Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 311850
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
150 PM CDT SUN AUG 31 2014

.DISCUSSION...
328 AM CDT

AFTER A FAIRLY QUIET DAY ACROSS THE AREA TODAY...ATTENTION QUICKLY
SHIFTS TO THE WEATHER FOR LABOR DAY INTO MONDAY NIGHT...DURING WHICH
PERIODS OF EXTREMELY HEAVY RAINFALL AND SEVERE WEATHER ARE POSSIBLE.

THUNDERSTORMS CONTINUED TO TRAIN ACROSS THE SAME AREAS ACROSS
PORTIONS OF FORD AND IROQUOIS COUNTIES IN ILLINOIS AND ALSO BENTON
COUNTY IN INDIANA YESTERDAY EVENING AND INTO THE OVERNIGHT HOURS.
THESE AREAS LIED IN THE RIGHT ENTRANCE REGION OF AN UPPER LEVEL JET
STREAK. THE COMBINATION OF THIS...AND THE CONTINUED LOW LEVEL
CONFLUENCE CONTINUED TO DRIVE THIS CONVECTION. RADAR ESTIMATES SOME
VERY HEFTY ISOLATED RAINFALL AMOUNTS IN EXCESS OF 4 INCHES ACROSS
PORTIONS OF THESE COUNTIES. ALTHOUGH SOME ISOLATED ACTIVITY
CONTINUES ACROSS THIS AREA AS OF THIS WRITING...IT IS EXPECTED THAT
ALL THIS ACTIVITY WILL SHIFT OUT OF THE AREA AROUND...OR JUST PRIOR
TO DAYBREAK AS THE UPPER LEVEL JET DEVELOPS EASTWARD.

WATER VAPOR IMAGERY EARLY THIS MORNING SHOWS THE NEXT UPPER LEVEL
SYSTEM CURRENTLY DIGGING ACROSS THE ROCKIES. AIRCRAFT AND UPPER AIR
OBSERVATIONS ACROSS WESTERN CANADA AND OVER THE NORTHEAST PACIFIC
INDICATE SOME IMPRESSIVE UPPER LEVEL WINDS...FOR THIS TIME OF
YEAR...IN EXCESS OF 110 KT WITHIN THE CORE OF THE JET STREAK
DIGGING OVER THE WESTERN CONUS. THE FIRST MID LEVEL IMPULSE IS
EXPECTED TO CONTINUE SHIFTING EASTWARD OVER THE UPPER MIDWEST INTO
MONDAY MORNING. AS IT DOES SO...IT WILL DRIVE A FAIRLY STOUT
SURFACE LOW NORTHEASTWARD FROM THE DAKOTAS TO ONTARIO. SOME VERY
SUBSTANTIAL CONVECTION IS EXPECTED TO DEVELOP AHEAD OF THE COLD
FRONT LATER TODAY ACROSS PORTIONS OF THE MID MISSOURI VALLEY AND
INTO MINNESOTA. THIS ACTIVITY WILL LIKELY SHIFT EAST-NORTHEASTWARD
OVERNIGHT TONIGHT...WITH SOME OF THE REMNANT ACTIVITY POSSIBLY
REACHING NORTH CENTRAL ILLINOIS BY AROUND...OR JUST PRIOR TO
DAYBREAK MONDAY. IT APPEARS THAT THIS INITIAL ROUND OF CONVECTION
COULD BE IN A WEAKENING PHASE AS IT TRIES TO PUSH OVER PORTIONS
OF NORTHERN ILLINOIS MONDAY MORNING. HOWEVER...RENEWED CONVECTIVE
DEVELOPMENT...POSSIBLY ALONG ANY OUTFLOW BOUNDARIES LAID DOWN BY
EARLY MORNING ACTIVITY...IS LIKELY ACROSS PORTIONS OF NORTHEASTERN
ILLINOIS AND NORTHWESTERN INDIANA BY MONDAY AFTERNOON.

MODEL SOUNDINGS ALL INDICATE FAVORABLE PROFILES FOR ACTIVE STRONG TO
SEVERE STORMS OVER THE AREA DURING THE AFTERNOON AND EARLY EVENING
MONDAY. DEEP LAYER BULK SHEAR LOOKS TO BE SOMEWHAT
UNIDIRECTIONAL...BUT WITH 0-6 KM SPEED SHEAR OF NEARLY 50
KT...ORGANIZED CONVECTIVE ACTIVITY APPEARS VERY POSSIBLE. THE MAIN
COMPLICATING FACTOR AT THIS POINT IS HOW MUCH ANY CLOUD DEBRIS OR
REMNANT MORNING CONVECTIVE ACTIVITY IMPACTS AFTERNOON HEATING AND
DESTABILIZATION ACROSS NORTHEASTERN ILLINOIS AND NORTHWESTERN
INDIANA. HOWEVER...IN SPITE OF THIS POSSIBILITY EARLY IN THE
DAY...IT DOES APPEAR THAT THE HIGH THETA E NEAR SURFACE AIR MASS
SHOULD BE ENOUGH TO PRODUCE AMPLE MLCAPE VALUES AT OR JUST IN
EXCESS OF 1500 J/KG...WHICH COMBINED WITH THE 45+ DEEP LAYER SHEAR
CERTAINLY POINTS TO A SEVERE THREAT. AT THIS TIME IT APPEARS
RATHER MODEST MID LEVEL LAPSE RATES WOULD RESULT IN THE MAIN
SEVERE THREAT BEING STRONG DAMAGING WIND GUSTS. HOWEVER...THERE IS
ALSO A SECONDARY THREAT OF LARGE HAIL...ESPECIALLY IF ANY SUPER
CELLS CAN FORM.

THERE IS ALSO A DECENT CONCERN FOR EXTREMELY HEAVY RAINFALL RATES
WITH THESE STORMS...WITH PWAT VALUES IN THIS JUICY ENVIRONMENT
LIKELY TO RESIDE AROUND 1.7 INCHES. NUMERICAL GUIDANCE CONTINUES TO
SUGGEST THAT A SECONDARY MID LEVEL IMPULSE WILL APPROACH THE AREA
MONDAY NIGHT AND ACT TO INDUCE/STRENGTHEN A SOUTHWESTERLY UPPER
LEVEL JET STREAK ACROSS THE GREAT LAKES REGION INTO MONDAY NIGHT.
THIS PROCESS COULD SET UP PORTIONS OF THE AREA WITHIN THE FAVORABLE
RIGHT ENTRANCE REGION OF A 100 KT UPPER LEVEL JET INTO TUESDAY
MORNING AS THE SURFACE COLD FRONTAL BOUNDARY SLOWLY TRIES TO SHIFT
SOUTHWARD ACROSS THE SOUTHERN HALF OF MY CWA. I HAVE INCREASING
CONCERNS THAT THIS COULD SET UP A VERY HEAVY RAIN EVENT MONDAY
NIGHT...WITH STORMS POTENTIALLY TRAINING TO THE EAST NORTHEAST
NEARLY PARALLEL TO THE SURFACE BOUNDARY. AT THIS TIME IT APPEARS THIS
HEAVY RAIN THREAT COULD END UP SOUTH OF INTERSTATE 80 MONDAY NIGHT.
HOWEVER...UNCERTAINTY IS STILL A BIT HIGH AS TO HOW FAR SOUTH THE
SURFACE FRONT WILL GET MONDAY NIGHT...AND HENCE WHERE THE MOST
FAVORED AREAS FOR TRAINING CELLS WILL OCCUR. AT THIS POINT...I HAVE
ADDED HEAVY RAIN WORDING TO THE FORECAST...MAINLY FOR AREAS SOUTH OF
INTERSTATE 80 INTO MONDAY NIGHT.

FOLLOWING THIS ACTIVE WEATHER MONDAY NIGHT IT APPEARS THE WEATHER
WILL BE DRY FOR THE MOST PART THROUGH LATE WEEK. THE MAIN STORY
HOWEVER...WILL BE THE INCREASING HEAT BY THURSDAY. FORECAST GUIDANCE
CONTINUES TO FORECAST MID LEVEL HEIGHTS TO BUILD SUBSTANTIALLY
ACROSS THE CENTRAL CONUS. THIS COULD LEAD TO A RUN AT
90 TEMPERATURES FOR THURSDAY AS 850 MB TEMPERATURES WARM TO 20 TO
22C. I HAVE STARTED CUTTING BACK ON THE SLIGHT CHANCE
POPS...ESPECIALLY ON WEDNESDAY AND WEDNESDAY NIGHT...BUT LEFT IN
SOME SLIGHTS FOR THURSDAY. ALTHOUGH SOME ISOLATED CONVECTIVE
ACTIVITY IS POSSIBLE DURING THIS TIME...IT APPEARS THAT THE BUILDING
HEIGHTS AND WARMING TEMPS ALOFT COULD END UP CAPPING THE AREA...AND
LIMITING THE THREAT FOR CONVECTIVE DEVELOPMENT THROUGH THURSDAY.

CONVECTIVE CHANCES DO LOOK TO RETURN TO THE AREA AGAIN BY FRIDAY.
ANOTHER SIGNIFICANT WAVE IS EXPECTED TO SHIFT EASTWARD ACROSS THE
NORTHERN CONUS AND SOUTHERN CANADA LATE IN THE PERIOD...AND THIS
LOOKS TO SHIFT A COLD FRONT SOUTHWARD ACROSS THE AREA SOMETIME
EITHER FRIDAY OR FRIDAY NIGHT AND THIS COULD SPARK OFF ACTIVITY
ACROSS THE AREA. COOLER WEATHER DOES LOOK TO OVERSPREAD TO AREA
UNDER NORTHERLY WINDS BY EARLY NEXT WEEKEND.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* LAKE BREEZE THIS AFTERNOON AND WHETHER OR NOT IT WILL PUSH
  ACROSS ORD/MDW.
* GUSTY SOUTHWEST WINDS DEVELOPING MID MORNING MONDAY.
* CHANCES FOR THUNDERSTORMS...WITH A LOWER CHANCE OF LINGERING
  PRECIP COMING IN TOMORROW MORNING...AND A BETTER CHANCE FOR NEW
  DEVELOPMENT OF STRONG THUNDERSTORMS MONDAY AFTERNOON.

BMD

//DISCUSSION...UPDATED 18Z...

THIS AFTERNOON...WEAK HIGH PRESSURE IS IN PLACE TODAY ALLOWING FOR
GENERALLY LIGHT SOUTHWEST FLOW. A LAKE BREEZE HAS ALREADY
DEVELOPED THOUGH REMAINS FAIRLY CLOSE TO THE SHORE AT THIS HOUR.
IT IS UNCERTAIN WHETHER IT WILL MAKE IT ACROSS THE TERMINALS BUT
MOST GUIDANCE IS LEANING AGAINST THIS OCCURRING...AND IF IT DOES IT
WOULD LIKELY BE LATE.

ATTENTION TURNS TO THE WEST TONIGHT AS AN UPPER LEVEL DISTURBANCE
PUSHES ACROSS THE PLAINS KICKING OFF THUNDERSTORMS. THESE
THUNDERSTORMS ARE EXPECTED TO PERSIST THROUGH THE OVERNIGHT HOURS
MOVING INTO NORTHERN ILLINOIS EARLY MONDAY MORNING IN A DISSIPATING
STAGE. RFD HAS BEST CHANCE FOR SEEING SOME FESTERING EARLY MORNING
ACTIVITY SO ADDED A PROB30 FOR TSRA. DEPENDING ON HOW FAR EAST THIS
PRECIP MAKES IT EARLY MONDAY...EXPECT THE ATMOSPHERE TO RECOVER
BECOMING MODERATELY UNSTABLE WITH ADDITIONAL THUNDERSTORMS EXPECTED
TO REDEVELOP MONDAY AFTERNOON. THESE STORMS COULD BE STRONG TO
SEVERE WITH GUSTY WINDS THE PRIMARY THREAT.

BMD

//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* LOW CONFIDENCE IN LAKE BREEZE MAKING IT THROUGH ORD/MDW TODAY.
* LOW-MEDIUM CONFIDENCE IN CIGS REST OF TODAY.
* MEDIUM-HIGH CONFIDENCE IN WIND SPEEDS AND DIRECTION MONDAY.
* HIGH CONFIDENCE IN AT LEAST A PERIOD OF TSRA BUT LOW CONFIDENCE
  IN SPECIFIC TIMING.

BMD

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

MONDAY NIGHT...CHANCE SHRA/TSRA.

TUESDAY AND WEDNESDAY...VFR. WX NIL.

THURSDAY THROUGH SATURDAY...PERIODIC CHANCES FOR SHRA/TSRA.

KREIN

&&

.MARINE...
149 PM CDT

THE WEATHER PATTERN ACROSS THE WESTERN LAKES WILL BE FAIRLY
PROGRESSIVE OVER THE NEXT SEVERAL DAYS...WITH A PAIR OF SURFACE LOWS
PASSING ACROSS OR NORTH OF THE NORTHERN LAKES AND TRAILING COLD
FRONTS ACROSS LAKE MICHIGAN.

THE FIRST LOW...OVER FAR NORTHWEST MINNESOTA THIS AFTERNOON...WILL
MOVE NORTHEAST ACROSS ONTARIO AND THE NORTHERN LAKES THROUGH MONDAY
WITH ITS TRAILING COLD FRONT PUSHING SOUTHEAST ACROSS THE LAKE
MONDAY AFTERNOON AND EVENING. TIGHTENING OF THE GRADIENT AHEAD OF
THE LOW AND FRONTAL TROUGH WILL RESULT IN SOUTHERLY WINDS INCREASING
LATE TONIGHT AND MONDAY...WITH GUSTS REACHING THE 20-25 KT RANGE AND
CONTINUING INTO EARLY MONDAY EVENING BEFORE THE GRADIENT WEAKENS
ALONG THE COLD FRONTAL TROUGH. THIS FRONT WILL PUSH SOUTHEAST ACROSS
THE LAKE MONDAY NIGHT...WITH LIGHTER WINDS OVERNIGHT AND TUESDAY AS
A BUBBLE HIGH MOVES ACROSS THE AREA BEHIND THE FRONT.

THE HIGH WILL MOVE OFF TO THE EAST WEDNESDAY MORNING...WITH WINDS
BECOMING SOUTH AND STRENGTHENING TO 20-25 KT AGAIN LATER IN THE
WEDNESDAY AND CONTINUING INTO THURSDAY BEFORE THE NEXT SURFACE LOW
PASSES NORTH OF THE LAKES AND ITS TRAILING COLD FRONT MOVES THROUGH
LATE THURSDAY NIGHT AND EARLY FRIDAY. WINDS DONT LOOK TO BE TOO
STRONG IMMEDIATELY BEHIND THIS SECOND FRONT...THOUGH A PERIOD OF
ENHANCED NORTHERLY WINDS IS POSSIBLE SATURDAY AS HIGH PRESSURE
BUILDS IN BEHIND THE FRONT.

WITH GUSTY SOUTH WINDS MONDAY...A SMALL CRAFT ADVISORY HAS BEEN
ISSUED FOR THE ILLINOIS AND INDIANA NEAR SHORE WATERS FROM
MID-MORNING THROUGH MORNING EVENING.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...10 AM MONDAY TO 10 PM MONDAY.

&&

$$

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