423  
FXUS63 KLOT 201227 AAA  
AFDLOT  
 
AREA FORECAST DISCUSSION...UPDATED  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
727 AM CDT THU JUL 20 2017  
   
MESOSCALE DISCUSSION
 
 
727 AM CDT  
 
THE FINAL LINE OF STORMS IS MOVING THROUGH THE AREA, WITH  
OCCASIONAL WIND SURGES SEEN IN VELOCITY DATA OR INFERRED IN  
REFLECTIVITY DATA WITH SHARP FRONT EDGE GRADIENTS AND BACK EDGE  
DRYING. SUSPECT GUSTS OF AROUND 50 MPH ARE MOST PROBABLE IN THESE  
SURGES, MOST NOTEWORTHY AT THE TIME OF THIS WRITING IN KENDALL AND  
LASALLE COUNTIES.  
 
RAINFALL HAS BEEN VERY EFFICIENT WITHIN THIS TROPICAL-LIKE AIR  
MASS AND REMAINS THE PRIMARY CONCERN. WHILE THE MOVEMENT OF THESE  
STORMS HAS NOT BEEN FAIRLY QUICK, ESPECIALLY THIS BACK LINE, THE  
RATES ARE HEAVY ENOUGH (1" IN 30-60 MINUTES) TO CAUSE AT LEAST  
LOCALIZED FLOODING ISSUES. THIS IS ESPECIALLY TRUE IN URBAN AREAS.  
 
THIS MCS SHOULD CONTINUE SOUTHEAST AND THROUGH THE FORECAST AREA  
BY MID-MORNING, WITH SOME SPOTTY STORM DEVELOPMENT REMAINING  
POSSIBLE IN ITS WAKE.  
 
MTF  
 

 
   
SHORT TERM
 
 
305 AM CDT  
 
THROUGH TONIGHT...  
 
CONCERNS WITH THE SHORT TERM CENTER ON 1.) ANY REDEVELOPING  
ISOLATED STORMS THIS AFTERNOON AND TONIGHT, WHICH IF THEY CAN  
WILL CERTAINLY HAVE A HEAVY RAIN THREAT AND POSSIBLY A SEVERE  
ONE, AND 2.) NEAR HEAT ADVISORY CRITERIA FOR THE SOUTHERN AND  
WESTERN PARTS OF THE FORECAST AREA THIS AFTERNOON IF UNIMPACTED  
BY STORMS.  
 
EARLY MORNING STORMS ALONG THE ELEVATED INSTABILITY GRADIENT AND  
A POOL OF PWATS AROUND 2 INCHES WILL SLOWLY MOVE SOUTHEAST OVER  
THE NORTHERN AND EASTERN CWA. WITH SOME COLD POOL ASSISTANCE, THIS  
SHOULD PROPAGATE THROUGH THE AREA BY 9-10 A.M. HEAVY RAIN AND  
ISOLATED POCKETS OF 40+ MPH WINDS WILL BE THE MAIN CONCERN.  
 
HOW LONG THE INFLUENCE OF THE CLOUDS AND THE RAIN-COOLED AIR  
PERSISTS IS A CHALLENGE TO SAY, AS OUTFLOW SHOULD PUSH SOUTH TO  
AROUND I-80 OR SO. THE REMNANT FRONTAL ZONE WILL LIKELY RESIDE  
SLIGHTLY NORTH OF THIS AND IN GENERAL THAT BROAD CORRIDOR COULD  
SERVE AS A TRIGGER FOR ISOLATED AFTERNOON STORMS IF ENOUGH  
CONVERGENCE OFFSETS THE UPPER RIDGING. WITH LOW-LEVEL MOISTURE SO  
HIGH AND MIXED LAYER INSTABILITY FORECAST TO POOL TO AROUND  
2000-3500 J/KG BY MID-AFTERNOON, IT MAY NOT TAKE MUCH FOR ISOLATED  
INITIATION. WHILE THE BETTER WESTERLIES WILL BE NORTH, ENOUGH  
SHEAR IS FORECAST TO SUPPORT THE ISOLATED SEVERE THREAT FOR HAIL  
AND WIND, AND OBVIOUSLY LOCALIZED VERY HEAVY RAINFALL WOULD BE  
SEEN UNDER ANY STORMS.  
 
SOUTH OF THE FRONTAL ZONE (SOUTH OF I-80 OR SO), AFTERNOON  
TEMPERATURES ARE PRESENTLY FORECAST TO CLIMB INTO THE 90S, BASED  
ON MORNING CONVECTIVE DEBRIS CLOUDS FADING, AND OUTFLOW GENERALLY  
REMAINING NORTH. MID 90S ARE ATTAINABLE IN THIS AREA AND IF THEY  
LOOK MORE LIKELY BY LATE MORNING -- GIVEN DEW POINTS IN THE MID TO  
UPPER 70S -- A HEAT ADVISORY FOR HEAT INDEX READINGS IN EXCESS OF  
105 WILL LIKELY NEED TO BE ISSUED. CONFIDENCE ON THIS IS JUST TOO  
LOW WITH THE CONVECTION AT THE MOMENT, AND THE DAY SHIFT SHOULD  
HAVE A CLEARER ASSESSMENT BY MID TO LATE MORNING.  
 
LATER TONIGHT, THE QUASI-STATIONARY BOUNDARY WILL BEGIN TO LIFT  
NORTH AS A WARM FRONT. THROUGH THE NIGHT, ISOLATED STORMS ALONG  
AND TO THE IMMEDIATE NORTH OF THE BOUNDARY ARE POSSIBLE WITH  
GRADUAL ISENTROPIC ASCENT ALONG IT. AGAIN A MARGINAL SEVERE  
THREAT WILL EXIST, BUT NOT SEEING ANY SIGNS OF A KEY SHORT WAVE  
DISTURBANCE TO TRIGGER ANYTHING WIDESPREAD.  
 
MTF  
 

 
   
LONG TERM
 
 
305 AM CDT  
 
FRIDAY THROUGH SUNDAY...  
 
FOCUS THEN SHIFTS TO WHAT APPEARS TO BE AN ACTIVE FRIDAY AND  
FRIDAY NIGHT, AS MULTIPLE MORE PRONOUNCED SHORT WAVE DISTURBANCES  
ARE FORECAST TO RESIDE IN THE WESTERLIES. THE FRONTAL ZONE WILL  
CONTINUE TO BE THE PRIMARY FOCUS MECHANISM FOR STORMS, AND  
CONFIDENCE IS INCREASING THAT THIS SHOULD RESIDE IN THE NORTHERN  
CWA BY MIDDAY FRIDAY. UNCERTAINTY IS STILL HIGH ON WHETHER THAT  
KEEPS THE HEAVIEST RAIN JUST NORTH OF THE AREA OR ACTUALLY COMING  
ACROSS IT, AS WAVES OF CONVECTION WOULD LIKELY MODULATE THINGS A  
LITTLE SOUTHWARD MORE SO INTO OUR AREA. CONCERNS CENTER ON HEAVY  
RAIN WITH STORM MOVEMENT PARALLEL TO THE BOUNDARY AND LOW SPEEDS  
OF FORECAST BACKBUILDING VECTORS. IN ADDITION, MODEL QPF BULLS  
EYES -- WHILE NOT PERFECTLY PLACED -- DO CORRELATE TO THE SIGNAL  
WE ARE SEEING. HAVE GONE AHEAD AND MENTIONED HEAVY RAIN IN THE  
FORECAST FOR NORTH OF I-80 FRIDAY NIGHT. IN ADDITION, STRONGER  
SHEAR WILL SUPPORT ORGANIZED STORMS AT TIMES IN THE REGION, WITH  
DEEPER CORES CAPABLE OF HAIL AND ANY BOWING SEGMENTS BRINGING A  
WIND THREAT. SUPERCELL STRUCTURES WOULD SEEM PLAUSIBLE FRIDAY  
EVENING ALONG THIS FRONTAL ZONE GIVEN DEGREE OF 850-500MB  
CROSSOVER AND HODOGRAPHS, HOWEVER IT IS JUST CHALLENGING TO SAY  
WHERE SPECIFIC CONVECTIVE THREATS WILL SHAPE UP GIVEN HOW MUCH  
EACH ROUND OF CONVECTION AFFECTS CONDITIONS FOR THE NEXT.  
 
LIKE A BROKEN RECORD, TEMPERATURES WILL AGAIN DEPEND ON CONVECTIVE  
INFLUENCES. THE SOUTHERN HALF OF THE AREA RUNS THE POTENTIAL OF  
REACHING HEAT ADVISORY CRITERIA AGAIN. DEW POINTS SHOULD RESIDE  
IN THE MID-UPPER 70S AREAWIDE GIVING A VERY MUGGY FEEL.  
 
SATURDAY WILL BE MORE OF THE SAME, THOUGH THE FORCING MECHANISM  
MAY BE MORE SO THE APPROACHING COLD FRONT, AS LOW PRESSURE IS  
EXPECTED TO BE MOVING EASTWARD OVER OR JUST NORTH OF THE AREA.  
STRONG, HEAVY-RAIN PRODUCING STORMS WILL AGAIN BE POSSIBLE, THOUGH  
THE EMPHASIS ESPECIALLY BY SATURDAY NIGHT, MAY BEGIN TO SHIFT MORE  
SO INTO THE SOUTHERN CWA. DEW POINTS AHEAD OF THE BOUNDARY MAY  
REACH THEIR PEAK ON SATURDAY, SO AGAIN WILL HAVE TO WATCH FOR  
HEAVY ADVISORY. OFFSHORE FLOW LOOKS TO POSSIBLY BE THE STRONGEST  
THIS DAY, OFFERING DOWNTOWN CHICAGO THE WARMEST DAY IN THE  
STRETCH, IF NOT IMPACTED BY CONVECTION.  
 
BY SUNDAY, THE FRONT IS FORECAST TO MOVE THROUGH, WITH NORTHWEST  
WINDS GRADUALLY CHANGING AIR MASSES TO START NEXT WEEK.  
 
MTF  
 
MONDAY THROUGH THURSDAY...  
 
A LARGE AREA OF HIGH PRESSURE WILL BE BUILDING ACROSS THE GREAT  
LAKES REGION MONDAY AND IS EXPECTED TO SLOWLY BUILD SOUTH AND  
EAST THROUGH THE MIDDLE OF NEXT WEEK. THIS IS NOW SLOWER THEN  
PREVIOUS FORECASTS WITH DRY WEATHER EXPECTED THROUGH AT LEAST  
WEDNESDAY MORNING. THE GFS CONTINUES TO SHOW A COLD FRONT MOVING  
SOUTH ACROSS THE REGION WEDNESDAY NIGHT INTO THURSDAY. THE ECMWF  
HAS SLOWED THIS FRONT AND THE CANADIAN IS GENERALLY DRY THROUGH  
THE END OF THE WEEK KEEPING THE HIGH OVER THE SOUTHERN LAKES  
REGION. SO CONFIDENCE REGARDING PRECIP CHANCES DURING THE MIDDLE  
OF NEXT WEEK IS LOW...BUT MAINTAINED CHANCE POPS FOR NOW. THIS  
PATTERN WOULD SUGGEST GENERALLY SEASONAL TEMPS WITH LOWER  
HUMIDITY...ALONG WITH LAKE BREEZES. THOUGH SOME INCREASE IN  
DEWPOINTS IS POSSIBLE AS FLOW EVENTUALLY TURNS MORE SOUTHERLY ON  
THE BACK SIDE OF THE HIGH. CMS  
 

 
   
HYDROLOGY
 
 
315 AM CDT  
 
THE HYDROLOGIC OUTLOOK (ESF) FOR ALONG AND NORTH OF I-88 HAS BEEN  
CONTINUED DUE TO CONCERNS OF ADDITIONAL ROUNDS OF STORMS THROUGH  
EARLY THIS WEEKEND. EARLY MORNING STORMS ARE PRODUCING ADDITIONAL  
RAINFALL UP TO OR OVER AN INCH, PRIMARILY IN NORTH CENTRAL  
ILLINOIS (ROCKFORD/BYRON AREAS). LESSER BUT STILL MODERATE RAIN IS  
EXPECTED OVER THE DES PLAINS AND FOX RIVER BASINS. JUST LAST  
NIGHT'S RAIN BROUGHT A STEADYING OF THE HYDROGRAPHS AT SEVERAL  
LOCATIONS, SHOWING THAT IT MAY NOT TAKE TOO MUCH TO CAUSE RISES  
GIVEN WET ANTECEDENT CONDITIONS. CONCERNS ARE PARTICULARLY  
HEIGHTENED ON FRIDAY AND FRIDAY NIGHT, WHERE THE ENVIRONMENT WILL  
BE EVEN MORE FAVORABLE FOR VERY EFFICIENT RAIN-PRODUCING STORMS  
(TRAINING STORMS, POSSIBLE BACKBUILDING, DEEP MOISTURE, ETC).  
WHILE THERE IS UNCERTAINTY ON LOCATION, IN COLLABORATION WITH WPC  
AND ADJACENT WFOS, THE FOCUS IS PRESENTLY MOST FAVORED IN SOUTHERN  
WISCONSIN AND FAR NORTHERN ILLINOIS. WPC HAS GONE AHEAD WITH A  
MODERATE RISK OF EXCESSIVE RAINFALL IN THAT PERIOD.  
 
MTF  
 

 
   
AVIATION
 
 
FOR THE 12Z TAFS...  
 
646 AM...MAIN FORECAST CONCERN AND CHALLENGE IS THUNDERSTORMS.  
 
BACK EDGE OF THE THUNDERSTORMS FOR THIS MORNING EXTENDS FROM UGN  
TO SQI AND SHOULD CLEAR ORD BY 13Z AND MDW BY 14Z. THERE MAY BE  
SOME FESTERING/LINGERING SHOWERS THROUGH MIDDAY BUT CONFIDENCE IS  
LOW. LINE OF THUNDERSTORMS APPROACHING WILL PRODUCE BRIEF  
TORRENTIAL RAIN WITH VIS POSSIBLY UNDER 1SM.  
 
CONFIDENCE FOR AFTERNOON CONVECTION HAS DECREASED SOME IN THE  
PAST 6 HOURS...IN PART BECAUSE OF HOW WIDESPREAD AND FURTHER  
SOUTHWEST THIS CURRENT ACTIVITY HAS BECOME. MODELS STILL SHOW  
SCATTERED THUNDERSTORM DEVELOPMENT MID/LATE AFTERNOON ACROSS THE  
TERMINALS BUT TIMING HAS SLOWED. MORE IMPORTANTLY...LOCATION OF  
NEW ACTIVITY IS UNCERTAIN AND IT COULD DEVELOP SOUTHWEST OF THE  
TERMINALS. GIVEN THE LOWER CONFIDENCE HAVE OPTED TO ONLY EXTEND  
THE TEMPO BY AN HOUR TO ACCOUNT FOR POSSIBLE LATER TIMING AND  
LATER SHIFTS CAN ADJUST ACCORDINGLY BASED ON TRENDS THIS  
AFTERNOON. THUNDERSTORM CHANCES TONIGHT WILL DEPEND ON AFTERNOON  
CONVECTION...WHICH IF IT DEVELOPS AS EXPECTED WOULD LIKELY MOVE  
SOUTH/SOUTHWEST WITH MAINLY DRY CONDITIONS TONIGHT. IF AFTERNOON  
CONVECTION DOESN/T MATERIALIZE...THE LOW LEVEL JET WILL LIKELY  
ALLOW DEVELOPMENT OVERNIGHT INTO FRIDAY MORNING. AND THIS MAY  
STILL TAKE PLACE ANYWAY...BUT CONFIDENCE IS TOO LOW TO INCLUDE  
THIS POTENTIAL IN THE TAFS EXCEPT AT RFD.  
 
WINDS WILL BE ERRATIC FOR THE NEXT FEW HOURS. OUTSIDE OF A STRONG  
NORTHWEST WIND WITH OUTFLOW/STORMS...THERE WILL LIKELY BE A PERIOD  
OF SOUTHEASTERLY WINDS WHICH SHOULD SLOWLY TURN SOUTH/SOUTHWEST  
LATER THIS MORNING AND MORE WESTERLY BY EARLY AFTERNOON. HIGHER  
WIND SPEEDS/GUSTS FROM PREVIOUS FORECAST ARE NOT EXPECTED TO  
MATERIALIZE GIVEN THE IMPACT OF THESE STORMS ON THE WIND FIELD. A  
SHIFT TO THE NORTHEAST IS STILL EXPECTED MID/LATE THIS AFTERNOON  
BUT LOWER CONFIDENCE ON TIMING AND THIS SHIFT MAY ALSO BE IMPACTED  
BY THUNDERSTORMS. WINDS ARE EXPECTED TO BECOME LIGHT SOUTHEASTERLY  
LATER THIS EVENING.  
 
FINAL CONCERN LATER TONIGHT INTO FRIDAY MORNING IS FOG WHICH IS  
DEPENDENT ON CLOUD COVER AND ANY THUNDERSTORMS. WITH LIGHT  
WINDS...MOIST LOW LEVELS AND A DISSIPATING BOUNDARY/FRONT IN THE  
AREA...ITS POSSIBLE FOG MAY DEVELOP IN THE USUAL AREAS AND PERHAPS  
BECOME LOCALLY DENSE. CONFIDENCE TOO LOW TO INCLUDE WITH THIS  
FORECAST. CMS  
 

 
   
MARINE
 
 
229 AM...REMAINING STRONGER WINDS FROM OUTFLOW/THUNDERSTORMS LATE  
WEDNESDAY EVENING SHOULD DIMINISH SOON OVER THE SOUTHERN PARTS OF  
LAKE MICHIGAN. CONFIDENCE REGARDING WIND SPEEDS AND DIRECTIONS IS  
SOMEWHAT LOW WITH ADDITIONAL THUNDERSTORMS EXPECTED NEAR/OVER  
SOUTHERN LAKE MICHIGAN THIS MORNING. A WEAK FRONTAL BOUNDARY IS  
EXPECTED TO MOVE SOUTH SHIFTING WINDS NORTHERLY BY LATE AFTERNOON.  
PRIOR TO THAT AND IN THE ABSENCE OF ANY THUNDERSTORMS...ITS  
POSSIBLE THE GRADIENT WILL TIGHTEN ENOUGH FOR SOUTHWEST WINDS INTO  
THE 15-25 KT RANGE. TRENDS WILL NEED TO BE MONITORED THROUGH THE  
MORNING. STRONG/ERRATIC WINDS CAN BE EXPECTED WITH ANY  
THUNDERSTORMS THAT MOVE OVER THE LAKE.  
 
A WEAK RIDGE OF HIGH PRESSURE WILL MOVE ACROSS THE LAKES REGION  
TONIGHT AND THEN LOW PRESSURE OVER THE NORTHERN AND CENTRAL  
PLAINS WILL MOVE EAST ACROSS THE REGION SATURDAY NIGHT INTO  
SUNDAY WITH A BROAD BUT WEAK AREA OF HIGH PRESSURE SETTLING OVER  
THE REGION EARLY NEXT WEEK. CMS  
 

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

 
 

 
 
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