Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 020511
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1211 AM CDT TUE SEP 2 2014

.DISCUSSION...
842 PM CDT

THUNDER HAS CEASED IN THE SHOWER ACTIVITY NORTH OF INTERSTATE 88.
EXPECT THESE SHOWERS TO SLOWLY DRIFT SOUTHEAST AND WEAKEN EVEN MORE WITH
THE LOSS OF DAYTIME HEATING. THERE ARE SOME SHOWERS MOVING
NORTHEAST THROUGH IOWA THIS EVENING AHEAD OF AN UPPER TROUGH AXIS
THAT MAY DRIFT THROUGH LATER TONIGHT...AND ADDED A SLIGHT CHANCE
OF SHOWERS TO ACCOUNT FOR THIS...BUT AT THIS TIME IT DOES NOT LOOK
TO BE TOO BIG OF A DEAL GIVEN THE LACK OF MOISTURE. MODELS
STILL SUGGEST THE THUNDERSTORM ACTIVITY FARTHER WEST IN SOUTH
DAKOTA AND NEBRASKA WILL ALSO WEAKEN WITH TIME AND GENERALLY PASS
BY INTO WISCONSIN TUESDAY.

THE BIGGER SHOW IS TAKING PLACE IN THOUGH MISSOURI AND EXTENDING
INTO SOUTHWEST ILLINOIS. THIS AREA IS SOUTH AND EAST OF A
STRENGTHENING UPPER JET AND WITH BETTER AXIS TO DEEPER MOISTURE.
STILL LOOKS LIKE SOME OF THIS WILL PROGRESS INTO OUR SOUTHEASTERN
ZONES AFTER MIDNIGHT. CURRENT GRIDS HAVE A DECENT HANDLE ON THIS
EVOLUTION.

KMD

//PREV DISCUSSION...
323 PM CDT

MAIN FORECAST CONCERNS ARE GENERALLY PRECIP CHANCES TONIGHT...AND
AGAIN LATE WEDNESDAY INTO FRIDAY WITH RETURN OF WARM/MORE HUMID AIR.
ATTENTION THEN TURNS TO MUCH COOLER AND DRIER WEATHER NEXT WEEKEND
INTO EARLY NEXT WEEK.

OUTFLOW FROM OVERNIGHT IOWA MCS AND PRE-FRONTAL TROUGH MOVING INTO
INDIANA THIS AFTERNOON HAS ALL BUT ENDED SIGNIFICANT THUNDERSTORM
THREAT FOR THE FORECAST AREA THIS AFTERNOON...WITH THE POTENTIAL FOR
ONLY A FEW ISOLATED SHOWERS AND THUNDERSTORMS MAINLY EAST OF A
PONTIAC TO VALPARAISO LINE. ADDITIONAL SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS OVER NORTHEAST IOWA/WESTERN WISCONSIN ARE
ASSOCIATED WITH UPPER LEVEL TROUGH AND TRUE SURFACE COLD FRONT...
THOUGH THIS ACTIVITY SHOULD BE AT ITS DIURNAL PEAK THIS AFTERNOON
AND HIGH-RES GUIDANCE INDICATE A WEAKENING/DISSIPATING TREND AS IT
APPROACHES AND MOVES INTO NORTHERN IL THIS EVENING. BASED ON THIS
HAVE BACKED OFF LATE AFTERNOON AND EVENING POPS EVEN FURTHER...WITH
ONLY A SLIGHT CHANCE MENTION OF A FEW SHOWERS MAINLY NORTH OF I-88
IN FAR NORTHERN IL INTO EARLY EVENING. ILX 18Z SOUNDING SHOWS DRY
RELATIVELY DRY COLUMN...WITH A FAIRLY STRONG CAPPING INVERSION NOTED
BETWEEN 800-850 HPA. NO WONDER CONVECTION ALONG PRE-FRONTAL TROUGH
IN OUR EASTERN CWA HAS STRUGGLED THIS AFTERNOON. PRECIP POTENTIAL
DOES STILL APPEAR TO INCREASE LATER TONIGHT ACROSS CENTRAL IL...AND
ACROSS OUR FAR SOUTHEASTERN CWA...AS DEEP LAYER FORCING INCREASES
WITH APPROACH OF MAIN UPPER TROUGH AXIS AND LOW LEVEL WINDS BACK
ALONG COLD FRONTAL BAROCLINIC ZONE. GUIDANCE HAS CONTINUED TO TREND
FARTHER SOUTH WITH THE AXIS OF PRECIP HOWEVER...AND HAVE FOLLOWED
SUIT OVERNIGHT WITH POPS AGAIN MAINLY SOUTH OF ROUGHLY A
PONTIAC-VALPO LINE. COLD FRONT CLEARS SOUTHEAST CWA EARLY TUESDAY
MORNING...WITH ANY LINGERING PRECIP IN OUR FAR SOUTHEAST BY
MID-MORNING.

FLOW BECOMES FAIRLY ZONAL ALOFT IN THE WAKE OF THE DEPARTING TROUGH
TUESDAY...WITH WEAK SURFACE HIGH PRESSURE MOVING ACROSS THE AREA.
MILD/DRY WEATHER EXPECTED FOR THE MOST PART TUES-WED.

A SHOT OF VERY WARM AND HUMID AIR THEN IS SET TO RETURN LATER
WEDNESDAY NIGHT INTO FRIDAY. STRONG SHORT WAVE MOVES ACROSS THE
PACIFIC NORTHWEST AND ACROSS THE CANADIAN BORDER DURING THE PERIOD
AND INDUCES SURFACE LOW PRESSURE DEVELOPMENT ACROSS THE NORTHERN
PLAINS. SOUTHERLY RETURN FLOW STRENGTHENS AS GRADIENT TIGHTENS
SOUTHEAST OF THE LOW...WITH SURFACE WARM FRONT LIFTING NORTHEAST
ACROSS THE FORECAST AREA LATE WEDNESDAY NIGHT INTO THURSDAY. SHOWER
AND THUNDERSTORM POTENTIAL INCREASES WITH THE WARM FRONT...THOUGH
THE BEST FORCING IS GENERALLY NORTH OF THE FORECAST AREA. GREATER
THUNDERSTORM POTENTIAL APPEARS TO BE WITH THE APPROACH/PASSAGE OF
THE COLD FRONT WHICH TRAILS THE LOW ON FRIDAY. DEPENDING ON CLOUD
COVER AND EFFECTS OF ANY PRECIP...THURSDAY LOOKS TO BE VERY WARM AND
HUMID...WITH DEW POINT TEMPS CLIMBING INTO THE 70S AND AFTERNOON
HIGH TEMPS IN THE UPPER 80S-LOW 90S. FRIDAY ALSO HAS THE POTENTIAL
TO BE SIMILARLY WARM...THOUGH SOME COOLING BEHIND THE FRONT ACROSS
FAR NORTHERN IL AND CLOUDS/PRECIP MAY BE LIMITING FACTORS.

A SPRAWLING AREA OF HIGH PRESSURE BUILDS IN BEHIND THE COLD FRONT
SATURDAY AND TAKES UP RESIDENCE INTO EARLY NEXT WEEK. DRY AND MUCH
COOLER WEATHER...HIGHS LOW-MID 70S AND LOWS IN THE
50S...CHARACTERIZE THE FORECAST FOR THE EXTENDED PERIOD
SATURDAY-MONDAY.

RATZER

&&

.AVIATION...

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

* NONE.

KJB

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

WIDELY SCATTERED SHOWERS ARE DEVELOPING AGAIN ACROSS NORTHERN
ILLINOIS IN ASSOCIATION WITH THE APPROACHING SURFACE COLD FRONT AND
UPPER LEVEL DISTURBANCE. OVERALL THIS ACTIVITY LOOKS TO BE
LIGHT...AND SHOULD ONLY PERSIST ACROSS THE AREA FOR A COUPLE
HOURS. THEREAFTER...A QUIET WEATHER DAY IS EXPECTED ACROSS THE
REGION. LIGHT WESTERLY FLOW DURING THE DAY WILL LIKELY ALLOW A
LAKE BREEZE TO DEVELOP. HOWEVER...AT THIS TIME IT APPEARS THAT IT
MAY STRUGGLE TO REACH KORD AND KMDW...WITH KGYY THE MOST LIKELY
CANDIDATE TO EXPERIENCE A WIND SHIFT THIS AFTERNOON.

KJB


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

* HIGH FOR ALL ELEMENTS.

KJB

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

WEDNESDAY AND THURSDAY...PREDOMINANTLY VFR...THOUGH THERE IS A
SLIGHT CHANCE OF A THUNDERSTORM.

FRIDAY...PRETTY GOOD CHANCE OF A PERIOD OR TWO OF TSRA.

SATURDAY AND SUNDAY...VFR. NORTHEAST WINDS LIKELY.

IZZI

&&

.MARINE...

142 PM...LOW PRESSURE OVER NORTHERN ONTARIO WILL LIFT NORTHEAST
ACROSS NORTHERN QUEBEC TUESDAY. A TRAILING COLD FRONT FROM THIS
LOW WILL MOVE ACROSS THE LAKE TONIGHT. SOUTHWEST WINDS 15-25KTS
WILL DIMINISH THIS EVENING THEN SHIFT NORTHWESTERLY OVERNIGHT.
WEAK RIDGING WILL MOVE ACROSS THE SOUTHERN LAKES TUESDAY AND
TUESDAY NIGHT AS WINDS SHIFT BACK SOUTHERLY ACROSS THE LAKE. A
LARGE AREA OF LOW PRESSURE WILL DEVELOP AND DEEPEN OVER THE
PLAINS WEDNESDAY AND THEN LIFT NORTHEAST INTO ONTARIO THURSDAY
NIGHT AND FRIDAY. THE GRADIENT WILL TIGHTEN AHEAD OF THIS SYSTEM
WEDNESDAY NIGHT AND THURSDAY. A TRAILING COLD FRONT FROM THIS
LOW WILL THEN MOVE ACROSS THE LAKE FRIDAY. CMS

&&

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

$$

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