166  
FXUS63 KILX 201101  
AFDILX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE LINCOLN IL  
601 AM CDT SUN AUG 20 2017  
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 255 AM CDT SUN AUG 20 2017  
 
CLOSELY MONITORING MCS JUST NORTH OF A STATIONARY FRONT  
STRETCHING EAST-WEST ACROSS NORTHERN MISSOURI AND CENTRAL  
ILLINOIS THIS MORNING. FORECAST FOR TODAY FOCUSES ON HOW FAR INTO  
THE DRY AIR ABOVE 850 MB THAT IS EVIDENT IN KILX AND KDVN 00Z  
SOUNDINGS. LATEST CAMS ARE MOVING IT INTO THE WESTERN FORECAST  
AREA THIS MORNING PRIOR TO DISSIPATION AND WILL GENERALLY FOLLOW.  
IMPACTS ON TEMPS DUE TO CLOUD COVER BLOW-OFF WILL LIKELY BE  
SIGNIFICANT OVER WESTERN HALF OF REGION LEADING TO SLIGHTLY COOLER  
TEMPS IN THE ILLINOIS RIVER VALLEY EAST TO I-55. EASTERN FORECAST  
AREAS WILL LIKELY SEE PLENTY OF SUN AND HEAT INDEX VALUES  
REACHING INTO THE LOWER 90S.  
 
SUBTLE WAVES CONTINUE TO MOVE THROUGH THE BROAD CYCLONIC FLOW IN  
PLACE OVER THE NORTHERN HALF OF THE CONUS. DESPITE THE WEAKNESS OF  
THE NEXT WAVE IN THE TRAIN, MODELS ARE CONSISTENT IN DEVELOPING  
THE NEXT NOCTURNAL MCS A BIT FURTHER EAST TOMORROW NIGHT. THERE IS  
SOME DIFFERENCES IN HOW THAT MCS IS FORECAST TO EVOLVE HOWEVER.  
NAM AND GFS TAKE THE CONVECTIVE SYSTEM SOUTHEAST INTO WESTERN  
ILLINOIS AND EASTERN MISSOURI, WHILE ECMWF SUGGEST A MORE EASTWARD  
MOVEMENT. WILL BRING CHANCE POPS INTO THE ILLINOIS RIVER VALLEY  
TONIGHT, BUT BE AWARE THAT THERE IS SUBSTANTIAL UNCERTAINTY.  
   
LONG TERM
 
(MONDAY THROUGH SATURDAY)  
ISSUED AT 255 AM CDT SUN AUG 20 2017  
 
MODELS CONSISTENT IN BUILDING RIDGE OVER WESTERN U.S. MONDAY AND  
DIGGING A 500MB TROUGH SOUTHWARD INTO THE MIDWEST. A COLD FRONT  
ASSOCIATED WITH THE MAIN THRUST OF ENERGY IS EXPECTED TO PUSH  
SOUTHEAST INTO THE REGION MONDAY AND THROUGH THE AREA TUESDAY.  
AHEAD OF THE FRONT, MOIST SOUTH FLOW WILL PUSH DEW POINTS INTO THE  
70S LEADING TO CAPES WELL ABOVE 1500 J/KG AND HEAT INDEX VALUES  
PUSHING 100. ONE POSSIBLE NEGATING FACTOR FOR VIGOUROUS CONVECTION IS  
THE POTENTIAL FOR CLOUD COVER REMNANTS FROM SUNDAY NIGHT MCS, BUT  
AS WAS DISCUSSED IN SHORT-TERM DISCUSSION THERE IS HIGH  
UNCERTAINTY TO HOW THAT SYSTEM WILL PLAY OUT. EITHER WAY, CLOUD  
COVER SHOULD BE ON THE RELATIVE HIGH SIDE NORTH OF I-70 MONDAY  
AFTERNOON DUE IN PART TO THE REMNANT CLOUD COMBINED WITH DIURNAL  
CU IN THE VERY MOIST ENVIRONMENT.  
 
PRECIPITATION AHEAD OF AND ALONG THE FRONT SHOULD MOVE INTO THE  
FORECAST AREA MONDAY NIGHT AND PUSH THROUGH THE AREA TUESDAY. THE  
PRECIPITATION SHOULD LIMIT INSTABILITY AND KEEP HIGH TEMPS IN THE  
LOWER TO MIDDLE 80S. PRECIPITABLE WATER IS FORECAST TO BE AROUND 2  
INCHES OVER MUCH OF THE FORECAST AREA BY 18Z, SO THERE WILL BE A  
THREAT OF HEAVY RAINFALL. POTENTIAL FOR LOCALIZED FLOODING WILL  
LIKELY DEPEND ON HOW MUCH AND WHERE RAIN OCCURS IN THE PREVIOUS 24  
HOURS TO SATURATE THE SOIL. MOST AREAS ARE CURRENTLY ON THE DRY  
SIDE AND CAN HANDLE QUITE A BIT OF PRECIP.  
 
ONCE THE FRONT CLEARS THE AREA LATE TUESDAY, MUCH DRIER AND  
COOLER CONDITIONS ADVECT INTO THE STATE. TEMPERATURES INTO NEXT  
WEEKEND SHOULD AVERAGE 5-10 DEGREES BELOW SEASONAL NORMALS.  
 

 
   
AVIATION
 
(FOR THE 12Z TAFS THROUGH 12Z MONDAY MORNING)  
ISSUED AT 600 AM CDT SUN AUG 20 2017  
 
WARMING CLOUD TOP TEMPERATURES IN THE NORTHERN PORTION OF THE  
LINEAR MCS IN IOWA SUGGEST WEAKENING TREND WITH THE EASTWARD  
ADVANCING PORTION OF THE LINE. AREA IS MOVING INTO WEAKER  
INSTABILITY AS IT MOVES EAST AND FURTHER FROM WARM FRONT. THE  
SOUTHERN PORTION OF THE MCS REMAINS STRONG AS IT MOVES SOUTHEAST  
ALONG THE CAPE AXIS OVER MISSOURI. STILL SOME QUESTION AS TO HOW  
FAR EAST STORMS WILL MAKE IT BEFORE DISSIPATING. LATEST HRRR (08Z)  
SUGGESTS A WIDELY SCATTERED STORM OR SHOWER MAY REACH KPIA AND  
KSPI LATE THIS MORNING. WILL INTRODUCE A BRIEF PERIOD OF VCTS/VCSH  
AT THOSE SITES. OTHERWISE...TERMINALS WILL BE INFLUENCED BY  
REMNANT CLOUD FROM THE SYSTEM TODAY. THERE REMAINS SIGNIFICANT  
UNCERTAINTY WITH NEXT NOCTURNAL MCS EXPECTED TO DEVELOP OVER IOWA  
THIS EVENING AND AS IT PUSHES EAST OR SOUTHEAST MAY IMPACT KPIA  
AFTER 06Z. WILL ADD A VCTS TO REFLECT CURRENT THINKING.  
 
HIGH CLOUDS ASSOCIATED WITH THIS MORNINGS MCS HAS LIMITED BR  
FORMATION AND CURRENT TEMPERATURE/DEW POINT SPREADS ARE 3-6  
DEGREES AT CENTRAL ILLINOIS TERMINALS. WE WILL REMOVE THE TEMPO  
GROUP THAT WAS PUT IN AT 06Z.  
 

 
   
ILX WATCHES/WARNINGS/ADVISORIES
 
 
NONE.  

 
 

 
 
SHORT TERM...BARKER  
LONG TERM...BARKER  
AVIATION...BARKER  
 
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