Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 300826
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
326 AM CDT SAT MAY 30 2015

.UPDATE...
842 PM CDT

FOR THE EVENING UPDATE...I HAVE TWEAKED POPS A BIT THROUGH THE
EVENING TO GO WITH LOW END CHANCES FOR MAINLY ISOLATED
SHOWERS/STORMS ALONG AND NORTHWEST OF I-55. IT APPEARS THE MAIN
FOCUS FOR HIGHER COVERAGE SHOWERS AND STORMS OVER THE NEXT COUPLE
HOURS WILL BE EAST OF I-55. LATER TONIGHT THE MAIN FOCUS WILL BE
WITH THE LIKELY DEVELOPMENT OF ADDITIONAL RAIN AND SOME POSSIBLE
STORMS IN ADVANCE OF AN APPROACHING COLD FRONT...NOW SHIFTING ACROSS
CENTRAL IOWA. THE 00 UTC SOUNDINGS AT BOTH KILX AND KDVN INDICATE
POOR MID-LEVEL LAPSE RATES OVER THE AREA...AND HENCE MINIMAL
MUCAPE. THEREFORE...IT APPEARS THE OVERALL THUNDER THREAT WILL
REMAIN ON THE LOW SIDE WITH THIS ACTIVITY LATER TONIGHT.

NO REAL CHANGES HAVE BEEN MADE BEYOND TONIGHT. HOWEVER...THERE ARE
SOME CONCERNS FOR SOME HEAVY RAINFALL POTENTIAL ON SATURDAY ACROSS
THE AREA AS STOUT MID-LEVEL DISTURBANCE OVER WESTERN ARKANSAS
INDUCES A LOWER LEVEL WAVE ALONG THE APPROACHING COLD FRONT ON
SATURDAY. THIS COULD RESULT IN A BAND OF FRONTOGENETICALLY INDUCED
RAIN ACROSS NORTHERN ILLINOIS...AND SHOWERS AND THUNDERSTORMS ACROSS
MY SOUTHERN COUNTIES. GIVEN THE PRECIPITABLE WATER VALUES ARE
EXPECTED TO BE AT OR EVEN ABOVE THE 00UTC RAOB VALUE OF 1.69 INCHES
AT KILX...HEAVY RAINFALL IS CERTAINLY A CONCERN. HOWEVER...IT
APPEARS THAT AREAS ALONG AND NORTH OF INTERSTATE 80 WILL REMAIN
RELATIVELY STABLE...SO THE POTENTIAL OF VERY HEAVY RAINFALL RATES
APPEARS TO BE LOWER GIVEN THE LACK OF BETTER CONVECTIVE POTENTIAL.
AREAS FARTHER SOUTH ACROSS NORTHWESTERN INDIANA AND MY EAST CENTRAL
ILLINOIS COUNTIES LOOK TO REMAIN CLOSER TO THE WARM SECTOR AND HENCE
WITHIN THE AREA OF BETTER CONVECTIVE POTENTIAL. THEREFORE...IT
APPEARS THAT THESE AREAS COULD HAVE A HIGHER THREAT FOR SOME HEAVY
RAIN RATES GIVEN THE ABUNDANCE OF ATMOSPHERIC MOISTURE IN PLACE.
OVERALL...IT LOOKS LIKE MOST OF THE AREA WILL SEE SOME GOOD AMOUNTS
OF RAIN IN EXCESS OF A HALF INCH. HOWEVER...SOME AREAS COULD
EXPERIENCE MUCH HIGHER AMOUNTS...ESPECIALLY SOUTH OF INTERSTATE 80.
WITH THIS IN MIND...QPF AMOUNTS MAY NEED TO BE RAISED IF THE 00 UTC
GUIDANCE CONTINUES TO SUPPORT HEAVY RAIN.

OTHERWISE...IT WILL BE TURNING MUCH COLDER AND WINDY DURING THE DAY
ACROSS NORTHERN ILLINOIS.

KJB

&&

.SHORT TERM...
248 PM CDT

THROUGH SATURDAY EVENING...

MESSY WEATHER PATTERN IN PLACE REST OF TODAY AND TONIGHT AS THE
REGION FALLS IN THE WARM SECTOR AHEAD OF A COLD FRONT DROPPING
ACROSS THE UPPER MIDWEST AND PLAINS THIS AFTERNOON. EXPECT
UNCAPPED MODEST INSTABILITY TO BE IN PLACE THROUGH LATE THIS
EVENING TONIGHT THOUGH FORCING IS FAIRLY WEAK RESULTING IN
UNFOCUSED ISOLATED TO WIDELY SCATTERED SHOWER ACTIVITY. MID LEVEL
LAPSE RATES CONTINUE TO BE WEAK IN THE 5-6 C/KM RANGE OVERNIGHT SO
DONT SEE A SIGNIFICANT INCREASE IN THE THUNDER CHANCES THROUGH
EARLY OVERNIGHT...AND SEVERE THREAT IS FAIRLY MINIMAL WITH AN
ISOLATED WET MICROBURST BEING THE MAIN CONCERN. THE COMBINATION OF
POOR LAPSE RATES...INCREASING MOISTURE AND RELATIVELY HIGH MELTING
LEVELS WILL GREATLY LIMIT THE HAIL THREAT DESPITE DEEP LAYER SHEAR
INCREASING THROUGH THE EVENING.

SYNOPTICALLY...WE ARE NOT UNDER A FAVORABLE AREA OF FORCING MOST
OF THE EVENING AND EARLY OVERNIGHT HOURS HOWEVER MODELS DO
CONTINUE TO DEVELOP CONVECTION OVER THE AREA. UNDERCUT MODEL POPS
THROUGH THE EARLY OVERNIGHT HOURS LEAVING ABOUT A 30-40 POP CWA-
WIDE...THINKING A LOT OF THE MODEL VORTICITY THAT IS GENERATED IS
A RESULT OF CONVECTIVE FEEDBACK. A MORE ROBUST UPPER LEVEL WAVE IS
PROGGED TO LIFT ACROSS THE MID-MISSISSIPPI VALLEY EARLY SATURDAY
MORNING AND THE COLD FRONT APPROACHES FROM THE NORTHWEST...SO DO
GRADUALLY INCREASE POPS THROUGH THE PREDAWN HOURS. THE COLD FRONT
SHOULD BE NEAR THE IL/WI STATE-LINE AROUND SUNRISE TOMORROW
MORNING. BUMPED UP LOWS FOR THE NORTHERN HALF OF THE CWA WITH A
SLOWER THAN EXPECTED ARRIVAL OF THE FRONT.

COLD FRONT SHOULD CLEAR THE SOUTHERN COUNTIES OF THE CWA BY EARLY
TO MID AFTERNOON. NORTHERN COUNTIES WILL HAVE EARLY MORNING
HIGHS THEN TEMPERATURES FALLING THROUGH THE DAY AS AN UNSEASONABLY
STRONG COLD FRONT PUSHES THROUGH. 30 KT WINDS COMING DOWN BEHIND
THE FRONT OVER A MOSTLY STABLE LAKE MICHIGAN ARE EXPECTED TO
INCREASE IN THE WARMER NEARSHORE WATERS. WINDS GUSTING TO AROUND
40 MPH ARE EXPECTED TO BLEED INLAND...WITH GUSTS OF 30 TO 35 MPH
EXPECTED FARTHER INLAND. STRONG WINDS AND THE ONSHORE COMPONENT
WILL DRIVE WAVES INTO THE 8 TO 12 FOOT RANGE SO THERE IS THE
POSSIBILITY OF FLOODING ALONG THE BIKE PATH IN CHICAGO AND OTHER
TYPICALLY FLOOD PRONE AREAS ALONG THE SOUTH END OF THE LAKE
MAINLY EARLY AFTERNOON THROUGH THE EVENING.

INSTABILITY DOES HAVE A LITTLE TIME TO BUILD IN THE SOUTHERN TWO
THIRDS TO HALF OF THE CWA AHEAD OF THE FRONT BY MID MORNING...SO
THERE MAY BE BETTER CHANCES FOR THUNDERSTORMS DURING THE DAY
TOMORROW AHEAD OF THE FRONT. INSTABILITY QUICKLY DIMINISHES
BEHIND THE FRONT AND SOME MODELS /ESPECIALLY GFS/ SHOW A STABLE
WAVE DEVELOPING ALONG THE FRONT WITH RAIN CONTINUING BEHIND THE
FRONT AS AN F-GEN BAND DEVELOPS. EXPECT A LOT OF THE FORECAST AREA
TO FALL INTO THE UPPER 40S BY MID TO LATE TOMORROW AFTERNOON...AND
SITES ALONG THE LAKE EXPERIENCING WIND CHILLS IN THE MID TO UPPER
30S...ONLY TWO DAYS BEFORE METEOROLOGICAL SUMMER BEGINS.

DEUBELBEISS

&&

.LONG TERM...
303 PM CDT

SATURDAY NIGHT THROUGH FRIDAY...

A PROLONGED PERIOD OF DRY AND SEASONABLY COOL WEATHER IS EXPECTED
WITH HIGH PRESSURE SETTING UP SHOP OVER THE GREAT LAKES REGION
INTO THE FIRST HALF OF NEXT WEEK. CHILLIEST CONDITIONS WILL LIKELY
BE SUNDAY WITH GRADUAL WARMING TREND THROUGH REMAINDER OF THE
EXTENDED PERIOD AS HIGH SLOWLY SHIFTS EASTWARD ALLOWING FOR
GRADUALLY MORE SOUTHERLY FLOW TO DEVELOP AS UPPER RIDGE BUILDS
EASTWARD INTO THE EASTERN HALF OF THE CONUS AGAIN BY END OF NEXT
WEEK ALLOWING TEMPS TO RETURN TO ABOVE AVERAGE LEVELS.
PROMINENT/STRONG LAKE ENHANCED NORTHEAST WINDS SUNDAY WILL BE
FOLLOWED BY MORE TYPICAL LAKE BREEZES NEXT WEEK...WITH LAKE
INFLUENCE LESSENING BY THE DAY AS GRADIENT TURNS MORE SOUTHERNLY
WITH TIME.

IZZI

&&

.AVIATION...

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

* WIND SHIFT TO NORTH/NORTHEAST MID MORNING.
* STRONG/GUSTY NORTH/NORTHEAST WINDS FROM MID MORNING THRU THE
  REMAINDER OF THE PERIOD.
* SHRA EARLY THIS MORNING...RAIN LATER THIS MORNING INTO THIS
  AFTERNOON.
* IFR/LIFR FROM MID MORNING THRU THIS EVENING. CMS

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

A STRONG COLD FRONT WILL MOVE ACROSS THE AREA LATER THIS MORNING.
SOUTHWESTERLY WINDS GENERALLY UNDER 10KTS WILL TURN MORE WESTERLY
BY MORNING WITH SPEEDS DIMINISHING JUST A BIT. WINDS WILL SLOWLY
TURN NORTHWEST JUST AHEAD OF A COLD FRONT WHICH WILL SHIFT WINDS
TO THE NORTH/NORTHEAST. INITIALLY SPEEDS WILL INCREASE INTO THE
12-16KT RANGE WITH GUSTS INTO THE LOWER/MID 20KT RANGE. BUT THESE
SPEEDS AND GUSTS WILL INCREASE THRU THE DAY WITH 20G30KT EXPECTED
BY LATE AFTERNOON INTO THIS EVENING.

SCATTERED SHOWERS WILL BE POSSIBLE FOR THE NEXT FEW HOURS WITH A
PERIOD OF SHOWERS/RAIN TOWARD DAY BREAK ASSOCIATED WITH THE
ACTIVITY NOW OVER CENTRAL IL. THERE COULD BE A LULL MID MORNING
BUT OVERALL...EXPECT PERIODS OF SHOWERS AND RAIN...POSSIBLY HEAVY
AT TIMES FOR MOST OF TODAY...SLOWLY ENDING INTO THIS EVENING.
INSTABILITY LOOKS RATHER MARGINAL FOR THUNDER ACROSS THE
TERMINALS. COULD BE A FEW THUNDERSTORMS WITH THE ACTIVITY EXPECTED
AROUND DAYBREAK...BUT POTENTIAL TOO LOW TO MENTION IN THE TAFS.
COULD BE THUNDERSTORMS JUST SOUTH OF THE TERMINALS LATER THIS
AFTERNOON HOWEVER.

AS RAIN SATURATES THE INCREASINGLY COLDER AIR SPREADING IN BEHIND
THE FRONT...CIGS SHOULD STEADILY LOWER TO IFR LATER THIS MORNING
WITH A PERIOD OF LIFR POSSIBLE. SOME UNCERTAINTY REGARDING HOW
LONG THESE LOWER CIGS WILL PERSIST BUT SHOULD SEE SOME IMPROVEMENT
BACK INTO IFR LATER THIS AFTERNOON AND INTO MVFR THIS EVENING. CMS

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

* HIGH FOR WINDS THRU THE PERIOD...MEDIUM FOR COLD FRONT/WIND
  SHIFT TIMING.
* HIGH FOR SHOWERS/RAIN.
* HIGH FOR IFR/LIFR. MEDIUM FOR TIMING/DURATION. CMS

//OUTLOOK FOR ORD/MDW FOR 12Z SUNDAY-12Z FRIDAY...UPDATED 00Z...

SUNDAY AND MONDAY...VFR. NORTHEAST WINDS.

TUESDAY...VFR. EAST WINDS.

WEDNESDAY...VFR. EAST-SOUTHEAST WINDS.

THURSDAY...VFR. SOUTH WINDS.

RATZER

&&

.MARINE...

326 AM...A STRONG COLD FRONT WILL MOVE SOUTH ACROSS LAKE MICHIGAN
THIS MORNING AS STRONG HIGH PRESSURE BUILDS ACROSS THE NORTHERN
PLAINS AND UPPER MIDWEST. THIS WILL TIGHTEN THE GRADIENT BEHIND
THE FRONT AND NORTHERLY WINDS WILL INCREASE TO 30 KT BEHIND THE
FRONT WITH GALES LIKELY OVER THE SOUTHERN PART OF THE LAKE. IN
ADDITION...WEAK LOW PRESSURE WILL DEVELOP OVER INDIANA AND OHIO
TONIGHT...AND THIS WILL MAINTAIN THE STRONG GRADIENT INTO SUNDAY
MORNING. AS THIS LOW DEPARTS...THE GRADIENT WILL SLOWLY WEAKEN
SUNDAY AFTERNOON INTO SUNDAY NIGHT. THE HIGH OVER THE UPPER
MIDWEST WILL MOVE ACROSS THE NORTHERN LAKES AND ONTARIO SUNDAY
THEN INTO QUEBEC MONDAY...WITH WEAKER RIDGING EXTENDING BACK
ACROSS THE LAKES REGION THROUGH TUESDAY. CMS

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...LAKESHORE FLOOD ADVISORY...ILZ006-ILZ014...1 PM SATURDAY TO 4 AM
     SUNDAY.

IN...LAKESHORE FLOOD ADVISORY...INZ001-INZ002...1 PM SATURDAY TO 4 AM
     SUNDAY.

LM...GALE WARNING...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...9 AM SATURDAY TO 10 PM SATURDAY.

&&

$$

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