519  
FXUS63 KLOT 071739  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
1239 PM CDT TUE JUL 7 2020  
   
SHORT TERM
 
 
313 AM CDT  
 
THROUGH TONIGHT...  
 
THE CONTINUATION OF THE IMPRESSIVE STRETCH OF ABOVE NORMAL  
TEMPERATURES DURING MID-SUMMER -- ADDING ONTO A DOZEN DAYS SO  
FAR -- AND THE LIKELIHOOD OF AFTERNOON SCATTERED THUNDERSTORMS  
ARE TODAY'S FORECAST MESSAGES. HEAT INDEX READINGS ARE FORECAST TO  
PEAK IN THE 95 TO 99 RANGE TODAY WITH MEDIUM-HIGH CONFIDENCE. AS  
FOR THUNDERSTORMS, THERE WILL BE A PROPENSITY TO PRODUCE LOCALLY  
HEAVY RAIN UNDER ANY DEEPER CORES ALONG WITH SOME GUSTY DOWNDRAFTS  
(30+ MPH). CONFIDENCE IN AT LEAST A FEW THUNDERSTORMS IN THE  
FORECAST AREA IS HIGH.  
 
AFTER BEING LOCATED MORE SOLIDLY UNDER THE UPPER AND MID-LEVEL  
RIDGING THE PAST SEVERAL DAYS, THIS MORNING FINDS US MORE TO THE  
NORTHEAST OF THE MAXIMUM HEIGHTS. USUALLY THAT MEANS SOME  
VULNERABILITY TO SHORT WAVE IMPULSES AND MORE READILY ERODING OF  
ANY CAPPING. WHILE NOT SIGNIFICANT BY ANY MEANS ON EITHER ASPECT  
TODAY, EACH ARE CONTRIBUTORS TO THE SCATTERED CONVECTIVE CHANCES.  
SATELLITE WATER VAPOR IMAGERY AND VAD PROFILERS EARLY THIS  
MORNING INDICATE A SHEARED SHORT WAVE FROM CENTRAL WISCONSIN INTO  
CENTRAL IOWA, WHICH IS EXPECTED TO DRIFT OVER THE FORECAST AREA  
DURING PEAK HEATING. BEHIND THAT IS AN ONGOING MCS ACROSS  
SOUTHEAST SOUTH DAKOTA AND A CONVECTIVELY-ENHANCED VORTICITY  
CENTER TO ITS NORTH, AND THE RAP SOLUTION BRINGS THAT REMNANT MCV  
TOWARD THE IL/WI BORDER AREA BY LATE DAY. IN ADDITION TO THESE  
TWO FEATURES FOR SOME MARGINAL UPPER SUPPORT AND FOCI, THE SURFACE  
CONVERGENCE OF A LAKE BREEZE WILL AGAIN BE PRESENT IN THE LAKE  
ADJACENT REGION OF NORTHEAST ILLINOIS AND NORTHWEST INDIANA.  
CONVERGENCE ALONG THIS IS LIKELY TO BE GREATER TODAY THANKS TO  
BOUNDARY LAYER WEST-SOUTHWEST FLOW AROUND 10 KT AS SEEN ON LAST  
EVENING'S 00Z DVN SOUNDING. ALSO THERE MAY BE A SUBTLE OUTFLOW  
LEFT OVER NEAR THE WI/IL STATE LINE FROM LAST NIGHT'S CONVECTION  
IN CENTRAL WISCONSIN.  
 
MOISTURE HAS EVER SO GRADUALLY BEEN INCREASING THE PAST 24 HOURS,  
AND SURFACE DEW POINTS EARLY THIS MORNING ARE IN THE UPPER 60S TO  
LOWER 70S. SOME MIXING WILL ALMOST SURELY HAPPEN AGAIN TODAY  
GIVEN COMMONALITIES TO RECENT DAYS THAT HAVE OBSERVED LOWERING  
AFTERNOON DEW POINTS, AS WELL AS MODEL SOLUTIONS OF TODAY'S  
BOUNDARY LAYER. THERE STILL SHOULD BE ENOUGH WITH THE HEATING TO  
RESULT IN THE CAP BEING BREACHED AND ANY OF THE AFOREMENTIONED  
AREAS OF FOCUS TO RESULT IN MORE TOWERING CUMULUS, AND GIVEN LESS  
MID-LEVEL DRY AIR, LIKELY A SCATTERING OF CONVECTIVE CELLS.  
AWAY FROM THE IMPETUS MECHANISMS, THERE WILL PROBABLY BE LIMITED  
(<15 PERCENT) COVERAGE, BUT PINPOINTING THAT AREA OR AREAS ARE  
SOMEWHAT TOUGH TODAY. SO HAVE AREAWIDE 20-40 PERCENT POPS IN THE  
FORECAST WITH THE HIGHEST BEING GENERALLY ALONG THE LAKE BREEZE  
AND WHERE THE WAVES ARE EXPECTED TO BE DURING PEAK HEATING INTO  
THE EARLY EVENING. WITH A HIGHER SOURCE ELEVATION OF DOWNDRAFT  
(CLOUD BASES) AS WELL AS STEEP LOW-LEVEL LAPSE RATES, A FEW GUSTY  
DOWNDRAFTS IN THE REGION ARE PROBABLE, WITH OUTFLOWS POTENTIALLY  
SPARKING ADDITIONAL CONVECTIVE CELLS. AS IS TYPICAL IN SUCH A  
PATTERN, A QUICK ONE INCH OF RAIN UNDER ANY OF THESE CELLS IS  
IN PLAY THIS AFTERNOON AND EARLY EVENING.  
 
FOR TEMPERATURES TODAY, THE TOP OF THE MIXED BOUNDARY LAYER IS  
ABOUT 1C WARMER (19-20C) THAN YESTERDAY, BUT MORE CUMULUS MAY  
STUNT THE ABILITY TO WARM ABOVE YESTERDAY IN MANY LOCATIONS. HAVE  
GONE WITH THE PERSISTENCE FORECAST OF 90-94 FOR INLAND LOCATIONS,  
WITH MID 80S LAKESIDE AS THE LAKE BREEZE MOVES IN BY EARLY  
AFTERNOON. WITH SOME MIXING OF DEW POINTS INTO THE MID 60S LIKELY,  
PEAK HEAT INDEX READINGS CONTINUE TO FALL IN THE 95-99 RANGE.  
AGAIN BELOW HEAT ADVISORY CRITERIA (105) BY SEVERAL DEGREES, BUT  
WILL CONTINUE TO NOTE BASIC HEAT SAFETY IN MESSAGING.  
 
ANY CONVECTION PERSISTING INTO EARLY TO MID EVENING IS MOST  
FAVORED NORTH OF I-80. THE SECOND MID-LEVEL WEAK IMPULSE IS LIKELY  
STILL TO BE DRIFTING OVER THE AREA THROUGH THE REST OF THE EVENING  
AND OVERNIGHT. CANNOT RULE OUT A FEW SPOTTY SHOWERS OR A COUPLE  
STORMS IN THAT TIME, BUT COVERAGE AFTER 10-11 P.M. SHOULD BE LESS  
THAN 10 PERCENT.  
 
MTF  
 

 
   
LONG TERM
 
 
330 AM CDT  
 
WEDNESDAY THROUGH MONDAY...  
 
WEDNESDAY AND THURSDAY MAY PRESENT A SLIGHTLY DIFFERENT CHARACTER  
TO THE PERSISTENT HEAT WE HAVE HAD, AS BOTH DAYS COULD EXPERIENCE  
LESS MIXING OF DEW POINTS, RESULTING IN HEAT INDEX READINGS  
POSSIBLY TOPPING 100 DEGREES FOR A FEW HOURS. IT ALSO MEANS A  
NON-ZERO CHANCE TO REACH HEAT ADVISORY LEVEL (HEAT INDEX = 105+),  
BUT STILL THAT'S ON THE LOWER END OF THE SOLUTION SPECTRUM.  
THUNDERSTORM CHANCES PEAK WITHIN THE LATER THURSDAY AFTERNOON  
THROUGH FRIDAY AFTERNOON PERIOD. IT IS NOT THE TYPE OF PATTERN  
WHERE WE EXPECT STORMS THROUGH THAT ENTIRE PERIOD NOR AN IDEAL  
TIMING FOR AN ELEVATED SEVERE THREAT, HOWEVER A COUPLE ROUNDS OF  
STORMS ARE POSSIBLE IN THAT TIME. THE WEEKEND CONTINUES TO FAVOR  
SLIGHTLY COOLER AND ALSO DRIER IN TERMS OF DEW POINTS, AND AS FOR  
PRECIPITATION THE MORE DRIER SOLUTION IS FAVORED. THAT'S NOT  
BECAUSE WE WANT A DRY WEEKEND (HA!), BUT DUE TO THE EC MODEL  
SOLUTION BEING MORE CONSISTENT THAN THE GFS FOR THIS WEEKEND AND  
WITH ITS 00Z SOLUTION IS IN FACT COMPLETELY DRY.  
 
FOR WEDNESDAY, GUIDANCE CONTINUES CONSISTENT IN BUILDING HEIGHTS  
BACK INTO THE AREA. THIS SHOULD STUNT MUCH FOR CONVECTIVE  
COVERAGE AND ALSO ENABLE SURFACE TEMPERATURES TO ACHIEVE 1-2  
DEGREES WARMER THAN TODAY. THE FOOTPRINT OF 90S IS PROBABLY GOING  
TO BE THE HIGHEST WE HAVE SEEN SO FAR IN THIS WARM STRETCH.  
CONFIDENCE IS NOT ALL THAT HIGH ON HOW FAR AFTERNOON DEW POINTS  
WILL MIX OUT. IF LESS MIXING THAN FORECAST AND DEW POINTS HANG  
AROUND 70, HEAT INDICES WILL SOLIDLY ENTER THE 100-105 RANGE.  
 
FOR THURSDAY-FRIDAY, THE INFLUENCE OF AN UPPER LOW PASSING  
EASTWARD ACROSS THE CANADIAN PRAIRIE PROVINCES WILL BE FELT THIS  
FAR SOUTH, WITH A SOUTHERN EXTENSION OF HEIGHT FALLS AND FORCING  
VIA SHORT WAVE(S) EXPECTED. HOWEVER, THIS AND ANY CLOUD INFLUENCE  
ARE LOOKING MORE LIKELY NOT TO ARRIVE UNTIL LATER THURSDAY, THUS  
ALLOWING FOR ANOTHER HOT AND HUMID DAY. THE STORY WILL BE SIMILAR  
TO WEDNESDAY, THOUGH FOR NOW HAVE NOT GONE AS WARM AS WEDNESDAY  
BECAUSE OF THE POTENTIAL OF INCREASING AFTERNOON CLOUDS. THE  
PRIMARY TIMING OF FOCUS CONTINUES TO BE THURSDAY NIGHT INTO FRIDAY  
MORNING. THE 00Z EC IS LATER THAN THIS, MORE SO FRIDAY, WITH A  
SLOWER SHORT WAVE. DETAILS WILL NATURALLY HAVE TO BE WORKED OUT IN  
TIME. ANY SEVERE THREAT MAY BE DEPENDENT ON IF AN MCS, LIKELY NOT  
A STRONG ONE THOUGH, CAN DEVELOP TO OUR WEST AND ENTER NORTH  
CENTRAL ILLINOIS THURSDAY NIGHT. ALSO A SLOWER SOLUTION LIKE THE  
EC COULD RESULT IN A FEW STRONG STORMS IN THE EASTERN CWA FRIDAY.  
 
AGAIN HAVE GONE WITH MORE OF AN EC SOLUTION FOR THE WEEKEND, WHICH  
LOWERS THE BLEND-PROVIDED POPS SOME. EITHER WAY, THE TEMPERATURES  
AND DEW POINTS LOOK TO BE SLIGHTLY LESS THAN THEY HAVE BEEN GIVEN  
LOW-LEVEL NORTHWESTERLY FLOW. A SECOND AND MORE OF A TRUE COLD  
FRONT IS POSSIBLE SATURDAY NIGHT-SUNDAY. THIS MAY TURN NORTHERLY  
WINDS WITH ENOUGH SPEED TO PRESENT SOME RIP CURRENT RISK FOR  
LAKE MICHIGAN BEACHES ON SUNDAY.  
 
BEYOND THIS CURRENT FORECAST, WE ARE KEEPING AN EYE ON THE  
POTENTIAL FOR HEAT TO RETURN AND POTENTIALLY MORE SUBSTANTIALLY  
DURING THE MID TO LATTER PART OF NEXT WEEK. THE 00Z GUIDANCE  
SIGNAL WAS A TAD MORE MUTED WITH THIS THAN YESTERDAY'S 12Z.  
 
MTF  
 

 
   
AVIATION
 
 
FOR THE 18Z TAFS...  
 
DEEP CONVECTION IS ATTEMPTING TO DEVELOP ON VARIOUS SURFACE  
BOUNDARIES ACROSS NORTHERN ILLINOIS AND NORTHWEST INDIANA AT TAF  
ISSUANCE, PRIMARILY ON A LAKE BREEZE IN NORTHWEST INDIANA AND A  
WEAK SURFACE TROUGH FROM NORTH-CENTRAL TO WEST-CENTRAL ILLINOIS.  
ISOLATED TO WIDELY SCATTERED ACTIVITY IS EXPECTED TO CONTINUE TO  
DEVELOP THROUGH THE AFTERNOON ON THESE FEATURES. CONFIDENCE IN TS  
DIRECTLY AFFECTING A GIVEN TERMINAL REMAINS LOW ENOUGH TO PRECLUDE  
A TEMPO TS GROUP IN THE TAF, BUT COVERAGE SHOULD BE HIGH ENOUGH  
THAT AIRSPACES AROUND THE TERMINALS MINIMALLY SEE SOME IMPACTS.  
WHILE MID-LEVEL DRY AIR ENTRAINMENT WILL LIMIT INTENSITY OF MOST  
CELLS, A FEW COULD BECOME DEEP ENOUGH TO PRODUCE GUSTY DOWNDRAFTS  
OF 20-30 KNOTS IMMEDIATELY UNDER AND NEAR THE CELL.  
 
A LOW TO MID-LEVEL WAVE MOVING SOUTHEAST ACROSS SOUTHEAST MN AND  
NORTH-CENTRAL IA IS EXPECTED TO FORCE SCATTERED CONVECTION THROUGH  
THIS AFTERNOON. IF COVERAGE BECOME HIGH ENOUGH TO FORM LARGER  
CLUSTERS OF STORMS OR ANY EXISTING STORMS ARE MAINTAINED INTO THE  
EVENING, RESIDUAL SHOWERS AND OUTFLOW WINDS MAY AFFECT THE  
CHICAGO- AREA TERMINALS MID TO LATE EVENING.  
 
OTHERWISE, CONDITIONS SHOULD BE RELATIVELY QUIET THROUGH THE  
MORNING HOURS ON WEDNESDAY, WITH VRB WINDS 5 KNOTS OR LESS  
(FAVORING THE SE QUADRANT THIS EVENING AND SW QUADRANT OVERNIGHT  
INTO WEDNESDAY MORNING.) ENOUGH CAPPING SHOULD EXIST FOR ORD/MDW  
ON WEDNESDAY AFTERNOON TO LIMIT CONVECTIVE POTENTIAL ON A LAKE  
BREEZE, BUT A WIDELY ISOLATED THUNDERSTORM CANNOT BE RULED OUT.  
 
KLUBER  
 

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

 
 

 
 
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