Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 310834
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
334 AM 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 06Z...

* SLIGHT CHANCE OF FOG/STRATUS EARLY SUNDAY MORNING.

* CHANCE FOR A LAKE BREEZE/WIND SHIFT TO EASTERLY LATE SUNDAY
  AFTERNOON.

KREIN

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

SCT TSRA WILL CONTINUE TONIGHT WELL TO THE SOUTHEAST OF THE
TERMINALS OVER EAST CENTRAL IL AND WEST CENTRAL INDIANA. AN OLD
BOUNDARY LIES JUST SOUTH OF THE TERMINALS WITH SLIGHTLY DRIER/LESS
HUMID AIR TO THE NORTH...MOSTLY NOTABLE IN THE DEWPOINT GRADIENT.
GIVEN THE LACK OF RAIN TODAY AND MIXING OUT OF LOW LEVEL
MOISTURE...PLUS THE URBAN HEAT ISLAND EFFECTS...NOT ANTICIPATING
FOG/STRATUS TO BE MUCH OF AN ISSUE FOR THE CHICAGO TERMINALS.
HOWEVER...SHORT RANGE MODEL GUIDANCE IS STILL INDICATING A HIGH
POSSIBILITY OF IFR CIGS/VSBY AWAY FROM THE HIGHLY URBANIZED
AREAS...SO WILL MAINTAIN MVFR/IFR FOG AT THE MORE FOG PRONE
OUTLYING TERMINALS...PARTICULARLY GYY/DPA/RFD. ANY VSBY
RESTRICTIONS THAT DO FORM OVERNIGHT WILL QUICKLY DISSIPATE BY MID
MORNING SUNDAY WITH VFR CONDITIONS EXPECTED THE REMAINDER OF THE
DAY. WINDS SHOULD REMAIN LIGHT ENOUGH TO ALLOW A LAKE BREEZE TO
DEVELOP...BUT THERE APPEARS TO BE LITTLE PUSH BEHIND THE
BOUNDARY...SO AT THIS POINT...FEEL THAT CHANCES ARE LOW THAT THE
LAKE BREEZE WILL PUSH INLAND OF ORD/MDW. HOWEVER...IF THE LAKE
BREEZE DOES MANAGE TO PUSH WEST OF ORD/MDW...IT WOULD NOT BE UNTIL
ARND 22-23Z AND ELY WINDS WOULD BE ARND 5-7KT.

KREIN

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

* MEDIUM-HIGH CONFIDENCE IN NO IFR AT ORD/MDW OVERNIGHT.

* MEDIUM TO HIGH CONFIDENCE IN LAKE BREEZE SUNDAY AFTERNOON...AND
  MEDIUM CONFIDENCE THAT THE BOUNDARY WILL NOT PUSH THROUGH
  ORD/MDW.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

KREIN

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

MONDAY...PERIOD OR TWO OF SHRA/TSRA...STRONG TSRA POSSIBLE.

TUESDAY AND WEDNESDAY...PREDOMINANTLY VFR.

THURSDAY...SLIGHT CHANCE TSRA...OTHERWISE VFR.

FRIDAY...CHANCE TSRA.

IZZI

&&

.MARINE...
300 AM CDT

A PERIOD OF CHANGEABLE WEATHER...IN PARTICULAR...WIND
DIRECTION...IS SETTING UP FOR THE NEXT SEVERAL DAYS. LOW PRESSURE
JUST NORTH OF GEORGIAN BAY WILL TRACK EAST TO SOUTHERN QUEBEC BY
LATE SUNDAY MORNING WHILE DRAGGING A WEAK COLD FRONT SOUTH ACROSS
THE LAKE AND CLEARING THE SOUTH END OF LAKE MICHIGAN BY EARLY
SUNDAY. THIS WILL SET UP A BRIEF PERIOD OF NWLY WINDS...BUT AS
HIGH PRESSURE BUILDS OVER THE NORTHERN LAKES BEHIND THE FRONT
SUNDAY AFTERNOON WINDS WILL BECOME LGT/VRBL FOR A SHORT TIME.
SLY FLOW WILL RETURN TO THE LAKE SUNDAY NIGHT AND
MONDAY...STRENGTHENING TO ARND 25KT AS ANOTHER LOW DEVELOPING OVER
THE NORTHERN PLAINS DEEPENS WHILE TRACKING TO HUDSON BAY MONDAY
NIGHT. THIS LOW WILL DRAG ANOTHER COLD FRONT SOUTH ACROSS LAKE
MICHIGAN MONDAY NIGHT. WEAK HIGH PRESSURE WILL BUILD ACROSS THE
LAKE TUESDAY BRINGING ANOTHER PERIOD OF LIGHT WINDS. BUT YET
ANOTHER LOW IS EXPECTED TO MOVE OUT OF THE NORTHERN ROCKIES AND
INTO THE NORTHERN PLAINS WEDNESDAY NIGHT...BRINGING ANOTHER PERIOD
OF BRISK SOUTH WINDS TO THE LAKE FOR WEDNESDAY AND THURSDAY. THIS
LOW IS FORECAST TO TRACK TO NEAR THE SOUTHERN TIP OF JAMES BAY
DRAGGING ANOTHER COLD FRONT ACROSS THE LAKE FRIDAY OR FRIDAY
NIGHT. ACCOMPANYING THESE FRONTAL PASSAGES WILL ALSO BE PERIODS OF
THUNDERSTORMS AS WELL.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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