Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 152209
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
509 PM CDT MON SEP 15 2014

.DISCUSSION...
221 PM CDT

TONIGHT...
FRONTAL BOUNDARY THIS AFTERNOON CONTINUES TO SLIDE SOUTHEAST ACROSS
THE FORECAST AREA. VISIBLE IMAGERY ALONG AND JUST AHEAD OF THE
BOUNDARY INDICATED SOME VERTICAL GROWTH TO THE CLOUDS...SUGGESTIVE
OF SOME INSTABILITY. THERE HAS BEEN SOME SPOTTY CONVECTION ACROSS
NORTHEAST MO STRETCHING EAST THROUGH IL/CENTRAL IN BUT SOUTH OF THE
CWFA. DESPITE CONSIDERABLE SOLAR SHIELDING HAS BEEN ABLE TO KEEP
TEMPS IN THE 50S...WITH A FEW LOCATIONS BEING ABLE TO NUDGE TO ARND
60. WINDS WILL SHIFT NORTHWEST/NORTH BEHIND THE BOUNDARY WITH MUCH
DRIER/COOLER AIR STEADILY FILTERING INTO THE REGION OVERNIGHT.
PRECIP WILL COME TO AN END LATER THIS AFTN/EARLY EVE...WITH JUST A
FEW LINGERING SHOWERS ALONG AND EAST OF GIBSON CITY TO FOWLER. TEMPS
OVERNIGHT WILL COOL INTO THE LOW/MID 40S...WITH A FEW OF THE
TRADITIONALLY COOL SPOTS POSSIBLY DIPPING INTO THE UPR 30S BEFORE
DAYBREAK TUE. MEANWHILE CLOSER TO DOWNTOWN CHICAGO...THE URBAN HEAT
ISLAND SHUD AID IN KEEPING TEMPS IN THE UPR 40S TO ARND 50. THE ONLY
WILDCARD FOR OVERNIGHT LOWS WILL BE SKY COVER/WINDS...WHICH WITH
HIGH PRESSURE BUILDING IN SHUD ERODE/DIMINISH.

THERE IS ALSO SOME CONCERN THAT WITH THE LACK MIXING THIS AFTN AND
EXPECTED TEMPS BEING COOLER THAN AFTN DEW PTS...COUPLED WITH LIGHT
WINDS...COULD POINT TOWARDS PATCHY FOG DEVELOPING AWAY FROM THE
METRO AREA.

TUESDAY THROUGH THURSDAY...
RATHER BENIGN PATTERN FOR A GOOD PORTION OF THE WEEK. BROAD
ANTI-CYCLONIC FLOW WILL BUILD INTO THE REGION TUESDAY...WITH THE
MID-LVL TROUGH PIVOTING EAST. THE LONGWAVE PATTERN WILL LIKELY KEEP
THE REGION IN A CONTINUED NORTHWEST FLOW AS MID-LVL RIDGING RETURNS
TO THE WESTERN CONUS. GUIDANCE SUGGESTS THE SFC RIDGE THAT ARRIVES
TUE WILL SHIFT EAST...HOWEVER ANOTHER DOME OF HIGH PRESSURE FROM
CANADA IS THEN POISED TO SLIDE SOUTH AND CONTINUE THE DRY/COOL
WEATHER WED/THUR. TEMPS WILL GENERALLY REMAIN IN THE MID/UPR
60S...BUT POSSIBLY REACH 70 DEG IN SEVERAL AREAS FOR THUR. ABUNDANT
SUNSHINE IS EXPECTED EACH OF THE NEXT SEVERAL DAYS AS WELL.
OVERNIGHT LOWS WILL ALSO DIP INTO THE 40S TUE NGT/WED NGT...THEN
WITH THE SFC RIDGE BEGINNING TO PUSH EAST THUR NGT TEMPS COULD HOLD
ARND 50.

CONFIDENCE...HIGH.

FRIDAY THROUGH SUNDAY...
WITH THE SFC RIDGE POISED TO PUSH EAST OF THE REGION FRI...FLOW WILL
BE TURNING SOUTHERLY AND THE MID-LVL WAVE STARTING TO FLATTEN.
ENSEMBLES CONTINUE TO SHOW CONSIDERABLE SPREAD ON TIMING OF
PRECIP/MOISTURE RETURNING...WITH MOST MEMBERS POINTING TOWARDS FRI
NGT. THE ATMOSPHERE WILL MODERATE BY FRI/SAT WITH TEMPS IN THE
MID/UPR 70S...OR MORE SEASONAL. SAT APPEARS TO BE THE MOST FAVORABLE
PERIOD TO SEE SPOTTY SHOWERS/THUNDERSTORMS AS A MID-LVL TROUGH
DEVELOPS OVER THE CENTRAL PLAINS AND NEARS THE REGION SAT AFTN/EVE.
THEN FOR THE SECOND HALF OF THE WEEKEND SFC RIDGING IS PROGGED TO
RETURN TO THE REGION.

CONFIDENCE...MEDIUM/HIGH.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 22Z...

* FRONTAL BOUNDARY APPROACHING FROM NW PASSING ORD BY 2245-2300Z
  AND MDW BY 2330Z OR SO SHIFTING WINDS TO VARIABLE 350-020 WITH
  SPEEDS 10-12 KT WITH A FEW INITIAL GUSTS TO 15-17 KT POSSIBLE FOR
  A SHORT TIME.

* WINDS LIKELY FALL BELOW 10 KT 00-01Z.

* FRONT WILL BRING RETURN TO MVFR WITH -RA/DZ.

* SKIES CLEAR EARLY THIS EVENING.

MDB

//DISCUSSION...UPDATED 22Z...

FRONTAL BOUNDARY NOW APPROACHING ORD/DPA HEADING SOUTHEAST.
CONDITIONS IMPROVE TO VFR FOR A SHORT TIME RIGHT AHEAD OF THE
FRONT BUT MVFR RE-DEVELOPS WITH FRONTAL PASSAGE ALONG WITH -RA/DZ.
WINDS SHIFT TO NORTH WITH SOME VARIABILITY BUT SPEEDS TO INCREASE
TO 10-12 KT FOR AT LEAST A SHORT TIME. LIGHT RAIN/DRIZZLE WILL
LINGER FOR 1-2 HOURS BEHIND THE FRONT KEEPING CIGS DOWN BUT
GREATER VARIABILITY IN CIGS WILL LIKELY OCCUR AS SUNSET
APPROACHES...WITH CLEARING EARLY THIS EVENING. HAVE MADE SOME
ADJUSTMENTS TO ADD DETAIL TO TIMING OF CHANGES BASED ON LATEST
TRENDS.

MDB

FROM 18Z...

A COLD FRONT WILL SAG SOUTHWARD ACROSS NORTHERN ILLINOIS THROUGH
THE AFTERNOON. THIS WILL LEAD TO A NORTHERLY WIND SHIFT ACROSS THE
TERMINALS BY MID TO LATE THIS AFTERNOON. THE EASTERN TAF SITES
COULD EXPERIENCE A BIT MORE OF A NORTHEASTERLY COMPONENT.
HOWEVER...WIND SPEEDS OVERALL APPEAR THAT THEY MAY REMAIN AT OR
UNDER 10 KT WITH THIS NORTHERLY WIND SHIFT.

SCATTERED SHOWERS ARE ALSO EXPECTED ACROSS THE REGION OVER THE
NEXT SEVERAL HOURS AS THIS FRONT...AND AN UPPER LEVEL DISTURBANCE
MOVES OVER THE AREA. ALTHOUGH THESE SHOWERS WILL BE LIGHT...SOME
LOWER CIGS INTO THE LOWER MVFR RANGE MAY DEVELOP FOR A PERIOD THIS
AFTERNOON.

SURFACE HIGH PRESSURE WILL BUILD OVER THE AREA TONIGHT INTO
TUESDAY. THIS WILL RESULT IN CLEARING SKIES THIS EVENING...AND
LIGHT WINDS INTO TUESDAY. SOME PATCHY FOG IS POSSIBLE
OVERNIGHT...MAINLY TO THE WEST AND SOUTH OF THE MAIN CHICAGO
TERMINALS. A LAKE BREEZE MAY DEVELOP ON TUESDAY AFTERNOON WITH THE
LIGHT WIND PATTERN. HOWEVER...CONFIDENCE ON HOW FAR INLAND THIS
FEATURE WOULD PUSH INLAND IS TOO LOW TO ADD A MENTION IN THE
LATEST 30 HOUR ORD TAF.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* HIGH CONFIDENCE IN TIMING OF FRONTAL PASSAGE. HIGH CONFIDENCE IN
  SHIFT TO 350-020 AND THAT WINDS WILL INCREASE TO 10-12 KT.
  MEDIUM CONFIDENCE IN DURATION/FREQUENCY OF 10+ KT WINDS.

* MEDIUM-HIGH CONFIDENCE IN WINDS FALLING BELOW 10 KT BY 01Z.

* HIGH CONFIDENCE IN CLEARING SKIES THIS EVENING...MEDIUM-HIGH
  CONFIDENCE IN TIMING.

MDB

//OUTLOOK FOR ORD/MDW FOR 00Z WEDNESDAY-12Z MONDAY...UPDATED 12Z...

TUESDAY NIGHT THROUGH FRIDAY...PRIMARILY VFR.
SATURDAY THROUGH SUNDAY...PERIODIC CHANCES FOR THUNDERSTORMS.

BMD

&&

.MARINE...
246 PM CDT

WEAK LOW PRESSURE WILL PUSH EAST THROUGH THE OHIO VALLEY AND
DISSIPATE THIS EVENING AS HIGH PRESSURE MOVES INTO THE MID
MISSOURI VALLEY TONIGHT. AHEAD OF THE HIGH A WEAK COLD FRONT WILL
SPREAD DOWN THE REMAINDER OF THE LAKE THIS EVENING AND BRIEFLY
INCREASE NORTH WINDS TO NEAR 20 KT THE HIGH WILL WEAKEN AND MOVE
OVER LAKE MICHIGAN ON TUESDAY. LOW PRESSURE OVER HUDSON BAY WILL
SPREAD INTO QUEBEC ON WEDNESDAY AS HIGH PRESSURE PUSHES SOUTH OUT
OF NORTHERN MANITOBA. AHEAD OF THE HIGH THE SOUTHWEST GRADIENT
INCREASES WITH WINDS 15 TO 25 KT FOR THE NORTH HALF OF THE LAKE. THE
HIGH REACHES LAKE SUPERIOR THURSDAY EVENING AND WILL PUSH A COLD
FRONT DOWN THE LAKE THURSDAY NIGHT. THIS HIGH WILL THEN MOVE TO
THE EASTERN LAKES BY LATE THURSDAY INTO FRIDAY AS SEVERAL LOWS
ORGANIZE IN THE CENTRAL PLAINS AND CANADIAN PRAIRIES.

KMD

&&

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

$$

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