171  
FXUS63 KIWX 220035  
AFDIWX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE NORTHERN INDIANA  
835 PM EDT MON AUG 21 2017  
   
SYNOPSIS
 
 
ISSUED AT 820 PM EDT MON AUG 21 2017  
 
RAIN CHANCES WILL RAMP UP OVERNIGHT AHEAD OF A STRONG COLD FRONT  
PROGRESSING OUT OF THE PLAINS. THE BEST CHANCES OF SHOWERS AND  
THUNDERSTORMS WILL EXIST LATE TONIGHT ACROSS THE NORTHWEST AND  
EARLY TUESDAY MORNING SOUTHEAST. OTHERWISE MUCH DRIER AND COOLER  
AIR WILL OVERSPREAD THE ARE IN BEHIND THIS SYSTEM MIDWEEK THROUGH  
NEXT WEEKEND.  
 

 
   
UPDATE
 
 
ISSUED AT 820 PM EDT MON AUG 21 2017  
 
LOW LEVEL MOISTURE TRANSPORT IS BEGINNING TO RAMP UP ACROSS THE  
MID MS VALLEY TO THE FAR WESTERN GREAT LAKES EARLY THIS EVENING AS  
SOUTHWEST FLOW GRADUALLY STRENGTHENS IN ADVANCE OF STRONGER UPPER  
LEVEL SHORT WAVE EJECTING OUT OF THE CENTRAL PLAINS. THE  
COMBINATION OF THE LEADING EDGE OF THIS STRONGER MOISTURE  
TRANSPORT AND A WEAKER SMALLER SCALE VORT WORKING ACROSS THE  
SOUTHERN GREAT LAKES THIS EVENING HAS BEEN ENOUGH TO ALLOW FOR AN  
INCREASE IN ISOLATED-SCATTERED THUNDERSTORMS ACROSS NORTHEAST  
ILLINOIS PAST FEW HOURS. SOME QUESTION AS TO THE LONGEVITY OF  
THESE SHOWERS AND THUNDERSTORMS EASTWARD INTO NORTH CENTRAL  
INDIANA OVER THE NEXT FEW HOURS. WANING OF SFC BASED INSTABILITY  
AND EASTWARD ADVECTION OF MIXED LAYER ALOFT MAY TEND TO TEMPER  
INSTABILITY MAGNITUDES INITIALLY THIS EVENING. HOWEVER, DURING THE  
OVERNIGHT HOURS, MORE SUBSTANTIAL LOW LEVEL MOISTURE SURGE WILL  
ALLOW MUCAPE AXIS ACROSS MID MS VALLEY TO RAPIDLY ADVECT NORTHEAST  
INTO THE AREA.  
 
WILL MAINTAIN TREND OF PREVIOUS FORECAST WITH RAMP  
UP OF LIKELY POPS ACROSS THE NORTHWEST HALF OF AREA COINCIDENT  
WITH GREATEST MUCAPE AXIS AS STRONGER UPPER SUPPORT MOVES IN  
OVERNIGHT. SEVERE THREAT DOES APPEAR TO BE ON THE MINIMAL SIDE,  
WITH A LOW END POTENTIAL OF AN ISOLATED INSTANCE OF HAIL/WIND  
GIVEN FAIRLY STEEP LAPSE RATES ALOFT AND MODERATE AMOUNT OF  
ELEVATED INSTABILITY. MAY NEED TO WATCH POTENTIAL OF SOME POCKETS  
OF HEAVIER RAINFALL ALSO OVERNIGHT, GIVEN PWATS INCREASING UP  
AROUND 2 INCHES AND STRENGTHENING LOW LEVEL JET ALLOWING FOR SOME  
UPWIND DEVELOPMENT. THE WINDOW OF GREATEST POTENTIAL OF ISOLATED  
STRONG STORMS/LOCALIZED HEAVY RAIN POTENTIAL APPEARS TO BE IN THE  
06Z-10Z TIMEFRAME. JUST SOME MINOR ADJUSTMENTS MADE TO POPS TO  
ACCOUNT FOR CURRENT TRENDS BUT OVERALL NO MAJOR CHANGES  
ANTICIPATED AT THIS TIME.  
 

 
   
SHORT TERM
 
(TONIGHT AND TUESDAY)  
ISSUED AT 200 PM EDT MON AUG 21 2017  
 
CHALLENGING FCST THIS PERIOD OWING TO MESSY MESOSCALE. DECAYING MCS  
OVER CNTRL IL ATTM WITH REMNANT MCV OVER NW IL. EXPECT THIS  
FEATURE WILL CONTINUE DOWNSTREAM AND INITIATE NEW CONVECTION THIS  
EVENING INVOF WELL ESTABLISHED LAKE BREEZY BOUNDARY OVER SW MI.  
SECONDARY ADDITIONAL DEVELOPMENT LIKELY WITHIN ELEVATED LOW LEVEL  
WARM ADVECTION ZONE THROUGH CNTRL IL WHICH WOULD THEN CARRY NEWD  
THROUGH WRN ZONES INTO LATE EVENING.  
 
OTHERWISE LARGE SCALE COLD FRONT IN ASSOCIATION WITH POTENT SW  
TROUGH DIGGING OUT OF CNTRL MB WILL CONTINUE SEWD OVERNIGHT. EWD  
PASSAGE OF PREFRONTAL TROUGH EXPECTED LATE TONIGHT WITH COLD FRONT  
TO FOLLOW TUE MORNING FOR MOST. FAR SE ZONES LAG WITH PERHAPS SOME  
ISOLATED DEVELOPMENT POSSIBLE TUE AFTN PENDING DEGREE OF CONVECTIVE  
OVERTURNING THAT OCCURS IN THE MORNING. REGARDLESS 12Z GUIDANCE  
ACROSS THE BOARD QUITE TEPID WITH POPS. BEST COVERAGE/MOST  
APPRECIABLE QPF EXPECTED THROUGH NW ZONES WITHIN ZONE OF MAXIMIZED  
LOW LEVEL MASS FLUX ALONG NOSE OF LOW LEVEL JET LATE. BUT GIVEN  
TIMING AND RELATIVE STABLE NEAR SFC LAYER SEVERE WEATHER THREAT  
LOOKS MINIMAL IF AT ALL.  
 

 
   
LONG TERM
 
(WEDNESDAY THROUGH MONDAY)  
ISSUED AT 200 PM EDT MON AUG 21 2017  
 
LITTLE OF NOTE THIS PERIOD AS BROAD SFC RIDGING BECOMES ESTABLISHED  
THROUGH THE WRN LAKES WITH AN EXTENDED PERIOD OF DELIGHTFUL LATE  
SUMMER WEATHER. SOME SEMBLANCE OF RETURN FLOW SEEN TOWARD MON WHERE  
SOME TOKEN LOW CHC POPS WERE RETAINED YET SYS ALOFT/RETURN FLOW  
BOTH LOOK PRETTY WEAK.  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z TUESDAY EVENING)  
ISSUED AT 820 PM EDT MON AUG 21 2017  
 
THE LEADING EDGE OF STRONGER LOW LEVEL MOISTURE TRANSPORT AND  
SMALL SCALE VORT MAX TRACKING ACROSS THE SOUTHERN GREAT LAKES HAVE  
ALLOWED ISOLATED/SCATTERED SHOWERS AND THUNDERSTORMS TO DEVELOP  
FROM SOUTHWEST LOWER MICHIGAN TO FAR WEST CENTRAL INDIANA.  
INSTABILITY MAGNITUDES AT LEAST INITIALLY THIS EVENING, MAY BE A  
BIT MORE UNFAVORABLE ACROSS NORTHEAST INDIANA, AND SOME QUESTION  
AS TO HOW THESE SHOWERS WILL HOLD TOGETHER OVER THE NEXT HOUR OR  
TWO. BETTER CHANCE OF SHOWERS AND THUNDERSTORMS SHOULD STILL HOLD  
OFF UNTIL THE OVERNIGHT HOURS WHEN INTENSIFYING LOW LEVEL JET  
ALLOWS FOR CONTINUED UPTICK IN MOISTURE TRANSPORT, AND AS STRONGER  
UPPER LEVEL SHORT WAVE APPROACHES FROM THE WEST. DID MAINTAIN  
MENTION OF MVFR VSBYS WITH PRECIP...AND THEN IN ASSOCIATION WITH  
THE FRONT ON TUESDAY. LLWS CRITERIA MAY BE MET FOR A SHORT TIME  
LATE TONIGHT INTO EARLY TUESDAY MORNING IN ADVANCE OF THE FRONT.  
OTHERWISE, TRANSITION BACK TO VFR CONDITIONS EXPECTED TUESDAY  
AFTERNOON WITH SOME NORTHWEST POST-FRONTAL GUSTS INTO THE 15 TO 20  
KNOT RANGE EXPECTED.  
 

 
   
IWX WATCHES/WARNINGS/ADVISORIES
 
 
IN...BEACH HAZARDS STATEMENT FROM TUESDAY MORNING THROUGH WEDNESDAY  
AFTERNOON FOR INZ003.  
 
MI...BEACH HAZARDS STATEMENT FROM TUESDAY MORNING THROUGH WEDNESDAY  
AFTERNOON FOR MIZ077.  
 
OH...NONE.  
LM...SMALL CRAFT ADVISORY FROM 5 AM TUESDAY TO 5 PM EDT WEDNESDAY FOR  
LMZ043-046.  
 

 
 

 
 
SYNOPSIS...MARSILI/T  
UPDATE...MARSILI  
SHORT TERM...T  
LONG TERM...T  
AVIATION...MARSILI  
 
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