905  
FXUS63 KLSX 212352  
AFDLSX  
 
AREA FORECAST DISCUSSION...UPDATED AVIATION  
NATIONAL WEATHER SERVICE SAINT LOUIS MO  
552 PM CST WED FEB 21 2018  
   
SHORT TERM
 
(THROUGH LATE THURSDAY AFTERNOON)  
ISSUED AT 331 PM CST WED FEB 21 2018  
 
FREEZING RAIN CONTINUES THIS AFTERNOON ACROSS PARTS OF SOUTHEAST MO.  
PRECIPITATION WILL SPREAD THROUGH SOUTHEAST MO AND SOUTHWEST IL THIS  
EVENING AS ANOTHER SURFACE WAVE MOVES NORTHEASTWARD ALONG THE  
STATIONARY FRONT THAT EXTENDS FROM LA NORTHEAST THROUGH EASTERN KY.  
THE PRECIPITATION SHOULD MOVE THROUGH THE ST LOUIS METRO AREA LATE  
THIS EVENING. THIS PRECIPITATION SHOULD SHIFT EAST OF OUR FORECAST  
AREA BY 12Z THURSDAY. THE PRECIPITATION TYPE SHOULD BE INITIALLY  
FREEZING RAIN, BUT MAY CHANGE TO LIQUID RAIN LATE TONIGHT AS THE  
SURFACE TEMPERATURE GRADUALLY WARMS ABOVE THE FREEZING MARK. ANOTHER  
AREA OF PRECIPITATION WILL MOVE NORTHEASTWARD THROUGH MAINLY  
NORTHEAST MO AND WEST CENTRAL IL LATE TONIGHT AND THURSDAY MORNING  
IN A REGION OF LOW-MID LEVEL WARM AIR ADVECTION AHEAD OF A SOUTHWEST  
FLOW SHORTWAVE. WITH AN ELEVATED WARM LAYER OVER THE SUBFREEZING  
SURFACE LAYER THE PRECIPITATION TYPE MAY BE INITIALLY A MIX OF SNOW  
AND SLEET, BUT WILL CHANGE OVER TO FREEZING RAIN. SURFACE  
TEMPERATURES IN THIS AREA WILL NOT WARM ABOVE THE FREEZING MARK  
UNTIL LATE THURSDAY MORNING. WILL CONTINUE THE WINTER WEATHER  
ADVISORY THIS EVENING FOR CENTRAL AND SOUTHEAST MO INTO PARTS OF  
SOUTHWEST IL, THEN FOR MOST OF OUR FORECAST AREA FOR LATE TONIGHT  
AND THURSDAY MORNING. DID NOT SEE MUCH PRECIPITATION LEFT FOR LATE  
THURSDAY MORNING AND AFTERNOON, PLUS BY THIS TIME THE TEMPERATURES  
ACROSS THE ENTIRE AREA SHOULD RISE ABOVE 32 DEGREES. MUCH OF THE  
FORECAST AREA WILL RECEIVE A GLAZING OF ICE ACCUMULATION, POSSIBLY  
UP TO ONE TENTH INCH IN ISOLATED SPOTS. NORTHEAST MO MAY PICK UP A  
LIGHT DUSTING OF SNOW AND SLEET LATE TONIGHT.  
 
GKS  
   
LONG TERM
 
(THURSDAY NIGHT THROUGH NEXT WEDNESDAY)  
ISSUED AT 331 PM CST WED FEB 21 2018  
 
(THURSDAY NIGHT - SATURDAY NIGHT)  
 
VERY ACTIVE 48 HOURS STILL ON TAP FROM THURSDAY NIGHT THROUGH  
SATURDAY NIGHT WITH MULTIPLE ROUNDS OF RAINFALL AND INCREASING  
CHANCES OF THUNDERSTORMS FRIDAY NIGHT AND ESPECIALLY ON SATURDAY.  
FIRST WAVE OF RAIN STILL LOOKS TO BE THURSDAY NIGHT AS AN UPPER-  
LEVEL DISTURBANCE INTERACTS WITH QUASI-STATIONARY FRONT TO THE SOUTH  
AND EAST OF THE AREA. STILL APPEARS TO BE A RATHER QUICK HIT OF  
RAINFALL ACROSS MUCH OF THE AREA WITH HEAVIEST AMOUNTS FOCUSED  
ACROSS PORTIONS OF SOUTHEAST MISSOURI AND SOUTHWEST ILLINOIS CLOSE  
TO THE FRONTAL BOUNDARY. FAIRLY BROAD/WEAK 850-HPA LOW-LEVEL JET AND  
A LACK OF ELEVATED INSTABILITY SUGGEST RAINFALL RATES SHOULD STAY  
ON THE LIGHT TO MODERATE SIDE. CONTINUE TO BELIEVE THIS PARTICULAR  
ROUND OF RAINFALL WILL LIKELY NOT CAUSE TOO MUCH IN THE WAY OF  
PROBLEMS, BUT WILL LIKELY CONTINUE TO SATURATE SOILS AND FURTHER  
ELEVATE SOME AREA RIVERS AND STREAMS. TEMPERATURES THURSDAY NIGHT  
ARE EXPECTED TO STAY ABOVE FREEZING ACROSS THE ENTIRE AREA, SO NOT  
EXPECTING ANY FREEZING RAIN.  
 
SOME GOOD NEWS ON FRIDAY INTO FRIDAY EVENING AS BI-STATE AREA SHOULD  
BE DRY BEHIND DEPARTING SHORTWAVE TROUGH. SURFACE FRONTAL BOUNDARY  
WILL ALSO SLIDE BACK SOUTHEASTWARD INTO THE LOWER MISSISSIPPI AND  
OHIO RIVER VALLEYS WITH SURFACE HIGH PRESSURE ACROSS THE NORTHERN  
PLAINS EXERTING ITS INFLUENCE INTO THE AREA. HIGHS ON FRIDAY WILL BE  
ON THE MILD SIDE FOR FEBRUARY IN THE 40S AND 40S.  
 
LOW-LEVEL WARM/MOIST ADVECTION INTENSIFIES ACROSS THE AREA FRIDAY  
NIGHT AS A STRONGER SHORTWAVE TROUGH BEGINS TO EJECT OUT OF THE  
ROCKY MOUNTAINS. WIDESPREAD RAINFALL IS EXPECTED ACROSS THE BI-STATE  
REGION NORTH OF THE DEVELOPING WARM FRONT. SOME ELEVATED INSTABILITY  
SO CONTINUE TO MENTION THUNDER CHANCES ACROSS SOUTHERN PORTIONS OF  
THE FORECAST AREA. STRONGER LOW-LEVEL FORCING, PRESENCE OF WEAK  
INSTABILITY ALOFT, DEEP WARM CLOUD DEPTHS, AND PRECIPITABLE WATER  
VALUES 200-300 PERCENT OF NORMAL FOR LATE FEBRUARY SUGGEST THE  
POTENTIAL FOR HEAVY RAINFALL RATES. AREA OF BIGGEST CONCERN FOR  
FRIDAY NIGHT AND INTO SATURDAY WILL LIKELY BE ON PORTIONS OF  
SOUTHEAST MISSOURI AND SOUTHWEST ILLINOIS WHERE 2-3 INCHES OF RAIN  
ARE FORECAST THURSDAY NIGHT THROUGH SATURDAY EVENING.  
 
ATTENTION THEN ON SATURDAY AFTERNOON AND EARLY EVENING WILL BE HOW  
FAR NORTH WARM FRONT REACHES AND TRACK OF SURFACE LOW ACROSS THE  
MISSOURI VALLEY. MODELS HAVE TRENDED A BIT SLOWER AND FURTHER  
NORTHWESTWARD WITH TRACK OF AFOREMENTIONED SURFACE LOW. INCREASED  
PROBABILITY OF THUNDERSTORMS ACROSS MUCH OF CWA AS APPEARS WARM  
FRONT WILL LIFT TO NEAR CENTRAL PORTIONS OF THE AREA BY MID  
AFTERNOON ON SATURDAY. VERY STRONG DEEP-LAYER SHEAR STRONG DYNAMICS  
ALOFT SUGGEST AN INCREASING THREAT FOR SEVERE WEATHER WITHIN THE  
WARM SECTOR. MAJOR UNKNOWN IS HOW FAR NORTH WARM FRONT GETS AND HOW  
QUICKLY AS WELL AS WIDESPREAD RAINFALL FRIDAY NIGHT AND EARLY  
SATURDAY MORNING MAY SLOW ITS NORTHWARD PROGRESS. IN ADDITION,  
DEGREE OF DESTABILIZATION WITHIN THE NORTHERN PORTION OF THE WARM  
SECTOR IS UNKNOWN DUE TO FORECAST LOW CLOUD COVER AND POTENTIAL  
SHOWER/THUNDERSTORM ACTIVITY CONTINUING THROUGH THE DAY ON SATURDAY.  
WILL HAVE TO KEEP A CLOSE EYE ON THIS PERIOD MOVING FORWARD GIVEN  
THE VERY STRONG DYNAMICS. PROBABLY WILL NOT TAKE TOO MUCH  
INSTABILITY TO POTENTIALLY YIELD SEVERE THUNDERSTORMS.  
 
(SUNDAY - NEXT WEDNESDAY)  
 
BROAD AREA OF SURFACE HIGH PRESSURE WILL FILTER INTO THE REGION ON  
SUNDAY THROUGH MONDAY. EXPECTING A WELCOME BREAK FROM THE RAIN ALONG  
WITH TEMPERATURES REMAINING SLIGHTLY ABOVE NORMAL DAY AND NIGHT.  
 
THE NEXT CHANCE OF RAIN APPEARS TO BE ON TUESDAY NIGHT AND NEXT  
WEDNESDAY AS ANOTHER UPPER-LEVEL DISTURBANCE EJECTS OUT OF THE  
DESERT SOUTHWEST.  
 
GOSSELIN  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING)  
ISSUED AT 550 PM CST WED FEB 21 2018  
 
DETERIORATING CONDITIONS WILL SPREAD BACK INTO THE TAF SITES THIS  
EVENING. BEGINNING WITH FZDZ AND IFR CIGS AT STL METRO SITES AND  
THIS WILL EXPAND NORTH AND WEST OVERNIGHT AND THURSDAY MORNING.  
LOOK FOR IFR CONDITIONS TO PREVAIL FOR MUCH OF THE DAY ON THURSDAY  
AND INTO THURSDAY NIGHT. FREEZING PCPN WILL TRANSITION TO ALL RAIN  
LATE TONIGHT OR THURSDAY MORNING, AS WARMER TEMPS BUILD BACK IN.  
NE SURFACE WINDS WILL VEER E AND SE BY THURSDAY NIGHT.  
 
SPECIFICS FOR KSTL: COULD TOGGLE SOME BETWEEN MVFR AND IFR CIGS  
BUT BELIEVE IFR WILL EVENTUALLY WIN OUT AND PREVAIL FOR MUCH OF  
THE VALID PERIOD. PCPN ALREADY IN PLACE WILL BECOME -RA OVERNIGHT  
AS WARMER AIR BUILDS IN AND WILL LESSEN OR BE MORE INTERMITTENT  
AFTER SUNRISE.  
 
TES  
 

 
   
LSX WATCHES/WARNINGS/ADVISORIES
 
 
MO...WINTER WEATHER ADVISORY UNTIL NOON CST THURSDAY FOR CRAWFORD MO-  
IRON MO-MADISON MO-REYNOLDS MO-SAINT FRANCOIS MO-SAINTE  
GENEVIEVE MO-WASHINGTON MO.  
 
WINTER WEATHER ADVISORY UNTIL NOON CST THURSDAY FOR AUDRAIN MO-  
BOONE MO-CALLAWAY MO-COLE MO-FRANKLIN MO-GASCONADE MO-  
JEFFERSON MO-LINCOLN MO-MONITEAU MO-MONTGOMERY MO-OSAGE MO-  
SAINT CHARLES MO-SAINT LOUIS CITY MO-SAINT LOUIS MO-WARREN  
MO.  
 
WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO NOON CST  
THURSDAY FOR KNOX MO-LEWIS MO-MARION MO-MONROE MO-PIKE MO-  
RALLS MO-SHELBY MO.  
 
IL...WINTER WEATHER ADVISORY UNTIL NOON CST THURSDAY FOR RANDOLPH IL.  
 
WINTER WEATHER ADVISORY UNTIL NOON CST THURSDAY FOR CALHOUN IL-  
GREENE IL-JERSEY IL-MACOUPIN IL-MADISON IL-MONROE IL-  
MONTGOMERY IL-SAINT CLAIR IL.  
 
WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO NOON CST  
THURSDAY FOR ADAMS IL-BROWN IL-PIKE IL.  
 

 
 

 
 
WFO LSX  
 
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