Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 012158
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
458 PM CDT WED OCT 1 2014

.DISCUSSION...
250 PM CDT

STRONG AND POSSIBLY SEVERE THUNDERSTORMS ARE EXPECTED
TOMORROW...AND AN UNSEASONABLY COOL AIRMASS WILL ARRIVE FOR THE
WEEKEND.

REST OF TODAY AND TONIGHT...
THIS AFTERNOON...A WARM FRONT STRETCHES FROM CENTRAL IOWA INTO
CENTRAL ILLINOIS AND IS THE FOCUS FOR SHOWERS AND THUNDERSTORMS
CURRENTLY OFF TO OUR WEST AND SOUTHWEST. THE PRECIP WILL GRADUALLY
SHIFT EAST ACROSS THE LOCAL FORECAST AREA THROUGH THE EVENING AS 30
KT LOW/MID LEVEL WINDS TRANSPORT 1.5-1.6 INCH PWATS INTO THE REGION
ALONG WITH SOME MODEST ELEVATED INSTABILITY. CANNOT RULE OUT SOME
ISOLATED THUNDER BUT THE BETTER CHANCES FOR THUNDERSTORMS SHOULD
REMAIN TO OUR SOUTH CLOSER TO THE FRONT WHERE THE INSTABILITY IS A
LITTLE STRONGER. MID LEVEL THETA-E ADVECTION SHIFTS OFF TO OUR EAST
BY THE EARLY MORNING/PREDAWN HOURS THURSDAY AND SHOULD RESULT IN A
PERIOD OF DRY WEATHER FOR A FEW HOURS THEREAFTER...THOUGH EARLY
MORNING PRECIP WILL PLAY A ROLE IN THE SEVERE THREAT TOMORROW AND
OUR ABILITY TO DESTABILIZE.

THURSDAY...
RELATIVELY UNORGANIZED SURFACE LOW IS PROGGED TO LIFT FROM OKLAHOMA
EARLY MORNING DEEPENING A FEW MILLIBARS BY LATE EVENING AS ITS
BECOMES A LITTLE BETTER ORGANIZED OVER FAR EASTERN IOWA INTO
WISCONSIN AHEAD OF A SHORTWAVE TROUGH LIFTING ACROSS THE MID
MISSISSIPPI VALLEY BY THURSDAY EVENING. A WARM FRONT IS PROGGED TO
LIFT ACROSS NORTHERN ILLINOIS DURING THE DAY THURSDAY WITH MID/UPPER
60 DEWPOINTS OVERSPREADING THE REGION. MID LEVEL LAPSE RATES WILL BE
UNIMPRESSIVE THROUGH THE DAY TOMORROW RESULTING IN MARGINAL
INSTABILITY DESPITE THE INFLUX OF MOISTURE...HOWEVER A VERY DYNAMIC
ENVIRONMENT IS EXPECTED AS THE RIGHT ENTRANCE REGION TO A 100-110KT
UPPER JET DEVELOPS OVERHEAD AND STRONG MID LEVEL HEIGHT FALLS
OVERSPREAD THE REGION IN ADVANCE OF THE COMPACT LEAD SHORTWAVE.
STRONG SYNOPTIC ASCENT SHOULD BE ABLE TO OVERCOME THE SHORTCOMINGS
OF THE THERMODYNAMIC ENVIRONMENT...AND STRONG SHEAR...ESPECIALLY IN
THE LOW LEVELS...COULD ALLOW FOR ROTATING UPDRAFTS PARTICULARLY IN
THE VICINITY OF THE WARM FRONT WHERE THERE IS A LOW BUT NON-ZERO
LOW-TOPPED SUPERCELL/TORNADO THREAT. NAM GUIDANCE MAY BE A BIT
OVERDONE WITH THE STRENGTH OF THE LOW AS IT IS A SLIGHTLY DEEPER
OUTLIER BUT DOES SHOW NICELY BACKED SURFACE WINDS NEAR THE WARM
FRONT RESULTING IN CURVED HODOGRAPHS AND AROUND 20KT 0-1KM SHEAR AND
45-50KT DEEP LAYER SHEAR. HIGH-SHEAR LOW-INSTABILITY ENVIRONMENTS
ARE OFTEN TRICKY TO PIN DOWN AND EVOLUTION OF TONIGHTS CONVECTION
ADDS AN ADDITIONAL DEGREE OF UNCERTAINTY TO THE FORECAST TOMORROW
MAINLY WITH RESPECT TO THE DEGREE OF SURFACE BASED INSTABILITY...
HOWEVER THIS SYSTEM WILL DEFINITELY BEAR WATCHING ESPECIALLY IF THE
LOW DEEPENS AS QUICKLY AS SOME MODELS INDICATE. THURSDAY EVENING AND
NIGHT...A COLD FRONT WILL SWEEP ACROSS THE REGION WITH THUNDERSTORMS
EXPECTED TO BECOME LINEAR OUT AHEAD. CONTINUED STRONG KINEMATICS
WILL RESULT IN AN ONGOING SEVERE THREAT WITH SEVERE WINDS BEING THE
PRIMARY CONCERN.

FRIDAY THROUGH SUNDAY...
LATE THIS WEEK INTO THE WEEKEND WILL BE CHARACTERIZED BY NORTHWEST
FLOW ALOFT AND MUCH COOLER TEMPERATURES. STRONG PV ANOMALY WILL DIG
ACROSS THE CENTRAL PLAINS AND MIDWEST RESULTING IN RAPID DEEPENING
OF THE SURFACE LOW AS IT LIFTS ACROSS THE GREAT LAKES FRIDAY
ENHANCING THE COLD ADVECTION BEHIND THE LOW. H85 TEMPS ARE PROGGED
FALL THROUGH THE DAY FRIDAY WITH THE 0C ISOTHERM PUSHING ACROSS THE
AREA AND TEMPS BOTTOMING OUT -3C TO -4C SATURDAY AFTERNOON/EVENING.
STEEP LOW/MID LEVEL LAPSE RATES AND THE MAIN AXIS OF THE UPPER
TROUGH PUSHING ACROSS THE REGION WILL SUPPORT SHOWERS ON FRIDAY...
THEN BEHIND THE UPPER WAVE SUBSIDENCE AND WARMING MID LEVELS SHOULD
CAP COLD AIR STRATOCUMULUS GROWTH SATURDAY AND SUNDAY WITH CHANCES
FOR PRECIP MUCH LOWER...THOUGH CANNOT RULE OUT A FEW DIURNALLY
DRIVEN SPRINKLES. TEMPERATURES WILL BE THE PRIMARY STORY THROUGH
THIS PERIOD...DROPPING WELL BELOW SEASONAL NORMS. HIGHS FRIDAY WILL
STRUGGLE TO HIT THE 60 MARK IN MOST AREAS...AND ARE ONLY EXPECTED TO
BE IN THE LOWER 50S SATURDAY UNDER THE THERMAL TROUGH. OVERNIGHT
LOWS ARE EXPECTED TO BE IN THE MID TO UPPER 30S OUTSIDE OF THE
THERMAL HEAT ISLAND OF CHICAGO WHERE TEMPS MAY STAY PROPPED UP IN
THE LOW 40S. FROST IN OUTLYING AREAS IS A POSSIBILITY ESPECIALLY
SATURDAY NIGHT WHEN WINDS ARE EXPECTED TO DIMINISH UNDER THE
INFLUENCE OF A WEAK SURFACE RIDGE AND THERE MAY BE SOME CLEARING TO
HELP RADIATE BETTER.

NEXT WEEK...
LONGWAVE TROUGH AXIS WILL GENERALLY REMAIN IN PLACE EARLY THROUGH
MIDDLE OF NEXT WEEK WITH THE LOCAL AREA REMAINING IN NORTHWEST FLOW
ALOFT. THERE WILL BE PERIODIC LOW CHANCES FOR PRECIP AS WAVES
TRANSLATE ACROSS THE REGION THOUGH TIMING THESE IS DIFFICULT THIS
FAR OUT SO CONFIDENCE IS LOW IN THE DETAILS. AT THE SURFACE...HIGH
PRESSURE IS EXPECTED TO BUILD ACROSS THE SOUTHEASTERN STATES
RESULTING IN LOW LEVEL SOUTHWEST FLOW AND WARM ADVECTION INCREASING
BY MIDWEEK AND ALLOWING TEMPERATURES TO MODERATE CLOSER TO NORMAL
VALUES.

DEUBELBEISS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 22Z...

* SOUTHEAST WINDS 10 TO 12 KT THROUGH EARLY EVENING.

* INCREASING CHANCES FOR SOME SHOWERS AND MVFR CIGS BY LATE
  TONIGHT INTO THURSDAY MORNING.

* CHANCES FOR THUNDERSTORMS INCREASE DURING THE MID AND LATE
  AFTERNOON ON THURSDAY...AND CONTINUE INTO THURSDAY NIGHT BEYOND
  THE CURRENT TAF PERIOD. PERIODS OF HEAVY RAINFALL AND LOWER VIS
  LIKELY.

KREIN

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

THE MAIN WEATHER CONCERNS DURING THE PERIOD WILL BE THE TIMING OF
SHOWERS AND THUNDERSTORMS...ESPECIALLY THURSDAY AFTERNOON.

A BROAD AREA OF LOW PRESSURE ACROSS THE PLAINS IS RESULTING IN A
SOUTHEASTERLY WIND COMPONENT ACROSS THE AREA THIS AFTERNOON. WIND
SPEEDS WILL GENERALLY BE AROUND 10 KT...WITH SOME OCCASIONAL GUSTS
UP AROUND 15 KT POSSIBLE.

THE BROAD AREA OF LOW PRESSURE OVER THE PLAINS WILL GRADUALLY
CONSOLIDATE AND SHIFT EAST-NORTHEASTWARD OVER THE MID AND PORTIONS
OF THE UPPER MISSISSIPPI VALLEY DURING THE DAY THURSDAY. THIS WILL
LIFT A WARM FRONTAL BOUNDARY OVER THE AREA BY EARLY THURSDAY
MORNING. THE ASSOCIATED INCREASE IN LOW LEVEL MOISTURE WITH THIS
SYSTEM COULD RESULT IN SOME LOWER MVFR CIGS AND VISBYS...ESPECIALLY
BY THURSDAY MORNING. A FEW SCATTERED SHOWERS ARE ALSO POSSIBLE
ACROSS THE AREA LATE TONIGHT INTO EARLY THURSDAY. THE THREAT FOR
THUNDER WITH THIS INITIAL ACTIVITY APPEARS TO BE LOW...BUT NOT
ZERO. MUCH BETTER CHANCES FOR SHOWERS AND THUNDERSTORMS...WITH
PERIODS OF HEAVY RAINFALL WILL BEGIN TO IMPACT THE AREA BY LATE
AFTERNOON AND EVENING ON THURSDAY...AND WILL LIKELY CONTINUE
BEYOND THE CURRENT 30 HOUR TAF PERIOD...INTO THURSDAY NIGHT.
OVERALL THE CONFIDENCE FOR THUNDERSTORMS OVER THE TERMINALS LATER
THURSDAY IS MODERATE TO HIGH...BUT CONFIDENCE ON THE EXACT TIMING
IS LOW TO MODERATE AT THIS TIME.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* MODERATE TO HIGH WITH THUNDERSTORMS BY LATE THURSDAY
  AFTERNOON/EVENING...BUT LOW TO MODERATE ON EXACT TIMING.

* LOW TO MODERATE WITH A PERIOD OF LOWER END MVFR CIGS BY EARLY
  THURSDAY.

KREIN

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

THURSDAY NIGHT...PERIODS OF TSRA. MVFR LIKELY. IFR POSSIBLE.

FRIDAY...CHANCE OF SHRA.

SATURDAY...VFR.

SUNDAY...CHANCE OF SHRA LATE.

MONDAY...SLIGHT CHANCE OF SHRA.

TUESDAY...SLIGHT CHANCE OF SHRA.

RODRIGUEZ

&&

.MARINE...
226 PM CDT

RELATIVELY QUIET WEATHER IS GOING TO COME TO AN END TONIGHT...AS
HIGH PRESSURE CONTINUES TO DEPART TO THE NORTHEAST WITH A STRONGER
LOW PRESSURE SYSTEM APPROACHING FROM THE SOUTHWEST. WINDS HAVE
REMAINED SOUTHEASTERLY THIS AFTERNOON...WITH A SLIGHT BUMP IN THE
GRADIENT CREATING OCCASIONAL GUSTS TO 20KT. WITH THE CONTINUED
SOUTHEAST FLOW...WAVES HAVE REMAINED ELEVATED IN THE ILLINOIS
NEARSHORE WATERS TO AROUND 2 TO 4 FT WHILE ALONG THE INDIANA
NEARSHORE WAVES ARE GENERALLY 1 TO 3 FT. AS HIGH PRESSURE DEPARTS
TONIGHT...WINDS WILL TURN SOUTHERLY AND A TIGHTENING GRADIENT IS
POISED TO INCREASE WINDS THURSDAY MORNING. AT THIS TIME WINDS ARE
EXPECTED TO REMAIN IN THE 15 TO 25 KT RANGE THRU THUR. AS THE LOW
ARRIVES OVER NORTHERN IL THUR EVENING...IT LOOKS LIKE A PERIOD OF
GALES MAY OCCUR ACROSS THE CENTRAL/SOUTHERN PORTIONS OF THE LAKE. AS
THIS LOW MOVES TO SOUTHERN ONTARIO WHILE STRENGTHENING...WITH
WINDS TO 30 KT LIKELY. A VERY TIGHT GRADIENT ON THE SOUTHERN END
OF THIS LOW PRESSURE SYSTEM AS WELL AS MUCH COLDER AIR MOVING OVER
THE LAKE WILL SUPPORT WINDS TO 30 KT FRIDAY AND FRIDAY NIGHT AS
WELL AS EVEN A PERIOD OF GALES POSSIBLE. THEN EXPECT A SLOW
DOWNWARD TREND IN SPEEDS THROUGH THE WEEKEND.

BEACHLER

&&

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

$$

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