230  
FXUS63 KLOT 230836  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
336 AM CDT SUN JUL 23 2017  
   
SHORT TERM
 
 
306 AM CDT  
 
THROUGH TONIGHT...  
 
TWO QUESTIONS FOR TODAY AND TONIGHT INVOLVE PRECIP CHANCES  
THROUGH DAYBREAK AND AGAIN TOWARD LATE MORNING INTO THE AFTERNOON.  
 
SHORT TERM CAMS HAVE HAD A DECENT HANDLE ON THE ACTIVITY OVER WI  
AND THE LAKE DURING THE OVERNIGHT HOURS ALTHOUGH THE RAP HAS SO  
FAR BEEN A LITTLE TOO EAGER TO DEVELOP CONVECTION ACROSS NORTHWEST  
IL. A COUPLE OF SMALL CELLS HAVE FINALLY STARTED TO POP UP SO A  
CONTINUED SLIGHT CHANCE FOR THAT AREA AND DOWNSTREAM SEEMS LIKE A  
PRUDENT APPROACH THROUGH DAYBREAK. WITH SUCH A LIGHT PRESSURE  
GRADIENT AT THE SURFACE THERE IS LITTLE IN THE WAY OF LOW LEVEL  
CONVERGENCE, AND MOISTURE TRANSPORT INTO THE AREA APPEARS TO BE  
MINIMAL AS WELL. SATELLITE WV LOOPS DO SUGGEST THE PRESENCE OF A  
WEAK SHORTWAVE ALOFT, SO MAYBE THAT WILL BE ENOUGH TO SUPPORT  
ISOLATED ACTIVITY THIS MORNING.  
 
LATER IN THE MORNING INTO EARLY AFTERNOON THE SURFACE BOUNDARY  
ASSOCIATED WITH THE PRESENT ACTIVITY OVER WI MAKES ITS WAY INTO  
THE AREA. AGAIN THE CONVERGENCE AT LOW LEVELS APPEARS TO BE WEAK  
THOUGH UPPER SHORTWAVE SUPPORT LOOKS TO BE A BIT MORE FAVORABLE.  
SCATTERED CONVECTION IS LIKELY TO DEVELOP IN NW IL TOWARD LATE  
MORNING AND TRAVERSE TOWARD THE SOUTHEAST THROUGH THE AFTERNOON  
AND INTO THE EVENING. GIVEN THE OVERALL LACK OF WIDESPREAD  
STRONG FORCING IT IS ALSO LIKELY THAT SOME AREAS WILL REMAIN DRY  
TODAY.  
 
IF CONVECTION DOES DEVELOP TODAY, SPC HAS THE SOUTHERN HALF OF  
THE FORECAST AREA IN A SLIGHT RISK. THIS LOOKS REASONABLE GIVEN  
THE ANTICIPATED LOCATION OF ACTIVITY AT THIS TIME, DECENT DEEP  
LAYER SHEAR, PLENTY OF LOW LEVEL MOISTURE AHEAD OF THE BOUNDARY,  
AND THE ADVANTAGE OF PEAK HEATING DURING THE AFTERNOON.  
 
BY THIS EVENING INTO THE OVERNIGHT HOURS THE BOUNDARY WILL BE  
PUSHING SOUTH AND OUT OF THE AREA. THIS WILL BRING AN END TO ANY  
LINGERING CONVECTION AND ALSO BEGIN TO USHER IN DRIER AND COOLER  
AIR WHICH WILL GIVE MONDAY A CONSIDERABLY DIFFERENT FEEL THAN  
WHAT WE HAVE BEEN EXPERIENCING THESE PAST FEW DAYS.  
 
LENNING  
 

 
   
LONG TERM
 
 
226 AM CDT  
 
MONDAY THROUGH SATURDAY...  
 
THE FIRST AND LIKELY LAST PARTS OF THIS COMING WEEK LOOK MUCH  
QUIETER AND LESS MUGGY THAN MUCH OF WHAT JULY HAS PROVIDED SO FAR.  
DURING MIDWEEK, PARTICULARLY SOMETIME LATER WEDNESDAY THROUGH THE  
DAY THURSDAY, A COLD FRONT WILL BRING A CHANCE FOR STORMS FROM  
NORTHWEST TO SOUTHEAST ACROSS THE AREA.  
 
NORTHEAST WINDS ON MONDAY WILL STEER IN COOLER BOUNDARY LAYER AIR,  
WITH 850MB TEMPERATURES OF 12C-13C. THIS WILL KEEP HIGHS IN THE  
MID TO UPPER 70S IN MOST AREAS, WHILE ONSHORE FLOW AND POSSIBLY  
AREAS OF MORNING LOW CLOUDS PROVIDE LOWER 70S ALONG THE IMMEDIATE  
LAKE SHORE. TUESDAY WILL SEE LESS SYNOPTIC WIND SPEED ALLOWING  
FOR A LAKE BREEZE PUSH. TEMPERATURES INLAND WILL WARM BACK UP TO  
AROUND OR A LITTLE ABOVE 80. MORE SUN SHOULD BE SEEN ON TUESDAY  
THAN MONDAY.  
 
THE UPPER RIDGE, WHILE DAMPENED FROM LAST WEEK, WILL ATTEMPT TO  
BUILD INTO THE AREA LATER TUESDAY INTO EARLY WEDNESDAY, BEFORE A  
STRONG UPPER WAVE ACROSS CANADA CUTS INTO IT. AN ASSOCIATED  
SURFACE COLD WILL BE MOVING SOUTHEAST WITH QUITE A BIT OF WARMING  
EXPECTED WEDNESDAY ACROSS OUR AREA. IF CURRENT FRONTAL TIMING  
STANDS AND NOT UNTIL WEDNESDAY NIGHT/EARLY THURSDAY, THINK  
FORECAST HIGHS ON WEDNESDAY COULD BE A TAD UNDERDONE, WITH  
POSSIBLY LOW 90S MORE PREVALENT. HIGH DEW POINT AIR, NOT TOUGH TO  
COME BY IN LATE JULY IN THE CORN BELT, WILL RETURN AHEAD OF THE  
FRONT, ASSISTING IN MODEST TO HIGH INSTABILITY. WHILE FRONTAL  
SPEED CANNOT BE LATCHED ONTO WITH A HIGH DEGREE OF CERTAINTY AT  
3-4 DAYS OUT, GUIDANCE IS IN FAIR AGREEMENT WITH THE BOUNDARY A  
WAYS NORTH OF THE CWA DURING PEAK HEATING WEDNESDAY. SO THE STORM  
CHANCES PRESENTLY PEAK WEDNESDAY NIGHT AND EARLY THURSDAY, WHICH  
MAY KEEP THE SEVERE WEATHER THREAT MORE LIMITED. THERE COULD BE  
SOMEWHAT OF A HEAVY RAIN THREAT WITH THIS DESPITE IT BEING COLD  
FRONTAL DRIVEN, AS THE STEERING WINDS BECOME UNIDIRECTIONAL TO THE  
BOUNDARY AS IT AND ITS ASSOCIATED HIGH PWAT PLUME PASS. ALSO SOME  
ANAFRONTAL RAIN CHARACTERISTICS OF THE FRONT INDICATED ON  
GUIDANCE AS IT SLOWLY CREEP SOUTH ON THURSDAY. SOMETHING WE WILL  
HAVE TO KEEP AN EYE ON, BUT 2-3 DAYS OF DRYING AHEAD OF TIME WILL  
BE WELL RECEIVED.  
 
BEYOND WE LOOK TO REPEAT MONDAY AND TUESDAY'S WEATHER LATER  
THURSDAY INTO THE START OF NEXT WEEKEND PER RECENT RUNS OF GLOBAL  
GUIDANCE.  
 
MTF  
 

 
   
AVIATION
 
 
FOR THE 06Z TAFS...  
 
VERY LIGHT WINDS OVERNIGHT INTO THE PREDAWN HOURS SHOULD  
EVENTUALLY TAKE ON A WEST TO NORTHWESTERLY COMPONENT TOWARD  
DAYBREAK AND INTO THE MORNING, THEN VERY GRADUALLY MOVE TOWARD  
A NORTHERLY DIRECTION BY EVENING. DEVELOPMENT OF A LAKE BREEZE  
ALSO IS A GOOD POSSIBILITY WITH SUCH A WEAK PRESSURE GRADIENT,  
BUT IT IS NOT CLEAR IF THIS WOULD PUSH ALL THE WAY TO THE  
TERMINALS. THE OTHER CONCERN WOULD BE DEVELOPMENT OF ISOLATED TO  
SCATTERED CONVECTION DURING THE LATE MORNING TO EARLY AFTERNOON  
HOURS AS THE FORCING FOR EXISTING ACTIVITY ACROSS WI MOVES TOWARD  
THE AREA. CONSIDERED ADDING A VCTS BUT ELECTED TO WAIT FOR  
ADDITIONAL INFORMATION GIVEN THE PRESENT DEGREE OF LOW CONFIDENCE  
ON TIMING AND EXTENT OF THIS ACTIVITY.  
 
LENNING  
 

 
   
MARINE
 
 
145 AM CDT  
 
BROAD LOW PRESSURE AND AN ASSOCIATED DIFFUSE AND MAINLY LIGHT WIND  
FIELD WILL PREVAIL OVER THE LAKE THIS MORNING. ISOLATED SHOWERS OR  
STORMS MAY CONTINUE OVER THE LAKE DURING TODAY ASSOCIATED WITH  
THIS LOW. AS THE LOW PRESSURE SHIFTS SOUTHEAST, AN ASSOCIATED COLD  
FRONT WILL PROGRESS SOUTH THIS AFTERNOON AND EVENING, TURNING  
WINDS FROM WESTERLY TO NORTHERLY. THIS FRONT/WIND SHIFT SHOULD  
REACH THE FAR SOUTHERN TIP OF THE LAKE BY MIDNIGHT.  
 
ON MONDAY, THE NORTHERLY WINDS LOOK TO OCCASIONALLY GUST OVER 20  
KT OVER THE OPEN WATER AND SOUTHERN NEARSHORES. THIS FETCH WILL  
BUILD WAVES TO AT LEAST CLOSE (3-5 FT) TO SMALL CRAFT ADVISORY  
CRITERIA IN THE ILLINOIS AND INDIANA NEARSHORE BEACHES LATE  
TONIGHT AND THROUGHOUT MONDAY.  
 
THE NEXT SIMILAR ORIENTED COLD FRONT WILL MOVE SOUTHWARD ACROSS  
THE LAKE ON OR NEAR WEDNESDAY EVENING. THIS COULD HAVE SOME GUSTY  
STORMS IN ADVANCE OF AND ALONG ITS PASSAGE. BEHIND THE PASSAGE,  
ANOTHER TIGHT PRESSURE GRADIENT LOOKS TO DRIVE NORTHERLY WINDS AND  
POTENTIAL SMALL CRAFT ADVISORY CRITERIA WAVES, AND MAYBE EVEN  
WINDS, FOR ILLINOIS AND INDIANA ON THURSDAY.  
 
MTF  
 

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

 
 

 
 
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