078
FOUS30 KWBC 291939
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
339 PM 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. 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. OFFSHORE
CONVECTION CONTINUES TO FADE OFFSHORE THE UPPER TX COAST WHILE THE
AXIS OF 850 HPA CONVERGENCE MOVING ACROSS LA AND ITS FORMER MCV
SPAWN NEW CONVECTION INLAND, WHICH WAS HINTED AT IN THE 12Z HREF
PROBABILITY FIELDS OF 0.5"+ IN AN HOUR WHICH SEEM TO BE HANDLING
THE SITUATION FAIRLY WELL, THUS FAR.
WHEN CELL TRAINING OR 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 SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
ARKLAMISS...
CENTRAL GULF COAST THROUGH THE OHIO VALLEY INTO THE GREAT LAKES...
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. BROAD INFLOW FROM THE GULF
WITH PRECIPITABLE WATER VALUES OF 1.25-1.75" AND ML CAPE
POTENTIALLY AS HIGH AS 4000 J/KG IS SUPPORTIVE OF DISCRETE AND
MULTI-CELL CLUSTERS INITIALLY, AND LIKELY TRANSITIONING TO MORE A
MORE LINEAR QLCS ORIENTATION ALONG THE FRONT INTO THE EVENING.
HOURLY RAIN TOTALS TO 2.5" ARE SUPPORTED BY THE INGREDIENTS ABOVE,
WITH THE 12Z CANADIAN REGIONAL INDICATING THE POTENTIAL FOR 6",
WHICH WOULD BE POSSIBLE WHERE STORMS TRAIN OR TWO OR MORE
MESOCYCLONES ALIGN. THE MODEL GUIDANCE IS COMING UP ON AMOUNTS AS
ANTICIPATED/WE GET MORE INTO THE MESOSCALE MODEL WINDOW, WITH
GLOBAL GUIDANCE IN THE 2-3" RANGE AND MESOSCALE GUIDANCE HIGHER.
WITH PROBABILITIES OF 3"+ MAXIMIZED IN THE VICINITY OF THE MID-
SOUTH, WENT AHEAD WITH AN UPGRADE TO A SLIGHT RISK FROM THE AR/LA
BORDER INTO CENTRAL MS.
FLORIDA PENINSULA/SOUTHEASTERNMOST GEORGIA...
A FRONTAL BOUNDARY AT 850 HPA SLOWLY LIFTS THROUGH THE STATE,
BRINGING PRECIPITABLE WATER VALUES TO 1.5-1.75". MU CAPE MAXIMIZES
ABOVE 2000+ J/KG IN SOUTH FLORIDA. THERE APPEARS TO BE ENOUGH
EFFECTIVE BULK SHEAR FOR THUNDERSTORM ORGANIZATION AS WELL. THE
MODEL GUIDANCE --- MOSTLY MESOSCALE -- INDICATES THE POTENTIAL FOR
LOCAL AMOUNTS IN THE 5" RANGE. THIS LED TO THE INTRODUCTION OF A
NEW MARGINAL RISK AREA.
ROTH/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 & NORTHEAST...
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 PARENT LONGWAVE TROUGH INTO THE MID-
ATLANTIC/NORTHEAST, WHILE THE SOUTHERN SHORTWAVE LIKELY HANGS BACK
INTO THE MID-SOUTH AND LOWER MS VALLEY. THE BEST MOISTURE AND
INSTABILITY LIES OVER PORTIONS OF THE SOUTHEAST (WHERE FORCING
FROM THE SECONDARY SHORTWAVE SHOULD STILL BE SUFFICIENT FOR
ORGANIZED CONVECTIVE ACTIVITY). THE INHERITED MARGINAL RISK WAS
GENERALLY 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.
NORTHEAST...
THUNDERSTORMS WITH HEAVY RAINFALL ARE EXPECTED DURING THE AFTERNOON
AND EVENING HOURS AHEAD OF A NORTHERN STREAM SHORTWAVE AS MU CAPE
PEAKS AROUND 1000 J/KG AND PRECIPITABLE WATER VALUES PEAK IN THE
1.25-1.5" RANGE. SUFFICIENT INFLOW AND EFFECTIVE BULK SHEAR IS
AVAILABLE FOR ORGANIZED CONVECTION FOR MUCH OF THE PERIOD, WITH
ACTIVITY EXPECTED TO BE IN DECLINE PAST TUESDAY 06Z. HOURLY RAIN
TOTALS UP TO 2" ARE POSSIBLE WHERE CONVECTION TRAINS, WHICH WOULD
CHALLENGE THE FLASH FLOOD GUIDANCE ACROSS URBAN PORTIONS OF THE
MID-ATLANTIC STATES. THIS LED TO THE INTRODUCTION OF A NEW MARGINAL
RISK AREA.
NEAR THE WESTERN CA/OR BORDER...
A CYCLONE BRING LOW-LEVEL INFLOW UP TO 40 KTS AND SOME LIMITED
INSTABILITY, UP TO 250 J/KG MU CAPE, INTO THE REGION. THE GUIDANCE
GENERALLY AGREES ON ~3" OF QPF THIS PERIOD. HOWEVER, PRECIPITABLE
WATER VALUES SHOULD STAY BELOW 0.75" AND FREEZING LEVELS ARE LOW,
GENERALLY 2000-4000 FEET ABOVE GROUND LEVEL IN WESTERNMOST OR AND
NORTHWESTERNMOST CA. MOSTLY AS A NOD TO CONTINUITY, A MARGINAL RISK
AREA WAS NOT RE-INTRODUCED HERE, BUT THERE IS A NON-ZERO CHANCE OF
0.5"+ RAIN TOTALS, WHICH WOULD BE PROBLEMATIC IN AND NEAR BURN
SCAR LOCATIONS.
ROTH/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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