377  
FXUS63 KIWX 201015  
AFDIWX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE NORTHERN INDIANA  
615 AM EDT THU SEP 20 2018  
   
SYNOPSIS
 
 
ISSUED AT 424 AM EDT THU SEP 20 2018  
 
ONE MORE HOT DAY WILL BE IN STORE FOR MANY AREAS AS HIGHS REACH  
THE UPPER 80S TO LOWER 90S. A CHANCE FOR A SHOWER OR STORM WILL  
EXIST MAINLY ALONG AND NORTH OF THE TOLL ROAD INTO THIS AFTERNOON.  
THERE WILL BE A CHANCE OF SHOWERS AND THUNDERSTORMS FRIDAY AFTERNOON.  
COOLER TEMPERATURES WILL FILTER IN BEHIND A COLD FRONT WITH HIGHS  
ONLY INTO THE 60S ON SATURDAY, THEN MAINLY IN THE 70S NEXT WEEK.  
 

 
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 424 AM EDT THU SEP 20 2018  
 
LOW CONFIDENCE FORECAST REGARDING IMPACTS OF UPSTREAM CONVECTION  
AND PRECIP CHANCES INTO THIS AFTERNOON AS WELL AS TEMPERATURES.  
 
ELEVATED FRONTAL BOUNDARY HAS BEEN ACTIVE FOR SEVERAL HOURS NOW  
FROM NE NEBRASKA INTO SOUTHERN WISCONSIN. RECENTLY DEVELOPED  
SHOWERS AND STORMS WERE ALSO BEING SEEN ACROSS PORTIONS OF WEST  
CENTRAL LOWER MICHIGAN. ACTIVITY SHOULD REMAIN NORTH OF THE AREA  
UNTIL POSSIBLY CLOSER TO THE 12Z AS OUTFLOW BOUNDARY PUSHES SOUTH  
AND ALLOWS FOR AT LEAST A CHANCE FOR A FEW SHOWERS OR STORMS. HAVE  
ATTEMPTED TO CAPTURE THIS AS BEST AS POSSIBLE WITH POPS EXPANDING  
SOUTHWARD FOR A PERIOD THIS AFTERNOON AS WARM SECTOR BECOMES  
RATHER UNSTABLE WITH MLCAPE 1000-2000 J/KG. BEST SHEAR WILL  
RESIDE WELL NORTH OF THE AREA RESULTING IN MORE PULSY STORMS THAT  
ARE ABLE TO DEVELOP WITH SPC MARGINAL RISK BEING PUSHED NORTH OF  
THE FORECAST AREA.  
 
CLOUD COVER TODAY WILL BE KEY IN DETERMINING FINAL AFTERNOON  
HIGHS WITH POTENTIAL FOR LOCATIONS IN THE SOUTHWEST TO GET QUITE  
WARM ONCE AGAIN. HIGHER DEWPOINTS MAY LIMIT OVERALL POTENTIAL FOR  
MID 90S BUT CAN'T FULLY RULE OUT. KEPT WITH PREVIOUS FORECAST WITH  
COOLEST READINGS NORTH AND EAST WHERE CLOUDS MAY LINGER LONGER.  
 
DRY FORECAST THIS EVENING AND MOST, IF NOT ALL OF TONIGHT AS LINE  
OF SHOWERS AND THUNDERSTORMS ADVERTISED TO DEVELOP WEST OF US  
ALONG THE COLD FRONT AND MOVE EAST. MOST MODELS WEAKEN THIS  
CONVECTION AS IT OUTRUNS BEST FORCING BUT LEFT SOME LOW POPS IN  
W/NW AREAS TOWARDS MORNING. WINDS WILL INCREASE AS LOW LEVEL JET  
RAMPS UP TO OVER 50 KTS AND AT LEAST SOME MIXING OF STRONGER WINDS  
OCCURS.  
 

 
   
LONG TERM
 
(FRIDAY THROUGH WEDNESDAY)  
ISSUED AT 424 AM EDT THU SEP 20 2018  
 
OUTFLOW BOUNDARY/PRE FRONTAL TROUGH FROM OVERNIGHT CONVECTION WILL  
BE THE MAIN FOCUS FOR BEST CONVECTION CHANCES. HI RES MODELS  
GENERALLY SPLIT ON EITHER THIS BOUNDARY RE-FIRING IN FAR SE AREAS  
OR POSSIBLY REMAINING SOMEWHAT ACTIVE WITH SHOWERS DURING THE  
MORNING THAT EVENTUALLY INTENSIFY. INSTABILITY AND INCREASING  
SHEAR WARRANT A MARGINAL RISK OF SEVERE FOR AT LEAST SE THIRD IF  
NOT A BIT FURTHER NW. SPC DAY 2 OUTLOOK EXPANDED MARGINAL NW INTO  
SE 2/3RD OR SO OF THE AREA. MAY BE SOMEWHAT OVERDONE BUT WITH SOME  
MODELS HINTING AT STORMS FURTHER NW DUE TO POSSIBLE SLOWER  
MOVEMENT OF FRONT CAN'T ARGUE.  
 
PRECIP SHOULD BE CLEAR OF THE AREA BY FRIDAY NIGHT WITH MUCH  
COOLER AIR FUNNELING IN. HIGHS IN THE 60S WILL DOMINATE SATURDAY  
WITH ONLY SOME MODERATION BACK TOWARDS NORMAL TEMPS INTO MID WEEK  
AS STRONGER TROUGH DIGS INTO THE PLAINS AND WORKS EAST WITH  
INCREASING CHANCES FOR SHOWERS AND POSSIBLY SOME STORMS STARTING  
AS EARLY AS MONDAY AFTERNOON BUT ESPECIALLY TUES AFTERNOON AND  
EVENING WHEN TIMING OF ENERGY AND PEAK HEATING ALL COMBINE. COLDER  
AIR WILL RETURN AGAIN BEHIND THE FRONT.  
 

 
   
AVIATION
 
(FOR THE 12Z TAFS THROUGH 12Z FRIDAY MORNING)  
ISSUED AT 559 AM EDT THU SEP 20 2018  
 
LOW LEVEL WARM FRONT IS BEGINNING TO LIFT NORTHWARD EARLY THIS  
MORNING, WITH SOME WARM ADVECTION INDUCED SHOWERS AND A FEW  
EMBEDDED STORMS HAVING DEVELOPED ACROSS SOUTHERN LOWER MICHIGAN.  
CYCLOGENESIS ACROSS NORTH CENTRAL US AND DIURNAL NORTHWARD MIXING  
OF THIS WARM FRONT WILL CONTINUE TO TAKE BETTER WARM FRONTAL  
FORCING NORTH OF TERMINALS THIS MORNING. FOR TODAY, WILL HAVE TO  
WATCH EVOLUTION OF WEAK OUTFLOW BOUNDARY ACROSS NORTHERN  
ILLINOIS FOR POTENTIAL REDEVELOPMENT LATER TODAY, ALTHOUGH SOME  
QUESTION AS TO HOW FAR SOUTHEAST THIS BOUNDARY WILL BE ABLE TO  
PROGRESS GIVEN BACKGROUND SYNOPTIC FLOW. SOME INDICATIONS NOTED  
IN HRRR/ARW MODEL GUIDANCE OF ISOLATED-SCATTERED SHOWERS/STORMS  
REDEVELOPING THIS AFTERNOON ACROSS FAR NORTHERN INDIANA. SOME  
SUPPORT FOR THIS IDEA COULD BE VERY WEAK VORT MAX LIFTING THROUGH  
UPSTREAM PORTION OF UPPER RIDGE FROM STL VICINITY, POSSIBLE  
CONTRIBUTION OF LINGERING SHALLOW WEAK OUTFLOW, AND SOME WEAK  
DIFFERENTIAL HEATING FORCING GIVEN MID/HIGH CLOUD SHIELD WORKING  
ACROSS THE SOUTHERN GREAT LAKES. STILL NOT QUITE ENOUGH CONFIDENCE  
FOR ANY PRECIP MENTION AT TERMINALS WITH 12Z TAFS GIVEN OVERALL  
LACK OF SUBSTANTIAL MID/UPPER FORCING, BUT THIS WILL BE MONITORED.  
 
NORTHWARD RETREAT OF WARM FRONT WILL ALLOW FOR MODERATE SOUTH-  
SOUTHWEST WINDS IN THE 10-15G20KT KNOT RANGE THIS AFTERNOON. DID  
INCLUDE MENTION OF LLWS AFTER 05Z TONIGHT AS NORTHERN INDIANA IS  
LOCKED IN WARM SECTOR WITH SOME DECOUPLING AND PRESENCE OF STRONG  
LOW LEVEL INVERSION. VFR CONDITIONS SHOULD HOLD THROUGH THIS  
PERIOD OUTSIDE OF ANY LOW PROB SHOWER/STORM CHANCES BOTH THIS  
AFTERNOON, AND THEN AGAIN TOWARD VERY END OF THE PERIOD AS PRE-  
FRONTAL TROUGH APPROACHES.  
 

 
   
IWX WATCHES/WARNINGS/ADVISORIES
 
 
IN...NONE.  
MI...NONE.  
OH...NONE.  
LM...SMALL CRAFT ADVISORY FROM 8 PM THIS EVENING TO 11 AM EDT  
SATURDAY FOR LMZ043-046.  
 

 
 

 
 
SYNOPSIS...MURPHY  
SHORT TERM...FISHER  
LONG TERM...FISHER  
AVIATION...MARSILI  
 
VISIT US AT WWW.WEATHER.GOV/IWX  
 
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WWW.TWITTER.COM/NWSIWX  
WWW.YOUTUBE.COM/NWSNORTHERNINDIANA  
 
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