Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 301728
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1228 PM CDT THU JUL 30 2015

.SHORT TERM...
356 AM CDT

TODAY THROUGH SUNDAY...

PATTERN REMAINS SIMILAR INTO THE WEEKEND...WITH DEEP LOW PRESSURE
OVER ONTARIO AND HUDSON BAY SLOWLY FILLING WHILE WEAK HIGH
PRESSURE RIDGE DEVELOPS FROM THE CENTRAL PLAINS INTO DOWNSTATE
IL/IN AND THE OHIO VALLEY. ALOFT...HIGH PRESSURE CENTERED OVER THE
SOUTHERN PLAINS RETROGRADES ACROSS THE SOUTHWEST/WESTERN CONUS
WHILE LARGE SCALE UPPER TROUGH AMPLIFIES OVER EASTERN NORTH
AMERICA. THIS MAINTAINS RELATIVELY FAST WEST-NORTHWEST FLOW ALOFT
ACROSS THE UPPER MIDWEST AND GREAT LAKES REGION. FORECAST
SOUNDINGS INDICATE 30-35 KTS WITH DEEP MIXING TO AROUND 800 MB
THIS AFTERNOON...WHICH SIMILAR TO WEDNESDAY IS EXPECTED TO PRODUCE
BREEZY/WINDY WEST SURFACE WINDS APPROACHING 30 MPH DURING THE
WARMEST MID-LATE AFTERNOON HOURS. DRY THERMODYNAMIC PROFILES
SHOULD ALSO KEEP SKIES RELATIVELY CLOUD-FREE OTHER THAN A FEW HIGH
BASED AFTERNOON CU...AND SHOULD HELP LOWER DEW POINT TEMPS INTO
THE MID-50S IN MANY SPOTS. 925-850 MB TEMPS SHOULD SUPPORT MID-
UPPER 80S ONCE AGAIN WITH STRONG MIXING.

GUIDANCE IN GOOD AGREEMENT IN DEPICTING A SMALL-AMPLITUDE SHORT
WAVE MOVING INTO THE UPPER MISSISSIPPI VALLEY LATE TONIGHT/EARLY
FRIDAY. WHILE HEIGHT FALLS AND VORTICITY ADVECTION WITH THIS
FEATURE APPEAR FAIRLY WEEK ACROSS THE FORECAST AREA...MODELS
CONTINUE TO INDICATE A SURFACE COLD FRONTAL TROUGH PUSHING
SOUTHEAST ACROSS THE CWA DURING THE DAY. SOME SPEED/TIMING
DIFFERENCES EXIST WITH THIS FEATURE... THOUGH IT APPEARS THAT IT
WILL BE ACROSS THE SOUTHERN HALF OR SO OF THE CWA DURING THE LATE
AFTERNOON PEAK DIURNAL INSTABILITY. MOST OF THE MODELS APPEAR TOO
MOIST IN THE LOW LEVELS WITH A DEEPLY MIXED BOUNDARY LAYER AGAIN
EXPECTED AND GENERALLY DRY THERMODYNAMIC PROFILES. LOW LEVEL
CONVERGENCE ALSO LOOKS TO BE RATHER WEAK WITH WESTERLY SFC FLOW
AHEAD OF THE BOUNDARY. WITH THAT SAID...VARIOUS MODELS DO DEPICT
ISOLATED CONVECTIVE DEVELOPMENT ALONG THE FRONT DURING THE LATE
AFTERNOON-VERY EARLY EVENING HOURS AS THE FRONT SLIDES SOUTHEAST
ACROSS OUR SOUTHERN COUNTIES. DEEP SHEAR OF 30-40 KTS COMBINED
WITH DRY PROFILES WOULD SUPPORT THE POTENTIAL FOR SEVERE WIND
GUSTS AND LARGE HAIL IN ANY ISOLATED STORMS THAT DO DEVELOP...AND
SPC HAS EXPANDED THE DAY 2 MARGINAL SEVERE RISK ACROSS MUCH OF OUR
SOUTHERN CWA. BASED ON ALL OF THESE FACTORS HAVE INDICATED A
SLIGHT CHANCE 21-03Z FRIDAY AFTERNOON/EVENING FOR THE POTENTIAL OF
AN ISOLATED STRONG/SEVERE STORM OR TWO. OTHERWISE... DEEP MIXING
AGAIN SHOULD SUPPORT BREEZY AND VERY WARM CONDITIONS WITH
AFTERNOON HIGHS IN THE UPPER 80S TO AROUND 90. DRIER AIR DOES
FILTER IN FRIDAY NIGHT AS THE FRONT PUSHES SOUTHEAST OF THE
FORECAST AREA.

PATTERN CONTINUES IN A SIMILAR FASHION THROUGH THE WEEKEND...WITH
PERIODIC SPEED MAXIMA OR OTHERWISE MINOR AMPLITUDE SHORT WAVES
RIPPLING THROUGH NORTHWEST FLOW ALOFT ACROSS THE REGION. WHILE
THESE FEATURES ARE RATHER SUBTLE IN THE GUIDANCE...IT DOES APPEAR
ONE APPROACHES SATURDAY NIGHT INTO SUNDAY. THIS LOOKS TO PRODUCE
THE NEXT CHANCE FOR SOME SCATTERED SHOWERS/THUNDERSTORMS LATER
SATURDAY NIGHT AND SUNDAY.

RATZER

&&

.LONG TERM...
356 AM CDT

MONDAY THROUGH WEDNESDAY...

GUIDANCE IN GOOD AGREEMENT IN A SHORT WAVE DIGGING ACROSS THE
GREAT LAKES EARLY IN THE PERIOD THAT REINFORCES TROUGH OVER THE
GREAT LAKES. WEAK SURFACE LOW AND ANOTHER COLD FRONT LOOK TO SAG
ACROSS THE AREA MONDAY AFTERNOON/NIGHT...AGAIN WITH A CHANCE OF
PRECIP. WEAK HEIGHT RISES THEN DEVELOP BEHIND THE DEPARTING MID-
LEVEL TROUGH AXIS TUESDAY ALLOWING SURFACE HIGH PRESSURE RIDGE TO
DEVELOP FROM THE UPPER MISSISSIPPI VALLEY ACROSS IL/WI/MI BY
TUESDAY NIGHT. ECMWF THEN WEAKENS SOUTHWESTERN CONUS UPPER RIDGE
MORE THAN THE GFS BY MID-WEEK...ALLOWING ANOTHER MID-LEVEL WAVE TO
PROPAGATE INTO THE MID-MISSISSIPPI VALLEY LATE TUESDAY NIGHT INTO
WEDNESDAY AND THUS BRINGS PRECIP THREAT ACROSS MO/IA AND WESTERN
IL TUES NIGHT AND INTO THE WFO LOT FORECAST AREA ON WEDNESDAY.

HIGH PRESSURE BUILDING ACROSS THE NORTHERN LAKES IN THE WAKE OF
THE COLD FRONT LATE MONDAY DOES RESULT IN A BIT OF A COOL DOWN AS
LOW LEVEL FLOW BECOMES MORE NORTH-NORTHEASTERLY TUE-WED...WITH
HIGHS AROUND 80/LOWER 80S AND COOLER 70S OFF OF THE LAKE.

RATZER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* WEST TO NORTHWEST WINDS GUSTING AROUND 25 KT THROUGH THE EARLY
  EVENING.

* GUSTY WEST WINDS FRIDAY WITH GUSTS UP TO 20 KT.

JEE

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

HIGH PRESSURE WILL REMAIN OVER THE REGION THROUGH FRIDAY EVENING.
 WEST TO NW WINDS WILL BE GUSTING INTO THE MID 20 KT RANGE AND
THEN DIMINISH TO LESS THAN 10 KT THIS EVENING. WEST WINDS THEN
BEGIN GUSTING AGAIN INTO THE LOW 20 KT RANGE MID FRIDAY MORNING
THROUGH THE AFTN. OTHERWISE VFR CONDITIONS ARE EXPECTED.

JEE


//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS.

JEE

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

* FRIDAY NIGHT AND SATURDAY...DRY/VFR. WESTERLY WINDS.
* SUNDAY...CHANCE OF TSRA. SOUTHWEST WINDS.
* MONDAY...SLIGHT CHANCE OF TSRA. NORTHWEST WINDS.
* TUESDAY...DRY/VFR. NORTHEAST WINDS.
* WEDNESDAY...CHANCE OF TSRA. EASTERLY WINDS.

&&

.MARINE...

233 AM...STRONG LOW PRESSURE OVER HUDSON BAY WILL REMAIN GENERALLY
STATIONARY INTO THIS WEEKEND AS IT SLOWLY WEAKENS. HIGH PRESSURE
OVER THE CENTRAL PLAINS WILL SLOWLY SPREAD EAST ACROSS THE OHIO
VALLEY TONIGHT AND FRIDAY. THIS WILL ALLOW A WESTERLY GRADIENT TO
PERSIST OVER THE LAKES REGION THROUGH FRIDAY. A WEAK COLD FRONT
WILL MOVE SOUTH ACROSS THE LAKE FRIDAY AFTERNOON/EVENING.

THE GRADIENT WILL BE STRONGEST ACROSS NORTHERN PORTIONS OF THE LAKE
THIS MORNING WITH SOME GUSTS TO 30KTS POSSIBLE. AS TEMPERATURES WARM
AND WINDS INCREASE OVER LAND THIS AFTERNOON...STRONGER WINDS WILL
LIKELY BE CONFINED TO THE WESTERN SHORE OF LAKE MICHIGAN WITH SPEEDS
DIMINISHING SOME OVER THE CENTRAL AND EASTERN PORTIONS OF THE LAKE
AND POSSIBLY TURNING MORE SOUTHWEST WITH LOW CONFIDENCE FOR SPECIFIC
SPEEDS/DIRECTIONS THIS AFTERNOON. SPEEDS/GUSTS THEN INCREASE A BIT
LATE TONIGHT INTO FRIDAY MORNING AHEAD OF THE FRONT. CMS

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 7 PM
     THURSDAY.

&&

$$

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