Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 271940
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
240 PM CDT MON JUL 27 2015

.SHORT TERM...
222 PM CDT

SHORT TERM...

TONIGHT THROUGH WEDNESDAY...

MAIN CONCERN IN THE SHORT TERM IS TIMING THE POTENTIAL FOR
THUNDERSTORMS FROM TUESDAY AFTERNOON THROUGH WEDNESDAY...AS WELL
AS VERY WARM AND HUMID WEATHER UNTIL COLD FRONT PASSES.

SURFACE HIGH PRESSURE ACROSS THE GREAT LAKES REGION WILL ONLY
SLOWLY DRIFT OFF TO THE EAST TUESDAY...AS DEEP LOW PRESSURE
DEVELOPS OVER THE NORTHERN PLAINS AND LIFTS NORTHEAST INTO
NORTHERN ONTARIO THROUGH WEDNESDAY IN RESPONSE TO A STRONG UPPER
TROUGH AND CLOSED LOW WHICH EJECTS FROM THE NORTHERN ROCKIES. IN
THE PROCESS...A DIFFUSE WARM FRONTAL ZONE CURRENTLY DRAPED ACROSS
THE LOWER MISSOURI VALLEY AND SOUTHERN IL/IN WILL LIFT NORTH
ACROSS THE FORECAST AREA LATE TUESDAY AND TUESDAY NIGHT...AHEAD OF
A COLD FRONT TRAILING THE CANADIAN LOW. WARM/HUMID AIR IN PLACE
ACROSS THE WARM FRONTAL ZONE WILL SPREAD INTO NORTHERN
IL/IN...INCREASING HUMIDITY AS WELL AS THE POTENTIAL FOR
THUNDERSTORMS...ESPECIALLY LATE TUESDAY NIGHT AND WEDNESDAY AHEAD
OF THE COLD FRONT.

SEVERAL CONVECTION ALLOWING MODELS DEPICT STORMS DEVELOPING
ACROSS THE CENTRAL PLAINS AND MID-MISSOURI RIVER VALLEY TO OUR
WEST TONIGHT...WITH DECAYING CONVECTION PERHAPS REACHING OUR
WESTERN CWA COUNTIES MIDDAY TUESDAY BEFORE DISSIPATING. HAVE
INDICATED SOME SLIGHT CHANCE POPS ACROSS PARTS OF NORTH CENTRAL
ILLINOIS TUESDAY AFTERNOON WITH THIS SCENARIO IN MIND. GREATER
POTENTIAL FOR THUNDERSTORMS LOOKS TO SPREAD IN AHEAD OF THE COLD
FRONT TUESDAY NIGHT AS INDICATED ABOVE. LATEST GUIDANCE RUNS ARE
SLIGHTLY FASTER WITH THE COLD FRONT THAN PREVIOUS...WITH THE FRONT
ALONG ROUGHLY A CHICAGO-ST LOUIS LINE WEDNESDAY MORNING.
THUNDERSTORM CHANCES WILL CONTINUE ACROSS THE SOUTHEAST HALF OF
THE CWA ALONG/AHEAD OF THE FRONT INTO THE AFTERNOON BEFORE THE
FRONT CLEARS THE FA.

IN ADVANCE OF THE FRONT AND THE SOUTHERN PERIPHERY OF THE UPPER
TROUGH AXIS...RIDGING CONTINUES TO STRENGTHEN ALOFT EARLY TUESDAY.
925 MB TEMPS IN THE +24/26 RANGE WOULD SUPPORT UPPER 80S TO LOWER
90S MAX TEMPS. CONSIDERING POSSIBLE HIGH CLOUD COVER FROM STORMS
TO OUR WEST...HAVE GONE MORE IN THE 87-90 RANGE ALONG THE LINES OF
A MAV/MET BLEND FOR TUESDAY HIGH TEMPS. SOUTH-SOUTHEAST SYNOPTIC
FLOW SHOULD PRODUCE A SOUTHEAST LAKE BREEZE ALONG THE IL SHORE
TUESDAY...WHERE TEMPS WILL BE HELD TO AROUND 80 DEGREES. AFTER A
WARM/HUMID NIGHT IN THE LOW-MID 70S TUESDAY NIGHT...COOLING TEMPS
FROM THE NORTHWEST BEHIND THE COLD FRONT WEDNESDAY SUPPORT HIGHS
FROM THE LOW-MID 80S NORTHWEST TO THE UPPER 80S SOUTHEAST. SURFACE
DEW POINTS IN THE MID-70S IN MOIST AXIS AHEAD OF THE FRONT
COMBINED WITH THESE WARM AIR TEMPS YIELD PEAK HEAT INDICIES AROUND
100 DEGREES. DEW POINTS AND HUMIDITY FALL OFF BEHIND THE FRONT
WEDNESDAY AFTERNOON.

RATZER

&&

.LONG TERM...
206 PM CDT

WEDNESDAY NIGHT THROUGH MONDAY...

FIRST PORTION OF THE EXTENDED PERIOD WILL FEATURE A BROAD SFC
RIDGE THAT STRETCHES FROM THE ROCKIES EAST THROUGH THE OHIO
VALLEY. THIS COUPLED WITH BROAD 500MB RIDGE COVERING MUCH OF THE
CENTRAL CONUS WILL HELP TO STEER WAVES FURTHER NORTH INTO THE
NORTHERN PLAINS/SOUTHERN CANADA. OPERATIONAL SOLUTIONS HAVE
DEMONSTRATED SOME VARIABILITY WITH RESPECT TO PLACEMENT OF THE
RIDGE AXIS.

THROUGH THE WEEKEND ENSEMBLES MAINTAIN A HIGH CONFIDENCE/MINIMAL
SPREAD PROG OF THE 500MB HEIGHTS...WITH SOME TROUGHING DEVELOPING
ACROSS THE GREAT LAKES LATE IN THE WEEKEND INTO EARLY NEXT WEEK.

OPERATIONAL SOLUTIONS BRING A FRONTAL BOUNDARY NEAR THE REGION
FRI...HOWEVER MOISTURE DOES NOT APPEAR TO BE ABUNDANCE WITH THIS
BOUNDARY AND MAY END UP BEING JUST A WIND SHIFT WITH SOME CLOUDS.
HAVE HELD ONTO A DRY FORECAST FRI/FRI NGT...THEN IT APPEARS A
SLIGHTLY BETTER CHANCE FOR SHOWERS/THUNDERSTORMS RETURNS SAT AND
COULD BE MORE DIURNALLY DRIVEN. THEN THE TROUGH BEGINS TO TAKE SHAPE
AND SAGS A BOUNDARY OVER THE REGION NEXT WEEKEND...WITH PERIODIC
CHANCES FOR SHOWERS AND THUNDERSTORMS.

TEMPERATURES THROUGHOUT THE EXTENDED PERIODS WILL REMAIN WITHIN
SEASONAL CONDITIONS...POSSIBLY NEARING THE UPR 80S SAT PRIOR TO THE
ARRIVAL OF THE FRONTAL BOUNDARY. OTHERWISE TEMPS WILL MAINLY BE IN
TEH MID 80S...WITH OVERNIGHT LOWS IN THE MID/UPR 60S.

BEACHLER

&&

.AVIATION...

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

* EAST WINDS TODAY...TURNING SOUTHEAST FOR TONIGHT AND TOMORROW.

LENNING

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

A DOME OF HIGH PRESSURE SITTING OVER THE GREAT LAKES WILL SHIFT
EAST AND ALLOW WINDS TO TURN SOUTHEAST OVERNIGHT. UPSTREAM OF THE
AREA OVER IOWA A COMPLEX OF THUNDERSTORMS HAS INCREASED IN
COVERAGE TODAY BUT IS EXPECTED TO FOLLOW THE UPPER LEVEL STEERING
FLOW AND DROP SOUTHWEST OF THE LOCAL AREA THIS EVENING AND
TONIGHT.

TOMORROW AGAIN LOOKS QUIET UNDER THE CONTINUED INFLUENCE OF THE
HIGH...BUT A DISTURBANCE MOVING IN LATE TOMORROW COULD BRING
STORMS TO THE LOCAL AREA TUESDAY NIGHT.

LENNING

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

* HIGH FOR ALL ELEMENTS.

LENNING

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

* TUESDAY NIGHT...CHANCE OF TSRA. SOUTHWEST WINDS.
* WEDNESDAY...CHANCE OF MORNING TSRA. STRONG/GUSTY WEST WINDS.
* THURSDAY...DRY/VFR. NORTHWEST WINDS.
* FRIDAY...DRY/VFR. SOUTHWEST WINDS.
* SATURDAY AND SUNDAY...CHANCE OF TSRA. WESTERLY WINDS.

&&

.MARINE...
240 PM CDT

SURFACE HIGH PRESSURE OVER THE GREAT LAKES THIS AFTERNOON WILL
SLIDE SLOWLY OFF TO THE EASTERN LAKES TUESDAY...AND ACROSS NEW
ENGLAND BY WEDNESDAY AS AN AREA OF LOW PRESSURE LIFTS NORTHEAST
INTO NORTHERN ONTARIO. A COLD FRONT TRAILING FROM THIS LOW WILL
PUSH SOUTHEAST ACROSS LAKE MICHIGAN WEDNESDAY MORNING THROUGH
EARLY AFTERNOON. LIGHT WINDS DOMINATED BY AFTERNOON LAKE BREEZE
CIRCULATIONS THIS AFTERNOON WILL BECOME MORE SOUTH-SOUTHEAST
TUESDAY...BEFORE SHIFTING TO THE WEST BEHIND THE FRONT WEDNESDAY.
A BRIEF PERIOD OF SOUTHWEST 15-20 KT IS POSSIBLE JUST AHEAD OF THE
FRONT EARLY WEDNESDAY...WITH WEST WINDS GENERALLY 15-20 KTS BEHIND
THE FRONT THROUGH WEDNESDAY NIGHT. OTHERWISE...A PROLONGED PERIOD
OF MODEST WEST-SOUTHWEST WINDS APPEARS LIKELY THROUGH THE END OF
THE WEEK AS THE LAKES SIT BEHIND LOW PRESSURE OVER HUDSON
BAY...AND WEAK HIGH PRESSURE THROUGH THE OHIO VALLEY.

RATZER

&&

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

$$

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