722
FOUS30 KWBC 291552
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1152 AM EDT SAT MAR 29 2025
DAY 1
VALID 16Z SAT MAR 29 2025 - 12Z SUN MAR 30 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR
SOUTHEAST LOUISIANA...
A CONVERGENCE ZONE/SEGMENT OF THE POLAR FRONT ACROSS THE NORTHERN
GULF AND SOUTHEAST TX IS EXPECTED TO LIFT NORTHEAST AND NORTH INTO
LOUISIANA WITH TIME AND ACT AS A FOCUS FOR CONVECTION TODAY INTO
TONIGHT. ALOFT, A MID- LEVEL SHORTWAVE IS MOVING ACROSS SOUTHEAST
AR AND ACTING AS A MECHANISM FOR DIVERGENCE ALOFT NEAR THE GULF
COAST. PRECIPITABLE WATER VALUES OF 1.75"+ ARE EXPECTED WITHIN AN
ENVIRONMENT OF 5640-5670 METER 1000-500 HPA THICKNESS, IMPLYING
COMPLETE SATURATION. ML CAPE IS EXPECTED TO RISE TO 2000 J/KG OR SO
DURING THE DAY TODAY...RECENT GOES-EAST IMAGERY SHOWS POCKETS OF
CLEARING OVER LOUISIANA AND SOUTHWEST MISSISSIPPI AT THE PRESENT
TIME WHICH SHOULD ALLOW FOR AN ADDITIONAL 1000 J/KG OR SO OF ML
CAPE TO FORM TODAY (SITTING PRESENTLY AT 500-1000 J/KG ACROSS
SOUTHERN LA AND THE SOUTHWEST HALF OF MS). A NORTH-NORTHEAST/SOUTH-
SOUTHWEST ORIENTED CONVERGENCE ZONE AT 850 HPA MOVES FROM WEST TO
EAST ACROSS LOUISIANA AND MISSISSIPPI TODAY AND TONIGHT, WHICH
SHOULD PROVIDE AN UPTICK TO LOW-LEVEL WINDS AND EFFECTIVE BULK
SHEAR, VALUES OF WHICH ARE ALREADY SUFFICIENT FOR ORGANIZED
CONVECTION IN AND NEAR SOUTHERN LA & SOUTHERN MS.
HOURLY RAIN TOTALS TO 2.5" SHOULD BE EASY TO ACHIEVE IN THIS
ENVIRONMENT -- SHOULD STORMS BACKBUILD/TRAIN FOR A PROLONGED
PERIOD, EVEN HIGHER HOURLY TOTALS WOULD BE POSSIBLE. THE 12Z HREF
SHOWED A 40%+ CHANCE OF 8"+ TOTALS ACROSS SOUTHEAST LA -- THERE IS
THE POTENTIAL FOR LOCAL AMOUNTS IN THE 10" RANGE. CONVECTION SOUTH
OF HIGH ISLAND TX AND WEST OF GALVESTON TX HAS BEEN DIFFICULT TO
DISLODGE, WITH BACK BUILDING CONVECTION EVIDENT. THE USUAL
EXPECTATION WOULD BE FOR THE OFFSHORE CONVECTION TO FADE IN THE
NEXT COUPLE HOURS AND SEND OUTFLOW BOUNDARIES ASHORE OR THAT THE
AXIS OF 850 HPA CONVERGENCE MOVING ACROSS LA WOULD FORCE NEW
CONVECTION INLAND, WHICH IS HINTED AT IN THE 12Z HREF PROBABILITY
FIELDS OF 0.5"+ IN AN HOUR AND THIS EXPECTATION MAY BE STARTING TO
MATERIALIZE IN RECENT RADAR IMAGERY.
THERE IS CONCERN ABOUT THE EXACT LOCATION AND TRACK OF ANY
ORGANIZED CONVECTIVE RAINFALL AREAS. WHEN CELL TRAINING/
BACKBUILDING ESTABLISHES ITSELF ACROSS SOUTHERN LA, AFTER ROUGHLY
FOUR HOURS IT COULD BORE A HOLE IN THE INSTABILITY FIELD. ONCE THIS
HAPPENS, BACKBUILDING WOULD BECOME MORE LIKELY AND THE 1000-500
HPA THICKNESS FIELD SUGGESTS A SOUTHEAST PROPAGATION TO SOME
PORTION OF THE MASS OF CONVECTION WOULD BE EXPECTED -- THIS SHOULD
CAUSE A NORTHWARD BOUND ON THE LOCATION OF HIGHEST RAINFALL
POTENTIAL. A SIGNIFICANT ENOUGH THREAT EXISTS FOR HIGH TOTALS THIS
AFTERNOON THROUGH TONIGHT THAT AN UPGRADE TO A MODERATE RISK
APPEARED PRUDENT BASED ON THE ABOVE, WHICH WAS COORDINATED WITH
LIX/THE SLIDELL LA FORECAST OFFICE AND ALSO SUPPORTS THEIR ONGOING
FLOOD WATCH. THE POTENTIAL FOR SIGNIFICANT IMPACT/INUNDATION FROM
FLASH FLOODING OVER URBAN AREAS CAN'T BE RULED OUT.
ROTH
DAY 2
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...
A MUCH BROADER INHERITED MARGINAL RISK AREA (RELATIVE TO DAY 1)
HAS BEEN LARGELY MAINTAINED, AS A PAIR OF SHORTWAVE TROUGHS
EMERGE FROM THE INTERMOUNTAIN WEST INTO THE CENTRAL U.S. ON
SUNDAY, EVENTUALLY PARTIALLY PHASING WITH THE POLAR JET (AND AN
ASSOCIATED LONGWAVE TROUGH) BY SUNDAY NIGHT. THIS COMPLEX
INTERACTION IS EXPECTED TO RESULT IN A BROAD AREA OF CONVECTION
ACROSS MUCH OF THE EASTERN HALF OF THE CONUS, INCLUDING PORTIONS OF
THE MS, TN, AND OH VALLEYS. DESPITE THE COMPLEXITY OF THE SYNOPTIC
AND MESOSCALE DETAILS, MODELS REMAIN IN FAIRLY GOOD AGREEMENT IN
INDICATING 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) SUPPORTIVE OF DISCRETE AND
MULTI-CELL CLUSTERS INITIALLY, AND LIKELY TRANSITIONING TO MORE A
MORE LINEAR QLCS ORIENTATION ALONG THE FRONT INTO THE EVENING (AS
BEST DEPICTED BY THE 00Z FV3 AND NAM-NEST, WHICH ARE THE ONLY CAMS
WHICH EXTEND THROUGH THE FULL PERIOD AT 00Z). WHILE WPC AREAL
AVERAGE QPF CALLS GENERALLY FOR ONLY 1-2" TOTALS, MOST
DETERMINISTIC SOLUTIONS FROM THE GLOBAL MODELS ARE LIKELY WELL
UNDERDONE ON LOCALIZED TOTALS, AS CAMS WILL BE NEEDED TO BETTER
MODEL THE DISCRETE CONVECTIVE INITIATION THAT IS ANTICIPATED PRIOR
TO THE MAIN QLCS PASSAGE (WITH THE FV3, NAM- NEST, AND CMC-REG ALL
INDICATING THE POTENTIAL FOR 3-5" TOTALS, WITH PROBABILITIES
LIKELY MAXIMIZED IN THE VICINITY OF THE MID-SOUTH AND MS DELTA
REGIONS WHERE INSTABILITY AND MOISTURE ARE MORE IDEAL... AND THIS
IS WHERE A TARGETED SLIGHT RISK IS MOST LIKELY TO BE INTRODUCED IN
FUTURE UPDATES, POSSIBLY AS EARLY AS LATER TODAY).
CHURCHILL
DAY 3
VALID 12Z MON MAR 31 2025 - 12Z TUE APR 01 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST...
THE AFOREMENTIONED PAIR OF SHORTWAVE TROUGHS ARE PROGGED TO FULLY
SEPARATE BY DAY 3, AS THE NORTHERN STREAM SHORTWAVE CONTINUES TO
PHASE WITH THE POLAR JET (AND ASSOCIATED PARENT LONGWAVE TROUGH)
INTO THE MID-ATLANTIC/NORTHEAST, WHILE THE SOUTHERN SHORTWAVE
LIKELY HANGS BACK INTO THE MID-SOUTH AND LOWER MS VALLEY. THIS IS
ANTICIPATED TO RESULT IN A WEAKER SETUP OVERALL FOR EXCESSIVE
RAINFALL ON MONDAY INTO MONDAY NIGHT, AS THE BEST DYNAMICS AND
UPPER-LEVEL FORCING OVER THE NORTHEAST AND MID-ATLANTIC IS
DISPLACED FROM THE BEST MOISTURE AND INSTABILITY OVER PORTIONS OF
THE SOUTHEAST (WHERE FORCING FROM THE SECONDARY SHORTWAVE SHOULD
STILL BE SUFFICIENT FOR ORGANIZED CONVECTIVE ACTIVITY). THE
INHERITED MARGINAL RISK WAS MAINTAINED FOR THIS PORTION OF THE
SOUTHEAST (INCLUDING MUCH OF AL/GA WITH SURROUNDING PORTIONS OF
TN/NC/SC AND MS), WHERE GEFS/ECENS EXCEEDANCE PROBABILITIES SUGGEST
THE BEST POTENTIAL FOR 2-4" LOCALIZED TOTALS. FARTHER NORTHEAST
INTO THE MID-ATLANTIC AND SOUTHERN PORTIONS OF NEW ENGLAND, WHILE
TOTAL TROPOSPHERIC MOISTURE WILL CERTAINLY BE LOWER (~1.25" VERSUS
1.5"+), THESE VALUES ARE EITHER EQUALLY OR MORE ANOMALOUS THAN IN
THE SOUTHEAST (90TH PERCENTILE OR HIGHER). DESPITE THIS, HAVE
DECIDED NOT TO INTRODUCE A MARGINAL RISK AT THIS TIME, DUE TO THE
COMBINATION OF MARGINAL INSTABILITY AND PROGRESSIVE STORM MOTIONS.
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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