369  
FOUS30 KWBC 220804  
QPFERD  
 
EXCESSIVE RAINFALL DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
403 AM EDT SUN APR 22 2018  
 
VALID 12Z SUN APR 22 2018 - 12Z MON APR 23 2018  
 
 
MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
25 S KXIH 20 SSE LBX 5R5 10 NE 62H 35 ESE ACT 10 NE 3T1  
30 NW LRF 15 E M19 15 W UTA 30 NNW TVR 25 NW MCB 10 ESE BTR  
15 WSW KVNP 15 N KCRH.  
 
SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
20 SSW PBF 15 WNW MLU 10 NE OCH 10 NNW JSO 10 W M89 20 SSW PBF.  
 
   
..SOUTHEAST U.S
 
 
A CLOSED LOW OF MODERATE STRENGTH AND MAINTAINING ITSELF ALONG ITS  
SLOW EASTWARD MARCH - CHARACTERISTICALLY A VERY APRIL SYSTEM -  
WILL SPREAD RAIN AND THUNDERSTORMS THROUGHOUT AREAS FROM THE LOWER  
OHIO VALLEY TO THE GULF COAST.  
 
MODEL GUIDANCE IS IN DECENT AGREEMENT, AND THE NOTION OF  
WIDESPREAD 1 TO 3 INCH RAINFALL DURING THE 24-HOUR PERIOD. THE  
MAIN QUESTION IS WHETHER RAIN RATES WILL BE INTENSE ENOUGH OR  
PERSISTENT ENOUGH OVER ANY GIVEN LOCATION TO YIELD MUCH IN THE WAY  
OF FLASH FLOODING. GENERALLY THE MOST PERSISTENT FORCING AND  
BETTER ALIGNMENT OF LOWER AND UPPER LEVEL FLOW SHOULD OCCUR DURING  
THE DAY FROM NORTHERN MISSISSIPPI INTO WESTERN TENNESSEE. THIS IS  
PERHAPS WHERE TRAINING WILL BE MOST PREVALENT, BUT INSTABILITY  
WILL BE MORE MARGINAL. AREAS FARTHER SOUTH SHOULD SEE SOMEWHAT  
GREATER INSTABILITY, BUT WITH MID LEVEL FLOW CUTTING ACROSS FROM  
WEST TO EAST, THE MEAN 0-6 KM WINDS ARE AT A SHARP ANGLE TO  
CORFIDI VECTORS, SUGGESTING TRAINING WILL NOT BE TOO COMMON. GIVEN  
FLASH FLOOD GUIDANCE VALUES ARE ALSO MUCH HIGHER, AND WILL BE  
DIFFICULT TO SURPASS WITH SOUTHWARD EXTENT, WPC TRIMMED BACK A BIT  
ON THE INHERITED SLIGHT RISK AREA FOR THE 12Z SUN- 12Z MON PERIOD.  
OTHERWISE, SLIGHT RISK IS MAINTAINED IN AREAS OF LOWER FFG ACROSS  
NORTHERN MS AL INTO SOUTHERN TN, AND INCLUDING THE ATLANTA METRO  
TO THE EAST, AS WELL AS THE MORE FAVORABLE TRAINING ENVIRONMENT IN  
WESTERN TN.  
 
THROUGHOUT THESE AREAS AREAL AVERAGE PRECIPITATION IS FORECAST AT  
1.5 TO AROUND 3.0 INCHES FOR THE 24-HOUR PERIOD, AND JUDGING BY  
MOISTURE AVAILABILITY AND HI-RES MODEL SOLUTIONS, HOURLY RATES  
COULD PEAK AROUND 1.50 INCHES, LEADING TO AT LEAST SOME RISK OF  
FLASH FLOODING, ESPECIALLY FROM THE TIME OF PEAK HEATING INTO THE  
EVENING HOURS.  
 
BURKE  

 
 
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