260  
FXUS63 KLOT 230820  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
320 AM CDT SAT JUN 23 2018  
 
   
SHORT TERM
 
 
319 AM CDT  
 
THROUGH TONIGHT...  
 
THE UPPER LOW THAT HAS BEEN DRIVING THE CLOUDY, RAINING AND COOL  
WEATHER ACROSS THE REGION FOR THE PAST COUPLE OF DAYS IS NOW  
CENTERED OVER NORTHEASTERN IN EARLY THIS MORNING. THIS SYSTEM IS  
FINALLY EXPECTED TO SHIFT TOWARDS NEW ENGLAND LATER TODAY AS IT  
BECOMES AN OPEN MID-LEVEL WAVE. THIS SHOULD SET THE STAGE TO LESS  
ACTIVE AND MILDER WEATHER ACROSS MOST OF THE REGION THROUGH THE  
WEEKEND.  
 
WHILE MOST OF THE AREA WILL REMAIN DRY BOTH TODAY AND ON SUNDAY,  
IT DOES APPEAR THAT ANOTHER MID-LEVEL DISTURBANCE, NOW LOCATED TO  
OUR WEST OVER NE, WILL QUICKLY SHIFT EASTWARD ACROSS NORTHERN  
PORTIONS OF IL THIS AFTERNOON. AS IT DOES SO, THERE LOOKS TO BE  
SOME SCATTERED SHOWER DEVELOPMENT, MAINLY TO THE SOUTH OF I-80.  
ALSO CANT RULE OUT A THUNDERSTORM OR TWO, BUT INSTABILITY SHOULD  
REMAIN RATHER SWALLOW, SO IT IS NOT LOOKING TO BE A BIG EVENT, BUT  
THERE DOES LOOK TO BE SOME SHOWERS ACROSS SOUTHERN SECTIONS OF THE  
AREA THIS AFTERNOON. ELSEWHERE, EXPECT SOME BREAKS IN THE CLOUDS  
TODAY, WITH HIGHS CLIMBING BACK INTO THE UPPER 70S, WITH THE  
EXCEPTION OF AREAS NEAR LAKE MICHIGAN, WHERE AN ONSHORE WINDS WILL  
RESULT IN COOLER CONDITIONS AROUND 70.  
 
KJB  
 

 
   
LONG TERM
 
 
319 AM CDT  
 
SUNDAY THROUGH FRIDAY...  
 
THE FORECAST CONCERNS DURING THE EXTEND PERIOD FIRST FOCUS ON THE  
INCREASING THREAT OF A COUPLE ROUNDS OF THUNDERSTORMS, ESPECIALLY  
MONDAY NIGHT THROUGH TUESDAY NIGHT. SOME OF THESE STORMS COULD BE  
APPROACHING SEVERE LIMITS AND ALSO BE VERY HEAVY RAINFALL  
PRODUCERS. THE FORECAST CONCERN LATER IN THE PERIOD, NAMELY  
THURSDAY INTO THE WEEKEND, WILL BE THE POTENTIAL FOR ANOTHER HEAT  
WAVE TO IMPACT THE AREA.  
 
THE MAIN FEATURE OF INTEREST FOR THE MONDAY NIGHT THROUGH  
TUESDAY NIGHT PERIOD IS THE UPPER TROUGH/LOW NOW DIGGING INTO THE  
PACIFIC NORTHWEST. THIS FEATURE IS FORECAST TO DIG OVER THE  
CENTRAL HIGH PLAINS BY LATE SUNDAY, THEN PROGRESS EASTWARD ACROSS  
THE UPPER MIDWEST AND WESTERN GREAT LAKES TUESDAY INTO EARLY  
WEDNESDAY. AS THIS OCCURS, AN ATTENDANT WEAK SURFACE LOW WILL  
DRIVE A SURFACE WARM FRONT NORTHEASTWARD ACROSS THE MID AND UPPER  
MISSISSIPPI VALLEY LATE MONDAY AND MONDAY NIGHT. THE AIRMASS TO  
THE WEST- SOUTHWEST OF THIS BOUNDARY IS EXPECTED TO BE VERY WARM  
AND MOIST, AND IS EXPECTED TO SET THE STAGE FOR THUNDERSTORM  
DEVELOPMENT WELL TO OUR WEST SUNDAY NIGHT AND MONDAY. THE THREAT  
FOR STORMS LOCALLY LOOK TO INCREASE ACROSS THE AREA MONDAY NIGHT  
AS WARM AIR ADVECTION INCREASES OVER THE AREA IN RESPONSE TO THE  
UPTICK IN THE SOUTHWESTERLY LOW-LEVEL JET. WE CAN'T RULE OUT A FEW  
STRONG STORMS WITH THIS ACTIVITY MONDAY NIGHT, BUT VERY HEAVY  
RAINFALL WILL ONCE AGAIN BECOME A CONCERN WITH THESE STORMS, AND  
WITH THE VERY WET CONDITIONS OF LATE, THIS COULD RESULT IN  
ADDITIONAL FLOODING CONCERNS OVER THE AREA.  
 
ANOTHER ROUND OF STORMS IS THEN POSSIBLE LATE TUESDAY INTO  
TUESDAY NIGHT AS THE MID/UPPER TROUGH SHIFTS TOWARDS THE WESTERN  
GREAT LAKES. GIVEN SOME INCREASED DEEP LAYER SHEAR AN ADDITIONAL  
THREAT FOR STRONG TO SEVERE STORMS WILL BE POSSIBLE, BUT  
CONVECTIVE TRENDS MAY BE DEPENDENT ON THE MONDAY NIGHT/EARLY  
TUESDAY ACTIVITY. MORE HEAVY RAIN WILL BE POSSIBLE WITH THESE  
STORMS AS WELL AS PRECIPITABLE WATER VALUES ARE LIKELY TO REMAIN  
AOA 1.7". THIS PERIOD WILL NEED TO BE WATCHED.  
 
THE MAIN THREAT OF STORMS LOOKS TO END ACROSS THE AREA EARLY  
WEDNESDAY AS THE SYSTEM SHIFTS TOWARDS THE EASTERN GREAT LAKES.  
THE NEXT CONCERN WILL BE THE BUILDING HEAT ACROSS THE CENTRAL  
PART OF THE COUNTRY MID TO LATE WEEK. FORECAST GUIDANCE CONTINUES  
TO BE IN DECENT AGREEMENT WITH THE GENERAL LARGER SCALE PATTERN  
ACROSS THE CONUS LATER IN THE WEEK TRANSITIONING TO A BUILDING  
UPPER RIDGE ACROSS THE CENTRAL AND EASTERN CONUS AS A TROUGH DIGS  
OVER THE WESTERN CONUS. THERE DOES, HOWEVER, REMAIN SOME DETAIL  
DIFFERENCES IN THE MAGNITUDE OF THE RIDGING OVER THE REGION INTO  
NEXT WEEKEND, AND THIS WILL ULTIMATELY DETERMINE IF WE ARE IN THE  
HEAT, OR ON THE EDGE OF THE HEAT WITH PERIODS OF THUNDERSTORMS.  
 
AT THE PRESENT TIME, IT DOES APPEAR THAT WE COULD FALL WITHIN  
ANOTHER PERIOD OF STORMS AS UPPER DISTURBANCES TOP THE PERIPHERY  
OF THE BUILDING RIDGE AND MOVE ACROSS OUR AREA. THIS LOOKS MOST  
PROBABLE THURSDAY INTO EARLY FRIDAY. THEREAFTER, THE RIDGE MAY  
BEGIN TO BUILD ENOUGH TO STEER THE MAIN THUNDERSTORM THREAT NORTH  
OF THE AREA. IF THIS DOES OCCUR, WE WOULD LIKELY BE SETTING UP  
FOR ANOTHER POSSIBLE PERIOD OF SIGNIFICANT HEAT (90S WITH HEAT  
INDICES AROUND 100) FRIDAY AND INTO NEXT WEEKEND. WHILE THERE ARE  
STILL UNCERTAINTIES TO IRON OUT WITH THIS POTENTIAL PERIOD OF  
HEAT NEXT WEEKEND, IT IS ANOTHER PERIOD TO WATCH CLOSELY.  
 
KJB  
 

 
   
AVIATION
 
 
FOR THE 06Z TAFS...  
 
PATCHY DRIZZLE/LIGHT RAIN SHOWERS WILL COME TO AN END EARLY THIS  
MORNING AS THE UPPER LEVEL LOW CONTINUES EASTWARD. MVFR CIGS WILL  
ALSO PUSH EAST WITH VFR CONDITIONS EXPECTED LATER TODAY. I HAVE  
HIGH CONFIDENCE IN VFR CONDITIONS, BUT LOW CONFIDENCE IN EXACT  
TIMING.  
 
A LAKE BREEZE WILL TURN WINDS NORTHEAST ARND 9KT THIS AFTERNOON,  
AND WINDS BECOME LIGHT AND VARIABLE THIS EVENING. I THINK THE  
LIGHT WINDS WILL KEEP A NORTHWEST DIRECTION CLOSER TO THE LAKE.  
 
JEE  
 

 
   
MARINE
 
 
212 AM CDT  
 
BUOYS INDICATE THAT WAVES NO LONGER POSE A THREAT TO SMALL CRAFT,  
SO I WILL LET THE SMALL CRAFT ADVISORY GO.  
 
THE SURFACE LOW OVER THE IN/OH STATE LINE WILL SLOWLY DRIFT EAST  
TODAY WHILE HIGH PRESSURE SETTLES OVER HUDSON BAY. NORTH TO  
NORTHEAST WINDS ARE EXPECTED TODAY AS IS A LAKE BREEZE THIS  
AFTERNOON. A COLD FRONT WILL SHIFT DOWN THE LAKE TONIGHT AND  
SUNDAY, AND NORTH WINDS ARE EXPECTED BEHIND THE FRONT.  
 
HIGH PRESSURE BUILDS OVER THE GREAT LAKES MONDAY, AND WINDS BECOME  
EAST AS THE HIGH SHIFTS TO THE EAST COAST. SOUTHEAST WINDS MAY  
INCREASE TO 15-25 KT MONDAY NIGHT-TUESDAY AS THE PRESSURE GRADIENT  
TIGHTENS BETWEEN THE DEPARTING HIGH AND LOW PRESSURE OVER THE  
PLAINS/UPPER MIDWEST. THE NEXT LOW PASSES OVER THE NORTHERN END OF  
THE LAKE WEDNESDAY MORNING, AND WINDS BECOME WESTERLY BEHIND ITS  
COLD FRONT.  
 
JEE  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...NONE.  
IN...NONE.  
LM...NONE.  

 
 

 
 
VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)  
 
FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:  
WWW.FACEBOOK.COM/NWSCHICAGO  
WWW.TWITTER.COM/NWSCHICAGO  
WWW.YOUTUBE.COM/NWSCHICAGO  
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab IL Page
The Nexlab IN Page Main Text Page