
TOTALS (2-4"). ULTIMATELY, A SLIGHT RISK MAY BE NEEDED IN FUTURE
UPDATES IF CONFIDENCE INCREASES BUT FOR NOW, CONFIDENCE IN EXACT
PLACEMENT IS LOW AS WELL AS THE FORWARD PROGRESSION OF CONVECTION
THAT MAY BE JUST FAST ENOUGH TO LIMIT DURATION OF HEAVY RAINFALL AT
ANY LOCATION FOR TOO LONG. PREVIOUS DISCUSSION FOLLOWS...
A MUCH BROADER INHERITED MARGINAL RISK AREA (RELATIVE TO DAY 2) HAS
BEEN LARGELY MAINTAINED (AND EXPANDED A BIT), AS MULTIPLE SHORTWAVE
TROUGHS EMERGING FROM THE INTERMOUNTAIN WEST AND PHASE WITH THE
POLAR JET AND AN ASSOCIATED LONGWAVE TROUGH. THIS COMPLEX
INTERACTION IS EXPECTED TO RESULT IN A BROAD AREA OF CONVECTION
ACROSS MUCH OF THE EASTERN CONUS, INCLUDING PORTIONS OF THE MS, TN,
AND OH VALLEYS. DESPITE THE COMPLEXITY OF THE SYNOPTIC AND
MESOSCALE DETAILS, GLOBAL MODELS ARE IN FAIRLY GOOD AGREEMENT IN
INDICATING A CONVECTION BREAKING OUT ACROSS A BROAD WARM SECTOR
(CHARACTERIZED BY PWATS OF 1.5-2.0", SB CAPE OF 1000-2000 J/KG, AND
EFFECTIVE BULK SHEAR OF 40-60 KTS). MOST DETERMINISTIC SOLUTIONS
DEPICT 1-3" LOCALIZED TOTALS (WITH 2-3" AMOUNTS MOST COMMONLY
LOCATED FARTHER SOUTH ACROSS THE LOWER MS VALLEY AND MID-SOUTH,
WHERE BETTER INSTABILITY AND MOISTURE TEND TO BE LOCATED) WITH BOTH
THE GEFS AND ECENS INDICATING 2" EXCEEDANCE POTENTIAL (WITH THE
GEFS FAVORING PORTIONS OF THE MID-SOUTH FROM NORTH MS INTO WEST AND
MIDDLE TN, WHILE THE ECENS FAVORS LOCATIONS FARTHER SOUTHWEST INTO
THE MS DELTA). THESE TYPES OF PROBABILITIES FROM THE ENSEMBLE
GUIDANCE SUGGEST THAT CAMS MAY WELL DEPICT 2-4" LOCALIZED TOTALS,
AND A TARGETED SLIGHT RISK INTRODUCTION MAY BE NECESSARY IN
SUBSEQUENT OUTLOOKS (ONCE BETTER ENSEMBLE AGREEMENT AND/OR A
STRENGTHENING TREND IS MORE EVIDENT IN THE GLOBAL GUIDANCE).
CHURCHILL/TAYLOR
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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