532  
FOUS30 KWBC 170722  
QPFERD  
 
EXCESSIVE RAINFALL DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
322 AM EDT SAT APR 17 2021  
 
DAY 1  
VALID 12Z SAT APR 17 2021 - 12Z SUN APR 18 2021  
 
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF  
THE CENTRAL-EASTERN GULF COAST AND SOUTHEAST...  
 
THE APPROACH OF A WEAK MID/UPPER LEVEL SHORTWAVE FROM TEXAS AND A  
JET STREAK OVER THE TENNESSEE VALLEY WILL PROVIDE THE NECESSARY  
FORCING FOR ASCENT FOR ANOTHER ROUND RAINFALL ACROSS PORTIONS OF  
THE CENTRAL-EASTERN GULF COAST AND SOUTHEAST SATURDAY-SATURDAY  
NIGHT. AT THE SURFACE, LOW PRESSURE WILL TRACK ALONG A  
QUASI-STATIONARY FRONT JUST OFFSHORE THE GULF COAST SATURDAY, THEN  
ACROSS NORTHERN FL/SOUTHERN GA SATURDAY NIGHT. INSTABILITY NORTH  
OF THESE FEATURES WILL REMAIN RATHER MARGINAL (UPWARDS OF 500  
J/KG) OVER PORTIONS OF SOUTHERN LA, SOUTHERN MS, AND SOUTHERN AL.  
THIS IS WITHIN AN AREA WHERE ANOMALOUSLY HIGH PRECIPITABLE WATER  
VALUES AND SUFFICIENT ONSHORE FLOW SHOULD REMAIN IN PLACE (1.6 TO  
1.6 INCHES; AROUND 2 STANDARD DEVIATIONS ABOVE NORMAL). FARTHER  
EAST ACROSS NORTH FL, THE LOW TRACK WILL LIFT THE WARM FRONT  
THROUGH MUCH OF THE AREA, ALLOWING THIS AREA TO TAP MORE ROBUST  
INSTABILITY WITHIN THE WARM SECTOR (MIXED LAYER CAPES 1000-2000  
J/KG) AHEAD OF FLAT UPPER SHORTWAVE AND SURFACE COLD FRONT.  
 
THE EXPANSIVE MARGINAL RISK AREA ACCOUNTS FOR THE POTENTIAL OF  
ISOLATED OR LOCALIZED FLASH FLOODING, BASED ON THE VARYING  
THERMODYNAMIC AND ANTECEDENT ENVIRONMENTS ACROSS THE OUTLOOK AREA.  
FARTHER WEST (ACROSS SOUTHERN LA/MS/AL), THE LACK OF SURFACE-BASED  
INSTABILITY WILL LIMIT HOURLY TOTALS TO 1" OR LESS OVER MOST  
AREAS. HOWEVER, THESE ARE AREAS WHERE SOILS REMAIN QUITE SATURATED  
PER THE LATEST NASA SPORT ANALYSIS (0-100 CM SOIL MOISTURE  
PERCENTILE 98+ PERCENT OVER EASTERN LA/SOUTHERN MS/FAR SOUTHERN  
AL), WITH LITTLE RECOVERY FROM A VERY WET WEEK. SO IT WOULDN'T  
TAKE AS MUCH RAINFALL TO CAUSE POTENTIAL RUNOFF ISSUES. FARTHER  
EAST (FL PANHANDLE/NORTH FL/SOUTHERN GA), STRONGER DEEP-LAYER  
INSTABILITY SHOULD ALLOW FOR HEAVIER SHORT-TERM RAINFALL RATES, AS  
THE HIGH-RES GUIDANCE (INCLUDING THE 00Z HRRR) INDICATES SPOTTY  
CLUSTERS WITH 1.5-2.0"/HR RATES LATER THIS AFTERNOON AND EVENING.  
SOILS ACROSS THESE AREAS AREN'T NEARLY AS SATURATED, NEVERTHELESS  
MUCH OF NORTH FL HAS OBSERVED 200-400+ PERCENT OF NORMAL RAINFALL  
OVER THE PAST WEEK.  
 
HURLEY/TAYLOR  
 
DAY 2  
 
THE DAY 2 OUTLOOK WILL BE UPDATED BY 0830Z.  
 
DAY 3  
 
THE DAY 3 OUTLOOK WILL BE UPDATED BY 0830Z.  
 
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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