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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KLOT 180537
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1237 AM CDT TUE JUN 18 2013

.DISCUSSION...
929 PM CDT

A SCATTERING OF SHOWERS WITH A FEW STORMS HAVE SEEN AN OVERALL
DIMINISHING IN COVERAGE SINCE 8 PM. ALMOST ALL NEW DEVELOPMENT IN
THAT TIME HAS BEEN ON OUTFLOW FROM PRECEDING SHOWERS/STORMS...WITH
THE 00Z DVN RAOB INDICATING INSTABILITY PRESENT FROM THE AFTERNOON
MIXED LAYER. SOME FESTERING ON OUTFLOWS IS LIKELY TO CONTINUE
ALTHOUGH EACH SUCCESSIVE REDEVELOPMENT SHOULD BE WEAKER. THE AREA
WILL REMAIN UNDER A FAVORABLE MID-LEVEL JET REGION INTO
OVERNIGHT...BUT THERE APPEARS TO BE A BREAK IN SHORT WAVES LET
ALONE THE DIMINISHING OF THE INSTABILITY WITH THE TIME OF DAY. SO
CONTINUE WITH NO SHOWERS FORECAST BY 1 AM.

OUTFLOWS BROUGHT RAIN-COOLED AIR ALONG WITH THE MARINE LAYER INTO
FAR NORTHEAST IL THAT CREATED A QUICK TEMPERATURE DROP. LOWS
STILL LOOK ON TRACK FOR MOST AREAS...ALTHOUGH DID BRING DOWN THE
FAR NORTHERN TWO TIERS OF COUNTIES A COUPLE DEGREES BASED ON THIS
LOWER STARTING POINT GOING INTO THE NIGHT. IF WINDS WERE LIGHTER
FOG MAY BE A CONCERN IN THOSE AREAS THAT SAW SHOWERS...BUT SPEEDS
WILL STAY UP TONIGHT AS THE SYNOPTIC COLD FRONT WORKS THROUGH THE
REGION.

THE GOES WV LOOP SHOWS THE NEXT SHORT WAVE BEING WELL-DEFINED IN
SOUTHEAST ND. THIS SMALL "BOWLING BALL" IN THE MID-LEVEL FLOW
LOOKS TO TRACK ACROSS THE SOUTHWEST FORECAST AREA DURING TUE
AFTERNOON. THE AMOUNT OF INSTABILITY ON GUIDANCE IS STRONGLY
CORRELATED TO THE SURFACE DEW POINT FORECAST...WHICH COULD BE HIGH
ON BOTH THE NAM AND GFS PER OBSERVATIONAL TRENDS AND INITIALIZATION
/AT LEAST ON THE 18Z RUNS/. STILL ENOUGH TO WARRANT A SLIGHT
CHANCE THOUGH...AS THERE CURRENTLY ARE STORMS WITH THE FEATURE IN
THE DAKOTAS AT THIS HOUR.

MTF

//PREV DISCUSSION...
251 PM CDT

TONIGHT...
VISIBLE IMAGERY EARLY THIS AFTERNOON DEPICTS A CIRRUS SHIELD FROM
SOUTHEAST IOWA STRETCHING NORTHEAST ALONG I-80 IN ILLINOIS INTO
NORTHCENTRAL INDIANA. WATER VAPOR IMAGERY ALSO INDICATED AN
ABUNDANCE OF UPR LEVEL MOISTURE ASSOCIATED WITH THE CLOUDS...HOWEVER
SOME SUBSIDENCE WAS OCCURRING LATE THIS MORNING INTO THE AFTERNOON
HOURS AS DEW POINTS AT THE SFC WERE DROPPING INTO THE MID/UPR 50S
ACROSS THE CWFA. LLVL FLOW UNDER THE CLOUD SHIELD WAS GENERALLY
SOUTHWESTERLY...WHILE FURTHER NORTH WINDS WERE WESTERLY. IT APPEARS
THAT WHERE SKIES ARE MOSTLY SUNNY SLIGHTLY BETTER MIXING WAS
OCCURRING...WHICH WAS ALSO AIDING IN THE SUBSIDENCE AT THE SFC.

THE FOCUS FOR LATER THIS AFTERNOON/EVENING IS ON THE APPROACHING
FRONTAL BOUNDARY AND POTENTIAL FOR ISOLATED SHOWERS/THUNDERSTORMS
ALONG/JUST AHEAD OF THIS BOUNDARY. VISIBLE IMAGERY DOES SHOW A
NARROW CHANNEL OF A CUMULUS FIELD DEVELOPING ACROSS CENTRAL WISC
THIS AFTERNOON...STRETCHING NORTHWEST THROUGH MINN. A WEAK MID-LVL
WAVE WAS SLIDING SOUTHEAST THE UPPER MIDWEST...WHICH WILL BE THE
DRIVER FOR SOME ADDED LIFT INTO THIS CUMULUS FIELD. AS EXPECTED
ISOLATED SHOWERS/THUNDERSTORMS WERE DEVELOPING UNDER THIS CHANNEL OF
CUMULUS ACROSS CENTRAL WISC/EASTERN MINN. MID-LVLS ARE MUCH
COOLER...WITH STEEP LAPSE RATES TO THE NORTH ACROSS WISC. THIS
SUGGESTS THAT A FEW THUNDERSTORMS COULD EASILY GROW VERTICALLY AND
POSSIBLY PRODUCE SOME HAIL. FEEL THAT MOST OF THE LATE AFTN/EARLY
EVENING WILL END UP BEING DRY FOR THE CWFA. GUIDANCE CONTINUES TO
SUGGEST THAT THE BEST CHC FOR ANY PRECIP WILL LIKELY OCCUR AFT 00Z.
WITH MUCH OF THE CONVECTIVE POTENTIAL BEING DIURNALLY
DRIVEN...EXPECT THAT ARND 2-3Z THE INSTABILITY TO GREATLY DIMINISH.
AT THIS TIME WILL HOLD ONTO THE SLIGHT CHC OF TSRA WORDING THRU 3Z.
ACROSS THE NORTH...THEN LINGER THE TSRA WORDING THRU 06Z FOR THE
SOUTHEAST CWFA.

EARLIER THINKING WAS THAT THIS BOUNDARY WOULD END UP BEING A BACK
DOOR COLD FRONT...AND THIS APPEARS TO BE THE CASE BASED ON CURRENT
CONDS TO THE NORTH. CLOUD COVER SHUD BE SLOWLY THINNING OVERNIGHT TO
A P-CLOUDY SKY...WITH WINDS TURNING NORTHERLY POST FRONTAL PASSAGE.
THIS WILL USHER IN MUCH COOLER AIR AND ALLOW TEMPS TO EASILY RADIATE
INTO THE MID/UPR 50S.

CONFIDENCE IN TEMPERATURES...HIGH.
CONFIDENCE IN PRECIPITATION/THUNDERSTORMS...LOW/MEDIUM.

TUESDAY THROUGH WEDNESDAY...
INITIAL GLANCE IS THAT TUE WOULD BE DRY...HOWEVER THE CONCERN IS ON
THE SOUTHEAST DIVING 500MB VORT THAT WILL BE ARRIVING ACROSS THE
WESTERN CWFA MID/LATE MORNING AND PIVOTING EAST BY THE MID-AFTERNOON
HOURS. THIS MAY PROVIDE ENOUGH LIFT...COUPLED WITH DIURNAL CUMULUS
TUE...AND SOME MOISTURE FROM THE BOUNDARY TO ALLOW A FEW
THUNDERSTORMS TO DEVELOP. WITH TEMPS ALOFT STEADILY COOLING ACROSS
THE CWFA...LAPSE RATES WILL STEEPEN AND A FEW STRONGER STORMS COULD
EASILY PRODUCE SOME PEA SIZE HAIL...BRIEF WIND GUSTS AND HEAVY
DOWNPOURS. THE FOCUS APPEARS TO BE ALONG AND SOUTH OF A ROCKFORD TO
BENTON COUNTY LINE.

OTHERWISE ANTI-CYCLONIC FLOW WILL CONTINUE TO SETTLE INTO THE GREAT
LAKES REGION...AND KEEP A NORTHEAST FLOW AT THE SFC. THIS WILL
EASILY AID IN THE MUCH COOLER MARINE ENVIRONMENT ADVECTING INLAND
ACROSS THE CWFA AND KEEP SFC TEMPS IN THE LOW/MID 70S...EXCEPT THE
FAR SOUTH/SOUTHWEST WHERE TEMPS MAY STILL TOUCH 80 DEG IN THE AFTN.
LOCATIONS IMMEDIATELY ADJACENT TO LAKE MICHIGAN MAY HAVE TEMPS
REMAIN IN THE MID 60S TUE.

OVERNIGHT TEMPS WILL BE MUCH COOLER IN THE LOW/MID 50S. A FEW
LOCATIONS ALONG AND NORTH OF A ROCKFORD...AURORA...WAUKEGAN LINE MAY
HAVE TEMPS IN THE UPR 40S EARLY WED MORNING. THEN HIGH PRESSURE
BEGINS TO DRIFT EAST. WHILE WINDS WILL BE MORE EASTERLY OFF OF THE
LAKE...TEMPS WILL BE SLIGHTLY WARMER WED AFTN IN THE UPR 70S TO LOW
80S. THE EXCEPTION WILL BE AREAS ADJACENT TO LAKE MICHIGAN...WHERE
TEMPS WILL LIKELY REMAIN IN THE 60S TO LOW 70S.

CONFIDENCE...HIGH.

THURSDAY THROUGH SUNDAY...
ENSEMBLES CONTINUES TO POINT TOWARDS A PATTERN CHANGE FOR THE END OF
THE WEEK INTO THE WEEKEND. MANY GUIDANCE MEMBERS ARE IN AGREEMENT
THAT THE 500MB TROUGH OVER THE GREAT LAKES REGION WILL PIVOT EAST
THUR...WITH A STRENGTHENING MID-LVL RIDGE ACROSS THE CENTRAL PLAINS
REPLACING THE DEPARTING TROUGH FRI. THE RIDGE DOES APPEAR TO FLATTEN
SLIGHTLY...WHICH MAY BE AN INDICATION THAT A SLIGHTLY ACTIVE PERIOD
MAY ACCOMPANY THE STEADY WARMING TREND. 850MB THERMAL RIDGE
CONTINUES TO BE RATHER WARM FOR THE WEEKEND...WITH TEMPS GENERALLY
AROUND 20-22 DEG C. THE EC/GEM HAVE TAKEN A SLIGHTLY COOLER APPROACH
WITH 18-20 DEG C...HOWEVER THE OVERALL THEME IS THAT TEMPS WILL BE
STEADILY WARMING. THE WILDCARD FOR MAX TEMPS SAT/SUN/MON...WILL BE
LLVL MOISTURE AND CLOUD COVER. GUIDANCE CONTINUES TO SUGGEST DEW
POINTS WILL GENERALLY BE AROUND 70 DEG. THIS MAY SLOW THE DIURNAL
CURVE FOR THE WEEKEND...HOWEVER TEMPS SHUD EASILY STILL WARM INTO
THE UPR 80S TO POSSIBLY LOWER 90S. AS A RESULT HAVE NUDGED TEMPS
WARMER THAN GUIDANCE FOR THE UPCOMING WEEKEND.

CONFIDENCE IN TEMPERATURES...MEDIUM/HIGH.
CONFIDENCE IN PRECIPITATION/THUNDERSTORMS...MEDIUM.

BEACHLER

&&

.AVIATION...

-- Changed Discussion --
//ORD AND MDW CONCERNS...UPDATED 06Z...

* POTENTIAL FOR LOW CIGS OVERNIGHT.

BMD

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

A COLD FRONT HAS DROPPED SOUTH ACROSS THE TERMINALS LEAVING
NORTHEAST FLOW OVER NORTHERN ILLINOIS AS HIGH PRESSURE BUILDS ACROSS
THE PLAINS AND NORTHERN LAKES REGION. THE HIGH IS EXPECTED TO VERY
GRADUALLY CONSOLIDATE AND SINK SOUTH TOWARDS NORTHERN LAKE MICHIGAN
THROUGH THE DAY TUESDAY LEADING TO VERY LITTLE CHANCE IN THE WEATHER
THROUGH THE FORECAST PERIOD. WINDS WILL PICK UP WITH ONSET OF MIXING
DURING THE DAY TUESDAY...THEN SHOULD DIMINISH AFTER SUNSET. VFR
CLOUDS EXPECTED THROUGH THE PERIOD...BUT CONTINUE TO MONITOR CLOSELY
THE CHANCE FOR LOWER CIGS DEVELOPING OVERNIGHT INTO THE EARLY
MORNING HOURS. NAM/RAP SOUNDINGS CONTINUE TO ADVERTISE THE POTENTIAL
FOR IFR CIGS TO DEVELOP AS THE COLUMN COOLS...BUT SEE NO SIGN OF
THIS YET UPSTREAM AND OTHER MODELS AS LESS BULLISH ON THE
DEVELOPMENT. NAM HAS A TENDENCY TO OVERDO LOW LEVEL MOISTURE SO FOR
NOW WILL MONITOR CLOSELY. IF LOW CIGS DO DEVELOP THE BEST TIME FRAME
WOULD BE ROUGHLY 09-14Z.

BMD


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

* LOW CONFIDENCE IN LOW CIGS DEVELOPING.
* HIGH CONFIDENCE IN OTHER FORECAST ELEMENTS.

BMD

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

WEDNESDAY...VFR.

THURSDAY...VFR.

FRIDAY...VFR EARLY...CHANCE OF TSRA OVERNIGHT.

SATURDAY AND SUNDAY...PERIODIC CHANCES FOR TSRA.

LENNING
-- End Changed Discussion --

&&

.MARINE...
245 PM CDT

A STRETCH OF RELATIVELY QUIET WEATHER IS STILL IN THE FORECAST
FOR LAKE MICHIGAN OVER MUCH OF THE UPCOMING WEAK. A TROUGH OF LOW
PRESSURE AND AN ASSOCIATED WEAK COLD FRONT...EXTEND FROM NORTHERN
MAINE WEST THROUGH CENTRAL LAKE MICHIGAN. AS THE LOW MOVES OFF TO
THE EAST...THE FRONT WILL CONTINUE TO MOVE SOUTH DOWN THE
LAKE...REACHING THE SOUTHERN TIP OF THE LAKE BY LATE THIS EVENING.
WINDS WILL SHIFT TO NORTHERLY BEHIND THE FRONT...WITH OCCASIONAL
GUSTS TO 25 KT TONIGHT. HIGH PRESSURE OVER THE NORTHERN PLAINS
WILL SPREAD EAST ACROSS THE GREAT LAKES LATE TONIGHT AND
TOMORROW...WITH HIGH PRESSURE REMAINING THE DOMINANT FEATURE OVER
THE LAKE THROUGH WEDNESDAY NIGHT WHEN THE HIGH WILL BEGIN TO SLIDE
OFF TO THE EAST. IN THE MEAN TIME...LOW PRESSURE DEVELOPING OVER
THE NORTHERN ROCKIES WEDNESDAY NIGHT AND WILL MOVE OVER THE
NORTHERN PLAINS BY FRIDAY MORNING. WITH THE HIGH PRESSURE TO THE
EAST AND THE DEVELOPING TROUGH TO THE WEST...WINDS WILL BECOME
SOUTHERLY BY WEDNESDAY OR WEDNESDAY NIGHT...THROUGH THE WEEKEND.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...BEACH HAZARDS STATEMENT...ILZ006-ILZ014...1 PM TUESDAY TO 4 AM
     WEDNESDAY.

IN...BEACH HAZARDS STATEMENT...INZ001-INZ002...1 PM TUESDAY TO 4 AM
     WEDNESDAY.

LM...NONE.
&&

$$

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