Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 292336
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
636 PM CDT WED JUL 29 2015

.SHORT TERM...
221 PM CDT

TONIGHT THROUGH FRIDAY...

COLD FRONT MOVING EAST OF THE FORECAST AREA THIS AFTERNOON...WITH
SURFACE HIGH PRESSURE RIDGE EXPANDING FROM THE CENTRAL PLAINS INTO
CENTRAL IL/IN. SOUTHERN PERIPHERY OF MID-LEVEL TROUGH ASSOCIATED
WITH CLOSED UPPER LOW OVER ONTARIO WILL SWEEP ACROSS THE REGION
TONIGHT...THOUGH GENERALLY DRY ATMOSPHERE UPSTREAM PER OAX 12Z
RAOB AND EXPECT ONLY A FEW PATCHY MID-HIGH CLOUDS WITH WEAK HEIGHT
FALLS AHEAD OF TROUGH AND BENEATH RIGHT ENTRANCE TO UPPER LEVEL
JET STREAK LIFTING THROUGH THE NORTHERN LAKES. SURFACE DEW POINTS
CONTINUE TO DROP OFF INTO THE 50S ON BREEZY WEST-NORTHWEST
WINDS...SETTING THE STAGE FOR A LESS HUMID NIGHT WITH LOWS NEAR
AVERAGE IN THE LOW-MID 60S.

UPPER PATTERN SETTLES INTO A RIDGE IN THE WEST/TROUGH IN THE EAST
CONFIGURATION THROUGH THE END OF THE WEEK...WITH A COUPLE OF MINOR
AMPLITUDE SHORT WAVES DEPICTED IN GUIDANCE FOR THURSDAY AND
FRIDAY. MODELS INDICATE A RATHER WEAK SURFACE TROUGH REFLECTION
WITH THE WAVE TOMORROW...WITH A LITTLE MORE OF A WEAK COLD FRONTAL
TROUGH INDICATED FRIDAY. GFS IS MORE BULLISH WITH DEVELOPING
CONVECTION ALONG THE FRONT FRIDAY AFTERNOON...THOUGH AM A LITTLE
SUSPICIOUS GIVEN OVERALL DRYNESS OF SOUNDINGS AND WEAK LOW-LEVEL
CONVERGENCE IN WESTERLY WINDS ALONG SOUTHWEST-NORTHEAST ORIENTED
BOUNDARY. SURFACE RIDGE TO OUR SOUTH PREVENTS ANY SIGNIFICANT
MOISTURE RETURN...WITH CURRENT UPSTREAM DEW POINTS OVER THE PLAINS
IN THE 30S AND 40S ACROSS NEBRASKA. THE WRF...WHICH OFTEN OVERDOES
LOW LEVEL MOISTURE/CAPES IS DRY WITH NO CAPE AND FOR NOW AM
MAINTAINING A DRY FORECAST. IT DOES LOOK FAIRLY BREEZY...WITH
WINDS IN THE 25 KT RANGE WITHIN THE MIXED LAYER FOR
FRIDAY...LIKELY GOOD FOR GUSTS 20+ MPH. 850 MB TEMPS RANGE FROM
+16/18 C THURSDAY TO AROUND +18 C FRIDAY...MIXING DOWN TO MID-
UPPER 80S FOR HIGHS BOTH DAYS AND A GOOD DEAL OF SUNSHINE.

RATZER

&&

.LONG TERM...
303 PM CDT

FRIDAY NIGHT THROUGH WEDNESDAY...

HOLDING ONTO THE IDEA THAT THE FRIDAY AFTERNOON ACTIVITY WILL BE
LIMITED AND HAVE CONTINUED A DRY FORECAST WITH THE COLD
FRONT/TROUGH PASSAGE FRIDAY EVENING...HOLDING SIMILAR REASONING TO
THE SHORT TERM DISCUSSION. SEVERAL PIECES OF GUIDANCE...INCLUDING
THE GFS AND GEM...ARE A BIT MORE AGGRESSIVE WITH PRECIPITATION ON
THE FRONT.

FOLLOWING THE FRONTAL PASSAGE...THE PATTERN CHANGES VERY LITTLE ON
SATURDAY WITH NORTHWEST FLOW ALOFT...SURFACE HIGH PRESSURE IN THE
PLAINS..AND LOW PRESSURE OVER HUDSON BAY. VERY SUBTLE DROP IN LOW
LEVEL THERMAL FIELDS SUGGEST A SLIGHTLY COOLER DAY THOUGH
TEMPERATURES HOLD AROUND NORMAL. ALSO A BIT HIGHER POTENTIAL FOR
ONSHORE WINDS/SOME LAKE BREEZE...MAINLY NEAR THE LAKESHORE. HIGHS IN
THE LOW TO MIDDLE 80S SEEM REASONABLE.

THE SURFACE HIGH PRESSURE AXIS SHIFTS EASTWARD SATURDAY NIGHT.
MEANWHILE...THE FLOW ALOFT AMPLIFIES WITH THE WESTERN CONUS RIDGE
BUILDING...AND THE HUDSON BAY TROUGH DIGGING DEEPER INTO THE EASTERN
CONUS. THIS PATTERN SUPPORTS OCCASIONAL WAVES PROPAGATING SW THROUGH
THE AREA AT TIMES...MOST NOTABLY LATE SATURDAY NIGHT INTO EARLY
SUNDAY...WITH MAYBE A SLIGHTLY STRONGER FRONT INTO EARLY MONDAY. EACH OF
THESE BRINGS A CHANCE OF SHOWERS AND THUNDERSTORMS. TEMPERATURES
HOLD AROUND NORMAL THROUGH THIS TIME AS WELL. AFTER THE FRONT ON
MONDAY...COOLER HIGH PRESSURE BUILDS IN FOR TUESDAY. THE
BAROCLINIC ZONE ASSOCIATED WITH THE FRONT GETS HUNG UP IN/OR MORES
LIKELY SOUTH OF OUR SOUTHERN ZONES TUESDAY BEFORE LIFTING BACK
NORTH INTO WEDNESDAY.

KMD

&&

.AVIATION...

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

* WEST-NORTHWEST WINDS GUSTING TO AROUND 25 KT ON THURSDAY FROM
  MID-DAY THROUGH THE AFTERNOON...POSSIBLY APPROACHING 30 KT AT
  TIMES DURING MID/LATE AFTERNOON.

RC

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

STABLE WEATHER PATTERN IN STORE FOR THE NEXT 24-30 HOURS...WITH
SPRAWLING HIGH PRESSURE CENTERED OVER LOWER MISSOURI VALLEY AND
UNSEASONABLY DEEP LOW PRESSURE OVER HUDSON BAY. WEST-NORTHWEST
GUSTS WILL SUBSIDE AROUND SUNSET THIS EVENING...AND THEN PRESSURE
GRADIENT AND DEEP MIXING WILL CAUSE WINDS TO PICK UP AGAIN OUT OF
THE WEST-NORTHWEST LATE THURSDAY MORNING. FORECAST SOUNDINGS
SUGGEST SUSTAINED SPEEDS IN MID TEENS/POSSIBLY UPPER TEENS AT
TIMES...AND GUSTS TO AROUND 25 KT/POSSIBLY APPROACHING 30 KT AT
TIMES DURING MID-LATE AFTERNOON. VFR CONDITIONS CAN BE EXPECTED
THROUGH THE PERIOD.

RC


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

* MEDIUM-HIGH IN SUSTAINED AND GUST SPEEDS THURSDAY AFTERNOON.

* HIGH IN REMAINING ELEMENTS.

RC

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

FRIDAY...PRIMARILY VFR BUT AN ISOLATED AFTERNOON-EVENING
SHRA/TSRA CANNOT BE COMPLETELY RULED OUT. GUSTY WEST WINDS.

SATURDAY...VFR THEN SLIGHT CHANCE FOR SHRA/TSRA SATURDAY NIGHT.
MAINLY NORTHWEST WINDS...BUT LAKE BREEZE WIND SHIFT POSSIBLE.

SUNDAY THROUGH TUESDAY...PRIMARILY VFR WITH PERIODIC CHANCE FOR
SHRA/TSRA. LAKE BREEZE WIND SHIFT POSSIBLE DURING
DAYTIME...ESPECIALLY ON TUESDAY.

RC

&&

.MARINE...
229 PM CDT

DEEP LOW PRESSURE OVER NORTHWESTERN ONTARIO THIS AFTERNOON WILL ONLY
SLOWLY DRIFT NORTHEAST ACROSS HUDSON BAY TONIGHT AND FRIDAY...BEFORE
BECOMING NEARLY STATIONARY AND SLOWLY FILLING OVER THE WEEKEND.
MEANWHILE...WEAK HIGH PRESSURE OVER THE CENTRAL PLAINS WILL EXTEND
INTO THE MIDWEST...EVENTUALLY WEAKENING AND ELONGATING THROUGH THE
OHIO VALLEY THROUGH SUNDAY. WITH THE LOW TO THE NORTH AND THE RIDGE
TO THE SOUTH...WEST-NORTHWEST SURFACE WINDS WILL PREVAIL ACROSS THE
LAKE...WITH THE GRADIENT STRONG ENOUGH FOR WINDS IN THE 25 KT RANGE
ACROSS MAINLY THE NORTHERN HALF OF THE LAKE. A WEAK COLD FRONT WILL
MOVE SOUTHEAST ACROSS THE LAKE FRIDAY...WITH THE GRADIENT WEAKENING
A BIT AT THAT TIME AND WINDS SHIFTING A BIT MORE NORTHWEST FRIDAY
NIGHT INTO SATURDAY BEFORE BACKING SOUTHWEST AGAIN BY SUNDAY.

RATZER

&&

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

$$

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