Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 040537
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1237 AM CDT SAT JUL 4 2015

.SHORT TERM...
200 PM CDT

THROUGH SATURDAY NIGHT...

SHARP UPPER TROUGHING IN NORTHERN ONTARIO...WITH BROAD TROUGHING
EXTENDING SOUTHWEST THROUGH NORTHERN ILLINOIS AND INTO THE ARKLATEX
REGION. A MORE WELL DEFINED COLD FRONTAL BOUNDARY WILL SINK
SOUTHEAST THROUGH THE NORTHERN GREAT LAKES TONIGHT ASSOCIATED WITH
THE TROUGH AXIS IN CANADA. MEANWHILE... SHORTWAVE ENERGY IS
TRACKING TO THE SOUTH OF THE SOUTHERN EXTENSION OF THE 500 MB
HEIGHT DEPRESSION OVER CENTRAL ILLINOIS...CLOSER TO THE LOW LEVEL
SURFACE FRONT IN SOUTHERN ILLINOIS. ADDITIONALLY...SMOKE ALOFT
FROM FIRES IN CANADA CONTINUE TO DRIFT TOWARD THE AREA UNDER NW
FLOW ALOFT. UPSTREAM OBSERVATIONS IN NORTH DAKOTA/WESTERN
MINNESOTA DEPICTING HAZE...WITH SMOKE REPORTED IN OBSERVATIONS IN
SOUTHERN MANITOBA. THIS FLOW DIRECTION WILL CONTINUE TO DRIFT THIS
SMOKE/HAZE TOWARD THE GREAT LAKES THROUGH AT LEAST EARLY SATURDAY
BEFORE THE UPPER FLOW BACKS TO A WSW DIRECTION. MORE ON THIS ON
OUR SOCIAL MEDIA OUTLETS.

BACK TO THE SENSIBLE WX FORECAST...THE BROAD UPPER TROUGH SLOWLY SHIFTS
EASTWARD THROUGH TONIGHT...TAKING THE MORE EXTENSIVE CLOUDINESS
WITH IT AS UPPER LEVEL HEIGHT RISES/NEGATIVE VORTICITY ADVECTION
SHIFTS THROUGH THE GREAT LAKES REGION. LIGHT PRECIP MAINLY SOUTH
OF OUR FAR SOUTHERN ZONES WILL TAPER THIS AFTERNOON AND EVENING AS
WELL. FARTHER NORTH...THE COLD FRONT WILL DRIVE AN AREA OF
PRECIPITATION...BUT THIS SHOULD LARGELY REMAIN CONFINED INTO
WISCONSIN AND OVER NORTHERN AND CENTRAL LAKE MICHIGAN TONIGHT AS
LOW LEVEL RH PROFILES ARE DRY.

WE WILL LIKELY SEE SOME DISSIPATING CLOUDS WITH THIS FRONT IN THE
MORNING...OTHERWISE DAYTIME HEATING LOOKS TO LEAD TO DECENT WARMING
ON THE INDEPENDENCE DAY ALONG WITH SOME AFTERNOON CUMULUS CLOUDS OUT
AHEAD OF THE WEAKENING FRONT. MOST GUIDANCE DECAYS THE FORCING WITH
THIS FRONT BEFORE EXITING SOUTHERN WISCONSIN...BUT THERE IS A SMALL
CHANCE THAT A SHALLOW POP-UP SHOWER SNEAKS INTO FAR NORTHERN
ILLINOIS GIVEN SOME WEAK INSTABILITY PRESENT AS SUGGESTED BY THE GFS
AND SPC WRF...LIKELY OVERDONE ON THE GFS GIVEN HIGH DEWPOINTS IN THE
RAW MODEL FORECAST. LOW LEVEL WIND FIELDS ARE GENERALLY 10 KT OR
LESS...SUGGESTING A SE LAKE BREEZE IS STILL A LIKELY SCENARIO IN THE
AFTERNOON...THOUGH WITH MUCH LESS OF A MAGNITUDE OF LAKE COOLING AND
INLAND PROGRESSION ANTICIPATED GIVEN THE PREDOMINANT WESTERLY FLOW.
THE UPPER FORCING IS BENIGN...SO WILL BE RELYING ON ANY SURFACE
CONVERGENCE...POSSIBLY AIDED BY THE LAKE BREEZE...TO TRIGGER ANY
SHOWERS. IF ANYTHING OCCURS...IT WOULD BE ISOLATED IN NATURE...WITH
THE MORE LIKELY SCENARIO JUST AFTERNOON CUMULUS CLOUDS...AND HAVE
CONTINUED WITH A DRY FORECAST GIVEN THE WRF-NAM/4KM NAM HAS HIGH
DEWPOINTS AND HAS NO PRECIP. WOULD NOT BE SURPRISED IF SOME LOW POPS
ARE INTRODUCED IN THE NORTH GIVEN A SIGNAL IN THE GFS/SREF/CANADIAN
AND SOME HI-RES GUIDANCE. OTHERWISE A SPLENDID FORECAST WITH
READINGS IN THE 80S INLAND...70S NEAR THE LAKE...AND MODEST HUMIDITY
LEVELS. BENIGN CONDITIONS WILL RULE THE EVENING PERIOD WITH
TEMPERATURES FALLING THROUGH THE 70S AND CLEARING SKIES.

KMD

&&

.LONG TERM...
325 PM CDT

SUNDAY THROUGH FRIDAY...

PRIMARY CONCERN IS WITH COLD FRONT PASSAGE MONDAY NIGHT AND
ASSOCIATED THUNDERSTORM POTENTIAL. ALSO FOCUSED ON WARM TEMPERATURES
FOR SUNDAY AND MONDAY.

AFTER DAYS OF NORTHWEST FLOW ALOFT...TREND OF RISING HEIGHTS ON
INDEPENDENCE DAY WILL CONTINUE INTO SUNDAY AS WE BECOME POSITIONED
UNDER SHORTWAVE RIDGING. A WEAK UPPER LOW WILL DEVELOP OVER THE TN
AND LOWER OH VALLEY...WITH ITS FORCING WELL REMOVED FROM OUR AREA.
OUTSIDE OF GFS...WHICH APPEARS TO BE AN OUTLIER WITH STRONGER
DISTURBANCE IN WEAK FLOW ALOFT KICKING OFF CONVECTION IN EASTERN
CWA ON SUNDAY...CONTINUE TO FAVOR A DRY FORECAST. BIG STORY WILL
BE MUCH WARMER TEMPERATURES IN THE UPPER 80S DUE TO 925 MB TEMPS
RISING TO THE LOWER 20S CELSIUS. EXPECTING A FEW SPOTS OF
90...INCLUDING INLAND PORTIONS OF CHICAGO HEAT ISLAND. WEAK
SOUTHERLY LOW LEVEL FLOW AND STRONG WARMING OF LAND WILL LIKELY
ENABLE LAKE BREEZE TO PUSH INLAND INTO NORTHEAST IL...CAPPING
HIGHS AROUND 80 NEAR THE IL SHORE...BUT INLAND PUSH WILL BE
LIMITED DUE TO FAIRLY DEEP MIXING OVER LAND. ITS UNCERTAIN WHETHER
LAKE BREEZE WILL PUSH INTO NW INDIANA SHORE AREAS.

SURFACE HIGH PRESSURE RIDGE WILL SLIDE EAST SUNDAY NIGHT AND
FRONTAL TROUGH TRAILING LOW PRESSURE MOVING INTO UPPER MIDWEST
WILL APPROACH...RESULTING IN SOUTHERLY WINDS INCREASING TOWARD
DAYBREAK MONDAY AND THEN BECOMING GUSTY DURING THE DAY ON
MONDAY...WITH GUST SPEEDS UP TO 30 MPH AT TIMES. SEASONABLY WARM
LOWS OF ~65-70 SUNDAY NIGHT WILL SET THE STAGE FOR TEMPS PERHAPS
EVEN WARMER THAN SUNDAY ON MONDAY AS 850/925 MB TEMPS WARM A FEW
MORE DEGREES. MODEL CONSENSUS HAS ALSO BEEN FOR SLOWER APPROACH OF
COLD FRONT TO WEST...ALSO SUPPORTIVE OF WARMER TEMPS. DID BUMP UP
MAX T TO UPPER 80S TO AROUND 90...BUT IF DAYLIGHT HOURS END UP DRY
AND SUNNIER AS ITS STARTING TO APPEAR THEY MIGHT...LOWER 90S TEMPS
WILL LIKELY BE MORE WIDESPREAD ESPECIALLY FOR EASTERN HALF OF CWA.
DEWPOINTS RISING INTO THE UPPER 60S TO LOWER 70S WILL YIELD HEAT
INDEX VALUES IN THE MID TO UPPER 90S. REGARDING CONVECTION AHEAD
OF AND ALONG THE FRONT...KEPT POPS IN SLIGHT TO CHANCE RANGE
MAINLY NW 1/3 OF CWA MONDAY AFTERNOON...WITH LIKELY TO CATEGORICAL
POPS MONDAY NIGHT.

MIDLEVEL LAPSE RATES LOOK TO BE FAIRLY MEAGER...WHICH COULD LIMIT
INSTABILITY SOME...AND THERE ARE QUESTIONS ON INTENSITY OF
CONVECTION IF FRONT SLOWS EVEN MORE GIVEN WHAT WOULD BE
UNFAVORABLE DIURNAL TIMING. DESPITE THESE ISSUES...SLIGHTLY BETTER
MIDLEVEL FLOW AND A STRONG LOW LEVEL JET COULD SUPPORT AT LEAST
SOME POTENTIAL FOR STRONG/SEVERE STORMS MONDAY EVENING AS WELL AS
HEAVY DOWNPOURS GIVEN PWAT VALUES POSSIBLY IN EXCESS OF 2 INCHES.
WINDS WILL FLIP NORTHERLY TUESDAY BEHIND COLD FRONT...BUT
SHOWER/TSRA CHANCES COULD CONTINUE TUESDAY MORNING INTO AFTERNOON
ESPECIALLY IN SOUTHEAST CWA CLOSER TO WAVE OF LOW PRESSURE ALONG
FRONTAL TROUGH. TEMPS WILL COOL DOWN INTO 70S TUESDAY...AND MAY
NOT GET OUT OF THE 60S LAKESIDE. FOR THE REST OF THE
PERIOD...GUIDANCE VARIES ON JUST HOW COOL AIRMASS WILL BE...WITH
LATEST ECMWF QUITE CHILLY UNTIL RECOVERY TO CLOSER TO NORMAL BY
THE END OF THE WORK WEEK. HAVE LOW POPS DAILY WEDS-FRIDAY...BUT
THESE MAY BE OVERDONE IF MORE AMPLIFIED GREAT LAKES TROUGHING
OCCURS AND KEEPS STORM TRACK SOUTH OF THE AREA.

RC

&&

.AVIATION...

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

* LIKELY LAKE BREEZE WIND SHIFT WITH SPEEDS POSSIBLY APPROACHING
  8-10KT.

RODRIGUEZ

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

HIGH PRESSURE SPANNING THE REGION WILL PROVIDE DRY AND VFR
CONDITIONS...WITH ONLY FEW CU/HIGH CLOUDS THROUGHOUT THE FORECAST
PERIOD. HAVE INTRODUCED A WIND SHIFT TO THE EAST LATER THIS
AFTERNOON WITH LAKE BREEZE PASSAGE. AT THIS TIME...SPEEDS SHOULD
REMAIN IN THE 6-8KT RANGE BUT THERE IS A SMALL CHANCE THAT SPEEDS
COULD BRIEFLY GET INTO THE 8-10KT RANGE WITH THE INTIAL PASSAGE.
HOWEVER...CONFIDENCE IS HIGHER IN THE LIGHTER SPEEDS.

RODRIGUEZ


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

* HIGH CONFIDENCE WITH VFR CONDITIONS.

* HIGH CONFIDENCE WITH WIND SPEED/DIRECTION AND LAKE BREEZE WIND
  SHIFT...MEDIUM CONFIDENCE THAT SPEEDS STAY BELOW 10 KT.

RODRIGUEZ

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

SUNDAY...VFR/DRY. SOUTH WINDS.

MONDAY...TSRA LIKELY MONDAY NIGHT. BREEZY SOUTH WINDS.

TUESDAY...CHANCE OF MORNING TSRA. NORTHEAST WINDS.

WEDNESDAY...VFR...CHANCE SHRA/TSRA WED NIGHT. EAST WINDS.

THURSDAY...SLIGHT CHANCE TSRA. WEST-SOUTHWEST WINDS.

RATZER

&&

.MARINE...
337 PM CDT

MODERATE SOUTHERLY WINDS OVER THE NORTHERN PORTION OF THE LAKE
AHEAD OF A COLD FRONT WILL DIMINISH AND GRADUALLY TURN NORTHERLY
WITH FRONTAL PASSAGE LATE TONIGHT. THE COLD FRONT WILL MAKE IT TO
MID LAKE ON SATURDAY AND THEN WASH OUT SATURDAY...WITH GENERALLY SOUTHERLY
WINDS FOR THE ENTIRE LAKE SATURDAY NIGHT THROUGH MONDAY. EXCEPTION
WILL BE SOME LAKE BREEZE INFLUENCE ON THE SOUTHERN PORTION OF THE
LAKE ON SUNDAY. SOUTHERLY WINDS WILL INCREASE AS HIGH PRESSURE
SLIDES EAST LATER SUNDAY AND ESPECIALLY MONDAY WITH APPROACH OF
LOW PRESSURE AND TRAILING COLD FRONT TROUGH FROM PLAINS.
CONDITIONS WILL BE STABLE GIVEN WARM AIRMASS OVER LAKE ON
MONDAY...BUT STILL LOOKING AT 20-25 KT WINDS NORTH HALF AND 15-25
KT SOUTH. COULD SEE AT LEAST OCCASIONAL GUSTS TO 30 KT UP NORTH
AND STRENGTHENING WINDS OVER LAND COULD RESULT IN SPEEDS CLOSE TO
SMALL CRAFT ADVISORY CRITERIA FOR IL AND IN NEARSHORE AREAS. COLD
FRONT WILL MOVE ACROSS LAKE LATE MONDAY NIGHT...WITH NORTHERLY
WINDS UP TO 20 KT BEHIND IT ON TUESDAY THEN LIGHTER NORTHEAST
WINDS ON WEDNESDAY AS HIGH PRESSURE BUILDS OVER THE LAKE.

RC

&&

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

$$

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