Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 300159
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
859 PM CDT SAT AUG 29 2015

.UPDATE...
859 PM CDT

MAIN FORECAST CONCERN OVERNIGHT CONTINUES TO BE THE POTENTIAL FOR
AREAS OF DENSE FOG TO DEVELOP. WITH WEAK LOW PRESSURE THAT WAS
OVER MID MS VALLEY DISSIPATING TO A TROUGH...WARM FRONT EXTENDING
NORTHWEST FROM THE LOW HAS BECOME BARELY DISCERNABLE. THIS FRONT
SERVED AS A MOISTURE CONVERGENCE AXIS COMBINED WITH FORCING FROM
THE UPPER TROUGH AXIS PIVOTING GRADUALLY SOUTHEAST ACROSS THE
REGION TO YIELD THE PERSISTENT SHOWER ACTIVITY INTO THIS
EVENING...BUT THE FORCING IS WEAKENING AS THE TROUGH SHIFTS
EAST...SO SHOWER ACTIVITY WILL FINALLY TAPER OFF...LATEST IN FAR
SOUTHEAST. AS OF THIS WRITING...THERE WAS A LOW TOPPED SUPERCELL
IN JASPER COUNTY WHERE BETTER INSTABILITY RESIDED AND ABOUT 20 KT
OF 0-1 KM SHEAR ACCORDING TO SPC MESOANALYSIS. THUNDER THREAT
SHOULD QUICKLY WANE IN EAST CENTRAL IL AND SOUTH OF KANKAKEE RIVER
IN INDIANA OVER NEXT FEW HOURS.

TURNING TO THE FOG POTENTIAL...THE WEAKENING OF THE LOW AND
EXTENSIVE HIGH PRESSURE COVERING THE EASTERN THIRD OF THE CONUS
HAS SET UP A VERY WEAK PRESSURE GRADIENT. DESPITE PLENTY OF
STRATUS...VERY MOIST LOW LEVELS AND LOW DEWPOINT DEPRESSIONS
SHOULD STILL SUPPORT FOG. BASED OFF OBS AND WEBCAMS ALONG
WISCONSIN SHORE TO THE ILLINOIS SHORE...VISIBILITY HAS BEEN ON A
DOWNWARD TREND THIS EVENING. HRRR-X SEEMS TO HAVE A GOOD HANDLE ON
FOG PROGRESSION...WHICH WOULD MEAN AREAS OF DENSE FOG ALONG THE
SHORE WOULD THEN SPREAD INLAND OVERNIGHT. DO NOT YET HAVE ENOUGH
CONFIDENCE TO ISSUE A DENSE FOG ADVISORY...BUT WILL ISSUE AN SPS
FOR THE LAKESHORE COUNTIES AND A GRAPHICAST TO HIT THE FOG
POTENTIAL ON OUR WEBSITE AND SOCIAL MEDIA. HAVE HIGHER CONFIDENCE
IN AREAS OF DENSE FOG FOCUSING ON COUNTIES ALONG AND NORTH OF
I-80...BUT WOULDNT BE SURPRISED TO SEE DENSE FOG ACROSS THE CWA IN
THE PREDAWN HOURS. UPDATED ZONES WILL BE SENT SHORTLY.

RC

&&

.SHORT TERM...

TONIGHT AND SUNDAY...

248 PM...FORECAST CONCERNS ARE DENSE FOG POTENTIAL TONIGHT...
LINGERING PRECIP TONIGHT AND ISOLATED THUNDERSTORMS SUNDAY
AFTERNOON.

WHILE MUCH OF THE PRECIP HAS ENDED THIS AFTERNOON WITH JUST SOME
SCATTERED SHOWERS ACROSS NORTHEAST IL AND NORTHWEST INDIANA...THERE
REMAINS THE POTENTIAL FOR SHOWERS INTO THIS EVENING MAINLY ACROSS
THE EASTERN HALF OR SO OF THE CWA. MAINTAINED CHANCE POPS SLOWLY
DROPPING OFF INTO THIS EVENING THOUGH OVERALL COVERAGE OF PRECIP
TRENDS IS A BIT UNCERTAIN. ANY LINGERING SHOWERS OVERNIGHT INTO
EARLY SUNDAY MORNING SHOULD BE CONFINED TO THE FAR SOUTHEAST CWA.

AS THE LOW NOW OVER WESTERN IL CONTINUES TO DISSIPATE...THE GRADIENT
WILL BECOME QUITE WEAK TONIGHT WITH WINDS ALREADY SHIFTING TO LIGHT
EAST/NORTHEAST ACROSS SOUTHEAST WI AND FAR NORTHEAST IL WHERE LOWER
CLOUDS AND SOME LIGHT FOG HAS DEVELOPED. MODEL FOG/VIS GUIDANCE ALL
SHOW FOG DEVELOPING IN THESE AREAS THIS EVENING THEN SPREADING TO
THE WEST/SOUTHWEST OVERNIGHT INTO SUNDAY MORNING. WITH MOS GUIDANCE
SHOWING MANY LOCATIONS TANKING OVERNIGHT. WITH THE LOW LEVELS
ALREADY SATURATED...THIS TREND SEEMS REASONABLE AND ADDED DENSE FOG
OVER NORTHWEST IL...MAINLY NORTHWEST OF AN ORD TO VYS LINE. WITH
LATE AUGUST SUNSHINE...EXPECT THIS TO BURN OFF BY MID MORNING
SUNDAY WITH CLOUDS SCATTERING OUT BY AFTERNOON. ASSUMING THIS
OCCURS...TEMPS SHOULD BE ABLE TO WARM INTO THE LOWER 80S. THOUGH A
LAKE BREEZE WILL LIKELY KEEP THE LAKESHORE AREAS COOLER.

AS TEMPS WARM...ESPECIALLY IN THE SOUTHEAST WHERE LESS FOG IS
EXPECTED...ENOUGH INSTABILITY IS PRESENT FOR ISOLATED SHOWERS AND
POSSIBLY A THUNDERSTORM. THIS SHOULD BE MAINLY DIURNALLY DRIVEN
FROM EARLY AFTERNOON TO EARLY EVENING THOUGH WITH MINIMAL
FORCING...PRECIP MAY END UP QUITE SPARSE. CMS

&&

.LONG TERM...
301 PM CDT

SUNDAY NIGHT THROUGH SATURDAY...

FOR THE LONGER TERM FORECAST PERIOD...A RATHER STAGNANT PATTERN IS
SETTING UP WITH HIGH PRESSURE SITTING OVER THE ERN CONUS AND A
TROUGH OF LOW PRESSURE REMAINING NEARLY STATIONARY OVER THE WESTERN
PLAINS.  THE UPPER LEVEL PATTERN WILL BE A LITTLE MORE
PROGRESSIVE...BUT THE PROGRESSION WILL LARGELY BE CONFINED TO NORTH
OF THE CANADIAN BORDER...WITH LITTLE EFFECT ON THE LOCAL AREA.

THE GENERAL TREND FOR THE LONG TERM FORECAST PERIOD WILL BE FOR A
WARMING TREND...BEGINNING ON SUNDAY AND PERSISTING THROUGH THE
REMAINDER OF THE WEEK.  BY MONDAY...A POSITIVE TILT UPPER LEVEL
RIDGE WILL EXTEND FROM THE DESERT SOUTHWEST THROUGH THE WESTERN
GREAT LAKES REGION...WITH GENERAL UPPER RIDGING PERSISTING OVER THE
REGION THROUGH THE WEEK.  BY MONDAY...TEMPERATURES SHOULD REACH THE
MIDDLE 80S ACROSS THE REGION...BUT A SOUTHEAST COMPONENT TO SFC
WINDS IS EXPECTED TO DEVELOP THROUGH THE DAY...WHICH COULD LIMIT
WARMING ALONG THE ILLINOIS LAKE MICHIGAN SHORE TO ARND 80F OR
POSSIBLE A LITTLE LESS.  THE REMNANTS OF WEAK TROPICAL SYSTEMS ARE
EXPECTED TO UNDERCUT THE UPPER RIDGE...TRACKING ACROSS THE SRN
MISSISSIPPI VALLEY...SETTING UP SLY-SWLY FLOW THROUGH THE LOWER
LEVELS...DRAWING SOME GULF MOISTURE INTO THE REGION.  THIS WOULD AID
IN THE DEVELOPMENT OF DIURNAL CU...WITH PERIODIC CHANCES FOR ISOLD-
SCT SHRA/TSRA...GENERALLY DURG THE DAYTIME HOURS.  THE LATEST LONG
RANGE MODEL RUNS CONTINUE THE TREND BEGUN YESTERDAY IN LIMITING
DAYTIME WARMING TO THE MIDDLE TO UPPER 80S THROUGH MUCH OF NEXT WEEK
WITH DEWPOINTS IN THE UPPER 60S TO LOWER 70S.  SO...WITH THE ABSENCE
OF ANY SIGNIFICANT FOCUSING MECHANISM...DO NOT EXPECT ANY
WIDESPREAD ORGANIZED PCPN THROUGH THE PERIOD WITH GENERALLY WARM AND
HUMID CONDITIONS.

KREIN

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* A FEW LINGERING SHOWERS THIS EVENING.
* LOWERING CIGS/VSBY OVERNIGHT.

BMD

//DISCUSSION...UPDATED 00Z...

A TROUGH IS DRAPED FROM NORTHEASTERN ILLINOIS INTO WEST CENTRAL
ILLINOIS THIS EVENING SERVING THE FOCUS FOR SCATTERED SHOWERS.
THESE SHOWERS ARE EXPECTED TO CONTINUE TO DIMINISH WITH PRIMARILY
DRY CONDITIONS EXPECTED OVERNIGHT AND THE REMAINDER OF THE TAF
PERIOD. THE TROUGH AXIS WILL SLOWLY SAG SOUTH OVERNIGHT WITH LIGHT
NORTHERLY WINDS OVERSPREADING THE TERMINALS. WIDESPREAD MVFR
STRATUS IS NOTED UPSTREAM WITH AN AREA OF IFR/LIFR CIGS OVER FAR
SOUTHEASTERN WISCONSIN. CIGS ARE EXPECTED TO LOWER AT THE
TERMINALS OVERNIGHT ALONG WITH VISIBILITY DROPPING WITH THE
DEVELOPMENT OF FOG. CONDITIONS SHOULD BE SLOW TO IMPROVE TOMORROW.
LIGHT WINDS WILL CONTINUE THROUGH THE DAY TOMORROW.

BMD

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* HIGH CONFIDENCE IN SHRA ENDING BY AROUND 01-02Z AT THE
  TERMINALS.
* MEDIUM-HIGH CONFIDENCE IN GENERAL CIG/VSBY TRENDS
  OVERNIGHT...BUT LOW-MEDIUM IN TIMING OF IFR AT 04Z. CONFIDENCE
  IN IFR INCREASES THRU AND AFTER 07Z.

BMD

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

MONDAY...VFR. SOUTH WINDS.
TUESDAY...VFR. SOUTHWEST WINDS.
WEDNESDAY...SLT CHC TSRA. PRIMARILY VFR. SOUTH WINDS.
THURSDAY...SLT CHC TSRA. PRIMARILY VFR. SOUTHEAST WINDS.
FRIDAY...SLT CHC TSRA. PRIMARILY VFR. SOUTHEAST WINDS.

BMD

&&

.MARINE...
301 PM CDT

A WEAK TROUGH OF LOW PRESSURE EXTENDS FROM THE MIDDLE MISSISSIPPI
VALLEY THROUGH SOUTHERN LAKE MICHIGAN AS HIGH PRESSURE SITS OVER
THE ERN CONUS AND A SLIGHTLY DEEPER TROUGH OF LOW PRESSURE
DEVELOPS OVER THE WESTERN PLAINS. THE WEAK TROUGH OVER SRN LAKE
MICHIGAN WILL PERSIST LONG ENOUGH INTO THIS EVENING FOR ELY WINDS
OVER THE SRN PORTIONS OF THE LAKE WHILE LOW PRESSURE MOVING
THROUGH THE CANADIAN PRAIRIES WILL KEEP WINDS OVER THE NORTHERN
PORTIONS OF THE LAKE MORE SLY TO SWLY. WEAK HIGH PRESSURE WILL BE
OVER THE SOUTH HALF OF THE LAKE SUNDAY...LEADING TO LAKE BREEZE
DEVELOPMENT AND ONSHORE WINDS OVER THE NEARSHORE WATERS AND LIGHT
AND VARIABLE WINDS OVER THE OPEN WATERS OF THE SOUTH HALF OF THE
LAKE. BY MONDAY...SLY-SWLY WINDS WILL COVER THE WHOLE LAKE BY
MONDAY AS A MODEST PRESSURE GRADIENT DEVELOPS OVER LAKE MICHIGAN
AS THE HIGH OVER THE ERN CONUS AND THE TROUGH OVER THE WRN PLAINS
REMAIN STATIONARY.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...DENSE FOG ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 UNTIL 9 AM SUNDAY.

&&

$$

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