Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 311145
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
645 AM CDT FRI JUL 31 2015

.SHORT TERM...
317 AM CDT

THROUGH SATURDAY...

IT SEEMS ODD TO BE TALKING ABOUT UNSEASONABLY DRY CONDITIONS OVER
THE AREA THIS MORNING AFTER THE COUNTLESS HEAVY RAIN EVENTS OVER THE
REGION OVER THE PAST COUPLE OF MONTHS WITH PWATS COMMONLY
APPROACHING 2". INSTEAD...PRECIPITABLE WATER VALUES ON YESTERDAYS
ILX RAOB SOUNDINGS INDICATED A VERY DRY ATMOSPHERIC COLUMN...WITH
VALUES UNDER AN INCH. THE 12 UTC THURSDAY RAOB EVEN HAD VALUES DOWN
AROUND 0.7 INCHES...WHICH IS NEAR THE CLIMATOLOGICAL MINIMUM FOR
THIS TIME OF YEAR. SO...THE ATMOSPHERIC COLUMN DOES NOT GET MUCH
DRIER THIS TIME OF YEAR. WITH SIMILAR PRECIPITABLE WATER VALUES
EXPECTED ACROSS MUCH OF THE AREA TODAY...IT APPEARS THE THREAT FOR
SHOWERS AND STORMS WILL BE VERY LOW.

A WEAK SURFACE BOUNDARY WILL SHIFT OVER THE AREA TODAY. HOWEVER...NO
BIG AIRMASS CHANGES ARE EXPECTED IN ITS WAKE. THEREFORE...IT APPEARS
THAT IT WILL BE ANOTHER VERY WARM...BUT UNSEASONABLY DRY AND BREEZY
DAY ACROSS THE REGION. DEW POINTS ACROSS THE AREA THIS MORNING
REMAIN IN THE UPPER 50S ACROSS MUCH OF NORTHERN ILLINOIS...WITH
LOWER TO MID 60S GENERALLY SOUTH OF INTERSTATE 80. THESE DRY DEW
POINTS WILL LIKELY DROP SEVERAL DEGREES AGAIN TODAY...ESPECIALLY
OVER NORTHEASTERN ILLINOIS AS MIXING TO...OR JUST ABOVE...800 MB IN
THIS EXTREMELY DRY AIRMASS...LIKELY RESULTING IN UPPER 40 TO LOWER
50 DEWPOINTS AT THE SURFACE OVER NORTHEASTERN ILLINOIS. GIVEN THAT
TEMPERATURES WILL LIKELY WARM WELL INTO THE 80S AGAIN TODAY...AS
THEY DID YESTERDAY...THIS WOULD SUGGEST THAT THE RELATIVE HUMIDITY
VALUES WILL DROP AS LOW AS 25% IN SOME PLACES ACROSS NORTHEASTERN
ILLINOIS. THESE DRY CONDITIONS IN COMBINATION WITH AFTERNOON WINDS
GUSTING UP TO NEAR 25 KT COULD RESULT IN NEAR RED FLAG CRITERIA
CONDITIONS BELIEVE IT OR NOT. HOWEVER...IT APPEARS THIS AREA WILL BE
CONFINED TO THE MORE URBAN AREAS IN AND AREA CHICAGO...WHERE THE
DRIEST CONDITIONS WILL DEVELOP.

WITH THESE DRY CONDITIONS...IT APPEARS THE THREAT FOR SHOWERS AND
STORMS WILL BE VERY LOW OVER NORTHERN ILLINOIS. SOME SLIGHTLY BETTER
LOW LEVEL MOISTURE SOUTH OF INTERSTATE 80 COULD SUPPORT SLIGHTLY
BETTER CHANCES FOR SHOWER/STORM DEVELOPMENT THIS AFTERNOON.
HOWEVER...I HAVE OPTED TO LEAVE THE MENTION OUT OF THE FORECAST. IF
ANY STORMS WERE ABLE TO DEVELOP ACROSS MY SOUTH THIS
AFTERNOON...THEY WOULD LIKELY REMAIN ISOLATED.

SATURDAY LOOKS TO BE ANOTHER WARM AND FAIRLY DRY DAY ACROSS THE
REGION. IT APPEARS OUR NEXT CHANCE OF STORMS WILL HOLD OFF UNTIL
LATER SATURDAY NIGHT. SEE LONG TERM DISCUSSION BELOW FOR MORE ON
THIS.

KJB

&&

.LONG TERM...
251 AM CDT

SATURDAY NIGHT THROUGH THURSDAY...

500MB TROUGH AXIS WILL BE PIVOTING EAST ACROSS THE EASTERN GREAT
LAKES SAT...WHILE RIDGING PERSIST ACROSS THE WESTERN CONUS. OPER
SOLUTIONS CONTINUE TO FEATURE A MID-LVL WAVE DEVELOPING SAT AFTN
ACROSS THE PLAINS...AND QUICKLY LIFTING EAST AND REACHING THE
FORECAST AREA LATE SAT NGT. INSTABILITY WILL BE ON THE INCREASE AS
THE WAVE SLIDES OVERHEAD...HOWEVER TIMING IS LESS FAVORABLE FOR
WELL ORGANIZED/DEVELOPED THUNDERSTORMS. SOME OF THE GUIDANCE
MEMBERS SUGGEST ARRIVAL OF THE WAVE WILL BE AFTER 6Z SUN.

THE 500MB TROUGH AXIS CONTINUES TO SEND A FEW WEAK LOBES OF
VORTICITY SOUTH ACROSS THE UPPER MIDWEST SUN/MON. MARGINAL
INSTABILITY WILL LINGER SUN/MON...SO WILL MAINTAIN THE MENTION OF
SHOWERS/THUNDERSTORMS. ALTHOUGH AT THIS TIME EXPECT THE BEST CHANCE
FOR THUNDERSTORMS WOULD OCCUR IN MORE OF A DIURNAL COMPONENT...IN
THE AFTERNOON/EVENING TIMEFRAME BOTH DAYS.

ENSEMBLES THEN TREND BACK TOWARDS THE WESTERN RIDGE
DEVELOPING...HELPING TO BRING A SLIGHTLY STRONGER TROUGH BACK ACROSS
THE GREAT LAKES REGION TOWARDS MID-WEEK. MINIMAL SPREAD CONTINUES TO
BE ADVERTISED AMONGST MOST ENSEMBLE MEMBERS...SUGGESTING A
RELATIVELY HIGH CONFIDENCE. A FRONTAL BOUNDARY WILL PUSH ACROSS THE
REGION TUE...WITH UNSEASONABLY COOLER AIR STEADILY ADVECTING SOUTH
WITH A THERMAL TROUGH SETTING UP OVER THE GREAT LAKES REGION. THIS
WILL TRANSITION TEMPS FROM THE MID/UPR 80S SUN/MON...TO AROUND 80
TUE...AND THEN MID/UPR 70S FOR THE REMAINDER OF THE EXTENDED
PERIODS. CHANCES FOR SHOWERS AND ISOLATED THUNDER LOOKS MINIMAL
BEYOND TUE...AND COULD END UP REMAINING DRY AS SOME GUIDANCE
SUGGESTS WEAK ANTI-CYCLONIC FLOW LINGERING OVERHEAD.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 12Z...

* WEST WINDS INCREASE BY MID-MORNING TO 12-14KT WITH GUSTS BETWEEN
  20-25KT. GUSTS DIMINISH SHORTLY AFTER SUNSET.

BEACHLER

//DISCUSSION...UPDATED 12Z...

HIGH PRESSURE WILL CONTINUE TO PROVIDE VFR CONDS ACROSS THE
REGION TODAY. MIXING WILL INCREASE BY MID- MORNING...RESULTING IN
GUSTY WEST WINDS DEVELOPING. CURRENT INDICATIONS ARE THAT GUSTS
WILL BE BETWEEN 20-25KT...HOWEVER IT IS POSSIBLE A FEW GUSTS COULD
APPROACH 26-28KT. MIXING WILL QUICKLY DISSIPATE SHORTLY AFTER
SUNSET...WITH WIND SPEEDS FALLING BACK TO ARND 10KT. MINIMAL CLOUD
COVER AGAIN TODAY...WITH EXCEPTION OF A THIN CIRRUS CLOUD DECK.

BEACHLER


//ORD AND MDW CONFIDENCE...UPDATED 12Z...

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS.

BEACHLER

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

SATURDAY NIGHT...MAINLY VFR. CHANCE OF SHOWERS/THUNDERSTORMS LATE.

SUNDAY...CHANCE OF TSRA. SOUTHWEST WINDS.
MONDAY...CHANCE OF TSRA. NORTHWEST WINDS.
TUESDAY...DRY/VFR. NORTHEAST WINDS.
WEDNESDAY...SLIGHT CHANCE OF TSRA. EASTERLY WINDS.
THURSDAY...SLIGHT CHANCE OF RAIN.

BEACHLER

&&

.MARINE...
255 AM CDT

THE GRADIENT TIGHTENS UP AGAIN TODAY...WITH WEST/NORTHWEST WINDS
CREATING SOME GUSTY CONDS MAINLY FOR THE SOUTHERN HALF. THIS WILL
CREATE HAZARDOUS CONDITIONS FOR SMALL CRAFT THROUGH THIS
AFTERNOON WITH 20-25KT GUSTS...THEN AFTER SUNSET THE GRADIENT
DIMINISHES ALLOWING WINDS TO BECOME LESS GUSTY. WITH HIGH PRESSURE
DRIFTING EAST ACROSS THE OHIO VALLEY SATURDAY...WINDS WILL BE
SLOWLY TURNING SOUTHWESTERLY. THE GRADIENT TIGHTENS LATE IN THE
WEEKEND AS ANOTHER AREA OF LOW PRESSURE APPROACHES LAKE MICHIGAN
FROM THE WEST. WINDS MAY BECOME GUSTY SUN AFTERNOON. A STRONGER
LOW AND FRONTAL BOUNDARY WILL ARRIVE MONDAY NGT INTO
TUESDAY...WITH WINDS TURNING NORTHWEST TO NORTH.

BEACHLER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743...10 AM
     FRIDAY TO 7 PM FRIDAY.

&&

$$

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