Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 030726
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
226 AM CDT WED SEP 3 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 06Z...

* VERY SMALL THREAT OF A SHORT PERIOD OF THUNDERSTORMS LATE
  WEDNESDAY NIGHT INTO EARLY THURSDAY MORNING.

* NONE.

KJB

//DISCUSSION...UPDATED 06Z...

LIGHT WINDS UNDER A SURFACE RIDGE OF HIGH PRESSURE WILL LIKELY
ALLOW FOR THE DEVELOPMENT FOR AT LEAST SOME PATCHY FOG AGAIN ACROSS
PORTIONS OF THE AREA OVERNIGHT. THEREFORE...THIS IS THE MAIN
AVIATION CONCERN THIS MORNING. HOWEVER...THIS FOG SHOULD NOT
IMPACT KMDW OR KORD.

WINDS WILL TURN SOUTHERLY DURING THE DAY TODAY AS THE SURFACE HIGH
SHIFTS TOWARDS THE EAST COAST. THESE SOUTHERLY WINDS WILL LIKELY
REMAIN ELEVATED WEDNESDAY NIGHT INTO THURSDAY AS A STOUT AREA OF
LOW PRESSURE MOVES EASTWARD ACROSS THE DAKOTAS. THIS SHOULD SET
THE STAGE FOR THUNDERSTORMS ACROSS MUCH OF THE UPPER MIDWEST AND
INTO WISCONSIN WEDNESDAY NIGHT INTO THURSDAY. THERE IS A SMALL
CHANCE (~20%) THAT A FEW OF STORMS COULD TRY TO SHIFT
SOUTHEASTWARD INTO FAR NORTHERN ILLINOIS LATE WEDNESDAY NIGHT INTO
EARLY THURSDAY MORNING BEFORE WEAKENING. HOWEVER...IT APPEARS THE
MAIN THREAT WILL BY FAR REMAIN NORTH OF THE AREA IN WISCONSIN...SO
I WILL HOLD OFF ON INTRODUCING ANY SHOWERS OR STORMS AT THIS TIME.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* LOW ON ANY STORMS IMPACTING THE TERMINALS LATE WED NIGHT.

* HIGH FOR ALL OTHER ELEMENTS.

KJB

//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...
225 AM CDT

A WEAK HIGH PRESSURE RIDGE WILL MOVE EAST THROUGH TODAY ALLOWING
LIGHT SOUTHWEST WINDS THIS MORNING TO BECOME MORE SOUTHERLY AND
INCREASE IN SPEED THIS AFTERNOON AND EVENING. AS LOW PRESSURE
MOVES FROM THE UPPER PLAINS TO NEAR LAKE SUPERIOR THURSDAY AND
THURSDAY NIGHT...SOUTHERLY WIND SPEEDS WILL BE AT THEIR HIGHEST
OVER THE LAKE. SUSTAINED SPEEDS OF 20 TO 25 KT ARE EXPECTED FOR
MUCH OF THAT PERIOD WITH GUSTS NOT FAR ABOVE THAT DUE TO A
PRONOUNCED STABLE MARINE LAYER. THE 30 KT GUSTS MAY BE MOST
REALIZED IN THE NEARSHORE AREAS OF ILLINOIS AND INDIANA WHERE THE
OFFSHORE COMPONENT WILL TAP INTO THE DEEPER MIXING OVER LAND. AN
ASSOCIATED COLD FRONT TO THE AFOREMENTIONED LOW WILL MOVE ACROSS
THE LAKE ON FRIDAY MORNING...TURNING WINDS NORTHERLY WHICH LAST
MOST OF THE WEEKEND. WIND SPEEDS BEHIND THIS FRONT VARY SOME ON
FORECAST MODEL PROFILES...HOWEVER THE SYNOPTIC SETUP SHOULD AT
LEAST ALLOW FOR PERIODS OF 15 TO 20 KT WINDS FRIDAY AFTERNOON INTO
SATURDAY MORNING. THIS WILL BUILD WAVES TO POSSIBLY NEAR SMALL
CRAFT ADVISORY ALONG THE ILLINOIS AND INDIANA NEARSHORE AREAS TO
START THE WEEKEND.

MTF

&&

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

$$

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