Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 231957
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
257 PM CDT THU APR 23 2015

.SHORT TERM...
257 AM CDT

THROUGH FRIDAY...

SATELLITE IMAGERY EARLY THIS MORNING SHOWS
MINIMAL CLOUD COVER OVER THE FORECAST REGION. JUST NORTH INTO
WISC...CLOUDS CONTINUE TO STREAM SOUTHEAST AS THE UPPER LEVEL VORT
CONTINUES TO INFLUENCE THE AREA. DRY AIR REMAINS LOCKED
OVERHEAD...WHICH IS EVIDENT IN THE LOW DEW POINTS GENERALLY IN THE
LOW 20S TO THE UPPER TEENS. FORTUNATELY JUST ENOUGH WIND ALOFT IS
HELPING TO PREVENT TEMPS FROM DROPPING TO MUCH BELOW
FREEZING...HOWEVER COULD SEE A FEW SITES DIP JUST PRIOR TO DAYBREAK
TO THE LOW 20S. WILL LEAVE THE FREEZE WARNING IN PLACE.

SFC RIDGE BECOMES CENTERED OVER THE FORECAST AREA TODAY...WHICH
SHOULD PROVIDE A BREAK IN THE BREEZY WINDS FROM THE PAST COUPLE OF
DAYS. IN ADDITION THE DAY SHOULD START MOSTLY SUNNY AND LIKELY
REMAIN...AS THE COLUMN CONTINUES TO BE LACKING MOISTURE. TEMPS WILL
LIKELY WARM INTO THE LOW 50S CLOSER TO LAKE MICHIGAN...HOWEVER HI-
RES GUIDANCE CONTINUES TO FEATURE A LAKE BREEZE DEVELOPING MIDDAY
AND PUSHING INLAND EARLY THIS AFTN. THIS COULD TRANSLATE INTO TEMPS
DROPPING IN THE AFTN TO THE MID 40S. ELSEWHERE TEMPS SHUD WARM INTO
THE MID/UPR 50S.

TONIGHT LOOKS TO BE ANOTHER COLD NIGHT...THE CHALLENGE IS HOW COLD.
THE SFC RIDGE AXIS WILL BE SHIFTING EAST AS THE DOWNSTREAM UPPER
LEVEL TROUGH DRIFTS EAST. WITH WINDS MUCH LIGHTER WITHIN THE
LOWEST FEW KM OF THE ATMOS...TEMPS JUST AFT SUNSET SHUD QUICKLY
RADIATE. HOWEVER...RETURN FLOW MOISTURE WILL BE STREAMING INTO THE
CENTRAL PLAINS AHEAD OF A DEVELOPING SHORTWAVE. THIS COULD HELP
BRING SOME WINDS BACK TO THE REGION AND SLOW THE RATE OF COOLING
EARLY FRI MORNING. WILL HOLD OFF ON ANY FROST/FREEZE HEADLINES FOR
TONIGHT AS SOME UNCERTAINTY REMAINS WITH LOWS...HOWEVER COULD SEE
A PORTION OF THE FORECAST AREA NEEDING A FREEZE WARNING.

THE LEADING EDGE OF MOISTURE SHUD ERODE AS IT INTERACTS WITH THE
VERY DRY DOWNSTREAM AIRMASS FRI...ALLOWING THE BULK OF FRI TO REMAIN
DRY. THEN AS THE DAY PROGRESSES...EXPECT ENOUGH MOISTURE TO OVERCOME
THE DRY ATMOS AND BRING LGT RAIN TO AT LEAST THE SOUTHERN CWFA BY
FRI NGT. TEMPS WILL GENERALLY WARM INTO THE MID/UPR 50S AGAIN
FRI...HOWEVER WITH AN EASTERLY FLOW OFF OF LAKE MICHIGAN...ADJACENT
LOCATIONS MAY STRUGGLE TO WARM BEYOND THE UPR 40S.

BEACHLER

&&

.LONG TERM...
257 AM CDT

SATURDAY THROUGH WEDNESDAY...

ENSEMBLES CONTINUE TO BE IN GOOD AGREEMENT WITH THE DOWNSTREAM
TROUGH PERSISTING THROUGH MID NEXT WEEK. HIGH LATITUDE BLOCKING
WILL REMAIN ACROSS THE NORTH ATLANTIC...KEEPING THE CWFA IN A
CONTINUED WEST/NORTHWEST FLOW.

GUIDANCE CONTINUES TO LIFT A DEVELOPING SHORTWAVE NORTHEAST EARLY
SAT ACROSS SOUTHERN IL...HOWEVER ABUNDANT MOISTURE IS PROGGED TO BE
WITHIN THIS WAVE. HAVE HELD ONTO LIKELY POPS SAT...AND CONSIDERING
THE LACK OF DIRECT SUNLIGHT...HIGHS SAT MAY STRUGGLE TO WARM BEYOND
THE UPR 40S CWFA-WIDE. THE GRADIENT WILL KEEP A NORTHEAST SFC FLOW
OFF OF LAKE MICHIGAN SAT...WHICH COULD KEEP AREAS ADJACENT TO LAKE
MI IN THE MID 40S SAT.

FORTUNATELY OPER GUIDANCE CONTINUE TO BE IN GOOD AGREEMENT WITH
EJECTING THIS SFC FEATURE EAST BY SUN...WITH A RETURNING SFC RIDGE
THRU MON. BY THE MIDDLE OF NEXT WEEK ENSEMBLES BEGIN TO DEVELOP A
RIDGE ACROSS THE WESTERN CONUS...HOWEVER THIS MAY BE FAR ENOUGH EAST
TO ALLOW THE GREAT LAKES REGION TO BE MORE SEASONAL WITH TEMPS IN
THE UPR 50S TO LOW 60S.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* CHANCE OF A WIND SHIFT TO NE LATE THIS AFTERNOON ORD...WIND
  SHIFT TO NE MORE LIKELY AT MDW

IZZI

//DISCUSSION...UPDATED 20Z...

WELL DEFINED LAKE BREEZE MAKING STEADY PROGRESS INLAND AND CHANCES
FOR LAKE BREEZE PASSAGE AT MDW SEEM TO BE INCREASING. HAVE
INCLUDED WIND SHIFT TO NE AT 2230Z AT MDW...BUT GIVEN THE STRENGTH
OF THE NW WINDS AHEAD OF THE BOUNDARY CONFIDENCE IS STILL NOT
TERRIBLY HIGH. CHANCES FOR A WIND SHIFT TO NE APPEAR A BIT HIGHER
THAN EARLIER FOR ORD...BUT STILL NOT HIGH ENOUGH TO ADD A WIND
SHIFT YET. WILL CONTINUE TO MONITOR TRENDS AND AMD IF CONFIDENCE
INCREASES.

IZZI

UPDATED 18Z...

DESPITE GUSTY NW WINDS THIS AFTERNOON ALREADY SEEING A LAKE BREEZE
BECOME FAIRLY WELL DEVELOPED NEAR THE SHORE AND LIKELY TO MAKE
SOME HEADWAY WESTWARD/INLAND THIS AFTERNOON. EMPIRICAL LOCAL
FORECAST RULE OF THUMB WOULD SUGGEST IT REMAINS EAST OF ORD OR
TERMINALS...THOUGH SOME HI-RES MODELS DO SUGGEST IT COULD MAKE IT
THROUGH MDW/ORD THIS AFTERNOON. LIKELY TO BE A CLOSE CALL...BUT
FOR NOW PLAN TO KEEP LAKE BREEZE JUST EAST OF ORD/MDW WITH THE
EXPECTATION IT COULD STALL OUT. ALSO POSSIBLE THAT AFTER
STALLING A DISSIPATING LAKE BREEZE COULD BEGIN MOVING WEST AGAIN
THIS EVENING AS GRADIENT WIND WEAKENS. OTHER THAN WINDS THIS
AFTERNOON FORECAST CONCERNS AND WEATHER NIL.

IZZI

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* MEDIUM CONFIDENCE IN MDW WIND DIRECTION FORECAST LATER THIS
  AFTERNOON...LOW CONFIDENCE IN ORD
* HIGH CONFIDENCE IN REMAINING FORECAST ELEMENTS

IZZI

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

* FRIDAY NIGHT...CHANCE SHRA. MVFR POSSIBLE. EAST WINDS.
* SATURDAY...SHRA LIKELY. IFR LIKELY. STRONG/GUSTY NORTHEAST WINDS.
* SUNDAY THRU TUESDAY...DRY/VFR. NORTHEAST WINDS.
* WEDNESDAY...MAINLY DRY/VFR.

KMD

&&

.MARINE...
147 PM CDT

ONSHORE FLOW IS EXPECTED TO DEVELOP ALONG THE ENTIRE ILLINOIS
NEARSHORE BY LATER THIS AFTERNOON AS A LAKE BREEZE INCHES INLAND.
WINDS SHOULD THEN BECOME VARIABLE AS HIGH PRESSURE PREVAILS INTO
FRIDAY. AN ONSHORE COMPONENT WILL DEVELOP AGAIN BY LATE FRIDAY
MORNING. THE PERIOD OF INTEREST IN THE MARINE FORECAST IS SATURDAY
AS FAIRLY DEEP LOW PRESSURE ADVANCES EASTWARD TO THE SOUTH OF THE
LAKE. THIS WILL RESULT IN STRENGTHENING EAST TO EAST-NORTHEAST
WINDS ACROSS THE SOUTHERN PART OF THE LAKE. THERE IS SOME
UNCERTAINTY ON HOW MUCH OF A PRESSURE GRADIENT WILL RESULT...BUT
EVEN GIVEN THAT SMALL CRAFT ADVISORY CRITERIA WINDS AND PROBABLY
WAVES ARE HIGHLY LIKELY. SOME GALE FORCE GUSTS ARE POSSIBLE IF
THIS LOW PRESSURE ENDS UP BEING FURTHER NORTH AND THUS CLOSER TO
THE LAKE. NORTHERLY WINDS ARE THEN EXPECTED TO PREVAIL WITH 30 KT
GUSTS LATER SUNDAY.

MTF

&&

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

$$

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