328  
FXUS63 KLSX 131926  
AFDLSX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAINT LOUIS MO  
226 PM CDT THU AUG 13 2020  
   
SHORT TERM
 
(THROUGH LATE FRIDAY NIGHT)  
ISSUED AT 226 PM CDT THU AUG 13 2020  
 
SLOW MOVING THUNDERSTORMS CONTINUE DEVELOP ALONG THE STATIONARY  
FRONT DRAPED FROM CENTRAL NORTH DAKOTA THROUGH CENTRAL IOWA INTO  
EASTERN MISSOURI AND SOUTHERN ILLINOIS. DRIVEN BY DAY TIME HEATING  
AND A MID-LEVEL LOW AND ASSOCIATED DISTURBANCE THAT RETROGRADED  
OVERNIGHT INTO EASTERN MISSOURI AND SOUTHWEST ILLINOIS, THESE STORMS  
WILL LARGELY LINGER WHERE THEY FORM. FLASH FLOOD CONTINUES TO BE A  
CONCERN AS LONG AS THESE SHOWERS AND THUNDERSTORMS PERSIST ACROSS  
THE AREA, PRODUCING PRODIGIOUS RAINFALL AMOUNTS ACROSS PORTIONS OF  
SOUTHEAST MISSOURI AND SOUTHWEST ILLINOIS THANKS TO 1.5-2" OF  
AVAILABLE PRECIPITABLE WATER.  
 
CONVECTION AND SKY COVER IS EXPECTED TO DECAY WITH NIGHTFALL,  
LEAVING MOIST CONDITIONS BEHIND. PATCHY FOG WILL BE POSSIBLE DURING  
THE EARLY HOURS OF THE MORNING TOMORROW IN AREAS WHERE THE SKY IS  
ABLE TO CLEAR SUFFICIENTLY OVERNIGHT. ANY FOG THAT DEVELOPS WILL  
DISSIPATE TOMORROW MORNING ONCE THE SUN COMES UP AND DAYTIME MIXING  
COMMENCES.  
 
OVERNIGHT AND INTO FRIDAY THE MID-LEVEL LOW CREEPS SLOWLY EASTWARD,  
CENTERING THE BEST LIFT OVER SOUTHEAST MISSOURI AND SOUTHWEST  
ILLINOIS. AS DAY TIME HEATING KICKS INTO GEAR ON FRIDAY, BLOSSOMING  
SHOWERS WILL BE ABLE TO TAP INTO THE 1500-2500 J/KG OF MUCAPE AND 1-  
1.5" OF PRECIPITABLE WATER TO GENERATE ANOTHER DAY OF SLOW MOVING  
THUNDERSTORMS PRODUCING LOCALIZED HEAVY RAINFALL OVER SOUTHEAST  
MISSOURI AND SOUTHWEST ILLINOIS. SHOWERS AND THUNDERSTORMS WILL DIE  
OFF WITH THE LOSS OF DAY TIME HEATING AND THE MOVEMENT OF THE MID-  
LEVEL LOW AND ASSOCIATED VORTICITY MAXIMA OUT OF THE IMMEDIATE AREA  
FRIDAY EVENING.  
 
WITH CONDITIONS ACROSS THE AREA LARGELY UNCHANGED BETWEEN WEDNESDAY  
AND FRIDAY, WE CAN EXPECT TEMPERATURES TO BE FAIRLY CONSISTENT WITH  
HIGHS IN THE MID TO UPPER 80S AND LOWS IN THE MID 60S TO LOW 70S.  
 
MRM  
   
LONG TERM
 
(SATURDAY THROUGH NEXT THURSDAY)  
ISSUED AT 226 PM CDT THU AUG 13 2020  
 
A LOW-LEVEL COLD FRONT WILL MOVE THROUGH THE REGION SATURDAY  
AFTERNOON. SREF PROBABILITIES AND OPERATIONAL MODELS INDICATE  
INSTABILITIES BETWEEN 1000-2000 J/KG ARE LIKELY WITH OPERATIONAL  
MODELS INDICATING AROUND 20 KTS OF 0-6 KM BULK SHEAR AHEAD OF AND  
ALONG THE FRONT. BUT A LACK OF DEEP MOISTURE CONVERGENCE ALONG THE  
FRONT WILL GREATLY LIMIT PRECIPITATION CHANCES, SO HAVE KEPT THE NBM  
FORECAST OF SCATTERED SHOWERS AND THUNDERSTORMS DURING THE DAY  
SATURDAY.  
 
A HIGH PRESSURE SYSTEM WILL BUILD IN BEHIND THE FRONT, LINGERING  
THROUGH THE END OF THE PERIOD AND HELPING TO KEEP THE EXTENDED  
PERIOD LARGELY DRY. OPERATIONAL AND ENSEMBLE GUIDANCE ALL POINT TO  
THE LINGERING SURFACE HIGH AND PERSISTENT NORTHERLY FLOW KEEPING THE  
AREA IN BELOW NORMAL TEMPERATURES THROUGH THE END OF THE PERIOD. THE  
CIPS ANALOGS FROM ST. LOUIS UNIVERSITY POINT TO BELOW NORMAL  
TEMPERATURES STICKING AROUND UNTIL THE END OF NEXT WEEK.  
 
MRM  
 

 
   
AVIATION
 
(FOR THE 18Z TAFS THROUGH 18Z FRIDAY AFTERNOON)  
ISSUED AT 1205 PM CDT THU AUG 13 2020  
 
SLOW MOVING THUNDERSTORMS HAVE FORMED ALONG A STATIONARY FRONT  
DRAPED THROUGH THE EASTERN HALF OF MISSOURI LATE THIS MORNING.  
MORE STORMS ARE ANTICIPATED TO FORM ALONG THIS BOUNDARY AND ARE  
EXPECTED TO IMPACT THE ST. LOUIS METRO TERMINALS, INCLUDING KSTL.  
UNCERTAINTY STILL EXISTS AS TO WHERE INDIVIDUAL THUNDERSTORMS WILL SET  
UP, HOWEVER CONFIDENCE IS HIGH THAT WHERE THUNDERSTORMS FORM,  
HEAVY RAIN WILL REDUCE VISIBILITIES TO IFR FLIGHT CONDITIONS AND  
COULD PRODUCE FLASH FLOODING. GIVEN THIS, HAVE INCLUDED HEAVY  
THUNDERSTORMS AND 3SM IN A TEMPO GROUP FOR THE STL METRO  
TERMINALS, INCLUDING KSTL.  
 
KUIN AND KCOU CAN EXPECT VFR FLIGHT CONDITIONS THIS AFTERNOON AND  
OVERNIGHT BEFORE THE POTENTIAL FOR FOG FORMATION TOMORROW MORNING.  
FOG IS LIKELY ACROSS THE AREA, PARTICULARLY THOSE THAT HAVE SEEN  
SIGNIFICANT RAINFALL IN THE PAST COUPLE OF DAYS. HAVE INCLUDED  
MVFR FOG AT KCOU AND KUIN WHERE HEAVY RAIN HAS NOT FALLEN  
RECENTLY, AND IFR FOG AT KSUS AND KCPS, WHICH ARE MORE MOIST AND  
MORE SUSCEPTIBLE TO FOG DEVELOPMENT. HAVE NOT INCLUDED FOG IN THE  
KSTL TAF YET, AS IT TENDS TO DEVELOP FOG LATER THAN THE OTHER  
TERMINALS, BUT WILL REEVALUATE AS NEEDED.  
 
MRM  
 

 
   
LSX WATCHES/WARNINGS/ADVISORIES
 
 
MO...FLASH FLOOD WATCH UNTIL 7 PM CDT THIS EVENING FOR CRAWFORD MO-  
FRANKLIN MO-IRON MO-JEFFERSON MO-LINCOLN MO-MADISON MO-  
REYNOLDS MO-SAINT CHARLES MO-SAINT FRANCOIS MO-SAINT LOUIS  
CITY MO-SAINT LOUIS MO-SAINTE GENEVIEVE MO-WARREN MO-  
WASHINGTON MO.  
 
IL...FLASH FLOOD WATCH UNTIL 7 PM CDT THIS EVENING FOR BOND IL-  
CALHOUN IL-CLINTON IL-FAYETTE IL-GREENE IL-JERSEY IL-  
MACOUPIN IL-MADISON IL-MARION IL-MONROE IL-MONTGOMERY IL-  
RANDOLPH IL-SAINT CLAIR IL-WASHINGTON IL.  
 

 
 

 
 
WFO LSX  
 
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