Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 060307
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1007 PM CDT TUE MAY 5 2015

.UPDATE...
856 PM CDT

MAIN CONCERNS FOR THE REMAINDER OF THIS EVENING AND OVERNIGHT WILL
BE ON THE POSSIBILITY OF DENSE FOG EXPANDING WEST. IR IMAGERY
INDICATES A FEW POCKETS OF THIN CLOUD COVER. GOES-R IMAGERY
DEPICTS THE LOW CLOUDS PUSHING BACK EAST OVER THE LAKE...WHICH
MATCHES UP WELL WITH SFC OBS INDICATING SOME IMPROVEMENT AT
ORD/MDW THE LAST COUPLE OF HOURS. SFC WINDS REMAIN SLIGHTLY
NORTHEASTERLY ARND 5-10MPH WITH VERY MINIMAL T/TD DEPRESSIONS.

LACK OF DRYING AND RECENT RAINFALL TODAY...COUPLED WITH LIGHT
WINDS SUGGESTS FOG WOULD DEVELOP. ADDITIONALLY...HI- RES GUIDANCE
CONTINUES TO INDICATE FOG RE-DEVELOPING AND EXPANDING WEST
OVERNIGHT ACROSS THE INLAND COUNTIES. WILL MAINTAIN THE DENSE FOG
ADVISORY FOR LAKE/COOK IN ILLINOIS...BUT AT THIS TIME GIVEN SOME
LOWER CONFIDENCE WILL JUST ISSUE AN SPS FOR FOG ACROSS THE
REMAINDER OF THE CWFA. THIS MAY NEED TO BE ADJUSTED LATER TONIGHT
TO AN ADVISORY.

BEACHLER

&&

.SHORT TERM...
308 PM CDT

THROUGH WEDNESDAY...

...ISSUED A FOG ADVISORY FOR LAKE AND COOK IN ILLINOIS THROUGH MID
MORNING WEDNESDAY...

WIDELY SCATTERED THUNDERSTORMS ARE DEVELOPING ACROSS MY SOUTHERN
COUNTIES...SOUTH OF I-80 THIS AFTERNOON IN CLOSE PROXIMITY TO THE A
SURFACE FRONTAL BOUNDARY. TEMPERATURES HERE ARE IN THE MID 70S TO
NEAR 80...WITH DEWPOINTS POOLING INTO THE UPPER 60S ALONG THE
BOUNDARY. THIS IS RESULTING IN A CORRIDOR OF AROUND 1500 MLCAPES
ACROSS THESE AREAS. VAD WIND PROFILES INDICATE THAT DEEP LAYER
EFFECT SHEAR IS ON THE ORDER OF 25 TO 30 KT. WITH THIS IN MIND...IT
DOES NOT APPEAR THAT THESE STORMS WILL PRESENT A SIGNIFICANT SEVERE
THREAT. HOWEVER...IF ANY STORMS CAN MAINTAIN AN UPDRAFT LONG
ENOUGH...THERE COULD BE SOME SMALL HAIL...THOUGH RADAR TRENDS
SUGGEST THAT ONCE THE STORMS TRY TO MOVE AWAY FROM THE LOW LEVEL
CONVERGENCE ALONG THE BOUNDARY THEY DECAY DUE TO THE LACK OF ANY
GOOD LARGER SCALE FORCING FOR ASCENT. AS SUCH...IT APPEARS THESE
STORMS WILL REMAIN CONFINED TO A RELATIVELY NARROW CORRIDOR ACROSS
MY SOUTHERN COUNTIES INTO EARLY THIS EVENING...BEFORE DISSIPATING
WITH SUNSET.

THE OTHER MAIN CONCERN TONIGHT INTO WEDNESDAY WILL BE THE THREAT OF
DENSE FOG. WEBCAMS NEAR THE LAKE HAVE BEEN SOCKED IN FOG WITH THE
VERY COOL ONSHORE FLOW...AND WITH THE HIGH DEW POINT AIR...I DONT
SEE MUCH OF AN IMPROVEMENT TONIGHT. IN FACT...WE COULD ACTUALLY SEE
THE DENSE FOG BEGIN TO DEVELOP INLAND ACROSS MUCH OF NORTHERN
ILLINOIS NORTH OF THE FRONTAL BOUNDARY THIS EVENING. I ISSUED A
DENSE FOG ADVISORY FOR LAKE AND COOK COUNTIES IN ILLINOIS.
HOWEVER...I WOULD NOT BE SURPRISED IF THIS ADVISORY NEED TO BE
EXPANDED INLAND THIS EVENING. AREAS OF FOG SHOULD LIFT INLAND AREAS
BY MID TO LATE MORNING AS THE WARM FRONT BEGINS TO SHIFT NORTHWARD.
HOWEVER...AREAS NEAR THE LAKE COULD STILL SEE SOME FOG AS IT APPEARS
THAT A LAKE BREEZE WILL DEVELOP AN ONSHORE WIND AGAIN ON WEDNESDAY.
THIS LAKE BREEZE BOUNDARY COULD ALSO SPARK OFF A FEW ISOLATED
SHOWERS OR STORMS LATER WEDNESDAY AFTERNOON ACROSS THE WESTERN
SUBURBS OF CHICAGO.

TEMPERATURES ACROSS MUCH OF THE AREA AWAY FROM LAKE MICHIGAN SHOULD
WARM TO NEAR 80 TO THE MID 80S SOUTH AS THE WARM FRONT LIFTS
NORTHWARD. HOWEVER...WITH A LAKE BREEZE AND ONSHORE FLOW...MUCH
COOLER CONDITIONS ARE EXPECTED NEAR THE LAKE.


KJB

&&

.LONG TERM...
320 PM CDT

WEDNESDAY NIGHT THROUGH TUESDAY...

THE PERIOD WILL BEGIN RELATIVELY QUIET WITH DRY CONDITIONS
EXPECTED WEDNESDAY NIGHT. ANY LINGERING PRECIP AND CLOUD COVER
FROM THE AFTERNOON WILL DIMINISH EARLY IN THE EVENING WITH CLEAR
SKIES EXPECTED FOR MOST AREAS THROUGHOUT THE NIGHT.
HOWEVER...UPPER LEVEL RIDGE WILL BEGIN TO SHIFT EAST OF THE AREA
DURING THIS TIME WHILE A MID/UPPER LEVEL TROUGH LIFTS NORTHEAST
ACROSS THE CENTRAL PLAINS. AS THIS OCCURS...DO ANTICIPATE SOME
PIECES OF ENERGY TO WORK TOWARDS THE REGION WHICH WILL TRY AND
WORK ANY PRECIP TO THE WEST NEAR THE CWA. THIS PRECIP WILL LIKELY
REMAIN TO THE WEST BUT DO EXPECT INCREASING CLOUDS FOR AREAS OVER
NORTHWEST ILLINOIS LATE.

MID/UPPER LEVEL TROUGH WILL CONTINUE LIFTING NORTHEAST THROUGH
THE NORTHERN PLAINS AND UPPER MIDWEST ON THURSDAY...WITH BROAD
SOUTHWEST FLOW ALOFT IN PLACE. SURFACE LOW WILL LIFT NORTH THROUGH
THE NORTHERN PLAINS/UPPER MIDWEST DURING THIS TIME AS WELL WITH
SURFACE PRESSURE GRADIENT OVER NORTHERN ILLINOIS TIGHTENING. THIS
SOUTHERLY FLOW AND PARTLY CLOUDY SKIES WILL HELP SET THE STAGE FOR
A WARM DAY ON THURSDAY...WITH LOW TO MID 80S EXPECTED AND EVEN FOR
AREAS NEAR THE LAKE. ENERGY TO THE WEST WILL SLOWLY BEGIN TO SHIFT
EAST THURSDAY NIGHT AND ALTHOUGH CHANCES FOR PRECIP INCREASE...IT
WILL LIKELY ONLY BE FOR AREAS ACROSS THE WESTERN CWA.

MOST GUIDANCE IN GENERAL AGREEMENT WITH SHIFTING THE ACTIVITY
EAST ACROSS THE CWA ON FRIDAY...WITH SHOWERS AND THUNDERSTORMS
APPEARING TO BE MORE LIKELY DURING THIS TIME. DEWPOINTS ARE
EXPECTED TO INCREASE INTO THE LOW TO MID 60S DURING THIS TIME WITH
INSTABILITY LIKELY INCREASING AS WELL. HOWEVER...IF PRECIP WERE TO
BE ONGOING AT THE START OF THE PERIOD...I WONDER HOW MUCH
INSTABILITY WILL BE PRESENT. ALSO...FLOW REMAINS RATHER WEAK
ALOFT. SO AT THIS TIME...AM NOT OVERLY CONCERNED WITH ANY
WIDESPREAD ORGANIZED CONVECTION. SIGNIFICANT INCREASES IN MOISTURE
WILL SUPPORT HEAVY RAINFALL POTENTIAL THOUGH. WILL NEED TO
CONTINUE MONITORING THIS PERIOD AS WELL AS GOING INTO FRIDAY NIGHT
WITH UPSTREAM FORCING AND DEVELOPMENT LIKELY MOVING NORTHEAST OVER
THE AREA. WITH STRONGER FORCING...MORE ORGANIZED DEVELOPMENT COULD
OCCUR WITH ADDITIONAL HEAVY RAINFALL. ACTIVE WEATHER PATTERN LOOKS
TO REMAIN IN PLACE INTO THE WEEKEND WITH SHOWERS AND THUNDERSTORMS
LIKELY.

RODRIGUEZ

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* IFR/LIFR CONDITIONS EXPECTED TO DETERIORATE TO LIFR/VLIFR IN LOW
  CIGS/FOG OVERNIGHT AND CONTINUE THROUGH EARLY WEDNESDAY
  MORNING.

* ISOLATED SHRA POSSIBLE OVERNIGHT...BUT LOW PROBABILITY AT
  TERMINALS.

* IMPROVEMENT TO MVFR/VFR EXPECTED BY MID-LATE WEDNESDAY MORNING.

* NORTHEAST WINDS LESS THAN 10 KT THIS OVERNIGHT BECOME SOUTHEAST
  WEDNESDAY.

RATZER

//DISCUSSION...UPDATED 03Z...

CIG/VIS BEGAN TO LOWER QUICKLY AFTER SUNSET...PARTICULARLY IN A
CORRIDOR FROM NORTH OF DOWNTOWN CHICAGO THROUGH AND NORTH OF
ORD/DPA AREA. HOWEVER...MORE NORTHERLY COMPONENT TO SFC WINDS
ALONG LAKE HAVE LOWERED DEW POINT A BIT WITH SOME IMPROVEMENT IN
CIG/VIS RECENTLY AND EVEN A HOLE IN THE LOW CLOUDS OVER DOWNTOWN
AND MDW...THOUGH CURRENT WEB CAM IMAGERY SHOWS LOW CLOUDS HAVE
MOVED BACK INTO DOWNTOWN. AVAILABLE GUIDANCE CONTINUES TO INDICATE
A DOWNWARD TREND OVERNIGHT...SO HAVE MAINTAINED IDEA OF GOING
FORECAST WITH A FEW HOURS OF HIGHER CONDITIONS LATE THIS EVENING
IN 03Z AMENDMENTS.

RATZER

PREVIOUS DISCUSSION FROM 00Z...

COOL...MOIST EAST-NORTHEAST ACROSS THE AREA NORTH OF A NEARLY
STATIONARY FRONTAL BOUNDARY OVER CENTRAL ILLINOIS CONTINUES TO
PRODUCE IFR/LIFR CIGS AND GENERALLY MVFR VIS IN FOG EARLY THIS
EVENING. THE FRONT WILL REMAIN TO OUR SOUTH OVERNIGHT...WITH
SUNSET AND DIURNAL COOLING EXPECTED TO REDUCE CEILING HEIGHTS AND
VISIBILITY AGAIN OVERNIGHT. GUIDANCE ALL INDICATES LIFR/VLIFR
CONDITIONS DEVELOPING WITH INVERSION LOWERING AND CEILINGS BELOW 500
FT OR LOWER AND FOG POTENTIALLY REDUCING SFC VIS TO 1/2SM OR LESS.
THESE CONDITIONS WILL LIKELY LAST INTO WEDNESDAY MORNING...BEFORE
THE FRONT BEGINS TO LIFT NORTH WITH WINDS SHIFTING SOUTHEAST AND
LOWEST LEVELS DRYING OUT. ISOLATED TSRA HAVE BEEN OCCURRING SOUTH
OF THE TERMINALS NEAR STALLED FRONT AND OUTFLOW BOUNDARIES...BUT
ARE EXPECTED TO FADE WITH SUNSET. AN ISOLATED SHRA IS POSSIBLE
MAINLY THIS EVENING/EARLY OVERNIGHT FARTHER NORTH...BUT VERY LOW
COVERAGE AND VERY LOW PROBABILITY OF DIRECTLY IMPACTING AN
INDIVIDUAL TERMINAL.

WITH WARM FRONT DEVELOPING NORTHWARD WEDNESDAY MORNING... CIG/VIS
ARE EXPECTED TO QUICKLY IMPROVE FROM LIFR/VLIFR TO VFR BY MID-LATE
MORNING. WINDS WILL LIKELY MAINTAIN AN EASTERLY COMPONENT FOR
CHICAGO TERMINALS...WITH EAST-SOUTHEAST WINDS GENERALLY 10 KT OR
LESS EXPECTED.

RATZER

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* MEDIUM IN CIG/VIS TRENDS THIS EVENING THROUGH WED MORNING.

* MEDIUM-HIGH IN SHRA/TSRA NOT DIRECTLY IMPACTING ORD/MDW
  TERMINALS.

* HIGH IN WIND TRENDS.

RATZER

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

THURSDAY...VFR. SOUTH WINDS.

FRIDAY...CHANCE TSRA/MVFR. SOUTH-SOUTHWEST WINDS.

SATURDAY...CHANCE TSRA/MVFR. NORTHEAST WINDS.

SUNDAY...CHANCE TSRA/MVFR. EAST-NORTHEAST WINDS.

MONDAY...CHANCE TSRA/MVFR. SOUTHEAST WINDS BECOMING NORTHWEST
LATE.

RATZER

&&

.MARINE...
347 PM CDT

MAIN CONCERNS OVER THE LAKE TONIGHT INTO WEDNESDAY...AND POSSIBLY
INTO WEDNESDAY NIGHT...WILL BE AREAS OF DENSE FOG. THIS DENSE FOG
IS ALREADY IN PLACE THIS AFTERNOON...WITH A DENSE FOG ADVISORY IN
PLACE MAINLY FOR THE SOUTH HALF TONIGHT INTO WEDNESDAY. WILL NEED
TO MONITOR THE POTENTIAL FOR POSSIBLY EXTENDING THIS ADVISORY IN
TIME...AS WELL AS EXPANDING IT FURTHER NORTH. NORTHEAST WINDS IN
PLACE WILL BECOME MORE EASTERLY TONIGHT INTO WEDNESDAY WHILE
DIMINISHING. AS A LOW OVER THE CENTRAL PLAINS LIFTS NORTHEAST
THROUGH THE NORTHERN PLAINS WEDNESDAY THROUGH THURSDAY AND THEN
TOWARDS WESTERN ONTARIO BY THURSDAY EVENING...WINDS WILL GENERALLY
BE SOUTHERLY DURING THIS TIME INCREASING TO THE 15 TO 25 KT RANGE
BY THE END OF THE WORK WEEK.

RODRIGUEZ

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...DENSE FOG ADVISORY...ILZ006-ILZ014 UNTIL 11 AM WEDNESDAY.

IN...NONE.
LM...DENSE FOG ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 UNTIL 4 PM WEDNESDAY.

&&

$$

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