450  
FXUS63 KLSX 270901  
AFDLSX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAINT LOUIS MO  
401 AM CDT MON MAR 27 2017  
   
SHORT TERM
 
(THROUGH LATE TONIGHT)  
ISSUED AT 400 AM CDT MON MAR 27 2017  
 
A LOW PRESSURE SYSTEM WILL MOVE THROUGH SOUTHERN MO/IL TODAY,  
BRINGING PERIODS OF SHRA/TSRA TO THE AREA. LLJ INTERACTIONS WITH THE  
SYSTEM'S LIFTING WARM FRONT HAD ALREADY PRODUCED A COUPLE OF MCSS  
EARLY THIS MORNING ACROSS OK/KS/MO. PRECIPITATION CHANCES WILL SHIFT  
EASTWARD THIS MORNING AS THE SURFACE LOW APPROACHES THE REGION. THE  
SYSTEM'S WARM FRONT CONTINUES LIFTING TODAY AND EVENTUALLY BISECTS  
MO/IL FROM SW TO NE, PUTTING THE SOUTHEASTERN HALF OF THE LSX CWA  
WITHIN THE WARM SECTOR BY AFTERNOON. SHRA/TSRA ARE EXPECTED TO  
REDEVELOP OR INCREASE IN COVERAGE THIS AFTERNOON, AND A FEW STRONG  
TO SEVERE THUNDERSTORMS WILL BE POSSIBLE BASED ON INSTABILITY AND  
SHEAR PROFILES. NEITHER INSTABILITY (~1000 J/KG) NOR SHEAR (~30KTS)  
LOOKS UNUSUALLY STRONG, AND WITH H7-H5 LAPSE RATES OF ~6.5-7 DEG  
C/KM, THE PRIMARY SEVERE WX THREAT APPEARS TO BE LARGE HAIL AND  
DAMAGING WIND GUSTS. THE SEVERE WEATHER THREAT IS GREATER FOR AREAS  
FARTHER SOUTHEAST OF THE LSX CWA WHERE BOTH INSTABILITY AND SHEAR  
VALUES ARE GREATER. PRECIPITATION SHOULD BE ENDING BY 03-06Z. THE  
WARM FRONT WILL SINK SOUTHWARD AS A COLD FRONT TONIGHT IN THE WAKE  
OF THE DEPARTING LOW PRESSURE SYSTEM.  
 
KANOFSKY  
   
LONG TERM
 
(TUESDAY THROUGH SUNDAY)  
ISSUED AT 400 AM CDT MON MAR 27 2017  
 
THE SYNOPTIC PATTERN AT THE START OF THE LONG TERM PERIOD WILL  
CONTINUE TO FEATURE AN ACTIVE FLOW REGIME DUE TO AN ACTIVE PACIFIC  
JET. THIS ACTIVE PATTERN WILL BRING TWO SYSTEMS OF NOTE INTO THE  
REGION THROUGH THE PERIOD, ONE WED/THUR AND ANOTHER ONE PERHAPS  
SATURDAY NIGHT INTO SUNDAY.  
 
SHORTWAVE RIDGING WILL DOMINATE ON TUESDAY AS THE MIDWEST LIES  
BETWEEN A TROUGH DEPARTING TO THE EAST AND THE NEXT ONE APPROACHING  
FROM THE SOUTHWEST. DESPITE THE RIDGING ALOFT, ENOUGH LOW-LEVEL  
MOISTURE TRAPPED BENEATH AN INVERSION WILL LEAD TO MOSTLY CLOUDY  
SKIES AND TEMPERATURES IN THE UPPER 50S TO LOW 60S.  
 
THE MAIN FOCUS OF THE PERIOD WILL BE ON THE SYSTEM WHICH WILL BEGIN  
TO IMPACT THE REGION AS EARLY AS TUESDAY NIGHT AND LINGER INTO  
THURSDAY NIGHT. AS THE UPPER-LEVEL LOW EJECTS OUT INTO THE CENTRAL  
PLAINS, A LOW-LEVEL JET OF 30-40 KNOTS WILL FOCUS INTO PORTIONS OF  
CENTRAL AND EASTERN MO. THE ASSOCIATED ISENTROPIC ASCENT WILL BE  
ENOUGH FOR A BAND OF SHOWERS TUESDAY NIGHT INTO WEDNESDAY.  
 
THINGS BECOME MORE INTERESTING WEDNESDAY NIGHT INTO THURSDAY AS THE  
UPPER-LEVEL LOW (NOW BECOMING MORE VERTICALLY STACKED WITH THE  
SURFACE REFLECTION) EJECTS CLOSER TO THE MIDWEST. A STRONGER PV  
ANOMALY WILL ROUND THE BASE OF THE UPPER-LEVEL LOW, WHICH WILL HELP  
TO ENHANCE LARGE-SCALE ASCENT AND ONCE AGAIN LEAD TO THE DEVELOPMENT  
OF A LOW-LEVEL JET, THIS TIME ON THE ORDER OF 40-50+ KNOTS. THIS  
LOW-LEVEL FLOW WILL HAVE AN OPEN GOFMEX TO WORK WITH LEADING TO  
EFFICIENT MOISTURE TRANSPORT INTO MO AND WESTERN IL. GIVEN THE  
INCREASING DYNAMICAL FORCING, IT IS LIKELY A LINE OF SHOWERS/STORMS  
WILL DEVELOP OVER EASTERN OK AND QUICKLY RACE EASTWARD WEDNESDAY  
NIGHT. THE SOUTHERN PART OF THE LSX CWA WILL LIKELY MARK THE  
NORTHERN EXTENT OF THE BETTER SURFACE-BASED INSTABILITY AHEAD OF  
THIS CONVECTIVE LINE. SPC HAS PLACED AREAS SOUTH OF I-70 IN A  
SLIGHT RISK WHERE IT APPEARS STORMS COULD REMAIN SURFACE-BASED INTO  
EARLY THURSDAY MORNING. THERE IS SOME UNCERTAINTY WITH THE NORTHERN  
EXTENT OF THIS SEVERE THREAT, WHICH WILL LIKELY BE DRIVEN BY LOW-  
LEVEL MOISTURE ADVECTION (I.E. HOW FAR NORTH THE RICHER SURFACE  
DEWPOINTS CAN GET) AHEAD OF THE LINE GIVEN FAIRLY POOR LAPSE RATES  
ALOFT. STAY TUNED.  
 
ON THURSDAY, THE STACKED LOW PRESSURE SYSTEM WILL MOVE THROUGH THE  
HEART OF MISSOURI. THE LATEST GUIDANCE IS IN MUCH BETTER AGREEMENT  
WITH THIS SCENARIO THAN PAST NIGHTS, THUS CONFIDENCE IS INCREASING  
IN ADDITIONAL SHOWERS/STORMS THURSDAY AFTERNOON INTO THURSDAY  
EVENING AHEAD OF THE SURFACE LOW BENEATH THE COLD CORE OF THIS  
SYSTEM. IN FACT, THIS SYNOPTIC SETUP IS FAIRLY SIMILAR TO WHAT WE  
EXPERIENCED THIS PAST SATURDAY. WILL ONCE AGAIN HAVE TO KEEP AN EYE  
ON THERMODYNAMICAL TRENDS, AS SUFFICIENT INSTABILITY DEVELOPING IN  
THE HEAT OF THE DAY AHEAD OF THE SURFACE LOW COULD SUPPORT ANOTHER  
SEVERE THREAT DURING THE DAY ON THURSDAY.  
 
THE UPPER-LEVEL LOW WILL FINALLY PUSH EAST THURSDAY NIGHT, LEAVING  
MUCH OF FRIDAY AND THE FIRST PART OF THE UPCOMING WEEKEND DRY. THE  
SECOND SYSTEM WILL THEN APPROACH SUNDAY INTO MONDAY, ALTHOUGH  
UNCERTAINTY IS HIGH GIVEN FAIRLY LARGE MODEL SPREAD. WILL CONTINUE  
WITH AN ENSEMBLE APPROACH FOR NOW UNTIL TRENDS BECOME MORE CLEAR.  
 
KD  
 

 
   
AVIATION
 
(FOR THE 06Z TAFS THROUGH 06Z MONDAY NIGHT)  
ISSUED AT 1045 PM CDT SUN MAR 26 2017  
 
SHOWERS AND THUNDERSTORMS ACROSS EASTERN PORTIONS OF KS AND OK  
AND WEST CENTRAL AND SOUTHWEST MO, AHEAD OF AN APPROACHING UPPER  
LEVEL AND SURFACE LOW WILL SPREAD INTO THE TAF SITES LATE TONIGHT  
AND EARLY MONDAY MORNING. THE CLOUD CEILING WILL DROP DOWN INTO  
THE IFR CATAGORY AS THE LOW LEVELS OF THE ATMOSPHERE SATURATE. THE  
SURFACE WIND WILL BECOME EASTERLY AS THE SURFACE LOW APPROACHES  
SOUTHWEST MO LATE TONIGHT, THEN BACK AROUND TO A NORTHWEST  
DIRECTION MONDAY AFTERNOON AS THE SURFACE LOW MOVES EASTWARD  
THROUGH SOUTHERN MO. THE SHOWERS AND STORMS SHOULD SHIFT SOUTHEAST  
OF THE TAF SITES BY LATE AFTERNOON OR EARLY MONDAY EVENING WITH  
IMPROVEMENT OF THE CLOUD CEILING INTO THE MVFR CATAGORY.  
 
SPECIFICS FOR KSTL: SHOWERS AND THUNDERSTORMS ACROSS EASTERN PORTIONS  
OF KS AND OK AND WEST CENTRAL AND SOUTHWEST MO, AHEAD OF AN  
APPROACHING UPPER LEVEL AND SURFACE LOW WILL SPREAD INTO THE STL  
AREA EARLY MONDAY MORNING. THE CLOUD CEILING WILL DROP DOWN INTO  
THE IFR CATAGORY AS THE LOW LEVELS OF THE ATMOSPHERE SATURATE. THE  
SURFACE WIND WILL BECOME EASTERLY AS THE SURFACE LOW APPROACHES  
SOUTHWEST MO LATE TONIGHT, THEN BACK AROUND TO A NORTHWEST  
DIRECTION MONDAY AFTERNOON AS THE SURFACE LOW MOVES EASTWARD  
THROUGH SOUTHERN MO. THE SHOWERS AND STORMS SHOULD SHIFT SOUTHEAST  
OF THE STL AREA BY EARLY MONDAY EVENING WITH IMPROVEMENT OF THE  
CLOUD CEILING INTO THE MVFR CATAGORY.  
 
GKS  
 

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

 
 

 
 
WFO LSX  
 
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