304  
FXUS63 KLOT 201706  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
1206 PM CDT MON AUG 20 2018  
   
UPDATE
 
 
1205 AM CDT  
 
NO SIGNIFICANT CONCERNS EXIST WITH THE CURRENT BATCH OF RAIN, WITH  
EMBEDDED THUNDER CONFINED ACROSS NORTHWEST INDIANA. ACROSS CENTRAL  
ILLINOIS SKIES HAVE CLEARED SOME, AND THIS AXIS WILL EVENTUALLY  
SHIFT INTO A PORTION OF THE AREA.  
 
ILX LIKELY SEND AN 18Z SOUNDING TO SAMPLE THIS ENVIRONMENT.  
MORNING SOUNDINGS DO SHOW SEASONABLY STRONG WIND PROFILE IN THE  
LOW LEVELS, AND WHILE LARGELY UNIDIRECTIONAL THROUGH THE MID  
LEVELS, THE LOW LEVEL DIRECTION COMPONENT MAY BE SIGNIFICANT NEAR  
THE FRONTAL BOUNDARY CURRENTLY JUST TO OUR SOUTH. LOW LEVEL  
VORTICITY AND INSTABILITY WILL INCREASE NEAR THIS BOUNDARY SUCH  
THAT THE CONCERNS FROM THE OVERNIGHT SHIFT REMAIN VALID. SUPERCELL  
COMPOSITE PARAMETERS DO INCREASE THIS AFTERNOON GIVEN THE LOW  
LEVEL TURNING AND EXPECTED MORE CELLUAR/SCATTERED DEVELOPMENT  
TODAY WILL MAKE THIS A HIGHLY CONDITIONAL EVENT, AND ALSO LIMITED  
TO ANY CELLS ON THE WARM FRONT.  
 
KMD  
 

 
   
SHORT TERM
 
 
331 AM CDT  
 
THROUGH TONIGHT...  
 
FAIRLY STRONG CYCLONE FOR AUGUST WILL CONTINUE ITS SLOW BUT STEADY  
TREK ACROSS THE REGION THE NEXT COUPLE DAYS BRINGING VARIOUS SORTS  
OF INCLEMENT WEATHER...  
 
1) ARC OF NON-SEVERE WAA (WARM AIR ADVECTION) DRIVEN  
SHOWERS/T-STORMS WILL MOVE ACROSS THE AREA TODAY  
2) POTENTIAL FOR ISOLATED- SCATTERED T-STORMS THIS AFTERNOON OVER  
WESTERN AND SOUTHERN CWA WITH SOME TORNADO POTENTIAL  
3) SCATTERED SHOWERS/T-STORMS WITH SOME LAKE EFFECT STORMS TUES  
INTO TUES EVENING  
4) LARGE WAVES, EXTREMELY DANGEROUS SWIM CONDITIONS, AND  
POTENTIAL FOR SOME MINOR LAKESHORE FLOODING TUESDAY INTO TUESDAY  
NIGHT, ESPECIALLY NW IN SHORE.  
 
TODAY, LOOKS FOR ARC OF SHOWERS AND SCATTERED T-STORMS CURRENTLY  
MOVING INTO SW IL TO CONTINUE TO SPREAD NORTH AND EAST AND RESULT  
IN SHOWERS AND STORMS OVERSPREADING THE CWA FROM SW TO NE THIS  
MORNING, LIKELY REACHING THE CHICAGO AREA AND EXTREME NW INDIANA  
MIDDAY/EARLY AFTERNOON. AIR MASS IS SEASONABLY MOIST AND CERTAINLY  
COULD BE SOME HEALTHY DOWNPOURS WITH THIS ACTIVITY, BUT FLOODING  
THREAT APPEARS LOW THANKS TO THE EXPECTED PROGRESSIVE NATURE OF  
THIS ACTIVITY.  
 
IN THE WAKE OF THIS INITIAL WAA DRIVEN BAND, MODELS ARE IN GOOD  
AGREEMENT IN PIVOTING A PRONOUNCED MID-UPPER LEVEL DRY INTRUSION  
INTO THE AREA THIS AFTERNOON. GREAT DEAL OF UNCERTAINTY EXISTS  
REGARDING THE MAGNITUDE OF DESTABILIZATION THAT WILL TAKE PLACE IN  
THE WAKE OF THE INITIAL WAVE OF PRECIP. IN A SPRING OR FALL EVENT,  
WOULD BE QUITE CONCERNED THAT STRATUS LINGERING IN THE WAKE OF THE  
INITIAL BAND WOULD LIMIT HEATING AND KILL ANY POTENTIAL SEVERE  
THREAT, BUT THIS IS MID-AUGUST WITH A STILL FAIRLY HIGH SUN ANGLE,  
SUGGESTING A BETTER CHANCE OF MIXING OUT THE STRATUS AND SEEING  
SOME INSOLATION THIS AFTERNOON. REASONABLY GOOD AGREEMENT AMOUNT  
THE SUITE OF CAMS AND HREF THAT THERE WILL BE AN AXIS OF >1000  
J/KG OF SBCAPE THIS AFTERNOON ARCING FROM EAST-CENTRAL IL NORTH  
AND WEST TOWARD EXTREME EAST CENTRAL IA.  
 
WHILE THE SHEAR/CAPE PROFILES REALLY DON'T SCREAM TORNADO  
POTENTIAL, THE AMBIENT VORTICITY IN ASSOCIATION WITH THE  
APPROACHING UPPER LOW COULD RESULT IN AN ISOLATED TORNADO THREAT  
THIS AFTERNOON WITH A BROKEN BAND OF ISOLD-SCT'D T-STORMS. BUNKERS  
RIGHT-MOVING SUPERCELL MOTION THIS AFTERNOON IS PROGGED TO BE  
TOWARD THE NORTHEAST AT AROUND 20KT, WHICH GIVEN THE ORIENTATION  
OF THE WARM FRONT RULES LIMITS THE THREAT OF ANYTHING RIDING THE  
BOUNDARY. IN ADDITION, THE INSTABILITY AXIS COULD END UP BEING  
FAIRLY NARROW LEADING TO A RELATIVELY SHORT WINDOW FOR TORNADO  
POTENTIAL WITH THE STORMS.  
 
DESPITE THE AFOREMENTIONED MITIGATING FACTORS, THERE IS REASON TO  
BE CONCERNED ABOUT A NON-TRIVIAL TORNADO THREAT THIS AFTERNOON  
OVER MAINLY OUR SOUTHERN AND WESTERN CWA. LOW LEVELS WILL BE QUITE  
MOIST WITH LOW-MID 70 DEGREE DEWPOINTS EXPECTED IN THE WARM  
SECTOR, WHICH MEANS BOTH LOW LCL HEIGHTS AS WELL AS LOW LFCS.  
GIVEN SUFFICIENT HEATING, FORECAST SOUNDINGS HAVE FAIRLY STEEP LOW  
LEVEL LAPSE RATES, WHICH IN CONJUNCTION WITH THE LOW LFCS RESULTS  
IN SIGNIFICANT 0-3KT CAPE VALUES (FAVORABLE FOR LOW LEVEL UPDRAFT  
ACCELERATION AND VORTICITY STRETCHING FOR STORM SCALE ROTATION  
POTENTIAL).  
 
THE MARGIN OF ERROR BETWEEN SEVERAL TORNADOES OCCURRING ACROSS  
PORTIONS OF NORTHERN AND CENTRAL ILLINOIS AND RUN OF THE MILL  
WEAKER CONVECTION WITH MINIMAL TORNADO THREAT OR JUST SOME FUNNEL  
CLOUDS IS QUITE SMALL. THIS POTENTIAL WILL NEED TO BE MONITORED  
QUITE CLOSELY TODAY. OUTSIDE OF THE TORNADO THREAT, THE POTENTIAL  
FOR DAMAGING WINDS AND/OR HAIL LOOKS QUITE LOW TODAY. THE  
NORTHWARD AND EASTWARD EXTENT OF THE TORNADO POTENTIAL TODAY  
SHOULD BE LIMITED BY THE NARROWNESS OF THE INSTABILITY AXIS, WITH  
THREAT LARGELY CONFINED TO AREAS SOUTH OF I-88 ARCING SOUTHEAST  
TO NEAR THE IL/IN BORDER (REFERENCE OUR ENHANCED HAZARDOUS  
WEATHER OUTLOOK (EHWO) PAGE FOR GRAPHICAL DEPICTION OF OUR  
CURRENT THINKING REGARDING AREAL EXTENT OF TORNADO POTENTIAL THIS  
AFTERNOON/EARLY EVENING. EHWO CAN BE FOUND AT:  
WWW.WEATHER.GOV/LOT/EHWO AND SHOULD BE UPDATED BY 430 AM).  
 
THIS AFTERNOON ACTIVITY IS LIKELY TO DISSIPATE AFTER SUNSET WITH  
THE MAJORITY OF THE SHOWERS AND T-STORMS LATE THIS EVENING INTO  
THE OVERNIGHT HOURS SHIFTING NORTH AND EAST OF OUR CWA. UPPER LOW  
IS PROGGED TO BEGIN WEAKENING AND TRANSITIONING TO AN OPEN WAVE  
TUESDAY, THOUGH LOOKS TO BE SUFFICIENTLY COOL AIR ALOFT TO SUPPORT  
SOME SCATTERED SHOWERS AND T-STORMS INTO TUESDAY MORNING AND  
PERHAPS INTO THE AFTERNOON. THERE DOES APPEAR TO BE A WINDOW OF  
OPPORTUNITY FOR SOME LAKE EFFECT SHOWERS AND T-STORMS TUESDAY  
AFTERNOON AND EVENING INTO NORTHWEST INDIANA, BUT DEVELOPING  
INVERSION AND DRIER AIR ADVECTING IN SHOULD PUT THE KIBOSH TO THIS  
ACTIVITY WELL BEFORE SUNRISE WEDNESDAY.  
 
- IZZI  
 

 
   
LONG TERM
 
 
331 AM CDT  
 
TUESDAY THROUGH SUNDAY...  
 
LONGER TERM PERIOD WILL START OUT WITH SOME VERY PLEASANT SUMMER  
WEATHER WITH SUNSHINE, SLIGHTLY BELOW AVERAGE TEMPERATURES, AND  
COMFORTABLE HUMIDITY LEVELS WEDNESDAY INTO THURSDAY. MEDIUM RANGE  
MODELS IN REASONABLY GOOD AGREEMENT IN NEXT SHORTWAVE TROUGH  
MOVING ACROSS THE REGION LATE IN THE WEEK WITH SHOWER/T-STORM  
CHANCES RAMPING UP LATER FRIDAY THROUGH EARLY SATURDAY MORNING  
IN ADVANCE OF WARM FRONT SLATED TO MOVE ACROSS THE REGION.  
 
HEADING INTO THE WEEKEND AND EARLY NEXT WEEK, THERE HAS BEEN A  
FAIRLY CONSISTENT SIGNAL FOR ABOVE TO MUCH ABOVE AVERAGE  
TEMPERATURES. IN FACT, IT IS LOOKING INCREASINGLY LIKELY THAT WE  
COULD BE IN LINE FOR A LATE SEASON HEAT WAVE. THERE IS ENOUGH  
UNCERTAINTY IN THE EXACT PLACEMENT OF THE UPPER RIDGE AND  
SUBSEQUENTLY THE POTENTIAL "RING OF FIRE" CONVECTION TO JUSTIFY  
NOT ADJUSTING FORECAST TEMPERATURES THIS WEEKEND INTO EARLY NEXT  
WEEK UPWARD YET. HOWEVER, IF GFS AND ECMWF PROGGED 925MB TEMPS IN  
THE MID-UPPER 20S CELSIUS WERE TO VERIFY AND CONVECTION DOESN'T  
SHOVE THE EFFECTIVE FRONTAL BOUNDARY FARTHER SOUTH, THEN THOSE  
925MB TEMPS WOULD CERTAINLY SUPPORT HIGH TEMPERATURES SOLIDLY  
INTO THE 90S. GLANCING BEYOND THE DAY 7 PERIOD, MEDIUM RANGE  
GUIDANCE WITH STRONG SUPPORT FROM THEIR RESPECTIVE ENSEMBLES,  
SUGGEST WE COULD SEE A PROLONGED LATE SEASON HEAT EPISODE THAT  
EXTENDS WELL INTO NEXT WEEK. MORE ON THIS IN THE DAYS TO COME...  
 
- IZZI  
 

 
   
AVIATION
 
 
FOR THE 12Z TAFS...  
 
COMPLEX WEATHER IS EXPECTED THROUGHOUT THE PERIOD AS A STRONG LOW  
PRESSURE SYSTEM BY LATE SUMMER STANDARDS TRACKS FROM NEAR KOMA TO  
ACROSS NORTHERN IL THROUGH TONIGHT. DRY AND VFR CONDITIONS ARE  
EXPECTED AT ALL SITES THROUGH THE MORNING AMIDST STRENGTHENING E/ESE  
WINDS.  
 
MULTIPLE ROUNDS OF SHRA AND, AT TIMES, TS ARE EXPECTED THIS  
AFTERNOON AND EVENING. A BROKEN LINE OF SHRA WITH EMBEDDED TS SHOULD  
MOVE NE ACROSS ALL SITES EARLY TO MID AFTERNOON, WITH THE MOST  
LIKELY TIMING OF 19-22Z AT ORD/MDW. A FEW SHOWERS MAY REACH THE  
TERMINALS PRIOR TO THAT AS A LINE OF CONVECTION CURRENTLY ACROSS  
CENTRAL IL DISSIPATES WHILE SHIFTING NORTHWARD THIS MORNING. AFTER A  
POTENTIAL LULL IN ACTIVITY ANOTHER CLUSTER OF SHRA WITH THE  
POSSIBILITY FOR ISOLATED TS SHOULD SHIFT E ACROSS NE ILLINOIS AND NW  
INDIANA AROUND THIS EVENING. CONFIDENCE ON TS COVERAGE THIS EVENING  
HAS DECREASE, SO WILL CARRY ONLY A VCTS AT THIS TIME. ANY SHRA OR TS  
THIS AFTERNOON AND EVENING HAS THE POTENTIAL TO PRODUCE VERY HEAVY  
RAIN AND SIGNIFICANTLY REDUCED VIS. ALSO, ANY TS WILL BE CAPABLE OF  
GUSTY WINDS.  
 
SHRA WILL DIMINISH AND TS POTENTIAL SHOULD END OVERNIGHT MONDAY INTO  
TUESDAY AS THE LOW-PRESSURE TRACKS NE OVER OR NEAR RFD. IFR CEILINGS  
WILL BE POSSIBLE LATE MONDAY NIGHT INTO TUESDAY MORNING AS THIS  
OCCURS, ESPECIALLY AT RFD. A COLD FRONT WILL THEN USHERING IN GUSTY  
NNW WINDS BY MID-MORNING TUESDAY.  
 
KLUBER  
 

 
   
MARINE
 
 
159 AM CDT  
 
AN UNSEASONABLY STRONG LOW PRESSURE SYSTEM WILL TRACK FROM NW  
MISSOURI EARLY THIS MORNING TO SOUTHERN LAKE MICHIGAN LATE TONIGHT,  
TO SOUTHEAST ONTARIO TUESDAY AFTERNOON. SE WINDS OF 20-25 KNOTS ARE  
EXPECTED FOR THE SOUTH HALF OF THE LAKE TODAY AND THIS EVENING, WITH  
A BRIEF PERIOD OF WINDS TO 30 KNOTS POSSIBLE EARLY THIS EVENING.  
ACROSS THE NORTH HALF OF THE LAKE, SOUTH WINDS TO 15 KNOTS WILL  
GRADUALLY BECOME NE TO 20 KNOTS THROUGH THIS EVENING. AFTER THE LOW  
PASSES OVER THE LAKE FROM ROUGHLY KENOSHA TO MUSKEGON OVERNIGHT, N  
WINDS TO 30 KNOTS WITH OCCASIONAL GALES TO 35 KNOTS CAN BE EXPECTED  
ACROSS MUCH OF THE LAKE LATE TONIGHT THROUGH TUESDAY EVENING.  
 
THIS AFTERNOON AND TONIGHT, THUNDERSTORMS ON THE FAR SOUTHERN  
PORTION OF THE LAKE WILL BE CAPABLE OF PRODUCING WATERSPOUTS.  
ADDITIONALLY, SHOWERS ON TUESDAY ACROSS MUCH OF THE LAKE MAY RESULT  
IN A FEW WATERSPOUTS.  
 
WINDS WILL GRADUALLY DIMINISH TO UNDER 20 KNOTS BY WEDNESDAY  
AFTERNOON AS HIGH PRESSURE BEGINS TO BUILD ACROSS THE REGION. SW  
WINDS WILL THEN INCREASE TO 25 KNOTS ACROSS THE NORTH HALF BY  
THURSDAY EVENING, AND BECOME SOUTHERLY TO 30 KNOTS ACROSS ALL OF THE  
LAKE FRIDAY AFTERNOON THROUGH INTO SATURDAY AHEAD OF ANOTHER LOW  
PRESSURE TRACKING NE ACROSS THE UPPER MS RIVER VALLEY.  
 
KLUBER  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...BEACH HAZARDS STATEMENT...ILZ006-ILZ014 UNTIL 1 AM TUESDAY.  
 
IN...NONE.  
LM...SMALL CRAFT ADVISORY...WINTHROP HARBOR TO NORTHERLY IS. UNTIL 1 AM TUESDAY.  
 

 
 

 
 
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