Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 030016
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
716 PM CDT TUE SEP 2 2014

.DISCUSSION...
324 PM CDT

COLD FRONT WHICH MOVED THROUGH THE FORECAST AREA LAST NIGHT HAS
SETTLED INTO THE OHIO RIVER VALLEY THIS AFTERNOON...WITH WEAK HIGH
PRESSURE CENTER DEVELOPING EASTWARD FROM THE LOWER MISSOURI VALLEY.
MODEST WEST-SOUTHWEST LOW LEVEL WINDS WERE NOTED ACROSS THE CWA...
SEVERAL WEAK CONVERGENT ZONES INCLUDING AN OUTFLOW BOUNDARY FROM
MORNING THUNDERSTORMS OVER NORTHWEST IL...THE LAKE BREEZE BOUNDARY
ALONG THE ILLINOIS SHORE AND EXTENDING EAST ACROSS NORTHERN
INDIANA...AND A SUBTLE TROUGH AXIS/MOISTURE GRADIENT ACROSS OUR
SOUTHEASTERN COUNTIES. VISIBLE SATELLITE IMAGERY HAS SHOWN CU/TCU
DEVELOPMENT ALONG THESE BOUNDARIES AT TIMES WITH A FEW ISOLATED
SHOWERS HAVING FORMED ALONG THE BOUNDARIES ACROSS FAR EAST CENTRAL
IL AND NORTHWEST INDIANA. ANOTHER WEAK CONVERGENCE AXIS WAS FROM
NEAR MILWAUKEE BACK INTO NORTHWEST IL...ALSO WITH SOME CU
DEVELOPMENT ALONG IT. WHILE AN ISOLATED SHOWER WILL BE POSSIBLE
ALONG ANY OF THESE BOUNDARIES DURING THE REMAINDER OF THE AFTERNOON
HOURS...THE HIGHER SURFACE DEW POINT AND MLCAPE AXIS OVER THE FAR
SOUTHEASTERN CWA WILL HAVE THE BEST CHANCE (AND ONLY ISOLATED
COVERAGE AT THAT) OF ANY MORE SIGNIFICANT SHOWERS OR ISOLATED
THUNDER. LOSS OF DIURNAL INSTABILITY SHOULD END ISOLATED PRECIP
THREAT BY EARLY EVENING. WEAK SURFACE PRESSURE GRADIENT NORTH OF
SURFACE HIGH PRESSURE CENTER WILL ALLOW FOR LIGHT WINDS OVERNIGHT...
AND WITH MAINLY CLEAR SKIES SHOULD SEE SOME PATCHY FOG DEVELOP
ESPECIALLY ACROSS AREAS SOUTH OF CHICAGO WHERE DEW POINT TEMPS WILL
BE A LITTLE HIGHER AND WHERE GROUND LIKELY REMAINS MOIST FROM RECENT
HEAVY RAINS.

ATTENTION THEN TURNS TOWARD THE WEST WEDNESDAY...AS SURFACE LOW
PRESSURE DEVELOPMENT TAKES PLACE ACROSS THE HIGH PLAINS IN RESPONSE
TO STRONG MID-LEVEL SHORT WAVE CROSSING THE NORTHERN ROCKIES.
GUIDANCE HAS TRENDED DEEPER WITH THE MID-LEVEL WAVE AND THE
RESULTING SURFACE LOW...WITH SOUTHERLY RETURN FLOW DEVELOPING AND
STRENGTHENING ACROSS THE MISSISSIPPI VALLEY AND WESTERN LAKES REGION
BY LATER WEDNESDAY AND WEDNESDAY NIGHT. THIS WILL RESULT IN THE COLD
FRONT TO OUR SOUTH RETURNING NORTH AS A WARM FRONT WEDNESDAY
NIGHT...AND BRINGING VERY WARM/HOT AND HUMID CONDITIONS BACK FOR
THURSDAY AND FRIDAY. THUNDERSTORM POTENTIAL INCREASES WEDNESDAY
NIGHT AND EARLY THURSDAY AS THE WARM FRONT PUSHES NORTH...BEFORE
CAPPING INVERSION DEVELOPS IN 900-700 MB LAYER WITHIN WARM SECTOR
THURSDAY. HOT AND HUMID CONDITIONS EXPECTED THURSDAY...WITH 850 MB
TEMPS IN EXCESS OF +20 C AND 950 MB TEMPS APPROACHING +28 C.
PROVIDED THAT ANY MORNING CLOUDS/PRECIP WITH EARLY MORNING WARM
FRONTAL PASSAGE SCATTER...AND NO SIGNIFICANT CONVECTIVE OUTFLOW
REMAINS...AFTERNOON HIGHS AROUND 90/LOWER 90S LOOK REASONABLE.
COMBINED WITH DEW POINT TEMPS CLIMBING INTO THE 70S... HEAT INDEX
VALUES OF 95-105 WOULD MAKE FOR UNCOMFORTABLE CONDITIONS.

DEEP SURFACE LOW CONTINUES TO DEEPEN AS IT MOVES EAST ACROSS ONTARIO
THURSDAY NIGHT...WITH A TRAILING COLD FRONT PUSHING INTO NORTHERN
ILLINOIS PRE-DAWN FRIDAY. SHOWER/THUNDERSTORM POTENTIAL INCREASES
EARLY FRIDAY WITH ARRIVAL OF FRONT...AND CONTINUES THROUGH FRIDAY
NIGHT AS FRONT SAGS SOUTH INTO DOWNSTATE IL/IN. WITH FRONT
BISECTING CWA DURING THE DAY...A FAIRLY LARGE TEMP SPREAD IS
ANTICIPATED WITH UPPER 70S/LOW 80S ALONG THE WISCONSIN BORDER...AND
UPPER 80S/NEAR 90 AGAIN ACROSS THE SOUTHERN COUNTIES OF THE FORECAST
AREA WITH MOST OF THE CLOUD COVER AND PRECIP EXPECTED ALONG/NORTH OF
THE ADVANCING FRONT AS CAPPING REMAINS IN PLACE SOUTH OF THE
BOUNDARY.

BROAD SURFACE HIGH PRESSURE THEN SPREADS ACROSS THE UPPER MIDWEST
AND GREAT LAKES REGION SATURDAY AND SUNDAY...WITH DRY AND MUCH
COOLER CONDITIONS WITH EASTERLY LOW LEVEL FLOW AND LAKE BREEZES.
NEXT CHANCE OF ORGANIZED PRECIP APPEARS TO ARRIVE AT THE END OF THE
FORECAST PERIOD ON TUESDAY.

RATZER

&&

.AVIATION...

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

* NONE

KMD

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

STILL HAVE SOME CLOUDS DRIFTING SOUTHEAST BUT THESE SHOULD
GENERALLY DISSIPATE WITH THE LOSS OF DAYTIME HEATING. WITH
CLEARING SKIES AND LIGHT WINDS...THERE MAY BE SOME PATCHY GROUND
FOG AWAY IN OUTLYING AREAS. DEWPOINTS ARE A BIT LOWER TONIGHT THAN LAST
NIGHT...THEREFORE DO NOT EXPECT IT TO BE QUITE AS EXTENSIVE.

HIGH PRESSURE REMAINS IN CONTROL FOR WEDNESDAY FOR RELATIVELY
QUIET CONDITIONS. JUST SOME INCREASE IN SOUTHERLY WINDS TO AROUND
10 KT OR SO LATER IN THE DAY. A WARM FRONT WILL BE LIFTING NORTH
OVERNIGHT AND THIS WILL BRING A CHANCE OF THUNDERSTORMS...BUT THIS
JUST OUTSIDE OF THE 30 HR TAF PERIOD.

KMD


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

* HIGH FOR ALL ELEMENTS.

KMD

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

THURSDAY...CHC OF MORNING TSRA.

FRIDAY...CHANCE OF TSRA.

SATURDAY THROUGH MONDAY...VFR WITH NORTHEAST TO EAST WINDS.

TUESDAY...CHANCE TSRA.

KMD

&&

.MARINE...
251 PM CDT

MAIN FORECAST CONCERN IS WITH STRONG SOUTHERLY WINDS AHEAD OF
DEEPENING LOW PRESSURE MOVING FROM THE NORTHERN PLAINS ON THURSDAY
TO JAMES BAY BY FRIDAY MORNING. TIGHTENING PRESSURE GRADIENT AND
PRESSURE FALLS WILL RESULT IN STRENGTHENING SOUTHERLY FLOW DURING
THE DAY WEDNESDAY AND ESPECIALLY WEDNESDAY NIGHT. SUSTAINED WINDS
WILL PEAK ON THURSDAY IN THE LOWER 20 KT RANGE...BUT HAVE LOWER
CONFIDENCE IN GUST SPEEDS DUE WARM AIR MASS OVERSPREADING THE
REGION. WILL CONTINUE TO MENTION 30 KT GUSTS IN THE GLF. ITS
POSSIBLE THAT NEAR SHORE WATERS COULD SEE MORE FREQUENT GUSTS UP TO
30 KT IN STRONG OFFSHORE FLOW...WITH FORECAST SOUNDINGS INDICATING
THAT POTENTIAL. SMALL CRAFT ADVISORY WINDS ARE LIKELY THURSDAY
MORNING INTO THURSDAY EVENING FOR THE ENTIRE NEARSHORE.

THEN A STRONG COLD FRONT TRAILING FROM THE LOW PRESSURE AREA WILL
SWEEP DOWN THE LAKE THURSDAY NIGHT THROUGH FRIDAY AFTERNOON. A
PERIOD OF DECENT NORTHERLY WINDS COULD SET UP ESPECIALLY ON THE
SOUTHERN HALF OF THE LAKE...BUT HIGH PRESSURE WILL QUICKLY SPREAD
EASTWARD AND REDUCE SPEEDS. THEREFORE...IT IS UNCERTAIN WHETHER
DURATION OF NORTHERLY WINDS WILL BE SUFFICIENT TO RESULT IN WAVES
BUILDING TO SMALL CRAFT ADVISORY CRITERIA. FOR NOW DO HAVE WAVES
ABOVE CRITERIA IN GRIDS. WINDS WILL BECOME RELATIVELY LIGHT OVER
THE WEEKEND WITH EXPANSIVE HIGH PRESSURE SLOWLY SLIDING
EASTWARD...THOUGH NORTHERLY.

RC

&&

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

$$

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