900
FOUS30 KWBC 302017
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
417 PM EDT WED APR 30 2025
DAY 1
VALID 16Z WED APR 30 2025 - 12Z THU MAY 01 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST OKLAHOMA, NORTH TEXAS, AND WESTERN ARKANSAS...
THE THREAT OF SIGNIFICANT FLASH FLOODING CONTINUES TODAY, WITH THE
FOCUS INCREASINGLY SHIFTING TO PORTIONS OF NORTH TEXAS BASED ON
OBSERVATIONAL TRENDS AND THE LATEST 12Z HI-RES MODEL GUIDANCE.
A BOWING CONVECTIVE LINE CONTINUES TO ADVANCE EASTWARD THROUGH
S OK, WITH A TRAILING LINE OF CONVECTION EXTENDING INTO N TX ON
THE SOUTHERN PERIPHERY OF STRONG COLD POOL AND MESOHIGH SITUATED
OVER SC OK. OVER THE PAST COUPLE HOURS, THE CONVECTIVE LINE IN N
TX HAS BECOME INCREASINGLY ORIENTED NE-SW, OR EVEN ENE-WSW; IN
OTHER WORDS, MORE PARALLEL TO THE MEAN FLOW VECTOR. THIS HAS
INCREASED THE POTENTIAL FOR TRAINING AND BACKBUILDING, ESPECIALLY
INTO THE AFTERNOON HOURS AS THE LLJ BEGINS TO WEAKEN AND LOW-LEVEL
WINDS START TO VEER TO THE SOUTHWEST. FOR A MORE DETAILED MESOSCALE
ANALYSIS, PLEASE REFER TO MPD #202 ISSUED AT 14Z FOR THIS REGION.
THE MODERATE RISK AREA WAS MAINTAINED IN SE OK AND W AR, EVEN
THOUGH CONFIDENCE IN FLASH FLOODING HAS DECREASED SOMEWHAT DUE TO
THE FAIRLY STEADY FORWARD PROGRESS OF THE BOWING LINE, AND HOW THIN
THE CONVECTIVE LINE HAS BECOME. HOWEVER, IT WAS MAINTAINED OUT OF
DEFERENCE TO CONTINUITY AND THE POTENTIAL TO REINVIGORATE THE
CONVECTION IN THE NEXT FEW HOURS WITH SOME CLEARING OCCURRING AHEAD
OF THE LINE, WHICH COULD INCREASE RAIN RATES AGAIN.
GREATER CONCERN NOW EXISTS IN N TX. THE FORWARD PROGRESSION OF THE
LINE AND ASSOCIATED COLD POOL IN S OK AND OVERALL CONVECTIVE
EVOLUTION HAS LED TO FAR MORE WEST-EAST ORIENTATION TO THE
CONVECTIVE LINE IN N TX THAN MODELS HAD ORIGINALLY BEEN
ADVERTISING. THIS CERTAINLY INCREASES FLASH FLOOD POTENTIAL AND
OVER THE NEXT FEW HOURS THIS PLACES THE DALLAS-FT. WORTH METRO AREA
SQUARELY IN THE GREATEST RISK. HOURLY RAINFALL COULD EXCEED 2
INCHES PER HOUR, WITH UNOBSTRUCTED INFLOW FROM A VERY MOIST AND
UNSTABLE AIR MASS (MLCAPE 1500-2000 J/KG WITH MINIMAL CINH; PWS
1.6 TO 1.8 INCHES PER RAP ANALYSIS.) IN AN URBAN ENVIRONMENT, THIS
COULD LEAD TO SIGNIFICANT AND LIFE-THREATENING FLASH FLOODING,
PARTICULARLY IF THE RAIN RATES ARE SUSTAINED FOR A COUPLE HOURS,
WHICH IS A DISTINCT POSSIBILITY IN THIS CASE.
AS THE AFTERNOON AND EVENING PROGRESS, THE HI-RES MODELS DO
INDICATE THE POTENTIAL FOR THE SOUTHWESTERN PERIPHERY OF THE
CONVECTIVE LINE TO CONTINUE TO SLOW OR STALL. WHILE THEY DON'T
NECESSARILY AGREE ON THE SPECIFICS, MANY HREF MEMBERS, AS WELL AS
THE RRFS, INDICATE BACKBUILDING AND TRAINING CONVECTION COULD
BECOME MORE LIKELY IN THE CORRIDOR FROM WACO TO LONGVIEW. THIS
SEEMS TO BE WELL SUPPORTED BY OBSERVATIONAL TRENDS, AND IN FACT
MANY HI-RES MEMBERS ARE: (1) ALREADY NOT EXTENDING THE CONVECTIVE
LINE FAR ENOUGH SOUTHWEST BASED ON CURRENT RADAR TRENDS, AND (2)
HAVE THE CONVECTIVE LINE ORIENTED FAR TOO MUCH IN A N-S FASHION.
THEREFORE, THEY MAY BE UNDERESTIMATING RAINFALL POTENTIAL A BIT,
AND ALSO UNDERSELLING THE POTENTIAL FOR A SIGNIFICANT RAINFALL AND
FLASH FLOOD EVENT FURTHER TO THE SOUTHWEST. PROBABILITIES AND
ASSOCIATED RISK CATEGORIES WERE INCREASED A BIT DOWN THE I-35
CORRIDOR BETWEEN DFW AND AUS, AND THIS WILL BE MONITORED FOR
FURTHER UPDATES BASED ON OBSERVATIONAL TRENDS. RAIN RATES IN MUCH
OF C/N TX COULD REACH OR EXCEED 2 INCHES PER HOUR GIVEN THE
AVAILABLE INSTABILITY AND MOISTURE.
THE OTHER BIGGEST CHANGE FOR THIS OUTLOOK WAS TO EXTEND THE SLIGHT
RISK A BIT FURTHER NORTHEAST THROUGH MO AND INTO W IL BASED ON MORE
ELEVATED HREF PROBABILITIES OF EXCEEDING 1HR AND 3HR FLASH FLOOD
GUIDANCE AND A FAIRLY CONSISTENT SIGNAL IN THE GUIDANCE FOR A
CORRIDOR OF ENHANCED RAINFALL ASSOCIATED WITH AN MCV AND
CORRESPONDING SURFACE LOW.
LAMERS
DAY 2
VALID 12Z THU MAY 01 2025 - 12Z FRI MAY 02 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN SOUTHERN
OKLAHOMA, FAR NORTH TEXAS, AND FAR WESTERN ARKANSAS...
THE MOST SIGNIFICANT CHANGE FOR THIS OUTLOOK PERIOD WAS TO
INTRODUCE A SLIGHT RISK PRIMARILY OVER SOUTHERN OKLAHOMA, WHERE
PREVIOUSLY THE PROBABILITIES WERE BELOW THE MARGINAL RISK (5
PERCENT) THRESHOLD. THIS WAS DUE TO AN EMERGING CONSENSUS IN MODEL
GUIDANCE THAT THUNDERSTORMS WILL BLOSSOM ON THURSDAY NIGHT AHEAD OF
A DIGGING SHORTWAVE IN AN AREA OF RELATIVELY STRONG UPPER LEVEL
DIVERGENCE. ALTHOUGH THE MID LEVELS WILL BE DRIER THAN IN RECENT
DAYS, LEADING TO LOWER PWS OVERALL, STRONG INSTABILITY WILL BE IN
PLACE WHICH SHOULD SUPPORT ORGANIZED CONVECTION WITH MORE INTENSE
RAIN RATES AT TIMES. A KEY UNCERTAINTY AT THE MOMENT IS THE OVERALL
STRUCTURE OF CONVECTIVE CLUSTERS AND LINES, AND WHETHER THEY WILL
BE MORE PROGRESSIVE OR INCLUDE SOME TRAINING. HOWEVER, THE
POTENTIAL FOR HEAVY RAIN RATES IN AN AREA THAT HAS RECEIVED A LOT
OF RAINFALL ALREADY IN RECENT DAYS DOES RAISE THE RISK OF FLASH
FLOODING. HOURLY RAINFALL ON THE ORDER OF 1-2 INCHES WILL BE
POSSIBLE IN THIS ENVIRONMENT, AND IN AREAS WITH ALREADY WET GROUND
CONDITIONS THIS WOULD BE MORE THAN SUFFICIENT TO CAUSE FLASH
FLOODING.
ELSEWHERE, THE BROAD MARGINAL RISK THAT WAS ALREADY IN PLACE WAS
MAINTAINED WITH FEW CHANGES. A BROAD PLUME OF MODERATE TO STRONG
INSTABILITY WILL FAVOR SCATTERED LINES AND CLUSTERS OF
THUNDERSTORMS IN ADVANCE OF A SURFACE LOW AND TRAILING COLD FRONT
WITH FAIRLY HIGH INSTANTANEOUS RAIN RATES. THE KEY UNCERTAINTY
WOULD BE THE PERSISTENCE OF THOSE RAIN RATES AND THE DEGREE TO
WHICH ANY TRAINING COULD OCCUR, BUT THE OVERALL ENVIRONMENT WITH
PWS AROUND 1.3 TO 1.4 INCHES RAISES THE PROBABILITY HIGH ENOUGH TO
WARRANT THE BROAD MARGINAL RISK. THE ONLY EXCEPTION TO THE
CONTINUATION OF THE RISK AREA WAS OVER MUCH OF IL, IN, AND MI,
WHICH SHOULD LARGELY BE BEHIND THE COLD FRONT BY 18Z AND SO MOST OF
THE QPF ON MODEL GUIDANCE WOULD BE ASSOCIATED WITH POST-FRONTAL
SHOWERS THAT WOULD TEND TO HAVE LOWER RAIN RATES OVERALL.
LAMERS
DAY 3
VALID 12Z FRI MAY 02 2025 - 12Z SAT MAY 03 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS FROM THE
SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY...
A FLASH FLOOD EVENT MAY BE ONGOING AT THE BEGINNING OF THE PERIOD,
WITH CONVECTION DEVELOPING ON THURSDAY NIGHT IN SOUTHERN OKLAHOMA
AND CONTINUING INTO FRIDAY MORNING. THIS MAY CONTINUE BEYOND 12Z
FRIDAY (INTO THIS DAY 3 PERIOD) IN SE OK AND AR IN THE REGION OF
STRONG UPPER-LEVEL DIVERGENCE. THE EXISTING SLIGHT RISK OVER TX WAS
EXPANDED NORTHWARD TO ACCOUNT FOR THIS SCENARIO. WHETHER THAT ROUND
OF CONVECTION WANES DUE TO MORE EXTENSIVE CLOUD COVER AND LIMITED
DAYTIME HEATING, OR PROGRESSES SOUTHEAST AND BECOMES REINVIGORATED
DURING THE AFTERNOON, THE THREAT OF HEAVY RAINFALL AND FLASH
FLOODING SHOULD EXPAND SOUTHEAST ON FRIDAY INTO THE LOWER
MISSISSIPPI VALLEY (S AR, N LA, W MS) AND MUCH OF C AND E TX. THIS
WOULD BE VIA PROGRESSION OF THE EARLY ROUND OF STORMS, OR
REDEVELOPMENT IN THE AFTERNOON AND EVENING HOURS.
STRONG INSTABILITY WILL REMAIN OVER MUCH OF THE WARM SECTOR WITH
CAPE VALUES POTENTIALLY IN EXCESS OF 3000 J/KG IN SOME AREAS, AND
PW VALUES ARE EXPECTED TO INCREASE ABOVE 1.5 INCHES AS SSE FLOW
INCREASES OFF THE GULF. THE COMBINATION OF STRONG INSTABILITY AND
HIGH PWS WILL FAVOR VERY HEAVY RAIN RATES IN ORGANIZED CONVECTION,
AND THIS IS SUPPORTIVE OF THE BROAD SLIGHT RISK ON THE NEW
OUTLOOK. AT THE MOMENT, MODEL GUIDANCE IS SHOWING THE MOST
CONSISTENT SIGNAL FOR HEAVY RAIN FROM TEXAS HILL COUNTRY IN C TX
EAST- NORTHEAST INTO THE PINEY WOODS REGION OF E TX. GIVEN THE
OVERALL ENVIRONMENT, IT IS POSSIBLE AN UPGRADE TO A MODERATE RISK
MAY BE NEEDED AT SOME POINT.
LAMERS
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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