423  
FOUS30 KWBC 250752  
QPFERD  
 
EXCESSIVE RAINFALL DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
352 AM EDT MON SEP 25 2017  
   
..VALID 12Z MON SEP 25 2017 - 12Z TUE SEP 26 2017
 
   
..REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL
 
 
MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
65 N MMCU 60 SE MMCS 20 SW CNM 50 NNW HOB 40 NNE TCC 35 N LAA  
40 SSW ITR 15 NW ITR 25 WSW TIF 30 E MHE CBG 20 W RGK ADU  
20 NNW MHK 15 NNE PTT 35 NNW CSM 10 NW CSM 30 SW SJT 100 SW DRT  
90 NNW MMTC.  
 
SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
25 SW GBD 20 NE PYX 20 S BGD EHA 50 WNW GCK 25 SSE HDE 10 N HSI  
15 WNW OLU 20 NW ORC 10 N GYL 10 ESE MKT 10 S MKT 20 ENE SLB OMA  
20 S BIE 30 ESE RSL 25 SW GBD.  
 
SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
65 NE MMCU 45 SSE GDP 20 SSE BPG 50 NE 6R6 75 SW 6R6  
150 WNW MMMV.  
 
   
..SOUTHWEST TEXAS
 
 
ONE OF THE MORE NOTEWORTHY AREAS OF PERSISTENT SYNOPTIC  
FORCING...ENHANCED INSTABILITY...AND GROWING MODEL QPF SIGNAL  
EXISTS OVER THE RIO GRANDE VALLEY...AROUND THE SERRANIAS DEL BURRO  
RANGE IN COAHUILA...WITH THE PRECIPITATION SIGNAL BLEEDING OVER  
INTO THE DEL RIO TEXAS AREA. ALTHOUGH THERE IS LITTLE MOVEMENT OF  
THE LARGE SCALE TROUGHING THROUGH TODAY...THE AREA OF FAR  
SOUTHWEST TEXAS WILL REMAIN IN A GENERALLY DIFLUENT REGION OF THE  
500-250 MB FLOW...AND THIS DIFLUENCE MAY BECOME ENHANCED LATE  
MONDAY NIGHT AS AN UPPER JET STREAK TIGHTENS AND STRENGTHENS  
UPSTREAM OVER EASTERN NEW MEXICO. THUS...AFTER ONE ROUND OF  
DIURNAL CONVECTION SUBSIDES LATER THIS  
MORNING...ANOTHER...POTENTIALLY MORE SIGNIFICANT ROUND...IS  
POSSIBLE FROM THE AFTERNOON INTO THE OVERNIGHT HOURS.  
 
A BALANCE OF SLOW CELL MOTIONS AND ENHANCED LOW LEVEL  
INFLOW...PARTICULARLY DURING THE NATURAL BACKING AND STRENGTHENING  
OF THE LOW LEVEL FLOW AT NIGHT...COULD LEAD TO VERY SLOW MOVING  
CONVECTION WHOSE RAINFALL EFFICIENCY IS PEAKED BY THE INVOLVEMENT  
OF HIGH LEVEL MOISTURE STREAMING OFF OF TROPICAL STORM PILAR.  
THEREFORE NUMEROUS HI-RES MODELS START TO PRODUCE PROLIFIC  
RAINFALL AMOUNTS APPROACHING 10 INCHES OR MORE...AT LEAST ON A  
SMALL SCALE...AS OF MONDAY NIGHT. THIS IS PRIOR TO WHAT IS  
EXPECTED TO BE A MORE WIDESPREAD HEAVY RAIN EVENT UP ONTO THE U.S.  
SIDE OF THE BORDER LATER ON TUESDAY. THE DIFFICULTY ON DAY 1 IS  
PREDICTING HOW MUCH WILL REACH THE U.S. SIDE. JUDGING BY MODEL  
CAPE FORECASTS AND LOW LEVEL INFLOW...IT IS POSSIBLE THE HEAVIEST  
CELLS REMAIN IN MEXICO THROUGH MONDAY NIGHT...OR AFFECT ONLY A  
SMALL PART OF TEXAS...PERHAPS TOO SMALL TO WARRANT MORE THAN A  
SLIGHT RISK OF EXCESSIVE RAINFALL. THEREFORE...AS WE LEAN INTO THE  
BEGINNING OF THIS MULTI-DAY EVENT...THE DAY 1 SLIGHT RISK AREA  
OVER SOUTHWEST TEXAS IS CONNECTED TO A LENGTHY SLIGHT RISK  
COVERING THE SURFACE TROUGH AND COLD FRONT REGIONS UP THROUGH THE  
CENTRAL PLAINS. DEPENDING HOW THE MORNING OBSERVATIONS AND OTHER  
DATA EVOLVE...HOWEVER...IT WOULD NOT BE OUT OF THE QUESTION FOR  
WPC TO INTRODUCE A SMALL MODERATE RISK OVER THE DEL RIO TEXAS AREA  
IN LATER UPDATES.  
 
   
..CENTRAL/SOUTHERN PLAINS TO THE UPPER MIDWEST
 
 
A SHORTWAVE TROUGH WILL EJECT FROM THE ROCKIES TO THE UPPER  
MIDWEST...BUT DEEP LAYER FORCING WILL ONCE AGAIN PRIMARILY OCCUR  
TO THE COOL SIDE OF THE ASSOCIATED SURFACE COLD FRONT. THIS LACK  
OF WESTERLY FLOW AND ABSENCE OF STRONG FORCING OVER THE WARM  
SECTOR HAS GENERALLY HAMPERED THE MAGNITUDE OF MID LEVEL LAPSE  
RATES AND RESULTING INSTABILITY. ALTHOUGH RAIN WAS WIDESPREAD WITH  
THIS SYSTEM...IT HAD NOT YET BEEN TOO FOCUSED OR HEAVY. THE MODEL  
QPF SIGNAL IS SOMEWHAT HEAVIER FOR THE MONDAY AND MONDAY NIGHT  
PERIOD...PARTICULARLY ALONG THE TX/OK/KS PORTION OF THE  
FRONT...BEING BRUSHED BY THE SHORTWAVE EJECTING OUT OF THE CENTRAL  
ROCKIES. MOST OF THE HI-RES MODELS PRODUCE WIDESPREAD 1 TO 2 INCH  
RAINFALL WITH SPOTTY EMBEDDED 3 TO 4 INCH AREAS FROM THE SOUTHERN  
HIGH PLAINS UP ACROSS CENTRAL AND NORTHEAST KANSAS. FLASH FLOOD  
GUIDANCE VALUES ARE RELATIVELY HIGH...BUT COULD BE EXCEEDED  
LOCALLY...AS THE ORIENTATION OF THE FLOW BECOMES A BIT MORE  
FAVORABLE FOR SOUTHWEST TO NORTHEAST TRAINING.  
 
FARTHER NORTH A CAPE AXIS WILL EXTEND UP THROUGH IOWA INTO PARTS  
OF MN/WI. WEAKER LOW LEVEL INFLOW SPEEDS AND MORE STRONGLY VEERED  
INFLOW...HOWEVER...MAY BE PART OF THE REASON THAT THE MODEL QPF  
SIGNAL IS MUCH LESS ALONG THIS PORTION OF THE COLD FRONT...AND THE  
RISK OF FLASH FLOODING APPEARS MARGINAL.  
 
BURKE  

 
 
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