Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 020102 AAA
AFDLOT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
802 PM CDT TUE SEP 1 2015

.UPDATE...
759 PM CDT

NO BIG FORECAST CHANGES FOR THIS MILD EARLY SEPTEMBER NIGHT. A
WARM AND MOIST AIR MASS IS OBSERVED OVER THE AREA THIS
EVENING...WITH 925MB DEW POINTS AOA 17C AND SURFACE DEW POINTS OF
71-75F. SLIGHT LOW-LEVEL CONVERGENCE FROM EARLIER OUTFLOW HAS
HELPED TO PROMPTED STORMS OVER THE I-74 AND 72 CORRIDORS IN
CENTRAL ILLINOIS. SOME OF THIS WILL LIKELY FESTER THROUGH THE REST
OF THE EVENING BUT STEERING FLOW SHOULD KEEP THE STORMS JUST SOUTH
OF THE CWA...POSSIBLY CLIPPING SOUTHERN FORD AND IROQUOIS
COUNTIES.

FURTHER TO THE WEST...WARM ADVECTION INTO THE AFOREMENTIONED
MOIST AIR WILL LIKELY PROMPT SOME SHOWER AND MAYBE STORM
DEVELOPMENT OVER IA AND SOUTHERN MN. ANY OF THIS SHOULD WEAKEN
WELL BEFORE REACHING OUR CWA WEDNESDAY MORNING...OR PROGRESS MUCH
FURTHER NORTH. AGAIN AN UNSTABLE DAY IS FORECAST WEDNESDAY WITH
SOME GUIDANCE PRODUCING SPOTTY RAIN...ALTHOUGH LESS CONVERGENCE
AND SLIGHTLY WARMER MID-LEVEL TEMPS THAN TODAY ARE FORECAST.
DEBRIS AND/OR WARM ADVECTION CLOUDS MAY SNEAK OVER THE AREA IN
THE AFTERNOON WHICH COULD LIMIT WARMING A COUPLE DEGREES...BUT AT
THIS POINT GOING FORECAST LOOKS GOOD FOR WHAT OCCURRED TODAY OVER
OUR AREA AND UPSTREAM.

MTF

&&

.SHORT TERM...
225 PM CDT

THROUGH WEDNESDAY...

AN UPPER LEVEL LOW IS OVER CENTRAL INDIANA AND IT WILL SLIDE
SOUTHEAST THROUGH WEDNESDAY MORNING.  THUNDERSTORMS FORMED ALONG THE
WESTERN EDGE OF THIS LOW EARLIER THIS MORNING AND AFTERNOON...AND
HAVE NOW SUBSIDED. A FEW ISOLATED THUNDERSTORMS ARE POSSIBLE SOUTH
AND EAST OF I-55 THROUGH THIS AFTERNOON BUT THE AFTERNOON SHOULD BE
DRY FOR MOST AREAS.  THE LARGE UPPER LEVEL RIDGE REMAINS IN PLACE
OVER THE CONUS TONIGHT ALLOWING SOUTHWEST WINDS TO WEAKEN. DID NOT
MAKE ANY CHANGES TO LOW TEMPS TONIGHT WITH LOWS IN THE UPPER 60S TO
LOWER 70S.

AN UPPER LEVEL WAVE MOVES OVER WISCONSIN WEDNESDAY WITH A TRAILING
VORTICITY STREAMER THROUGH IOWA. EXPECTING CONVECTION TO BLOSSOM TO
OVER WISCONSIN AND IOWA AND REMAIN OUT OF IL AND INDIANA DURING THE
DAYTIME HOURS TOMORROW.  WINDS WILL BECOME WEST AND THERE IS CHANCE
OF A LAKE BREEZE TOMORROW. WINDS WILL BE AT 20KT OR LESS IN THE
LOWEST 1000 FT OF THE ATMOSPHERE IN THE MORNING AND THEN WEAKEN AS
THE DAY GOES ON. AS SUCH...IF A LAKE BREEZE DOES FORM...IT WOULD BE
LATE IN THE AFTERNOON OR EARLY EVENING AS THE WINDS RELAX.
THEREFORE...RAISED HIGH TEMPS DOWNTOWN INTO THE LOW 90S...WITH HIGH
TEMPS ACROSS THE REGION IN THE LOW 90S.  DEWPOINTS WILL ONCE AGAIN
BE IN THE 70S LEADING TO HEAT INDICES IN THE MID TO UPPER 90S.

ALSO ISSUED AN AIR QUALITY STATEMENT FOR LAKE AND PORTER COUNTIES IN
INDIANA AS AN AIR QUALITY DAY WAS FORECAST FOR PARTS OF NW INDIANA
BY THE INDIANA DEPARTMENT OF ENVIRONMENTAL MANAGEMENT. OZONE WILL BE
THE PRIMARY POLLUTANT WEDNESDAY.

JEE

&&

.LONG TERM...
225 PM CDT

WEDNESDAY NIGHT THROUGH TUESDAY...

A WEAK MID-LVL WAVE WILL BE SLIDING EAST ACROSS THE UPPER MIDWEST
WEDNESDAY EVENING...AND MAY BRING AN ISOLATED SHOWER/THUNDERSTORM TO
SOUTHERN WISC AND COULD PUSH INTO NORTHERN IL EARLY THUR. GUIDANCE
THEN PROGRESSIVELY BUILDS THE MID-LVL HEIGHTS OVER THE PLAINS AND
EVENTUALLY ARRIVES OVER THE FORECAST AREA THUR EVE. THIS SHOULD THEN
LIMIT THE POTENTIAL FOR CONVECTION/PRECIP LATE IN THE WEEK. THE BULK
OF CONVECTION WILL BE DISPLACED WELL NORTH INTO CANADA AND PERHAPS
NORTHERN WISC.

WARM THERMAL RIDGE REMAINS IN PLACE THROUGH THE END OF THE WEEK INTO
THE WEEKEND...SUPPORTING HIGHS IN THE UPR 80S TO ARND 90. WITH
GENERALLY SOUTHERLY FLOW AT THE SFC...LOW LEVEL MOISTURE WILL
CONTINUE TO ADVECT NORTH AND LINGER OVER THE REGION. THIS WILL KEEP
DEW POINTS ELEVATED IN THE UPR 60S TO ARND 70 LATE IN THE WEEK THRU
THE WEEKEND.

ENSEMBLES CONTINUE TO POINT TOWARDS A MORE ROBUST MID-LVL SHORTWAVE
ARRIVING ACROSS THE PAC-NW SAT/SUN AND STEADILY SLIDING EAST.
GUIDANCE HAS BEEN LEANING TOWARDS A WAVE DEVELOPING ALONG THE LEE-
SIDE OF THIS SHORTWAVE EARLY NEXT WEEK...WITH AN INCREASED CHANCE
FOR SHOWERS/THUNDERSTORMS FOR THE FIRST HALF OF NEXT WEEK. THIS MAY
BRING ABOUT A PATTERN SHIFT...AS ENSEMBLES INDICATE THE ROBUST
THERMAL RIDGE WILL BE PUSHING EAST AND THE ARRIVAL OF COOLER AIR
BEYOND THE CURRENT EXTENDED PERIODS.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* POTENTIAL FOR VSBY RESTRICTIONS IN BR LATE TONIGHT.

* LOW CHANCE FOR LAKE BREEZE WIND SHIFT LATE WEDNESDAY.

* LOW CHANCE FOR TSRA LATE WEDNESDAY.

RC

//DISCUSSION...UPDATED 00Z...

SHOULD OVERALL BE A FAIRLY QUIET TAF PERIOD. TSRA OVER CENTRAL IL
WILL STAY WELL SOUTH OF TERMINALS AND DISSIPATE THIS EVENING.
MOIST AIRMASS AND LIGHT WINDS COULD AGAIN SUPPORT LIGHT FOG/BR
OVERNIGHT INTO EARLY WEDS...BUT THE SETUP IS LESS CONDUCIVE THAN
THE PREVIOUS TWO NIGHTS. FAVORING DPA AND RFD FOR MVFR BR
CURRENTLY. WILL MONITOR TRENDS FOR ORD/MDW/GYY.

EXPECTING MODEST WEST-SOUTHWEST WINDS ON WEDNESDAY THAT MAY NOT
BE STRONG ENOUGH TO PREVENT A LAKE BREEZE FROM FORMING...BUT AT
THIS TIME THINKING IT WILL NOT PUSH THROUGH TERMINALS WITH GYY
PERHAPS HAVING BEST SHOT. FORECAST SOUNDINGS INDICATE UNSTABLE
CONDITIONS AND MINIMAL CAPPING WEDNESDAY AFTERNOON. HOWEVER...ASIDE
FROM LAKE BREEZE...A TRIGGER FOR CONVECTION WILL BE LACKING UNTIL
AN UPPER DISTURBANCE OVER WI POSSIBLY SHIFTS SOUTHEAST LATE
WEDNESDAY NIGHT. HAVE GONE WITH A DRY FORECAST IN TAFS...INCLUDING
ORD 30-HR TAF.

RC

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* LOW IN MVFR VSBY RESTRICTIONS LATE TONIGHT INTO EARLY WEDS.

* MEDIUM-HIGH THAT LAKE BREEZE WILL NOT MOVE THROUGH ORD/MDW LATE
  WEDNESDAY AFTERNOON/EARLY EVENING.

* LOW IN TSRA POTENTIAL ON WEDNESDAY AND WEDNESDAY EVENING.

RC

//OUTLOOK FOR ORD/MDW FOR 12Z THURSDAY-12Z TUESDAY...UPDATED 00Z...

THURSDAY...MAINLY VFR. LOW CHANCE FOR SHRA/TSRA AND ASSOCIATED
VSBY IMPACTS. EAST-SOUTHEAST WIND SHIFT EXPECTED IN AFTERNOON.

FRIDAY...VFR. SOUTHEAST TO EAST WINDS.

SATURDAY AND SUNDAY...VFR. EAST-SOUTHEAST WIND SHIFT POSSIBLE
DURING AFTERNOON.

MONDAY...LOW CHANCE TSRA AND ASSOCIATED VSBY IMPACTS. SOUTHWEST
WINDS.

RC

&&

.MARINE...
230 PM CDT

HIGH PRESSURE WILL REMAIN OVER THE EASTERN OHIO
VALLEY...ALLOWING SOUTHERLY TO AT TIMES SOUTHWESTERLY FLOW ACROSS
LAKE MICHIGAN. A WEAK FRONTAL BOUNDARY WILL TRY TO PUSH SOUTH ACROSS
FAR NORTHERN LAKE MICHIGAN AND TURN WINDS NORTHERLY...BUT THEN TURN
SOUTH AGAIN LATE IN THE WEEK. THE GRADIENT WILL REMAIN LIGHT...WHICH
WILL ALLOW FOR THE DEVELOPMENT OF AFTERNOON LAKE BREEZES. VERY WARM
AND MOIST AIR WILL CONTINUE TO FLOW ACROSS THE LAKE...AND COULD
ALLOW FOR PATCHY FOG TO REDEVELOP LATE TONIGHT. CURRENTLY WINDS ARE
JUST STRONG ENOUGH THAT THIS DOES NOT APPEAR TO BE FORMING. SO
CONFIDENCE IS CURRENTLY LOW.

BEACHLER

&&

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

$$

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