Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 040900
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
400 AM CDT FRI SEP 4 2015

.SHORT TERM...
338 AM CDT

TODAY AND TONIGHT...

MAIN CONCERNS ARE WITH CONTINUED VERY WARM TO HOT AND HUMID
CONDITIONS AND THUNDERSTORM POTENTIAL. MORNING W/V IMAGERY SHOWS A
WELL DEFINED UPPER LOW OVER SOUTHEAST WISCONSIN AND ANOTHER UPPER
LOW IN CLOSE PROXIMITY OVER LOWER MICHIGAN TRAPPED UNDER THE
PERSISTENT AND STRONG UPPER RIDGE THAT HAS BEEN BRINGING US JULY
IN SEPTEMBER. NONEXISTENT GRADIENT WILL EASILY ALLOW A LAKE BREEZE
TO PUSH INLAND TODAY...LIMITING HIGHS NEAR THE LAKE TO THE MID AND
UPPER 70S. SLIGHTLY COOLER 925 MB TEMPS THROUGH MID DAY NEAR THE
UPPER LOW WILL RESULT IN HIGHS INLAND NORTH OF I-80 TO THE MID AND
UPPER 80S...BUT WARMER 925 MB TEMPS SOUTH WILL YIELD ANOTHER DAY
IN THE UPPER 80S TO LOWER 90S. DEWPOINTS AROUND 70 WILL RESULT IN
AN UNSTABLE AND WEAKLY CAPPED ENVIRONMENT.

THE UPPER LOWS PIVOTING AROUND AND EVENTUALLY MERGING OVER
SOUTHERN LAKE MICHIGAN WILL PROVIDE SOME UPWARD MOTION...HOWEVER
LIFTING WILL BE COUNTERED BY GRADUAL HEIGHT RISES OVERALL. HI-RES
GUIDANCE ADMITTEDLY NOT TOO EXCITED WITH SHOWER/TSRA COVERAGE
TODAY...BUT GUIDANCE HONING IN ON FAIRLY ROBUST MOISTURE
CONVERGENCE INVOF LAKE BREEZE. THIS AND MINOR FORCING FROM THE
UPPER LOW APPEARS IT WILL BE THE IMPETUS FOR ISOLATED TO WIDELY
SCATTERED SHRA/TSRA DEVELOPMENT. HAVE KEPT POPS IN 20 TO 30
PERCENT RANGE...HIGHEST IN AFTERNOON NEAR THE INLAND PUSH OF THE
LAKE BREEZE. MANY AREAS ARE LIKELY TO STAY DRY TODAY...BUT GIVEN
WEAK FLOW ALOFT...ANY THUNDERSTORM WILL BE SLOW MOVING AND CAPABLE
OF HEAVY DOWNPOURS DUE TO MOIST AIRMASS. LOSS OF INSTABILITY WITH
NIGHTFALL THIS EVENING AND CONTINUED HEIGHT RISES WILL QUICKLY
SHUT OFF CONVECTION. CANNOT RULE OUT A VERY ISOLATED SHOWER OR
STORM LATE EVENING/OVERNIGHT...BUT HAVE GONE WITH A DRY FORECAST
DURING THIS TIME. IT WILL BE YET ANOTHER WARM AND MUGGY NIGHT...IN
THE UPPER 60S TO LOWER 70S.

RC

&&

.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 06Z...

* MVFR CIGS AND VSBYS IN THE AREA OVERNIGHT.

* LIGHT AND VARIABLE WINDS WITH FAVORED EASTERLY DIRECTION.

* ISOLATED TSRA FRIDAY AFTERNOON.

LENNING

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

A PRETTY TOUGH BUT HOPEFULLY LOW IMPACT FORECAST OVERNIGHT. MVFR
CIGS ARE BEING REPORTED AT SEVERAL UPSTREAM STATIONS AND
SATELLITE PICS SEEM TO SHOW THIS LOWER AREA OF CLOUDS SPREADING
TOWARD THE MDW/ORD TERMINALS. DESPITE MOSTLY CLEAR METAR AT TAF
TIME...WENT AHEAD AND INCLUDED A 3KT BKN CEILING. THIS CERTAINLY
WILL LOOK LIKE AN ODD DECISION IF TRENDS DO NOT PROGRESS AS
EXPECTED...BUT MAY ALSO BE TOO HIGH BASED ON A FEW POCKETS OF IFR
CIGS OUT THERE AS WELL. AS FOR VSBYS...OPTED TO DELAY THE LOWER
VALUES TILL AROUND SUNRISE SINCE TRENDS ARE NOT AS CLEAR AND FAR
FEWER OBS ARE HEADING THAT DIRECTION AT THIS HOUR.

FRIDAY AFTERNOON IS THE NEXT PERIOD OF CONCERN...WITH SUPPORT FOR
ISOLATED TSRA ACTIVITY...PERHAPS WITH A BIT MORE COVERAGE THAN
THURSDAY AFTERNOON BUT WITH LOW CONFIDENCE ON TIMING AND LOCATION.

LENNING

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

* LOW CONFIDENCE ON CIGS AND VSBYS OVERNIGHT.

* HIGH CONFIDENCE REGARDING LIGHT WIND SPEEDS EARLY IN TAF PERIOD.

* LOW CONFIDENCE ON TIMING AND COVERAGE OF TSRA FRIDAY AFTERNOON.

LENNING

//OUTLOOK FOR ORD/MDW FOR 12Z SATURDAY-12Z THURSDAY...UPDATED 00Z...

SATURDAY...SLIGHT CHANCE OF TSRA IN THE AFTERNOON.

SUNDAY...SLIGHT CHANCE OF TSRA LATE.

MONDAY...CHANCE OF TSRA.

TUESDAY...CHANCE OF TSRA.

WEDNESDAY...CHANCE OF TSRA.

KREIN

&&

.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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