Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 041938
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
238 PM CDT FRI SEP 4 2015

.SHORT TERM...
235 PM CDT

THROUGH SUNDAY...

SHOWERS AND THUNDERSTORMS HAVE BEEN MOST CONCENTRATED...BUT STILL
LARGELY SCATTERED...IN NORTHWEST INDIANA...AND EAST INTO EASTERN
ILLINOIS BUT WELL SOUTH...CLOSER TO A REMNANT FRONTAL BOUNDARY FROM
LAST NIGHT THAT STALLED OUT SOMEWHERE IN THE ROCHELLE TO JOLIET TO
VALPARAISO LINE.  MOST OF THIS ACTIVITY HAS CONTINUED EAST. THIS
BOUNDARY STILL REMAINS...BUT WE ALSO NOW HAVE A LAKE BREEZE BOUNDARY
THAT IS NOW MAKING STEADY BUT NOW SLOWING INLAND PROGRESS THIS
AFTERNOON. THIS COMBINED WITH UPPER LEVEL LOW PRESSURE ATOP THE AREA
1500-2500 J/KG OF MIXED LAYER CAPE AWAY FROM THE LAKE IS RESULTING
IN ADDITIONAL ISOLATED DEVELOPMENT THIS AFTERNOON.  THE BETTER
INSTABILITY AND CONVERGENCE SLIGHTLY COOLER TEMPS ALOFT STILL RESIDE
FROM THE KANKAKEE RIVER EASTWARD WILL ADDITIONAL DEVELOPMENT WILL
LIKELY CONTINUE. THESE STORMS STILL POSE A HEAVY RAINFALL THREAT IF
THEY CAN CONGEAL AS THEY ARE SLOW MOVING...WITH A LOW CONCERN FOR
GUSTY WINDS IN FORD/IROQUOIS COUNTY ILLINOIS INTO
BENTON/NEWTON/JAPSER COUNTIES OF INDIANA...WITH THE HIGHER
CONCERN EAST INTO CENTRAL INDIANA.

LIKE YESTERDAY...THE LOSS OF DAYTIME HEATING...WE SHOULD SEE
ACTIVITY DIMINISH QUICKLY. THE UPPER LOW LINGERS IN THE REGION
OVERNIGHT. ANOTHER WEAK WAVE WILL ATTEMPT TO MOVE INTO THE UPPER
LEVEL RIDGE THAT WILL BUILD IN FROM THE WEST OVERNIGHT. MANY
GUIDANCE MEMBERS SUGGEST THAT THIS COUPLED WITH SOME RAMPING UP OF
ISENTROPIC LIFT AHEAD OF THIS FEATURE MAY TRIGGER SOME ISOLATED
SHOWERS OR THUNDERSTORMS LATE TONIGHT INTO EARLY SATURDAY. HAVE
MOVED TOWARD THE DRIER NAM SOLUTION AT THIS POINT IN TIME WHICH IS
REFLECTED IN THE EXPERIMENTAL HRRR AND HI-RES NMM/ARW AS WE DO NOT
FEELING FORCING IS STRONG ENOUGH TO TAP INTO ANY ELEVATED
INSTABILITY BUT WILL BE A PERIOD TO WATCH.

THE UPPER RIDGE IS OVER WESTERN ILLINOIS...WITH THE UPPER LOW
INCHING EASTWARD. LOW LEVEL SOUTHWESTERLIES SHOULD ALLOW WARMING ALL
THE WAY TO THE LAKEFRONT WITH 90S INLAND AND UPPER 80S AT THE LAKE.
A WEAKENING OF THE PRESSURE GRADIENT NEAR THE LAKE WOULD SUPPORT A
SE WIND SHIFT NEAR THE LAKE LATER IN THE DAY TO SUPPORT SOME
COOLING. HEAT INDICES WILL GET INTO THE MID TO UPPER 90S IN MANY
SPOTS. HAVE HELD ONTO THE SLIGHT CHANCE POPS FOR TOMORROW
AFTERNOON GIVEN THE MOIST/UNSTABLE ENVIRONMENT...BUT AT THIS POINT
MOST AREAS LOOK TO REMAIN DRY AS THE MAIN TRIGGER IS LACKING.

KMD

&&

.LONG TERM...
350 AM CDT

SATURDAY THROUGH THURSDAY...

THE BIG STORY OVER THE HOLIDAY WEEKEND WILL BE THE CONTINUED
UNSEASONABLY HOT AND HUMID CONDITIONS. MID/UPPER RIDGING WILL
BECOME EVEN MORE PRONOUNCED AS THE WEAK UPPER LOW COMPLETELY
WASHES OUT. THIS WILL MEAN THAT CONVECTIVE DEVELOPMENT WILL BE
ISOLATED AT BEST ON SATURDAY...AS THERE WILL BE SOME MINOR
CAPPING. WEAK LAKE BREEZE INTO ILLINOIS COULD PROVIDE SOME ADDED
LIFT. OVERALL KEPT POPS IN SLIGHT CHANCE RANGE DURING AFTERNOON
SIMILAR TO PREVIOUS FORECAST. HIGHS IN THE UPPER 80S TO LOWER 90S
AND DEWPOINTS IN THE UPPER 60S TO LOWER 70S WILL SUPPORT HEAT
INDEX VALUES IN THE MID TO UPPER 90S. MINOR EXCEPTION EXPECTED TO
BE IMMEDIATE IL SHORE KEPT SLIGHTLY COOLER BY LAKE BREEZE. SUNDAY
HAS THE POTENTIAL TO BE THE HOTTEST DAY OF THIS STRETCH...WITH
SOLIDLY LOW 90S LIKELY AND MID 90S POSSIBLE IN SPOTS THANKS TO 925
MB TEMPS IN 25-27 CELSIUS RANGE. DEWPOINTS COULD BE A COUPLE
DEGREES LOWER...SO CURRENTLY HEAT INDEX VALUES FORECAST ARE
SIMILAR TO SATURDAY. HAVE MAINTAINED DRY FORECAST FIRMLY UNDER
UPPER RIDGE. MINOR UNCERTAINTY IS WHETHER WINDS WILL TURN
SOUTHEAST ENOUGH ALONG IMMEDIATE IL SHORE TO PROVIDE SOME MINOR
COOLING.

ON SUNDAY NIGHT INTO LABOR DAY...A COLD FRONT WILL BE SLOWLY
APPROACHING FROM THE NORTHWEST AS THE UPPER RIDGE FINALLY GETS
SHOVED EAST BY A FAIRLY DEEP TROUGH ALONG THE CANADIAN BORDER.
SUNDAY EVENING LOOKS TO BE DRY AND THEN HAVE CHANCE POPS MAINLY
WEST OF A PERU TO WAUKEGAN LINE OVERNIGHT. THE BOUNDARY WILL TAKE
ITS SWEET TIME MOVING THROUGH ON LABOR DAY MONDAY INTO TUESDAY AND
THEN A WAVE OF LOW PRESSURE LOOKS TO FORM SOMEWHERE OVER THE MID
MS VALLEY ON TUESDAY AND SHIFT EAST INTO WEDNESDAY. HIGH PRESSURE
OVER THE NORTHERN PLAINS WILL NUDGE THE BAROCLINIC ZONE SOUTH ON
WEDNESDAY...BRINGING COOLER AIR IN AND THEN COOLER AND DRIER AIR
FINALLY ON THURSDAY. HAVE LARGELY STUCK WITH CHANCE POPS FROM
BLENDED GUIDANCE...THOUGH SYNOPTIC SETUP WITH SLOW MOVING FRONT
AND WAVE/S OF LOW PRESSURE ALONG IN AND STRONG MOISTURE TRANSPORT
AHEAD THE BOUNDARY COULD SUPPORT SOME HEFTY RAIN TOTALS IN THE
MONDAY THROUGH WEDNESDAY PERIOD IN PERIODIC SHRA/TSRA. LABOR DAY
ITSELF MAY NOT BE A TOTAL WASHOUT...BUT PLAN FOR A CHANCE FOR
SHOWERS/T-STORMS AND GENERALLY WARM AND MUGGY CONDITIONS FOR
OUTDOOR BBQS. IF YOURE LOOKING FORWARD TO SOME MORE AUTUMN-LIKE
TEMPS...THE WHOLE AREA IS EXPECTED TO BE IN THE 70S ON
THURSDAY...WITH ONLY LOW 70S ALONG THE LAKESHORE.

RC

&&

.AVIATION...

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

* CHANCE FOR ISOLATED TSRA/SHRA THIS AFTERNOON.

* EAST WINDS LESS THAN 10 KT BECMG LGT/VRBL OVERNIGHT.

KREIN

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

LATEST WEB CAMS NEAR LAKE MICHIGAN AND DOWNTOWN
CHICAGO...HOWEVER...UGN HAS IMPROVED TO MVFR AND PWK HAS IMPROVED
TO VFR. WITH LITTLE EVIDENCE THAT THE IFR CIGS WILL PENETRATE
INLAND VERY FAR...HAVE REMOVED THE IFR CIGS.

SCT TSRA/SHRA ARE DEVELOPING ALONG AN OLD BOUNDARY EXTENDING FROM
SCNTRL WISCONSIN THROUGH NERN IL AND INTO NWRN INDIANA. EXPECT
THAT SCT TS WILL CONTINUE INTO THE EARLY EVENING...BUT REMAIN SCT
ENOUGH TO MAINTAIN THE VCTS MENTION AS CONFIDENCE IN DIRECT IMPACT
A THE TERMINALS IS LOW. ANTICIPATE THAT CONVECTIVE ACTIVITY SHOULD
WANE WITH SUNSET. A WEAK LAKE BREEZE IS STILL EXPECTED TO DEVELOP
THIS AFTERNOON...WITH LGT/VRBL WINDS BECOMING ELY AT LESS THAN 10
KT. WINDS SHOULD BECM LGT/VRBL OVERNIGHT AGAIN.

SATURDAY SHOULD BE RELATIVELY QUIET THROUGH MUCH OF THE
DAY...THOUGH WITH TROUGHING OVER THE CNTRL PLAINS SHIFTING A
LITTLE EAST...WINDS TOMORROW SHOULD BE PREVAILING SLY AT LESS THAN
10 KT. WITH TEMPS EXPECTED TO REACH NEAR 90F AND DEWPOINTS IN THE
UPPER 60S TO LOWER 70S...THERE SHOULD BE ANOTHER CHANCE FOR SCT
TSRA/SHRA AGAIN TOMORROW AFTERNOON.

KREIN

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

* MEDIUM CONFIDENCE FOR ISOLD TSRA CHANCES THIS AFTERNOON AND
  TOMORROW AFTERNOON...LOW CONFIDENCE IN COVERAGE AND DIRECT
  IMPACT ON THE TERMINALS.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

KREIN

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

SATURDAY NIGHT...VFR LIKELY.

SUNDAY...SLIGHT CHANCE OF TSRA LATE.

MONDAY THROUGH WEDNESDAY...CHANCE OF TSRA.

THURSDAY...VFR LIKELY.

LENNING

&&

.MARINE...
325 AM CDT

QUIET WEATHER AND GENERALLY LIGHT WINDS PERSIST OVER THE LAKE FOR
MOST OF THE HOLIDAY WEEKEND. WAVES COULD GROW A BIT BY LATE SUNDAY
OR MONDAY ACROSS THE NORTH HALF WITH SUSTAINED SOUTHERLY
FLOW...BUT STABLE CONDITIONS WITH COOL WATER AND WARM AIR WILL NOT
SUPPORT SIGNIFICANT WAVE GROWTH. A COLD FRONT CROSSING THE LAKE
FROM WEST TO EAST EARLY NEXT WEEK WILL BRING A CHANGE TO THIS
PATTERN.

LENNING

&&

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

$$

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