Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 032341
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
641 PM CDT THU SEP 3 2015

.SHORT TERM...
254 PM CDT

THROUGH SATURDAY...

MAIN FORECAST CONCERNS OVER THE NEXT COUPLE DAYS WILL BE CONTINUED
UNSEASONABLY WARM AND HUMID CONDITIONS...AS WELL AS PRECIPITATION
CHANCES.

THIS AFTERNOON...SATELLITE IMAGERY AND SURFACE OBS SHOW A WEAK
TROUGH DRAPED FROM NEAR LSE THROUGH MSN AND MKE AND ACROSS LAKE
MICHIGAN. THIS TROUGH WILL CONTINUE TO SAG SOUTH THE THE REMAINDER
OF THE AFTERNOON AND INTO THE EVENING WITH WINDS TURNING FROM WEST-
SOUTHWEST AHEAD TO NORTHEAST BEHIND. THIS BOUNDARY IS EXPECTED TO
ENHANCE/BECOME COUPLED WITH A LAKE BREEZE THIS AFTERNOON WHICH
SHOULD HELP PUSH TEMPERATURES BACK INTO THE 70S ALONG THE LAKE
FRONT. THE BOUNDARY COULD ALSO SERVE AS THE FOCUS FOR
ISOLATED/WIDELY SCATTERED THUNDERSTORM DEVELOPMENT. LATEST SPC MESO-
ANALYSIS PAGE SHOWS AXIS OF MODERATELY STRONG INSTABILITY ACROSS
NORTHERN ILLINOIS...WEAKLY CAPPED OR UNCAPPED...HOWEVER LACK OF ANY
FORCING AND WEAK MIDLEVEL HEIGHT RISES APPEAR TO BE KEEPING
CONVECTION IN CHECK FOR THE MOMENT. AS THE AFOREMENTIONED TROUGH
MOVES INTO THE AREA THIS EVENING...IT SHOULD BE SUFFICIENT TO KICK
OF A FEW THUNDERSTORMS THOUGH CHANCES WILL THEN BEGIN TO DIMINISH
LATE THIS EVENING AS THE BOUNDARY LAYER BEGINS TO STABILIZE
NOCTURNALLY. WITH THE BOUNDARY IN PLACE...ISOLATED DEVELOPMENT MAY
LINGER OVERNIGHT...BUT NOT EXPECTING MUCH COVERAGE.

FOR FRIDAY...

TROUGH AXIS SETTLES OVER THE AREA ON FRIDAY WITH FAIRLY WEAK FLOW AT
THE SURFACE. LOW PRESSURE OVER THE PLAINS WILL ADVANCE EASTWARD WITH
MODEST WAA SPREADING INTO PORTIONS OF NORTHERN ILLINOIS. ALSO...A
LAKE BREEZE FORMS IN THE AFTERNOON. ANOTHER WARM AND HUMID DAY IS
ON TAP...THUS AN UNSTABLE ENVIRONMENT WILL DEVELOP AGAIN. GUIDANCE
IS MIXED WITH CONVECTIVE DEVELOPMENT...BUT HAVE LEANED AWAY FROM
THE MORE AGRESSIVE GUIDANCE AT THIS POINT...AND HAVE KEPT THE
HIGHEST POPS...STILL IN THE CHANCE CATEGORY...FOR THE LATER
AFTERNOON ALONG THE LAKE BREEZE.

BMD

&&

.LONG TERM...

SATURDAY THROUGH THURSDAY...

308 PM...MULTIPLE FORECAST CONCERNS INCLUDING HIGH TEMPS SUNDAY
AND MONDAY...PRECIP CHANCES/TIMING SUNDAY NIGHT THRU WEDNESDAY
THEN COOLER/LESS HUMID END OF NEXT WEEK.

THE UPPER RIDGE WILL BE BUILDING ACROSS THE AREA ON SATURDAY AND
MODELS ARE IN GENERAL AGREEMENT WITH DECREASING POPS THRU THE DAY
AND LIKELY DRY CONDITIONS INTO SATURDAY EVENING. WINDS WILL LIKELY
BE LIGHT ENOUGH THAT A LAKE BREEZE TURNING WINDS SOUTHEASTERLY
ALONG THE IL SHORE SHOULD DEVELOP. NOT SURE IF THIS WILL BE STRONG
ENOUGH TO AID ANY ISOLATED CONVECTION TO DEVELOP. POSSIBLE THAT
THERE COULD STILL BE SOME ISOLATED SHOWERS OR THUNDERSTORMS
SATURDAY AFTERNOON SO MAINTAINED SLIGHT CHANCE POPS WITH DRY
CONDITIONS EXPECTED SATURDAY NIGHT AND SUNDAY.

HIGH TEMPS IN THE UPPER 80S/AROUND 90 LOOK REASONABLE SATURDAY...
COOLER ALONG THE IL SHORE. SUNDAY LOOKS TO BE WARMER WITH HIGHS
GENERALLY IN THE LOWER 90S MOST LOCATIONS. PERHAPS SLIGHTLY
COOLER ALONG THE IL SHORE WITH SOUTH/SOUTHEAST WINDS. WINDS MAY BE
A BIT STRONGER ON SUNDAY...PERHAPS IN THE 10-20 MPH RANGE
ESPECIALLY ACROSS NORTHWEST IL BUT WITH DEWPOINTS IN THE UPPER
60S/LOWER 70S BOTH DAYS...AFTERNOON HEAT INDICES IN THE MID 90S
POSSIBLE BOTH DAYS.

A FRONTAL BOUNDARY WILL MOVE SOUTHEAST ACROSS THE MIDWEST SUNDAY
INTO SUNDAY NIGHT AND 12Z GFS/ECMWF ARE IN GOOD AGREEMENT WITH
PRECIP HOLDING OFF UNTIL AFTER MIDNIGHT...PERHAPS NOT UNTIL CLOSER
TO DAWN MONDAY MORNING SO SUNDAY EVENING LOOKS DRY. AS THIS FRONT
SAGS INTO THE CWA MONDAY...FORCING BEGINS TO WEAKEN SO CONFIDENCE
REGARDING PRECIP COVERAGE AND TIMING IS FAIRLY LOW. COULD BE
DECAYING SHOWERS/CONVECTION IN THE MORNING AND THEN NEW
ADDITIONAL DEVELOPMENT WITH DAYTIME HEATING LATER MONDAY
AFTERNOON/EVENING. AT THIS POINT...MAINTAINED CHANCE POPS BOTH
LATE SUNDAY NIGHT AND ON MONDAY. HIGH TEMPS WILL BE TRICKY FOR
MONDAY. COOLER ACROSS NORTHERN AREAS SEEMS ON TRACK BUT SOUTHEAST
AREAS COULD EASILY GET BACK INTO THE UPPER 80S/LOWER 90S.

THIS FRONT THEN LAYS OUT ACROSS THE AREA AS IT WEAKENS MONDAY NIGHT
INTO TUESDAY WHILE A STRONGER COLD FRONT PUSHES ACROSS THE NORTHERN
PLAINS TUESDAY AND REACHES THE AREA TUESDAY NIGHT INTO WEDNESDAY.
WHILE THIS IS STILL SEVERAL DAYS OUT...GIVEN THE HIGH DEWPOINT AIR
LIKELY POOLING ALONG THIS BOUNDARY...ITS POSSIBLE THERE COULD BE
PERIODS OF HEAVY RAIN AT LEAST ACROSS SOME PARTS OF THE REGION.
ONCE THE SECOND STRONGER COLD FRONT MOVES ACROSS THE AREA ON
WEDNESDAY...PRECIP SHOULD COME TO AN END WITH COOLER AND LESS
HUMID AIR MOVING BACK INTO THE REGION. CMS

&&

.AVIATION...

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

* ISOLD/SCT TSRA/SHRA THROUGH EARLY EVENING...WITH ISOLATED
  ACTIVITY POSSIBLY LINGERING OVERNIGHT.

* ABRUPT WIND SHIFT TO NNE-NELY ARND 00Z WITH WINDS OCNL HIGHER
  GUSTS FOR A SHORT TIME FOLLOWING THE PASSAGE OF A LAKE BREEZE
  BOUNDARY.

* MVFR VIS POSSIBLE OVERNIGHT.

KREIN

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

A VERY WARM...MOIST AIRMASS IS IN PLACE OVER NRN IL/NWRN. AN
EAST-WEST ORIENTED BOUNDARY IS SLOWLY SAGGING SOUTH THROUGH SRN WI
AND HAS TOUCHED OFF A FEW TS AND SHOULD CONTINUE TO DO SO INTO THE
EVENING AS THE BOUNDARY PUSHES INTO NCNTRL/NWRN IL. A LAKE
BREEZE/BACK COLD FRONT IS PUSHING INLAND OFF OF THE LAKE INTO NERN
IL AND AS OF 23Z HAS ALREADY PUSHED THROUGH UGN/PWK AND SHOULD
REACH ORD JUST BEFORE 00Z AND THEN DPA/MDW RIGHT ARND 00Z. WINDS
WILL ABRUPTLY SHIFT TO NELY WITH OCNL GUSTS ARND 15KT FOR A SHORT
TIME FOLLOWING THE PASSAGE OF THE BOUNDARY. TEMPERATURES ARE ALSO
DROPPING OFF QUICKLY FOLLOWING THE BOUNDARY PASSAGE. THROUGH THE
EVENING...WINDS SHOULD DROP OFF TO LGT/VRBL AS HIGH PRESSURE STILL
COVERS THE WESTERN GREAT LAKES AND A VERY WEAK PRESSURE GRADIENT
WILL SET UP OVERNIGHT. DIMINISHING WINDS AND LOWERING DEWPOINT
DEPRESSIONS SHOULD HELP VIS DROP TO MVFR OR IFR LEVELS OVERNIGHT.

CONDITIONS SHOULD BE RELATIVELY QUIET THROUGH THE DAY TOMORROW AS
HIGH PRESSURE PERSISTS OVER THE REGION. VERY WARM CONDITIONS AND
WEAK WINDS SHOULD ALLOW FOR ANOTHER LAKE BREEZE TO DEVELOP
TOMORROW. TIMING OF THE LAKE BREEZE IS A BIT UNCERTAIN AT THIS
POINT...THROUGH DO FEEL THAT IT WILL PUSH INLAND OF THE CHICAGO
AREA TERMINALS SETTING UP ELY WINDS...BUT EXPECT THAT WINDS SHOULD
REMAIN UNDER 10KT. THE LAKE BREEZE MAY BE THE FOCUS FOR TSRA
TOMORROW AFTERNOON...SO HAVE INTRODUCED A PROB30 TS FOR LATE
TOMORROW AFTERNOON.

KREIN


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

* MEDIUM CONFIDENCE IN TSRA/SHRA CHANCES/COVERAGE THROUGH THIS
  EVENING. LOW CONFIDENCE OF DIRECT IMPACT AT THE TERMINALS.

* HIGH CONFIDENCE IN WIND SPEED/DIRECTION AND SHORT PERIOD OF
  HIGHER GUSTS THROUGH EARLY THIS EVENING.

* MEDIUM CONFIDENCE IN LAKE BREEZE DETAILS TOMORROW AFTERNOON.

* MEDIUM CONFIDENCE IN MVFR VIS DEVELOPING OVERNIGHT AND DURATION.

* LOW TO MEDIUM CONFIDENCE IN TS CHANCES TOMORROW AFTERNOON. LOW
  CONFIDENCE IN CHANCES OF DIRECT IMPACT AT THE TERMINALS.

KREIN

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

SATURDAY...SLIGHT CHANCE OF TSRA IN THE AFTERNOON.

SUNDAY...SLIGHT CHANCE OF TSRA LATE.

MONDAY...CHANCE OF TSRA.

TUESDAY...CHANCE OF TSRA.

WEDNESDAY...CHANCE OF TSRA.

KREIN

&&

.MARINE...

308 PM...A WEAK FRONTAL BOUNDARY WILL MOVE SOUTH OF THE LAKE THIS
EVENING WITH A PERIOD OF NORTH/NORTHEAST WINDS EXPECTED INTO
FRIDAY WHICH WILL SLOWLY DIMINISH FIRST ACROSS THE SOUTHERN PART
OF THE LAKE. WINDS WILL SLOWLY TURN SOUTHERLY FRIDAY NIGHT INTO
SATURDAY. THESE SOUTHERLY WINDS WILL SLOWLY INCREASE INTO THE
10-20 KT RANGE BY SUNDAY AS THE GRADIENT TIGHTENS SOME BETWEEN A
LARGE TROUGH OF LOW PRESSURE OVER THE PLAINS AND LARGE HIGH
PRESSURE OVER ATLANTIC COAST. ITS POSSIBLE WINDS MAY INCREASE INTO
THE 15-25 KT RANGE... ESPECIALLY ON THE NORTHERN PART OF THE LAKE.
A WEAKENING COLD FRONT WILL MOVE ACROSS THE LAKE SUNDAY NIGHT INTO
MONDAY MORNING WITH WINDS SHIFTING NORTHERLY BY MONDAY EVENING. IT
APPEARS WINDS WILL SHIFT BACK TO SOUTHERLY TUESDAY UNTIL A
STRONGER COLD FRONT ARRIVES LATE TUESDAY NIGHT INTO WEDNESDAY...
WHICH WILL SHIFT WINDS TO THE NORTHWEST. CMS

&&

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

$$

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