546  
FXUS63 KLOT 231135  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
635 AM CDT SUN APR 23 2017  
   
SHORT TERM
 
 
303 AM CDT  
 
THROUGH MONDAY NIGHT...  
 
BIG BUBBLE, NO TROUBLE. WITH LOW PRESSURE ACROSS THE OHIO VALLEY,  
WATER VAPOR SATELLITE DEPICTING MOISTURE ALOFT IS NICE AND DRY  
ACROSS OUR REGION CLEAR BACK TO COLORADO, WITH SURFACE HIGH  
PRESSURE SPRAWLED OUT FROM PLAINS OF COLORADO CLEAR ACROSS THE  
THE GREAT LAKES REGION. A COLD FRONT IS FOUND ACROSS THE UPPER  
GREAT LAKES WITH A VERY DRY LOW LEVEL AIRMASS AHEAD OF IT.  
 
THE HIGH WILL ONLY SLOWLY MIGRATE EAST TODAY WITH THE UPPER LEVEL  
BLOCK STILL IN PLACE, WHICH WILL DELIVER ANOTHER DRY AND MOSTLY SUNNY  
DAY. LOWER LEVEL WARMING WILL ALSO SUPPORT TEMPERATURES BACK UP  
ABOVE NORMAL TODAY, UPPER 60S TO NEAR 70 WEST. WITH THE WEAK  
PRESSURE GRADIENT, EXPECT A LAKE BREEZE THIS AFTERNOON WHICH WILL  
KEEP TEMPS AT OR BELOW 60 LAKESIDE. THE COLD FRONT WILL CRUISE  
SOUTH DOWN LAKE MICHIGAN TODAY WITH SOME HIGH CLOUD COVER  
SPREADING AHEAD OF IT INTO OUR AREA, BUT THE COOLING AND THICKER CLOUD  
COVER WILL LARGELY REMAIN NORTH OF THE AREA. WE WILL STILL HAVE TO  
WATCH AS THE FRONT WILL LIKELY SNEAK INTO NORTHERN/NORTHEAST  
ILLINOIS ACROSS MCHENRY/LAKE COUNTIES LATE THIS AFTERNOON AND  
EVENING. THE FRONT WILL NOT MAKE MUCH HEADWAY SOUTH AS LOW LEVEL  
FLOW BEGINS TO TURN SOUTHWESTERLY AS THE RIDGE SHIFTS EAST AND  
MORE PROGRESSIVE FLOW DEVELOPS UPSTREAM. IT WILL BE DRY AND MOSTLY CLEAR  
AGAIN TONIGHT, WITH SLIGHTLY WARMER OVERNIGHT TEMPS.  
 
THE FIRST IN A SERIES OF LOWS WILL EJECT FROM THE NORTHERN ROCKIES  
MONDAY, BUT THE UPPER FLOW REMAINS LESS PROGRESSIVE WIT THE UPPER  
LOW OVER THE SOUTHEAST. MORE PRONOUNCED SOUTHERLY LOW LEVEL FLOW  
(SOUTHEAST AT THE SURFACE) WILL BRING SOME ADDITIONAL WARMING, BUT  
STILL COOLER AT THE LAKEFRONT IN NORTHEAST ILLINOIS. THE DAY WILL  
START SUNNY WITH SOME INCREASE IN HIGHER CLOUDS LATE IN THE DAY.  
 
KMD  
 

 
   
LONG TERM
 
 
303 AM CDT  
 
MONDAY THROUGH SUNDAY...  
 
DEWPOINTS REMAIN VERY LOW INTO THE EXTENDED, WHICH WILL LIMIT  
PRECIPITATION IMPACTS THROUGH AT LEAST TUESDAY EVENING, BUT A  
SIGNIFICANT PATTERN CHANGE WILL TAKE PLACE WITH A SERIES OF LOW  
PRESSURE SYSTEMS TRACKING FROM THE WEST TOWARD OUR REGION, WHILE A  
BIG AREA OF HIGH PRESSURE WILL TAKE SHAPE ACROSS THE SOUTHEAST  
COASTLINE.  
 
DECENT MODEL AGREEMENT ON THE LOW FOR TUESDAY NIGHT FROM THE  
SOUTHERN PLAINS MIDDAY TUESDAY AND SHIFTING INTO CENTRAL  
WISCONSIN BY DAYBREAK WEDNESDAY, WHICH WILL BRING OUR FIRST CHANCE  
FOR RAIN AFTER MIDNIGHT. THERE IS SOME WEAK ELEVATED INSTABILITY  
TO WARRANT KEEPING A VERY LOW T-STORM MENTION, BUT NOT A GREAT  
THUNDER SETUP. GUIDANCE IS IN BETTER AGREEMENT WITH THE COLD  
FRONTAL TIMING FOR WEDNESDAY, BRINGING IT THROUGH THE HEART OF THE  
AREA IN THE AFTERNOON. THERE IS DECENT STRENGTHENING OF AN UPPER  
LEVEL JET, ACTUALLY SEVERAL JET SEGMENTS DURING THIS TIME WITH A  
SOUTHERN AND NORTHERN STREAM SYSTEM, AND THEREFORE LEADING TO A  
STRENGTHENING OF A SERIES OF LOWS INTO ONE ACROSS SOUTHERN OR  
CENTRAL LOWER MICHIGAN. THESE FORCING MECHANISMS SUGGEST AN UPTICK  
IN SHOWER ACTIVITY, AND WITH A LITTLE BETTER INSTABILITY THIS ALSO  
WARRANTS A LITTLE HIGHER CHANCE T-STORM MENTION. SURFACE BASED  
INSTABILITY MAY BE HARD TO COME BY GIVEN THE CLOUDY PATTERN.  
 
THE COLD FRONT WILL PUSH THROUGH THE AREA WEDNESDAY NIGHT. THERE  
COULD BE SOME LINGERING LIGHTER RAIN AS THE UPPER TROUGH AXIS WILL  
STILL BE SHIFTING THROUGH. WEAK HIGH PRESSURE WILL BUILD IN ON  
THURSDAY WHICH SHOULD DRY THINGS OUT, BUT SOME LINGERING CLOUDS  
AND ONSHORE FLOW/POSSIBLE LAKE BREEZE WILL MAKE FOR COOLER  
CONDITIONS THURSDAY. THE COLD FRONT WILL GET SHOVED WELL SOUTH TO  
THE OHIO VALLEY DURING THIS TIME. WITH HIGH PRESSURE STRETCHED  
OUT ACROSS THE NORTHERN GREAT LAKES LATER THURSDAY INTO FRIDAY,  
MUCH OF THE REGION WILL REMAIN IN A COOL ONSHORE FLOW PATTERN,  
POSSIBLY WITH LINGERING CLOUDS. WHILE THERE STILL REMAIN  
SIGNIFICANT MODEL DIFFERENCES REGARDING THE EVENTUAL EVOLUTION  
AND PROGRESSION OF A DEEP UPPER LOW ACROSS THE WEST. ELEVATED  
SHOWERS AND STORMS CAN BE EXPECTED AT TIMES DURING THE WEEKEND  
WITH THE GFS/GEM HAVING A BIT MORE PRECIP WITH A POTENTIAL FIRST  
ROUND FRIDAY AFTERNOON. THERE IS BETTER AGREEMENT WITH THE EC IN  
AT A LEAST A PORTION OF THE SLOPED WARM FRONTAL BOUNDARY LIFTING  
THROUGH FRIDAY NIGHT INTO SATURDAY MORNING WHERE A BETTER CHANCE  
OF SHOWERS AND MAYBE A STORM WOULD RESULT. THESE SAME MODELS AND  
THE GEM SHOW THE MAIN UPPER LOW NOT CLEARING THE REGION INTO LATE  
SUNDAY IF NOT EARLY MONDAY. THEREFORE SATURDAY NIGHT INTO SUNDAY  
IS ANOTHER WINDOW WHERE RAIN IS EXPECTED, WITH A SLIGHTLY BETTER  
CHANCE OF THUNDERSTORMS AS WELL. MODEL BLEND POPS ARE BIT WISHY-  
WASHY GIVEN THE INCONSISTENCIES IN TIMING FROM THE EXTENDED  
GUIDANCE, SO TRIED AT LEAST NUDGE THEM A BIT MORE IN PERIODS OF  
CONCERN. EXPECT ADDITIONAL MODEL SHIFTING GIVEN THE EVOLUTION OF  
DEEP UPPER LOWS CAN CERTAINLY GIVE THE MODELS FITS AT THIS  
DISTANCE.  
 
KMD  
 

 
   
AVIATION
 
 
FOR THE 12Z TAFS...  
 
635 AM...ONLY CONCERN THIS PERIOD IS LAKE BREEZE THIS AFTERNOON.  
 
HIGH PRESSURE OVER THE AREA THIS MORNING WILL SLOWLY MOVE SOUTH  
TODAY AND TONIGHT. LIGHT/CALM WINDS TO START WILL BECOME LIGHT  
WESTERLY THIS MORNING. THE WEAK GRADIENT WILL ALLOW A LAKE BREEZE  
TO DEVELOP WHICH WILL LIKELY MOVE THROUGH GYY LATE THIS MORNING  
AND THEN THROUGH ORD/MDW EARLY THIS AFTERNOON. CONFIDENCE ON  
TIMING IS ONLY MEDIUM AND TRENDS WILL NEED TO BE MONITORED AS THE  
LAKE BREEZE MAY ARRIVE SOONER THAN CURRENTLY EXPECTED. SPEEDS WILL  
ALSO NEED TO BE MONITORED AS THERE COULD BE A PERIOD OF 10KTS WITH  
THE LAKE BREEZE PASSAGE.  
 
A COLD FRONT WILL MOVE SOUTH ACROSS LAKE MICHIGAN TODAY AND MAY  
REACH FAR NORTHEAST IL LATE THIS AFTERNOON OR EARLY THIS EVENING.  
CONFIDENCE REGARDING HOW FAR SOUTH THIS FRONT WILL MOVE IS LOW BUT  
IF IT WERE TO REACH THE CHICAGO TERMINALS...IT WOULD SHIFT AN  
EAST/SOUTHEAST WIND BEHIND THE LAKE BREEZE MORE TO THE NORTHEAST.  
 
THERE WERE A FEW AREAS OF SHALLOW GROUND FOG EARLY THIS MORNING  
AND THAT MAY OCCUR AGAIN EARLY MONDAY MORNING IN THE USUAL  
LOCATIONS. OTHERWISE...DRY/VFR THROUGH THE PERIOD WITH FEW/SCT  
HIGH CLOUDS. CMS  
 

 
   
MARINE
 
 
213 AM...WEAK LOW PRESSURE NEAR JAMES BAY EARLY THIS MORNING WILL  
MOVE INTO NORTHERN QUEBEC TODAY. A TRAILING COLD FRONT WILL MOVE  
SOUTH ACROSS NORTHERN LAKE MICHIGAN THIS MORNING AND THEN INTO  
CENTRAL PORTIONS OF THE LAKE THIS AFTERNOON. THERE REMAINS SOME  
UNCERTAINTY AS TO HOW FAR SOUTH THIS FRONT MAY PUSH. ITS POSSIBLE  
THIS FRONT MAY MOVE AS FAR SOUTH AS THE IL/WI STATE LINE BY LATE  
THIS AFTERNOON...ON THE WEST SIDE OF THE LAKE. A RIDGE OF HIGH  
PRESSURE OVER THE SOUTHERN LAKES REGION WILL SLOWLY SAG SOUTH  
TODAY AND TONIGHT.  
 
AN AREA OF LOW PRESSURE WILL MOVE FROM THE WESTERN PLAINS TONIGHT  
TO LAKE SUPERIOR TUESDAY...AS IT DISSIPATES. A TRAILING COLD FRONT  
WILL EXTEND FROM THE WESTERN LAKES TO THE A SECOND AREA OF LOW  
PRESSURE OVER THE SOUTHERN PLAINS TUESDAY. THIS LOW WILL LIFT  
NORTHEAST ACROSS THE WESTERN LAKES REGION WEDNESDAY INTO WEDNESDAY  
NIGHT. CMS  
 

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

 
 

 
 
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