Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 291807
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
107 PM CDT FRI AUG 29 2014

.DISCUSSION...
326 AM CDT

THE MAIN FORECAST CONCERNS WILL CENTER AROUND CONVECTIVE CHANCES AND
TIMING...AS WELL AS TEMPERATURES TODAY...AND THROUGH THE HOLIDAY WEEKEND.

CURRENTLY...THE REMNANT MCV FROM YESTERDAY EVENINGS SHOWERS AND
STORMS CONTINUES TO SHIFT ACROSS MY SOUTHERN COUNTIES EARLY THIS
MORNING. HOWEVER...ONLY A FEW LIGHT SHOWERS/SPRINKLES REMAIN ACROSS
THIS AREA...WITH DRIER AND FAIRLY STABLE CONDITIONS IN PLACE TO THE EAST.

ANOTHER AREA OF CONVECTION IS ONGOING TO OUR WEST ACROSS IOWA AND
INTO SOUTHERN MN. THIS ACTIVITY IS OCCURRING WITHIN A REGION OF GOOD
LOWER LEVEL WARM AIR ADVECTION/ISENTROPIC ASCENT ON THE DOWNSTREAM
PERIPHERY OF A REMNANT MID/UPPER LEVEL TROUGH OVER THE PLAINS STATES.
DOWN STREAM UPPER RIDGING OVER THE GREAT LAKES REGION IS LEADING TO A
STEERING FLOW TO THE NORTHEAST. THEREFORE...MOST IF NOT ALL OF THIS
CONVECTIVE ACTIVITY WILL SHIFT INTO SOUTHERN WISCONSIN AND PERHAPS
FAR NORTHWESTERN ILLINOIS THIS MORNING. IN SPITE OF THIS...I HAVE
LEFT SLIGHT CHANCE POPS IN THIS MORNING AS IT IS POSSIBLE THAT SOME
ADDITIONAL CELLS COULD DEVELOP IN ASSOCIATION WITH AN EASTWARD MOVING
OUTFLOW BOUNDARY...AS EVIDENT OFF OF THE DMX RADAR MOVING TOWARDS
EASTERN IOWA. THIS ACTIVITY...IF IT OCCURS IN ANY OF NORTHERN
ILLINOIS EARLY THIS MORNING...WOULD LIKELY BE ISOLATED TO WIDELY
SCATTERED...WITH THE MAIN FOCUS REMAINING NORTH.

A SURFACE WARM FRONT IS EXPECTED TO LIFT NORTHWARD OVER THE AREA
INTO THIS AFTERNOON...WITH A WARMER AIRMASS EXPECTED IN ITS WAKE. IN
SPITE OF THIS...HOWEVER...HIGH TEMPERATURES ACROSS THE AREA WILL BE
TRICKY AS IT APPEARS THAT SOME SCATTERED THUNDERSTORMS COULD DEVELOP
ACROSS PORTIONS OF THE AREA THIS AFTERNOON...AND HAMPER THE MAGNITUDE
OF THE WARMTH ACROSS SOME AREAS. MODEST HEIGHT FALLS ALOFT ARE FORECAST
ACROSS NORTHERN ILLINOIS IN ASSOCIATION WITH THE AFOREMENTIONED UPPER
TROUGH SHIFTING EASTWARD OVER THE MID MISSOURI VALLEY THIS
AFTERNOON. IN SPITE OF THE FACT THAT THE BEST LARGE SCALE FORCING
FOR ASCENT WITH THIS DISTURBANCE SHOULD REMAIN WEST OF THE AREA THIS
AFTERNOON...THE EXPECTED INCREASE IN LOW-LEVEL THETA E IS EXPECTED
TO PRODUCE AN UNCAPPED ATMOSPHERE FEATURING MLCAPES OF AROUND 1500
J/KG BY THIS AFTERNOON. A MAJORITY OF THE GLOBAL MODELS...AS WELL AS
THE HIRES CONVECTIVE ALLOWING MODELS FIRE UP ACTIVITY WITHIN THIS
REGIME THIS AFTERNOON. IT APPEARS WE WILL HAVE TO WATCH MORNING
CLOUD COVER...HOWEVER...AS THIS COULD INHIBIT THE DEGREE OF
WARMING A BIT THROUGH THE MORNING...AND POSSIBLY LEAD TO WEAKER
INSTABILITY THAN WHAT THE MODELS ARE CURRENTLY ADVERTISING.

IF THE CONVECTION DOES DEVELOP AS  EXPECTED...MARGINAL DEEP LAYER
SHEAR OF 25 TO 30 KT COULD ALLOW FOR A FEW STRONGER STORMS CAPABLE
OF SOME ISOLATED HIGH WIND GUSTS. SOME TORRENTIAL DOWNPOURS WOULD
ALSO BE A THREAT FROM THESE STORMS GIVEN THE JUICY AIRMASS FEATURING
PWATS UP AROUND 1.75 INCHES. HOWEVER...EXTREME RAINFALL AMOUNTS
APPEAR UNLIKELY GIVEN STORM MOTIONS SHOULD BE TO THE NORTHEAST
AROUND 30 KT. SO THE STORMS SHOULD NOT BE OVER A GIVEN AREA TOO
LONG. MUCH OF THIS ACTIVITY APPEARS TO BE HIGHLY DIURNAL IN
NATURE...SO A RAPID DOWNWARD TREND IS EXPECTED AFTER SUNSET THIS
EVENING.

ANOTHER ROUND OF THUNDERSTORMS APPEARS LIKELY ON
SATURDAY...ESPECIALLY DURING THE MORNING AS THE UPPER LEVEL
TROUGH...AND AN ASSOCIATED SURFACE LOW AND COLD FRONT SHIFTS
EASTWARD ACROSS ACROSS SOUTHERN WI AND NORTHERN IL. THIS ACTIVITY
COULD LINGER INTO SATURDAY EVENING ACROSS MY FAR SOUTHEASTERN
COUNTIES. HOWEVER...IT APPEARS THAT FOLLOWING THIS FRONTAL PASSAGE
WE WILL BE GREETED WITH A NICE DRY SUNDAY ACROSS THE AREA.

UNFORTUNATELY...LABOR DAY APPEARS THAT IT COULD BE ANOTHER ACTIVE
WEATHER DAY ACROSS THE REGION. ANOTHER POTENT UPPER LEVEL
DISTURBANCE AND SURFACE FRONT IS FORECAST TO SHIFT OVER THE AREA BY
MONDAY. WE WILL ALSO HAVE TO WATCH FOR THE THREAT OF SEVERE
THUNDERSTORMS ON MONDAY AND MONDAY NIGHT AS A 45 TO 50 KT MID LEVEL
JET IS PROGGED TO DEVELOP ACROSS THE AREA. THIS COULD RESULT IN DEEP
LAYER SHEAR STRONG ENOUGH TO PRODUCE MORE ORGANIZED STRONG TO SEVERE
CONVECTION. VERY HEAVY RAINFALL WOULD AGAIN BE A CONCERN WITH THIS
ACTIVITY...ESPECIALLY IF THE FRONTAL ZONE STALLS OUT OVER THE AREA
MONDAY NIGHT. AT THIS TIME...HOWEVER...UNCERTAINTY IS HIGH ON WHERE
THE SURFACE FRONT MAY STALL AND HENCE WHAT AREAS COULD BE FAVORED FOR
EXTENDED PERIODS OF VERY HEAVY RAINFALL.

THE ACTIVE WEATHER PATTERN COULD SHIFT SOUTH OF THE AREA FOR A DAY
OR TWO INTO MID TO LATE WEEK BEFORE MORE PERIODS OF CONVECTION ARE
POSSIBLE. OVERALL...IT APPEARS TEMPERATURES WILL REMAIN ON THE WARM
SIDE OF AVERAGE...WITH THE POTENTIAL FOR SOME VERY WARM AND HUMID DAYS
LATER NEXT WEEK.

KJB

&&

.AVIATION...

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

* ISOLATED-SCATTERED SHRA/TSRA THIS AFTERNOON THRU EARLY EVENING.

* SOUTH WINDS WITH GUSTS 15-20 KT THIS AFTERNOON.

* ANOTHER PERIOD OF SHRA/TSRA LIKELY AFTER MIDNIGHT INTO SATURDAY
  MORNING.

* SOUTHWEST WINDS GUSTING AROUND 20 KT SATURDAY.

RATZER

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

FAIRLY LOW CONFIDENCE CONVECTIVE FORECAST THIS PERIOD...WITH
TERMINALS WITHIN A BROAD AREA OF SOUTH WARM/MOIST ADVECTION
DOWNSTREAM OF AN UPPER TROUGH OVER THE CENTRAL PLAINS BUT WITH
OTHERWISE LITTLE ORGANIZED FOCUS FOR TSRA.

SATELLITE EVIDENCE OF WEAK MID-LEVEL DISTURBANCE OVER NORTHEAST
IL EARLY THIS AFTERNOON...WITH MORNING ELEVATED SHRA/TSRA DECAYING
OVER NORTH CENTRAL IL. DIURNAL TSRA WERE FORMING IN LOW LEVEL
INSTABILITY AXIS ALONG IL/IN BORDER REGION GENERALLY ALONG A DEC-
GIJ-MKG LINE...AND THIS LOOKS TO REMAIN THE FOCUS REGION FOR
HIGHEST COVERAGE TSRA THIS AFTERNOON. HIGH-RES GUIDANCE STILL
DEVELOPS SOME ISOLATED SHRA/TSRA INTO THE TERMINAL REGION IN
ASSOCIATION WITH AFOREMENTIONED MID-LEVEL DISTURBANCE AND NORTH
OF SURFACE WARM FRONT WHICH IS ACROSS DOWNSTATE IL AT THIS TIME...
THOUGH COVERAGE IS EXPECTED TO REMAIN LOW. A GREATER CHANCE OF
SHRA/TSRA APPEARS TO BE LATE TONIGHT AND EARLY SATURDAY AS THE
MAIN UPPER TROUGH APPROACHES. WEAK COLD FRONT APPROACHES FROM THE
WEST SATURDAY MIDDAY AND AFTERNOON...WITH THUNDERSTORMS EXPECTED
TO DEVELOP ALONG THE FRONT BY EARLY AFTERNOON. CURRENT SUITE OF
HIGH-RES GUIDANCE SUGGESTS TSRA POTENTIAL MAY BE GREATER JUST
EAST/SOUTH OF CHICAGO TERMINALS SATURDAY AFTERNOON. OUTSIDE OF ANY
PRECIP VIS REDUCTION EARLY SATURDAY...SOME MVFR HAZE POSSIBLE
OVERNIGHT IN MOIST WARM SECTOR AIR.

WINDS...GENERALLY SOUTH 10-12 KT WITH GUSTS TO 20 KT OR SO
POSSIBLE THIS AFTERNOON. WIND MAY BACK A BIT TO THE SOUTH-
SOUTHEAST LATE THIS AFTERNOON/EARLY EVENING AS SURFACE WARM
FRONT APPROACHES...BEFORE SHIFTING SOUTHWEST OVERNIGHT AND SUNDAY
AHEAD OF THE COLD FRONT. DEEPER MIXING SUNDAY SUGGESTS GUSTS AGAIN
IN THE 20 KT RANGE OR A LITTLE HIGHER DURING THE LATE MORNING AND
AFTERNOON.

RATZER


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

* MEDIUM IN SHRA/TSRA BEING LOW IN COVERAGE IN IMMEDIATE TERMINAL
  AREA THIS AFTERNOON...GREATER COVERAGE EAST/SOUTH OF CHICAGO.

* LOW IN TSRA TIMING/COVERAGE LATER TONIGHT/SATURDAY.

* MEDIUM-HIGH IN WIND TRENDS THROUGH PERIOD.

RATZER

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

* SATURDAY NIGHT THROUGH SUNDAY NIGHT...VFR. WX NIL.

* MONDAY...CHANCE OF TSRA.

* TUESDAY...VFR. WX NIL.

* WEDNESDAY THROUGH THURSDAY NIGHT...SLIGHT CHANCE OF TSRA

KREIN

&&

.MARINE...
315 AM CDT

HIGH PRESSURE OVER THE EASTERN GREAT LAKES WILL MOVE EAST OFF THE
NEW ENGLAND COAST FRIDAY EVENING...WHILE A TROUGH OF LOW PRESSURE
DEVELOPS ACROSS THE CENTRAL PLAINS AND UPPER MIDWEST. THIS WILL
SET UP A PERIOD OF SOUTHERLY WINDS AND AS THE TROUGH MOVES
EAST...THE GRADIENT OVER THE LAKE WILL STRENGTHEN AND WINDS SHOULD
INCREASE TO THE 20-25 KT RANGE. A DEEPENING LOW CENTER WITHIN THE
TROUGH WILL MOVE TO WISCONSIN SATURDAY AND ACROSS NORTHERN LAKE
MICHIGAN SATURDAY EVENING. AS THE LOW TRACKS TO THE NORTHEAST...AN
ASSOCIATED COLD FRONT WILL MOVE DOWN THE LAKE SATURDAY AFTERNOON
AND EVENING...QUICKLY SHIFTING WINDS TO NWLY FOLLOWING THE FROPA.
THE LATEST MODEL GUIDANCE SUGGESTS THAT THE FRONT WILL WASH OUT
LATE SATURDAY NIGHT BEFORE REACHING THE SOUTH END OF THE LAKE AND
WINDS THERE SHOULD NEVER TRULY SHIFT NWLY...BUT RATHER BECOMING
LIGHT AND VARIABLE LATE SATURDAY NIGHT THROUGH SUNDAY MORNING. A
DEEPER LOW IS EXPECTED TO DEVELOP OVER THE DAKOTAS AND LIFT
THROUGH WESTERN ONTARIO SUNDAY NIGHT AND MONDAY BRINGING A RETURN
TO BRISK SOUTH TO SOUTHWEST WINDS TO THE LAKE FOR MONDAY AND
TUESDAY.

KREIN

&&

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

$$

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