015  
FXUS63 KLSX 170521  
AFDLSX  
 
AREA FORECAST DISCUSSION...UPDATED AVIATION  
NATIONAL WEATHER SERVICE SAINT LOUIS MO  
1221 AM CDT SAT APR 17 2021  
   
SHORT TERM
 
(THROUGH SUNDAY AFTERNOON NIGHT)  
ISSUED AT 255 PM CDT FRI APR 16 2021  
 
KEY MESSAGES THROUGH THE SHORT-TERM PERIOD:  
 
1) WIDESPREAD LIGHT RAIN IS STILL EXPECTED TO DEVELOP, STARTING IN  
CENTRAL MO AND SPREADING EAST THROUGH THE EVENING. TIMING WAS SLOWED  
SLIGHTLY IN LIGHT OF LATEST MODEL GUIDANCE.  
 
2) RAIN WILL BECOME LESS WIDESPREAD TOMORROW MORNING, BECOMING  
SHOWERY AND INTERMITTENT.  
 
3) ANOTHER ROUND OF RAIN IS EXPECTED SUNDAY, WITH A CHANCE FOR  
ISOLATED THUNDERSTORMS.  
 
HIGH CLOUDS CONTINUE TO OVERSPREAD THE MID-MISSISSIPPI VALLEY IN  
ADVANCE OF AN INVERTED SURFACE TROUGH, CURRENTLY ALONG THE KS/MO  
BORDER, SPARKING SHOWERS UP TO THE MO/IL BORDER. ALOFT, BROAD  
LONGWAVE TROUGHING EXISTS ACROSS MOST OF THE CONUS, WITH THE  
EXCEPTION OF WEAK RIDGING IN THE NORTH-CENTRAL U.S. IN BETWEEN TWO  
AMPLIFIED ULLS. THE PASSAGE OF TWO CLOSED ULLS WILL BE THE CATALYST  
FOR OUR SHORT-TERM PRECIPITATION CHANCES AND AN OVERALL CRUMMY  
WEEKEND.  
 
THE FIRST ROUND OF LIGHT PRECIPITATION WILL ACCOMPANY A WEAK  
SHORTWAVE PASSAGE LATER THIS AFTERNOON. THE LATEST HIGH-RESOLUTION  
ENSEMBLE GUIDANCE POINTS TO A SLOWER PRECIPITATION ONSET ACROSS THE  
AREA TONIGHT THAN IN PREVIOUS FORECASTS, AND DETERMINISTIC  
MODELS/CAMS ARE ALSO SUPPORTING THIS SCENARIO. THE LIMITING FACTORS  
FOR PRECIPITATION WILL BE A COMBINATION OF WEAK FORCING AND POOR  
MOISTURE AVAILABILITY. PVA ASSOCIATED WITH THE SHORTWAVE IS NOT VERY  
ROBUST ACCORDING TO THE DETERMINISTIC GUIDANCE, AND LOW-LEVEL  
FORCING IS EQUALLY UNIMPRESSIVE. ALSO, MODEL SOUNDINGS ARE  
CONSISTENTLY SHOWING A RELATIVELY DRY BOUNDARY LAYER THAT THE  
PRECIPITATION WILL NEED TO OVERCOME BEFORE REACHING THE GROUND. ALL  
THAT BEING SAID, CAMS AND DETERMINISTIC GUIDANCE DOES EVENTUALLY  
HAVE WIDESPREAD PRECIPITATION BEGINNING LATER THIS AFTERNOON IN  
CENTRAL MO. BY LATE THIS EVENING, LIGHT STRATIFORM RAIN IS EXPECTED  
ACROSS MOST OF MO AND COULD VERY WELL BE INTO WESTERN IL.  
 
BY TOMORROW MORNING, AS THE INVERTED TROUGH/LEADING SHORTWAVE PASS  
TO OUR EAST, PRECIPITATION SHOULD BECOME LESS WIDESPREAD. AMIDST THE  
BASE OF THE SHORTWAVE, LOW-LEVEL LAPSE RATES SHOULD START TO  
INCREASE WITH DIURNAL HEATING SATURDAY AFTERNOON. MARGINAL  
INSTABILITY, MAINLY SHALLOW IN NATURE, SHOULD REDEVELOP SHOWERS  
ACROSS THE AREA LATE SATURDAY MORNING THROUGH THE AFTERNOON. POPS  
WERE INCREASED WHERE AN AXIS OF NON-ZERO MUCAPE IS RESOLVED IN THE  
MODELS (MAINLY ALONG/NORTH OF I-70), BUT PRECIPITATION CHANCES EXIST  
AREAWIDE THROUGH THE EVENING SATURDAY. ANY PRECIPITATION WILL REMAIN  
SHOWERY AND INTERMITTENT IN THE ABSENCE OF ORGANIZATION OR STRONG  
DYNAMIC FORCING. WITH THE SHALLOW NATURE OF THE CONVECTION,  
THUNDERSTORMS ARE NOT EXPECTED TO DEVELOP. THE WIDESPREAD CLOUD  
COVER AND SHOWERS WILL HELP KEEP HIGH TEMPERATURES IN THE UPPER 50S  
AREAWIDE TOMORROW, WHICH IS AROUND 10F BELOW CLIMATOLOGY.  
 
THE SECOND ROUND OF PRECIPITATION WILL ACCOMPANY A MORE ROBUST  
CLOSED ULL THAT DROPS SOUTH INTO THE REGION LATE SATURDAY NIGHT.  
GUIDANCE CONTINUES TO STRUGGLE WITH PRODUCING RAIN WITH THIS  
SYSTEM, GIVEN THE LACK OF APPRECIABLE MOISTURE ASSOCIATED WITH THE  
SYSTEM. THE MORE ROBUST FORCING SHOULD PRODUCE ANOTHER ROUND OF  
LIGHT RAIN, DESPITE THE MOISTURE-STARVED SYSTEM. POPS WERE  
INCREASED AREAWIDE ON SUNDAY, BUT ANY RAIN THAT FALLS WILL AGAIN  
BE SHOWERY IN NATURE. LAPSE RATES IN THE LOW AND MID LEVELS ARE  
MORE IMPRESSIVE ON SUNDAY, AND MUCAPE VALUES AMONG THE  
DETERMINISTIC AND SHORT-RANGE ENSEMBLE GUIDANCE ARE SLIGHTLY  
HIGHER THAN SATURDAY'S SYSTEM. WHILE ISOLATED, THUNDERSTORMS  
CANNOT BE RULED OUT THROUGH THE DAY ON SUNDAY. AS THE SYSTEM  
DEPARTS TO THE EAST LATER SUNDAY AFTERNOON, PRECIPITATION CHANCES  
DIMINISH AS WELL.  
 
MRB  
   
LONG TERM
 
(SUNDAY NIGHT THROUGH NEXT FRIDAY)  
ISSUED AT 255 PM CDT FRI APR 16 2021  
 
KEY MESSAGES THROUGH THE EXTENDED PERIOD:  
 
1) PRECIPITATION RETURNS TO THE AREA TUESDAY WITH THE PASSAGE OF A  
COLD FRONT. EXACT TIMING OF THE FRONT IS STILL UNCERTAIN, WHICH WILL  
IMPACT PRECIPITATION TIMING, HIGH TEMPERATURES, AND PRECIPITATION  
TYPE.  
 
2) BEHIND THE COLD FRONT, TEMPERATURES NEAR 15F BELOW NORMAL ARE  
FORECAST. SUB-FREEZING TEMPERATURES WEDNESDAY MORNING INTRODUCE THE  
POTENTIAL FOR WIDESPREAD FROST.  
 
MONDAY WILL REMAIN DRY AMIDST ZONAL FLOW AND AN ABSENCE OF ANY  
FORCING, GIVING US A REPRIEVE FROM THE DREARY WEEKEND WEATHER.  
CLEARING SKIES AND SOUTHERLY WINDS AHEAD OF A COLD FRONT WILL BRING  
TEMPERATURES BACK TO THE NORMAL MID/UPPER 60S WE'D EXPECT FOR MID  
APRIL. HOWEVER, THE NICE WEATHER LOOKS TO BE SHORT-LIVED: A STRONG  
COLD FRONT LOOKS TO SAG SOUTH ACROSS THE REGION AND SPARK WIDESPREAD  
PRECIPITATION. WPC CLUSTER ANALYSIS SHOWS EXCELLENT AGREEMENT  
REGARDING THE OCCURRENCE OF PRECIPITATION ON TUESDAY, BUT DIFFERS ON  
THE TIMING. ONE CLUSTER, COMPRISED ALMOST EXCLUSIVELY OF EPS  
MEMBERS, IS SLOWER TO BRING THE FRONT THROUGH THAN THE OTHER THREE.  
POPS RIGHT NOW DO NOT EXCEED 55% TO ACCOUNT FOR THIS TIMING  
UNCERTAINTY, BUT THE CHANCE FOR MEASURABLE PRECIPITATION ON TUESDAY  
LOOKS HIGH. IF THE SLOWER SOLUTION IS REALIZED, HIGH TEMPERATURES  
WOULD BE WARMER ON TUESDAY THAN CURRENTLY FORECAST. RIGHT NOW,  
GUIDANCE POINTS TO A QUICKER FRONTAL PASSAGE, THUS TUESDAY'S HIGH  
TEMPERATURES WERE LOWERED USING OUR CURRENT FORECAST AND THE 25TH  
PERCENTILE OF THE NATIONAL BLEND OF MODELS.  
 
ANOTHER AREA OF UNCERTAINTY SURROUNDING THE FRONTAL PASSAGE IS THE  
POTENTIAL FOR WINTRY PRECIPITATION. DETERMINISTIC GUIDANCE IS  
SUGGESTING THAT THE PRECIPITATION WITH THIS FRONT WILL BE LARGELY  
ANAFRONTAL, WHICH INTRODUCES A NARROW WINDOW OF TIME FOR  
PRECIPITATION TO INTERACT WITH MUCH COLDER AIR. SAY IT ISN'T SO, BUT  
THE POTENTIAL DOES EXIST FOR SNOWFLAKES TO MIX WITH RAIN EARLY  
TUESDAY MORNING. WHILE DETERMINISTIC GUIDANCE, PARTICULARLY THE GFS,  
IS BULLISH IN BRINGING MEASURABLE SNOW AS FAR SOUTH AS ST. LOUIS,  
THAT WILL BE QUITE DIFFICULT AND ENSEMBLE GUIDANCE DOES NOT SUPPORT  
IT. THE BEST CHANCES FOR ANY FROZEN PRECIPITATION EXIST IN FAR  
NORTHEAST AND CENTRAL MO, WHERE PRECIPITATION WILL HAVE LONGER  
RESIDENCE TIME IN SUFFICIENTLY-COLD AIR. AS OF NOW, NO SNOW  
ACCUMULATIONS ARE FORECAST.  
 
BEHIND THE FRONT TUESDAY, VERY COLD AIR WILL ENTER THE MID-  
MISSISSIPPI VALLEY. CIPS ANALOG GUIDANCE FROM SAINT LOUIS  
UNIVERSITY, WHICH HAS BEEN SIGNALING THIS COOLDOWN FOR A WEEK,  
CONTINUES TO SHOW 850MB TEMPERATURES LESS OVER 1.5 SIGMA BELOW  
CLIMATOLOGY FOR WEDNESDAY. IT ALSO SHOWS THAT OVER 70% OF THE TOP 15  
HISTORICAL ANALOGS FOR WEDNESDAY FEATURE MINIMUM TEMPERATURES BELOW  
FREEZING ACROSS MOST OF THE REGION. WITH THIS MAGNITUDE OF COLD, THE  
POTENTIAL FOR WIDESPREAD FROST IS BECOMING HIGHER WEDNESDAY MORNING.  
VEGETATION ACROSS THE AREA REMAINS SUSCEPTIBLE AFTER WE FLIRTED WITH  
FROST EARLIER THIS WEEK. IF CLOUDS ARE ABLE TO CLEAR OUT ON TUESDAY  
EVENING, FORECAST LOW TEMPERATURES COULD STILL BE TOO HIGH, WHICH  
INTRODUCES A FREEZE POTENTIAL.  
 
AFTER TUESDAY'S PRECIPITATION, THE COLD AIR WILL KEEP US DRY THROUGH  
THE MAJORITY OF THE WEEK. WHILE MORE PRECIPITATION COULD BE ON TAP  
FOR US THIS WEEKEND, THERE IS STILL CONSIDERABLE TIMING AND LOCATION  
UNCERTAINTY WITH THE SYSTEM.  
 
MRB  
 

 
   
AVIATION
 
(FOR THE 06Z TAFS THROUGH 06Z SATURDAY NIGHT)  
ISSUED AT 1220 AM CDT SAT APR 17 2021  
 
SHOWERS HAVE FINALLY SPREAD INTO THE ST. LOUIS REGION TERMINALS AND  
THE MAIN CONCENTRATION OF SHOWERS OVERNIGHT SHOULD BE FROM CENTRAL  
THROUGH SOUTHEAST AND EAST CENTRAL MO INTO SOUTHWEST IL, IMPACTING  
KCOU AND KSTL/KSUS/KCPS. IT'S UNCLEAR HOW MUCH RAIN WILL MAKE IT  
INTO KUIN BUT I THINK THEY WILL SEE AT LEAST SOME LIGHT RAIN  
OVERNIGHT. COVERAGE OF SHOWERS IS EXPECTED TO WANE BETWEEN 10-12Z  
WITH SPOTTY/SCATTERED COVERAGE SATURDAY MORNING AND THEN SCATTERED  
SHOWERS AGAIN IN THE AFTERNOON. FLIGHT CONDITIONS WILL GRADUALLY  
DETERIORATE OVERNIGHT INTO EARLY SATURDAY MORNING AND THEN IMPROVE  
AGAIN LATE MORNING AND INTO THE AFTERNOON.  
 

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

 
 

 
 
WFO LSX  
 
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