Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 012351
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
651 PM CDT MON SEP 1 2014

.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 00Z...

* BRIEF SHOWER POSSIBLE MID EVENING

IZZI

//DISCUSSION...UPDATED 00Z...

COLD FRONT APPROACHING FROM THE NORTHWEST WITH NARROW BAND OF
SHRA/ISOLD TSRA ALONG THE FRONT. GREATEST COVERAGE LOOKS TO STAY
NORTH OF CHICAGO METRO TAF SITES...THOUGH STILL COULD BE A BRIEF
SHOWER IN THE 02-04Z TIME FRAME. WINDS SHOULD SUBSIDE FAIRLY
QUICKLY THIS EVENING AND REMAIN WESTERLY TONIGHT AND INTO THE DAY
TUESDAY. LAKE BREEZE LIKELY TO DEVELOP AND PROBABLY MOVE THROUGH
GYY BY EARLY AFTERNOON...BUT CURRENTLY EXPECT IT TO REMAIN EAST OF
ORD/MDW. CLOUDINESS LATE TONIGHT WILL PROBABLY BE ENOUGH TO
PREVENT ANY SIGNIFICANT VSBY RESTRICTIONS IN FOG EARLY TUES AM.

IZZI


//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS

IZZI

//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...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 10 PM MONDAY.

&&

$$

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