805  
FXUS63 KILX 291746  
AFDILX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE LINCOLN IL  
1246 PM CDT THU JUL 29 2021  
   
SYNOPSIS
 
 
ISSUED AT 156 AM CDT THU JUL 29 2021  
 
OPPRESSIVE HEAT IS EXPECTED TODAY AS HEAT INDEX VALUES SURGE INTO  
THE TRIPLE DIGITS. SHOWERS AND THUNDERSTORMS WILL LIKELY DEVELOP  
LATER THIS AFTERNOON AND EXTEND INTO EARLY EVENING ALONG AND AHEAD  
OF A COLD FRONT. A FEW OF THESE STORMS MAY BECOME SEVERE WITH  
DAMAGING WINDS, LARGE HAIL, LOCALIZED FLASH FLOODING, AND EVEN A  
TORNADO POSSIBLE. COOLER TEMPERATURES ARRIVE FRIDAY AND WILL  
EXTEND INTO EARLY NEXT WEEK.  
 

 
   
UPDATE
 
 
ISSUED AT 1043 AM CDT THU JUL 29 2021  
 
STORMS STILL ONGOING IN THE NORTH AND SLOWLY MOVING SOUTH-  
SOUTHEAST AND DISSIPATING WHILE MOVING SOUTHEAST. LOTS OF CIRRUS  
OVER THE AREA AS WELL AND THIS COULD HOLD TEMPS DOWN A BIT.  
HOWEVER, ANY BREAKS IN THE CLOUDS AND TEMPS SHOULD STILL QUICKLY  
RISE WHERE THE HOLES ARE. SO ONLY DROPPED TEMPS BY 1-2 DEGREES.  
WITH DWPTS STILL IN THE MID 70S OVER MOST OF THE AREA, HEAT INDEX  
VALUES ARE STILL EXPECTED TO REACH 105 OR HIGHER OVER THE AREA AS  
WELL...SO NOT CHANGE TO THE ADVISORY PLANNED. SO, OVERALL HAVE  
MADE ADJUSTMENTS TO THE POP/WX GRIDS TO ACCOUNT FOR CURRENT  
CONDITIONS AND FORECAST FOR THIS AFTERNOON...STILL EXPECTING MORE  
CONVECTION, AND SMALL UPDATE TO TEMPS. UPDATE WILL BE SENT  
SHORTLY.  
 

 
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 156 AM CDT THU JUL 29 2021  
 
IMPACTFUL WEATHER IS EXPECTED TODAY ACROSS THE REGION AS A DUAL  
THREAT BEARS DOWN ON THE MIDWEST. DANGEROUS HEAT HAS TRIGGERED A  
HEAT ADVISORY FOR THIS AFTERNOON, WHILE A SEVERE WEATHER EPISODE  
LOOMS BY EVENING.  
 
DANGEROUS HEAT:  
THE PERIPHERY OF A 594MB HEAT RIDGE CONTINUES TO BUILD EASTWARD  
INTO PORTIONS OF THE MISSISSIPPI AND OHIO RIVER VALLEYS.  
TEMPERATURES ARE EXPECTED TO WARM INTO THE LOWER 90S THIS  
AFTERNOON JUXTAPOSED TO SURFACE DEWPOINTS CLIMBING INTO THE UPPER  
70S. THIS COMBINATION OF HEAT AND HUMIDITY WILL PRODUCE HEAT INDEX  
VALUES BETWEEN 105F - 110F, AND HAS PROMPTED A HEAT ADVISORY FOR  
OUR ENTIRE OUTLOOK AREA UNTIL 8 PM CDT.  
 
SEVERE WEATHER POTENTIAL:  
ALL EYES REMAIN ON THE MCC PUSHING ACROSS WI/IA/IL EARLY THIS  
MORNING. WHILE CONVECTION MAY LINGER INTO THE DAYLIGHT HOURS  
ACROSS NORTHERN IL, THE EXPECTATION IS FOR THIS COMPLEX OF STORMS  
TO WEAKEN AS THE LLJ BEGINS TO RECEDE AND THE MAIN MID- LEVEL  
SHORTWAVE DEPARTS TOWARD THE EASTERN GREAT LAKES REGION. A QUICK  
GLANCE AT LOT/DVN RADAR IMAGERY REVEALS A PAIR OF OUTFLOW  
BOUNDARIES THAT ARE SAGGING SOUTHWARD ACROSS NORTHERN ILLINOIS  
AHEAD OF A WEAKLY FORCED COLD FRONT. THIS CONVECTIVE DEBRIS WILL  
PROVE CRITICAL FOR STORM RE- INITIATION LATER THIS AFTERNOON.  
 
THE BOUNDARY LAYER WILL BECOME VERY UNSTABLE TODAY AHEAD OF THE  
COLD FRONT, AND WILL BE AIDED BY AN ELEVATED MIXED-LAYER THAT  
ADVECTS IN FROM THE NORTHWEST. UPPER 70S SURFACE DEWPOINTS AND  
STEEP MID- LEVEL LAPSE RATES WILL YIELD HIGHLY BUOYANT PARCELS  
(4000-5000 J/KG SBCAPE) AHEAD OF THE FRONT AS A CAPPING INVERSION  
ERODES BY 18Z. AT THIS TIME, IT IS NOT CLEAR WHETHER CONVECTIVE  
INITIATION WILL OCCUR ALONG A PRE-EXISTING OUTFLOW BOUNDARY, OR  
THE FRONT, AS BOTH FEATURES FIGURE TO LIE IN FAIRLY CLOSE  
PROXIMITY TO EACH OTHER EARLY THIS AFTERNOON.  
 
AS WITH ANY SEVERE WEATHER SETUP, THERE ARE A FEW LIMITING  
FACTORS. TODAY, THE MAIN ONE IS INSUFFICIENT DEEP LAYER SHEAR.  
WITH THE UPPER- LEVEL JET CORE DISPLACED NORTHWARD ACROSS THE  
GREAT LAKES REGION, CENTRAL ILLINOIS WILL ONLY HAVE 20-25 KTS OF  
DEEP LAYER SHEAR TO WORK WITH. THIS FIGURES TO LIMIT THE AMOUNT OF  
UPDRAFTS THAT CAN BECOME ORGANIZED/SEVERE THIS AFTERNOON. OTHER  
LIMITING FACTORS INCLUDE WEAK SURFACE CONVERGENCE AND THE LACK OF  
A DEFINABLE MID- LEVEL SHORTWAVE TROUGH TO HELP ENHANCE FORCING.  
NEVERTHELESS, WE DO ANTICIPATE ISOLATED INSTANCES OF SEVERE  
WEATHER TO DEVELOP LATER THIS AFTERNOON GIVEN THE SUPREME  
INSTABILITY IN PLACE. STORM- RELATIVE HODOGRAPHS FAVOR MOSTLY  
MULTI-CELL CLUSTERS, BUT A FEW DISCRETE CELLS CANNOT BE RULED OUT,  
ESPECIALLY EAST OF I-57 WHERE SHEAR WILL BE HIGHEST. THUS, ALL  
SEVERE WEATHER HAZARDS REMAIN POSSIBLE GOING INTO EARLY EVENING.  
LOCALIZED FLASH FLOODING IS ALSO A CONCERN GIVEN THE VERY HUMID  
AIR MASS. THE HREF IS SIGNALING SOME 1"+/HR RATES ACROSS THE  
AREA.  
 
THE FRONT WILL PUSH SOUTH OF OUR OUTLOOK AREA BY 03Z, AND SKIES  
WILL BEGIN TO CLEAR FROM NORTH TO SOUTH IN ITS WAKE. OVERNIGHT  
LOWS WILL COOL INTO THE MID-TO-UPPER 60S.  
   
LONG TERM
 
(FRIDAY THROUGH WEDNESDAY)  
ISSUED AT 156 AM CDT THU JUL 29 2021  
 
COOLER AND DRIER CONDITIONS WILL RETURN BY FRIDAY AS SURFACE HIGH  
PRESSURE BEGINS TO BUILD IN FROM THE NORTH. THE NBM KNOCKS  
TEMPERATURES DOWN INTO THE MID-TO-LOWER 80S HEADING INTO THIS  
WEEKEND AS THE 594MB HEAT RIDGE GETS SQUEEZED WESTWARD BY A  
STRONG UPPER- LEVEL LOW DEVELOPING OVER THE HUDSON BAY.  
 
SHOWERS AND THUNDERSTORMS WILL REMAIN A POSSIBILITY LATE FRIDAY  
EVENING INTO SATURDAY, MAINLY SOUTH OF A GALESBURG TO  
LAWRENCEVILLE LINE, AS AN MCS SKIRTS THE MID-MISSISSIPPI VALLEY.  
A STRETCH OF DRIER BUT CONTINUED COOL WEATHER THEN LOOKS LIKELY BY  
SUNDAY AS ANOTHER COLD FRONT PUSHES ACROSS THE REGION. THESE  
COOLER CONDITIONS WILL REMAIN IN PLACE THROUGH THE MIDDLE OF NEXT  
WEEK WITH DAILY HIGHS IN THE LOW 80S AND OVERNIGHT LOWS NEAR 60.  
 

 
   
AVIATION
 
(FOR THE 18Z TAFS THROUGH 18Z FRIDAY AFTERNOON)  
ISSUED AT 1246 PM CDT THU JUL 29 2021  
 
MODELS NOT HANDLING THE CURRENT STORM SITUATION VERY WELL SO  
LOCATION OF MESOSCALE FEATURES WILL BE THE KEY TO THE FORECAST.  
SCATTERED STORMS STILL POPPING IN THE NORTH NEAR PIA AND MOVING  
TOWARD BMI THIS AFTERNOON BUT REST OF THE AREA IS JUST SEEING  
EITHER LIGHT RAIN OR HIGH BASED CLOUDS. ATMOSPHERE IS BECOMING  
INCREASINGLY MORE UNSTABLE, BUT IT REMAINS CAPPED OVER MOST OF THE  
AREA. THE OUTFLOW BOUNDARY IS DOWN NEAR I-72 WHILE THE FRONT IS  
STILL IN NORTHERN IL. ONCE CAP ERODES AND FRONT ARRIVES, STORMS  
WILL DEVELOP AGAIN...FIRST AT PIA, THEN BMI AND THEN MOVING SOUTH  
TOWARD SPI/DEC/CMI. HAVE INCLUDED TEMPO GROUPS FOR ALL SITES AT  
BEST GUESS OF 3HR PERIOD WHEN CONVECTION IS POSSIBLE AT EACH SITE.  
OTHERWISE VCTS FOR 1-2HRS AFTER THE TEMPO GROUPS. AFTER PRECIP,  
CLOUDS SHOULD SCATTER OUT AND FOG SHOULD DEVELOP GETTING AS LOW AS  
4SM WITH 2SM TEMPO IN THE MORNING. WINDS WILL BE WEST TO  
SOUTHWESTERLY AND THEN BECOMING NORTHWEST AFTER PRECIP AND THE  
LIGHT AND VARIABLE OVERNIGHT THROUGH MORNING.  
 

 
   
ILX WATCHES/WARNINGS/ADVISORIES
 
 
HEAT ADVISORY UNTIL 8 PM CDT THIS EVENING FOR ILZ027>031-036>038-  
040>057-061>063-066>068-071>073.  
 

 
 

 
 
UPDATE...AUTEN  
SYNOPSIS...MJA  
SHORT TERM...MJA  
LONG TERM...MJA  
AVIATION...AUTEN  
 
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