622  
FXUS63 KLSX 201211  
AFDLSX  
 
AREA FORECAST DISCUSSION...UPDATED AVIATION  
NATIONAL WEATHER SERVICE SAINT LOUIS MO  
711 AM CDT MON AUG 20 2018  
   
SHORT TERM
 
(THROUGH LATE TONIGHT)  
ISSUED AT 350 AM CDT MON AUG 20 2018  
 
THE CURRENT ACRHING BAND OF SHOWERS AND THUNDERSTORMS CENTERED  
ACROSS WESTERN IL IS OCCURING IN ASSOCIATION WITH LOW LEVEL  
MOISTURE CONVERGENCE VIA A SOUTHWESTERLY LLJ ALONG WITH A VORT MAX  
ROTATING THRU EASTERN MO. PRESENT INDICATIONS ARE THE BAND ALONG  
WITH SOME TRAILING MORE SCATTERED TYPE ACTIVITY WILL CONTINUE TO  
MOVE NORTHEAST AS THE VORT MAX ROTATES NORTHEAST AND THE LLJ  
VEERS, PLACING THE BULK OF THE ACTIVITY JUST EAST OF THE CWA BY  
12Z. THE LINGERING MORE SCATTERED SHOWERS/STORMS IN THE WAKE OF  
THE MAIN BAND SHOULD EXIT THE CWA BY MID-MORNING AS WELL, ALTHOUGH  
SCATTERED SHOWERS COULD CONTINUE IN NORTHEAST MO AND WEST CENTRAL  
IL OFF AND ON THIS MORNING.  
 
LIKE DISCUSSED YESTERDAY, I THINK FROM MID MORNING UNTIL EARLY  
AFTERNOON WE WILL BE IN A LULL WITH POSSIBLY SOME SPOTTY SHOWERS.  
STRATUS WILL OVERSPREAD CENTRAL AND NORTHEAST MO THIS MORNING,  
HOWEVER TO THE EAST OF THAT ACROSS EAST CENTRAL AND SOUTHEAST MO  
INTO WESTERN IL WE SHOULD SEE SOME DECENT DIURNAL HEATING. THIS  
SHOULD RESULT IN MODEST DESTABILIZATION WITH SBCAPE VALUES  
POSSIBLY AROUND 1500 J/KG BY EARLY AFTERNOON. PRESENT THINKING IS  
THAT LARGE SCALE ASCENT ASSOCIATED WITH THE MIGRATORY UPPER TROF  
ALONG WITH LIFT ALONG/AHEAD OF THE ADVANCING COLD FRONT WILL  
RESULT IN THE DEVELOPMENT OF SCATTERED THUNDERSTORMS BY 19-20Z.  
THE MAJORITY OF THE MODELS ARE SHOWING AN AXIS OF MORE PRONOUNCED  
CONVERGENCE FROM ST. LOUIS TO STEELVILLE AND IN THIS REGION THE  
COVERAGE OF STORMS SHOULD BE A BIT GREATER, EVOLVING INTO A BROKEN  
LINE AND MOVING EAST THROUGH THE MID-LATE AFTERNOON ACROSS  
SOUTHEAST MO AND SOUTHWEST IL. DEEP LAYER SHEAR ON THE ORDER OF 35  
KTS WOULD SUPPORT BOTH MULTICELL AND SHORT-LIVED SUPERCELL MODES,  
WITH A PREDOMINANT LARGE HAIL AND DAMAGING WIND THREAT. THE  
BROKEN LINE OF STORMS SHOULD BE LOCATED ACROSS THE FAR EASTERN CWA  
AT 00Z, AND EXITING EAST BY MID-EVENING WITH SCATTERED COVERAGE  
OF SHOWERS AND STORMS FURTHER WEST. THE THREAT OF STORMS WILL END  
WITH WANING INSTABILITY BY MID-LATE EVENING AND WITH THE PASSAGE  
OF THE COLD FRONT. STRATUS WILL THEN OVERSPREAD THE AREA THROUGH  
THE REMAINDER OF TONIGHT WITH SOME SPOTTY SHOWERS IN THE POST-  
FRONTAL/POST SURFACE LOW CYCLONIC FLOW.  
 
GLASS  
   
LONG TERM
 
(TUESDAY THROUGH SUNDAY)  
ISSUED AT 350 AM CDT MON AUG 20 2018  
 
COOLER WEATHER IN GENERAL IS EXPECTED ON TUESDAY AND IT STILL  
APPEARS THERE WILL BE AN EXTENSIVE AMOUNT OF CLOUD COVER, ESPECIALLY  
IN THE MORNING. CYCLONIC FLOW WILL DOMINATE IN THE WAKE OF THE  
DEPARTING SYSTEM ALONG WITH HIGH LEVELS OF 925-850 MB RH, SUGGESTING  
THAT STRATUS/STRATOCU WILL BE PROMINENT. THERE COULD ALSO BE SOME  
LINGERING SPOTTY LIGHT SHOWERS ACROSS THE EASTERN PART OF THE  
CWA.  
 
BELOW NORMAL TEMPERATURES STILL LOOK ON TRACK FOR WEDNESDAY AND  
THURSDAY AS AN IMPRESSIVE 1020+ MB SURFACE HIGH PRESSURE SYSTEM  
DOMINATES THE SENSIBLE WEATHER. THE PATTERN ALOFT HOWEVER WILL  
BEGIN A TRANSITION TO MORE PROGRESSIVE LATE THURSDAY INTO FRIDAY.  
THE COMBINATION OF A MIGRATORY TROF TRANSLATING ACROSS THE  
MID-UPPER MS VALLEY AND THE DEVELOPMENT OF RETURN FLOW AND  
SOUTHWESTERLY LLJ IN THE WAKE OF THE RETREATING ANTICYCLONE, WILL  
RESULT SHOWERS AND THUNDERSTORMS LATE THURSDAY NIGHT THROUGH  
FRIDAY INTO FRIDAY NIGHT. THE RETURN OF ABOVE AVERAGE TEMPS/SUMMER  
HEAT STILL LOOKS ON TRACK FOR THE WEEKEND AS HEIGHTS BUILD ALOFT  
ACCOMPANYING THE DEVELOPMENT OF AN EXPANSIVE UPPER HIGH/RIDGE  
CENTERED IN THE LOWER MS VALLEY.  
 
GLASS  
 

 
   
AVIATION
 
(FOR THE 12Z TAFS THROUGH 12Z TUESDAY MORNING)  
ISSUED AT 641 AM CDT MON AUG 20 2018  
 
MVFR FLIGHT CONDITIONS/CEILINGS HAVE OVERSPREAD MUCH OF THE AREA  
THIS MORNING. THERE WILL BE SOME IMPROVEMENT FOR EAST CENTRAL AND  
SOUTHEAST MO AND SOUTHWEST IL BY EARLY THIS AFTERNOON, WHICH  
INCLUDES THE ST. LOUIS REGION TERMINALS. ELSEWHERE I THINK THAT  
MVFR FLIGHT CONDITIONS WILL GENERALLY CONTINUE. PRECIPITATION WILL  
BE AT A MINIMUM THIS MORNING, SPOTTY SHOWERS PERHAPS, BUT BY  
EARLY AFTERNOON WE WILL SEE SCATTERED SHOWERS AND THUNDERSTORMS  
AHEAD OF AN ADVANCING COLD FRONT AND LOW PRESSURE SYSTEM. THE ST.  
LOUIS TERMINALS AND POINTS SOUTH AND EAST WILL HAVE THE HIGH  
PROBABILITY. THE LOW PRESSURE SYSTEM AND COLD FRONT WILL PASS TO  
THE EAST OF THE AREA DURING THE EVENING WITH LOWERING MVFR  
CEILINGS OVERSPREADING THE ENTIRE REGION IN ITS WAKE. GUSTY  
SOUTH-SOUTHWEST SURFACE WINDS ARE EXPECTED TODAY, WITH WINDS  
SHIFTING TO THE WEST AND NORTHWEST BEHIND THE FRONT TONIGHT.  
 
SPECIFICS FOR KSTL:  
 
MVFR FLIGHT CONDITIONS/CEILINGS HAVE OVERSPREAD THE AREA EARLY THIS  
MORNING. THEY SHOULD PERSIST THROUGH THE MORNING WITH IMPROVEMENT  
TO VFR BY EARLY THIS AFTERNOON. PRECIPITATION WILL BE AT A MINIMUM  
THIS MORNING, A SPOT SHOWER PERHAPS, BUT BY 19-20Z WE WILL SEE  
SCATTERED SHOWERS AND THUNDERSTORMS AHEAD OF AN ADVANCING COLD  
FRONT IMPACT THE TERMINAL. THE LOW PRESSURE SYSTEM AND COLD FRONT  
WILL PASS TO THE EAST OF THE AREA DURING THE EVENING WITH LOWERING  
MVFR CEILINGS SPREADING INTO THE TERMINAL OVERNIGHT. GUSTY SOUTH-  
SOUTHWEST SURFACE WINDS ARE EXPECTED TODAY, WITH WINDS SHIFTING TO  
THE WEST BEHIND THE FRONT LATER TONIGHT.  
 
GLASS  
 

 
   
LSX WATCHES/WARNINGS/ADVISORIES
 
 
MO...NONE.  
IL...NONE.  

 
 

 
 
WFO LSX  
 
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