Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 291958
AFDLOT

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

.DISCUSSION...
221 PM CDT

SYNOPSIS...UNSETTLED WEATHER CONTINUES THROUGH TOMORROW WITH
SCATTERED THUNDERSTORMS EXPECTED.  CONFIDENCE REMAINS LOW IN WHEN
AND WHERE THUNDERSTORMS WILL FORM.  SUNDAY STILL LOOKS DRY BUT
MULTIPLE CHANCES FOR STORMS RETURN MONDAY THROUGH THE END OF THE
WEEK. ABOVE NORMAL TEMPERATURES ARE ALSO EXPECTED THROUGH THE PERIOD
AS WELL.

//NEAR TERM FORECAST//

THROUGH TONIGHT...
A LARGE RIDGE IS OVER THE EAST COAST WHILE A TROUGH IS OVER THE
PLAINS.  THE TROUGH WILL WEAKEN AS IT APPROACHES OUR AREA AND PASS
OVERHEAD LATE SATURDAY NIGHT INTO SUNDAY MORNING. AT THE SURFACE THE
HIGH HAS PUSHED EAST WHILE A DEVELOPING LOW PRESSURE SYSTEM IS OVER
MINNESOTA.  THE LOWS WARM FRONT LIES TO OUR SOUTH ACROSS SOUTHERN
IL...AND IT WILL LIFT NORTH THROUGH THE REGION THIS AFTERNOON.

HAVE VERY LOW CONFIDENCE IN EXACT TIMING AND LOCATION OF
THUNDERSTORMS THROUGH THIS EVENING.  THERE IS NO CLEAR FORCING
MECHANISM BUT WITH AROUND 2000 J/KG OF CAPE AVAILABLE...STORMS ARE
CERTAINLY POSSIBLE. PWAT VALUES ARE AROUND 2 INCHES SO THE MAIN
CONCERN REMAINS HEAVY DOWNPOURS. THE STRONGEST STORMS MAY PRODUCE
WIND GUSTS TO 50 KT BUT NO SEVERE WEATHER IS EXPECTED AS WIND SHEAR
IS BASICALLY NIL.

FOR POPS USED A BLEND OF NEAR TERM GUIDANCE WHICH HAS BEEN DOING
REASONABLY WELL THIS MORNING.  HOWEVER...KEPT A CHANCE OF STORMS
ACROSS THE REGION THIS AFTERNOON AND EVENING DUE TO THE SCATTERED
NATURE OF THE STORMS THAT ARE EXPECTED.  A WASH OUT IS NOT
EXPECTED.  THE BEST CHANCE FOR WIDESPREAD STORMS WILL BE OVERNIGHT
AS THE UPPER LEVEL TROUGH APPROACHES. IN FACT THE STORMS ASSOCIATED
WITH THE TROUGH STRETCH FROM MINNESOTA THROUGH ARKANSAS.  MODELS
DIFFER IN HOW WELL THIS AREA OF PRECIP WILL HOLD TOGETHER SO LIMITED
POPS TO THE LIKELY CATEGORY.

LOWS WILL BE IN THE UPPER 60S TO LOWER 70S TONIGHT AND FOG IS
POSSIBLE. GIVEN THAT WE HAVE NOT SEEN MUCH RAIN TODAY...HAVE LIMITED
CONFIDENCE SO LEFT FOG OUT OF THE FORECAST FOR NOW.

SATURDAY AND SATURDAY NIGHT...
THE SURFACE LOW PASSES OVER LAKE MICHIGAN SATURDAY AFTERNOON WHILE
ITS COLD FRONT GETS DRAGGED THROUGH THE AREA SATURDAY
AFTERNOON/EVENING.  SCATTERED STORMS ARE AGAIN POSSIBLE IN THE
MORNING IN THE WARM AND MOIST ENVIRONMENT WITH LINGERING 2 INCH PWAT
VALUES. STORMS WILL THEN FORM ALONG THE COLD FRONT SOUTH OF A GARY
IN TO PONTIAC IL LINE. STORMS ARE POSSIBLE NORTH AND WEST OF THIS
LINE...BUT CONFIDENCE IS SIGNIFICANTLY LOWER.

CAPE VALUES WILL BE AROUND 1000 J/KG TOMORROW WITH WEAK SHEAR ONCE
AGAIN. THEREFORE EXPECTING STORMS TO PRODUCE HEAVY DOWNPOURS AND THE
STRONGEST MAY BE CAPABLE OF GUSTY WINDS.  SEVERE WEATHER IS NOT
EXPECTED.

TOMORROW LOOKS LIKE ANOTHER WARM AND STICKY DAY AHEAD OF THE COLD
FRONT.  LOWS WILL BE IN THE 60S AND FOG IS POSSIBLE OVERNIGHT.

JEE

//EXTENDED FORECAST//

SUNDAY...
EXPECT A BRIEF LULL IN THE PRECIP CHANCES DURING MUCH OF THE DAY
SUNDAY AS THE FIRST COLD FRONT DEPARTS TO THE EAST AND ANOTHER LOW
TAKES SHAPE OVER THE NORTHERN PLAINS. IN BETWEEN THESE TWO
SYSTEMS...WEAK INDUCED SURFACE HIGH SHOULD RESULT IN QUIET WEATHER
LOCALLY...WITH THE H85 THERMAL RIDGE ADVECTING OVERHEARD THROUGHOUT
THE DAY. RETURN FLOW WILL HELP PUSH THE DEWPOINTS BACK UP TO AROUND
70 WITH AFTERNOON HIGH TEMPS EXPECTED TO BE IN THE MID 80S. STEEP
LOW/MID LEVEL LAPSE RATES WILL RESULT IN MODERATE INSTABILITY
DEVELOPING DURING THE AFTERNOON...BUT MODEST SBCIN IS PROGGED TO
STAY IN PLACE CAPPING ANY SURFACE BASED CONVECTION. IN ADDITION...
THE AREA SHOULD BE SQUARELY UNDER AREA OF MID LEVEL HEIGHT RISES
WHICH SHOULD HELP REINFORCE THE INVERSION...AND NO SIGNIFICANT
FORCING NOTED OVER THE AREA.

MONDAY...
TROUGHING CURRENTLY MOVING ONSHORE OVER THE PAC NORTHWEST TODAY WILL
MOVE ACROSS THE CENTRAL PLAINS THIS WEEKEND...THEN BEGIN TO LIFT
ACROSS THE UPPER MIDWEST EARLY NEXT WEEK. SURFACE LOW IS PROGGED TO
DEEPEN FROM THE NORTHERN PLAINS SUNDAY AS IT LIFTS TO NEAR HUDSON
BAY LATE ON MONDAY. THIS WILL DRAG A COLD FRONT ACROSS THE REGION ON
MONDAY. WITH THE LOW LIFTING AWAY FROM THE AREA...CONVERGENCE ALONG
THE FRONT IS PROGGED TO WEAKEN AS IT COMES ACROSS NORTHERN ILLINOIS
EARLY MONDAY WITH THE BETTER DYNAMICS STAYING TO OUR NORTH.
MEANWHILE...THE LOCAL AREA APPEARS TO LIE ON THE NORTHERN PERIPHERY
OF THE BETTER INSTABILITY TO OUR SOUTH. GFS SHOWS SOME HINTS THAT
PARTICULARLY NORTH CENTRAL ILLINOIS MAY BE SPLIT BY THE
PRECIP...ESPECIALLY CONSIDERING THE CAPPING EARLY IN THE DAY...BUT
WILL HOLD OFF ON FINE TUNING THIS DETAIL FOR NOW. IN ADDITION...THE
REGION DOES FALL WITHIN AN OVERLAPPED AREA OF FAVORABLE
SHEAR/INSTABILITY THAT COULD POTENTIALLY SUPPORT A SEVERE WEATHER
THREAT.

TUESDAY THROUGH FRIDAY...
FRONTAL BOUNDARY IS PROGGED TO STALL ACROSS/NEAR CENTRAL ILLINOIS
TUESDAY GRADUALLY LIFTING BACK NORTH ACROSS THE FORECAST AREA BY THE
END OF THE WORK WEEK AS A SERIES OF UPPER LEVEL DISTURBANCES MOVE
ACROSS THE REGION. THIS WILL RESULT IN PERIODIC CHANCES FOR SHOWERS
AND THUNDERSTORMS ACROSS THE AREA...THOUGH TIMING AND LOCATION
DETAILS WILL BE DIFFICULT TO PIN DOWN THIS FAR OUT GIVEN THE
PATTERN. THE ATMOSPHERE REMAINS QUITE SOUPY IN THE VICINITY OF THE
FRONT WHERE PWATS REMAIN AROUND 2 INCHES. THUS THE HEAVY RAIN AND
FLOODING THREAT WILL HAVE TO BE CLOSELY MONITORED THROUGH MUCH OF
THE UPCOMING WEEK.

DEUBELBEISS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* 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 20Z...

ISOLATED TSRA DEVELOPING FROM NORTHEAST OF DOWNTOWN CHICAGO SOUTH
INTO IKK AREA. THINK COVERAGE WILL REMAIN FAIRLY LOW...THOUGH HAVE
ADDED VCTS FOR THE NEXT FEW HOURS FOR CHICAGO TERMINALS AS THEY
WILL BE SPARSE BUT IN THE NEIGHBORHOOD SO TO SPEAK.

RATZER

PREVIOUS DISCUSSION FROM 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 20Z...

* 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...
221 PM CDT

MODERATE SOUTHERLY WINDS ARE IN PLACE TODAY AHEAD OF LOW PRESSURE
OVER THE NORTHERN PLAINS. SOUTH TO SOUTHWEST WINDS 20 TO 25 KT
WITH AN OCCASIONAL 30 KT GUSTS POSSIBLE WILL BE IN PLACE THROUGH
TOMORROW...THEN A COLD FRONT WILL START TO PUSH ACROSS THE LAKE
TURNING WINDS NORTHWESTERLY. HIGH PRESSURE WILL PROVIDE A BRIEF
LULL IN THE WINDS SUNDAY ALLOWING LAKE BREEZES TO DEVELOP...BUT
THEN SOUTHERLY WINDS RAMP BACK UP TO 25 TO 30 KT SUNDAY NIGHT INTO
MONDAY AHEAD OF ANOTHER LOW. THIS LOW IS PROGGED TO DEEPEN AS IT
LIFTS FROM THE NORTHERN PLAINS SUNDAY TO HUDSON BAY LATE MONDAY.

BMD

&&

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

$$

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