Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 080616
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
116 AM CDT WED JUL 8 2015

.UPDATE...

823 PM CDT

THE ONLY CHANGES TO THE GOING FORECAST WAS TO INCREASE POPS AND QPF
AMOUNTS ON WEDNESDAY AFTERNOON AND EVENING WITH THE NEXT STORM
SYSTEM EXPECTED TO IMPACT THE REGION. HEAVY RAINFALL APPEARS TO BE A
GOOD BET WITHIN AND POSSIBLY EVEN A BIT NORTH OF THE CURRENT FLASH
FLOOD WATCH...SO I HAVE ALSO INCLUDED THE MENTION OF HEAVY RAINFALL
IN THE FORECAST FOR THE SOUTHERN HALF OR SO OF THE AREA.

MODEL FORECAST GUIDANCE IS IN GOOD AGREEMENT IN FORECASTING A
SYNOPTICALLY FORCED RAINFALL EVENT OVER THE AREA LATER WEDNESDAY
THROUGH WEDNESDAY NIGHT. THE MAIN SYSTEM OF INTEREST IN EVIDENT IN
WATER VAPOR IMAGERY OVER NORTHERN TEXAS INTO OKLAHOMA EARLY THIS
EVENING. THIS SYSTEM IS EXPECTED TO INDUCE A SURFACE WAVE OF LOW
PRESSURE TONIGHT ALONG A SOUTHWEST TO NORTHEAST ORIENTED SURFACE
FRONTAL BOUNDARY...CURRENTLY STRETCHING FROM THE SOUTHERN PLAINS TO
THE EASTERN GREAT LAKES REGION. THIS SYSTEM IS THEN FORECAST TO LIFT
OVER NORTHERN ILLINOIS WEDNESDAY EVENING.

THE TRACK OF THIS SYSTEM FAVORS AN AREA OF GOOD FRONTOGENESIS AND
MOISTURE CONVERGENCE RIGHT ACROSS THE SOUTHERN HALF OF MY CWA
WEDNESDAY EVENING...ROUGHLY FROM EAST CENTRAL ILLINOIS NORTHEASTWARD
ACROSS NORTHWESTERN INDIANA. ALTHOUGH CONVECTIVE INSTABILITY WILL BE
VERY LOW...THE PRESENCE OF STRONG MESOSCALE FORCING IN COMBINATION
WITH THE GOOD SYNOPTIC SUPPORT...ALONG WITH THE PRESENCE OF AROUND 2
INCH PWATS...SHOULD PRODUCE A LARGE AREA OF MODERATE RAIN...WITH
SOME EMBEDDED BANDING OF HEAVY RAINFALL AND POSSIBLE THUNDERSTORMS.
CURRENTLY IT STILL APPEARS THE CURRENT FLASH FLOOD WATCH WILL COVER
MUCH OF THE MOST SUSCEPTIBLE AREAS TO HEAVY RAINFALL AND POSSIBLE
RENEWED FLOODING. HOWEVER...I DID CONSIDER ADDING A FEW COUNTIES TO
THE NORTH OF THE CURRENT WATCH...ESPECIALLY LAKE AND PORTER IN
NORTHWESTERN INDIANA. HOWEVER...GIVEN THIS IS STILL 24 HOURS AWAY I
HAVE OPTED TO HOLD OFF ON ANY CHANGES TO THE WATCH...AS A FARTHER
SOUTH TRACK OF THE LOW COULD MEAN LESS RAIN FARTHER NORTH.

GIVEN ALL OF THE ABOVE...I HAVE UPPED POPS TO 80 TO 90 PERCENT SOUTH
OF INTERSTATE 80...WITH LIKELY POPS EXTENDING OVER MUCH OF THE
CHICAGO METRO AREA WEDNESDAY EVENING. TO COMPLEMENT THIS...I HAVE
ALSO INCREASED QPF AMOUNTS WEDNESDAY EVENING. OVERALL...THESE
CHANGES RESULT IN 1.5 TO 2 INCHES SOUTH OF INTERSTATE 80...THOUGH
ISOLATED HIGH AMOUNTS ARE CERTAINLY POSSIBLE UNDER ANY MESOSCALE
BANDS THAT DEVELOP. AS MENTIONED ABOVE...I HAVE OPTED TO NOT MAKE
ANY CHANGES TO THE AREAS IN THE WATCH...HOWEVER...IF CURRENT MODEL
TRENDS CONTINUE...THE WATCH MAY NEED TO BE EXPANDED TO THE SOUTHERN
TIP OF LAKE MICHIGAN.

KJB

&&

.SHORT TERM...
234 PM CDT

THROUGH WEDNESDAY...

THE COLD FRONT THAT MOVED THROUGH EARLY TODAY NOW LIES ACROSS
CENTRAL INDIANA AND SOUTHERN IL WITH THE RAIN ALSO LOCATED OVER
THOSE REGIONS. EXPECTING THE DRY CONDITIONS TO CONTINUE OVERNIGHT
AS HIGH PRESSURE MOVES OVER CENTRAL LAKE MI LATE TONIGHT. GUSTY
NORTH WINDS WILL DIMINISH FIRST FOR AREAS WEST OF I-55 AS THE
GRADIENT WILL WEAKEN QUICKEST THERE. GUSTY WINDS ALONG THE LAKE
WILL TAKE THE LONGEST TO DIMINISH BUT THINKING GUSTS WILL FINALLY
BE BELOW 20 MPH LATE THIS EVENING.

CUMULUS CLOUDS HAVE FORMED IN THE COOL AIR AS THE STRATUS ASSOCIATED
WITH THE FRONT SHIFTS TO THE SOUTHEAST. THE CU WILL BE DIURNAL IN
NATURE SO EXPECTING SKIES TO CLEAR OVERNIGHT ALLOWING TEMPS TO FALL
INTO THE LOW TO MID 50S. SOME LOCATIONS ALONG THE IL/WI STATE LINE
MAY EVEN DROP INTO THE UPPER 40S.  CHICAGO AND THE INDIANA SHORE
LINE WILL BE THE WARMEST WITH LOWS AROUND 60 DUE TO FLOW OFF OF
THE LAKE.

ATTENTION THEN SHIFTS TO TOMORROW/S RAINFALL. SOME AREAS SOUTH OF
I-80 RECEIVED UP TO 3 INCHES OF RAIN MONDAY AND MONDAY
NIGHT...AND WHEN THAT IS COMBINED WITH THE RECENT WET PATTERN WE
ARE IN...THERE ARE CONCERNS ABOUT FLOODING TOMORROW. BOTTOM LINE
UP FRONT...ISSUED A FLASH FLOOD WATCH FOR LASALLE
COUNTY...KANKAKEE COUNTY...THROUGH JASPER COUNTY SOUTH FROM
WEDNESDAY AFTERNOON THROUGH WEDNESDAY NIGHT.

A LOW PRESSURE SYSTEM WILL FORM OVER OKLAHOMA LATE THIS EVENING AND
DEEPEN AS IT MOVES OVER THE MID IL/IN STATE LINE WEDNESDAY EVENING.
AN UPPER LEVEL VORT STREAMER ASSOCIATED WITH THE LOW SHIFTS NORTH
INTO NORTHERN IL AND INDIANA WEDNESDAY MORNING...AND THEN PARKS OVER
THE AREA WEDNESDAY AFTERNOON. 12Z GUIDANCE IS IN BETTER AGREEMENT
THAN 06Z RUNS ABOUT PRECIP FALLING ALONG AND SOUTH OF THE I-88
CORRIDOR...SO HAVE MEDIUM-HIGH CONFIDENCE IN HOW FAR NORTH PRECIP
GETS WEDNESDAY. THE NAM IS THE MAIN OUTLIER WITH MUCH LESS PRECIP
THAN THE GFS AND ECMWF. PWAT VALUES INCREASE TO 1.5-2 INCHES
DURING THE DAY WEDNESDAY. HOWEVER...MODEL SOUNDINGS FEATURE A LOT
OF DRY AIR ALOFT THAT THE RAIN WILL HAVE TO OVERCOME...SO HAVE
MEDIUM CONFIDENCE IN HOW QUICKLY SHOWERS PUSH NORTH WEDNESDAY.
GUIDANCE ALSO FEATURES VERY LITTLE CAPE...SO ONLY HAVE A SLIGHT
CHANCE OF THUNDERSTORMS IN THE FORECAST.

THINKING THE MAJORITY OF PRECIP WILL FALL AS STRATIFORM RAIN WITH A
FEW EMBEDDED STRONGER RAIN SHOWERS OR THUNDERSTORMS. THE TOTAL
AMOUNT OF RAIN IS THE CONCERN...WITH 0.75 TO AN INCH OF ADDITIONAL
RAIN FORECAST FOR AREAS ALONG AND SOUTH OF I-80. LOCALLY HIGHER
AMOUNTS ARE POSSIBLE. ONCE AGAIN...WE ISSUED A FLASH FLOOD WATCH
FOR THE SOUTHERN PORTION OF THE CWA SINCE SIGNIFICANT RAINFALL
AMOUNTS ARE POSSIBLE OVER AREAS THAT ALREADY RECEIVED AMPLE
AMOUNTS OF RAIN FOR WEDNESDAY AFTERNOON THROUGH WEDNESDAY NIGHT.

WITH THE INCREASED CLOUD COVER...LOWERED HIGH TEMPS FOR WEDNESDAY BY
A DEGREE OR TWO. EXPECTING HIGH TEMPS AROUND 70 WITH UPPER 60S ALONG
THE LAKE.

JEE

&&

.LONG TERM...
305 PM CDT

WEDNESDAY NIGHT THROUGH TUESDAY...

SURFACE LOW WILL TRACK FROM DOWNSTATE ILLINOIS INTO NORTHEASTERN
INDIANA WEDNESDAY NIGHT WITH DEFORMATION AXIS PIVOTING ACROSS THE
SOUTHEASTERN TWO THIRDS OF THE CWA BRINGING STEADY RAINFALL
OVERNIGHT. BASED ON THE LOW TRACK...CURRENT EXPECTATIONS ARE FOR
THE HEAVIEST RAINFALL EAST OF THE I-55 CORRIDOR WHERE THREE
QUARTERS TO AN INCH OF RAIN IS EXPECTED...WITH LOCALLY HIGHER
AMOUNTS POSSIBLE. MOISTURE IS DEEP RESULTING IN FAIRLY POOR LAPSE
RATES THROUGH THE COLUMN. GIVEN THE LACK OF ANY SIGNIFICANT
INSTABILITY...WILL ONLY MENTION A SLIGHT CHANCE OF EMBEDDED
THUNDER WITHIN THE CORRIDOR OF HIGHEST POPS.

BY THURSDAY...SURFACE LOW BEGINS TO FILL AS IT MOVES TO NEW ENGLAND
WITH A WEAK TRAILING SURFACE TROUGH SAGGING SOUTH INTO CENTRAL
ILLINOIS. MEANWHILE...HIGH PRESSURE BUILDS FROM THE UPPER MIDWEST
INTO NORTHERN ILLINOIS AND SHOULD PROVIDE QUIET WEATHER FOR MOST OF
THE CWA...WITH TEMPERATURES IN THE MID 70S AWAY FROM THE LAKE.
NORTHEAST WINDS OFF OF LAKE MICHIGAN THROUGH THE DAY WILL KEEP
TEMPERATURES IN THE UPPER 60S TO AROUND 70 ALONG THE LAKE FRONT.
MODELS DEVELOP VERY LIGHT QPF ACROSS THE SOUTHERN PORTIONS OF THE
CWA NEAR THE AFOREMENTIONED TROUGH...HOWEVER SOUNDINGS SHOW FAIRLY
SHALLOW MOISTURE AND THERE IS NO UPPER LEVEL SUPPORT WITH A BUILDING
MID/UPPER LEVEL RIDGE...SO OPTED TO CAP POPS AT SILENT 14S THURSDAY
AFTERNOON AND EVENING.

FROM FRIDAY ON...BERMUDA HIGH WILL BEGIN TO SHIFT WEST WITH STRONG
RIDGE DEVELOPING AND BECOMING CENTERED OVER TEXAS AND THE PLAINS
OVER THE WEEKEND. MID LEVEL WARM ADVECTION WILL RESULT IN A WARMING
TREND FRIDAY INTO THE WEEKEND WITH MID 80S EXPECTED CWA WIDE
SATURDAY AND SUNDAY AND SHOULD CONTINUE INTO NEXT WEEK. WITH THE
WARMER TEMPERATURES THOUGH WILL ALSO COME PERIODIC CHANCES FOR
SHOWERS AND THUNDERSTORMS GIVEN THE PERSISTENT MID LEVEL WAA ALONG
WITH SEVERAL WEAK UPPER LEVEL DISTURBANCES RIDING DOWN THE LEE SIDE
OF THE RIDGE.

DEUBELBEISS

&&

.AVIATION...

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

* RA/SHRA LIFTING NORTH ON WEDNESDAY AFTERNOON...WITH FALLING
  VSBY AND CIGS INTO THE EVENING.

* EAST WINDS MAY BE AROUND 10 KT LATE WEDNESDAY AFTERNOON INTO
  WEDNESDAY EVENING.

KMD

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

SHOULD REMAIN FAIRLY QUIET UNTIL AFTER DAYBREAK TODAY. DEWPOINT
DEPRESSIONS ARE DECLINING IN THE LIGHT WIND REGIME...BUT STILL NOT
EXPECTING ALOT OF FOG AS THE HIGH CLOUD CANOPY SPREADS OVERHEAD.

LEAD WAVE OF RAIN IS PUSHING INTO WESTERN ILLINOIS. LATEST SHORT
TERM GUIDANCE TRACKS THIS WAVE STRAIGHT THROUGH NORTHEAST ILLINOIS
THIS MORNING. SATURATION WILL BE A CONCERN AS THE WAVE MOVES
THROUGH WITH TIME HEIGHT INDICATING SOME LOWER LEVEL SATURATION
COMBINED WITH SOME HIGH CLOUDS. CARRIED A VCSH WITH THIS LEAD
WAVE...AND THIS MAY JUST BE SOME SPRINKLES OR SOME VERY LIGHT RAIN
GIVEN THE SATURATION CONCERNS. NEAR TERM GUIDANCE WOULD ALSO HINT
AT GETTING NEAR MVFR CONDITIONS AS EARLY AS 17-18Z. BETTER
SATURATION ARRIVES WITH THE MORE ORGANIZED LOW MOVING UP FROM THE
SOUTHWEST. RAIN WILL SPREAD OVER THE AREA LATE AFTERNOON AND MORE
SO IN THE EVENING WITH A DETERIORATION TO MVFR CIGS AND
VSBY...WITH IFR CONDITIONS LIKELY TO SPREAD NORTHWARD AS WELL AS
RAIN CONTINUES A GOOD PORTION OF THE NIGHT. STILL SOME
UNCERTAINTIES WITH HOW FAR NORTH/WEST THE MAIN PRECIPITATION
SHIELD WILL SPREAD WITH BETTER MOISTURE TRANSPORT FOCUS EAST OF
THE LOW CENTER TRACK BUT THERE IS STILL A DECENT DEFOMRATION AXIS
THAT SETS UP FARTHER NORTH AND WEST. THUNDER CHANCES LOOK
LOW...WITH SLIGHTLY HIGHER CHANCES IN NORTHWEST INDIANA AND POINTS
SOUTH.

EAST WINDS LOOK TO HOLD JUST A SHADE UNDER 10 KT...WITH SOME
INCREASE OVERNIGHT AS THE SURFACE LOW TRACKS NEARBY AND WINDS
SHIFT NORTHEASTERLY.

KMD


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

* HIGH IN OVERALL WIND TRENDS...MEDIUM IN WIND SPEEDS WEDNESDAY
  AFTERNOON AND EVENING.

* MEDIUM-HIGH IN RAIN ARRIVING LATE WEDNESDAY AFTERNOON...MEDIUM
  IN TIMING AND ASSOCIATED CIG AND VSBY IMPACTS.

KMD

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

THURSDAY...MVFR CIGS PSBL EARLY...OTHERWISE VFR. NORTHEAST WINDS.

FRIDAY...CHANCE SHRA/TSRA AND AND ASSOCIATED VSBY IMPACTS. MAINLY
VFR CIGS.

SATURDAY THROUGH MONDAY...PERIODIC CHANCE FOR SHRA/TSRA AND
ASSOCIATED VSBY IMPACTS. MAINLY VFR CIGS.

RC

&&

.MARINE...
305 PM CDT

WIND GUSTS HAVE BEGUN TO DIMINISH ALONG THE IL SHORE...BUT AM STILL
SEEING GUSTS UP TO 30 KT OVER THE SOUTHEAST END OF THE LAKE. HIGH
PRESSURE CURRENTLY OVER THE PLAINS WILL MOVE OVER THE LAKE BY
WEDNESDAY MORNING WITH WINDS DIMINISHING THROUGH THE NIGHT. WAVE
OBSERVATIONS ARE HIGHER THAN FORECAST WITH SITES ON THE SOUTHEAST
END OF THE LAKE SHOWING 9 FT. THE LATEST WAVE MODEL RUN ALSO CAME IN
WITH HIGHER WAVES ALONG THE NSH WATERS SO WILL EXTEND THE SMALL
CRAFT ADVISORY FOR WINTHROP HARBOR TO WILMETTE HARBOR TO MID EVENING
AS WAVES WILL REMAIN HAZARDOUS TO SMALL CRAFT LONGER THAN ORIGINALLY
THOUGHT.  NO CHANGES WILL BE MADE TO THE CURRENT SMALL CRAFT
ADVISORY FOR THE OTHER NEARSHORE ZONES.

WITH HIGH PRESSURE OVERHEAD TOMORROW...WINDS WILL BE LESS THAN 10 KT
OUT OF THE NORTH TO NORTHEAST OVER THE SOUTHERN HALF AND VARIABLE
OVER THE NORTHERN HALF.  NORTHEAST WINDS INCREASE TO 10-15 KT OVER
THE FAR SOUTHERN END OF THE LAKE AS A LOW PRESSURE SYSTEM PASSES
OVER NORTHERN IL AND INDIANA WEDNESDAY NIGHT. WINDS THEN BECOME
SOUTHWEST AS ANOTHER RIDGE SHIFTS EAST OF THE LAKE THURSDAY NIGHT
INTO FRIDAY WITH SOUTHERLY WINDS INTO EARLY NEXT WEEK.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...FLASH FLOOD WATCH...ILZ023-ILZ032-ILZ033-ILZ039...1 PM WEDNESDAY
     TO 7 AM THURSDAY.

     BEACH HAZARDS STATEMENT...ILZ014 UNTIL 4 AM WEDNESDAY.

IN...BEACH HAZARDS STATEMENT...INZ001-INZ002 UNTIL 4 AM WEDNESDAY.

     FLASH FLOOD WATCH...INZ010-INZ011-INZ019...1 PM WEDNESDAY TO 7
     AM THURSDAY.

LM...SMALL CRAFT ADVISORY...NEARSHORE WILMETTE HARBOR TO MICHIGAN CITY IN UNTIL
     4 AM WEDNESDAY.

&&

$$

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