Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290843
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
343 AM CDT FRI MAY 29 2015


.SHORT TERM...
339 AM CDT

THROUGH SATURDAY...

A WARM FRONT EXTENDING FROM NE IOWA INTO THE SOUTHERN SUBURBS OF
CHICAGO AND INTO INDIANA CONTINUES TO MAKE NORTHWARD PROGRESS DURING
THESE EARLY MORNING HOURS. THE RELATIVELY COMFORTABLE MID 50S
DEWPOINTS IN NORTHERN ILLINOIS WILL SOON BE REPLACED BY MUGGY 60S
DEWPOINTS.  ALOFT...A CLOSED MID LEVEL CIRCULATION IS NOTED IN WATER
VAPOR IMAGERY OVER WYOMING...WITH MULTIPLE SHORTWAVE TROUGHS/VORT
MAXES EMBEDDED IN BROAD SOUTHWESTERLY FLOW ACROSS THE PLAINS AND
EXTENDING INTO THE LOCAL AREA. SOME SHOWER ACTIVITY IS ONGOING
ACROSS EASTERN IOWA AND EXTENDING BACK INTO MISSOURI IN
CONDITIONALLY UNSTABLE AND MOIST (PWS OF 1.5 OR SO). SOME
ISOLATED SHOWERS/EARLIER T-STORMS NEAR KGYY ARE OCCURRING OVER OUR
AREA ASSOCIATED WITH A FAST MOVING UPPER JET SEGMENT...ALONG WITH
AN INCREASE IN LOW LEVEL FLOW PER OVERNIGHT AMDAR SOUNDINGS AND
LOCAL VWP ANALYSIS. A FEW HUNDRED JOULES OF ELEVATED INSTABILITY
DO EXIST ACROSS THE AREA THIS MORNING.

THE CHALLENGE TODAY WILL BE IN TIMING AND COVERAGE OF SHOWERS AND
EMBEDDED THUNDERSTORMS GIVEN THE PASSING OF SUBTLE SHORTWAVES IN
THE SW FLOW ALOFT. THE ENTIRE AREA...WITH THE EXCEPTION OF OVER
THE LAKE...WILL BE FIRMLY ENTRENCHES IN THE WARM SECTOR BY
DAYBREAK. WE WILL BE UNCAPPED MUCH OF THE DAY...BUT INSTABILITY IS
EXPECTED TO BE TEMPERED. WEAK UPGLIDE ON 305K ISENTROPIC SURFACES
ARE NOTED OVER NE ILLINOIS...SLIGHTLY BETTER OFF TO OUR WEST.
THEREFORE EXPECT COVERAGE WILL REMAIN LIMITED OVER THE AREA
INITIALLY. THIS LIFT IS EXPECTED TO INCREASE OVER THE REMAINDER OF
THE AREA THROUGH THE MORNING/EARLY AFTERNOON AS THESE SHORTWAVES
PROGRESS THROUGH THE AREA. ATTEMPTING TO LATCH ONTO THESE WAVES IN
IOWA/MISSOURI AND AT LEAST MODERATE CONFIDENCE FROM DECENT
AGREEMENT IN HI-RESOLUTION GUIDANCE IN PROGGING SHOWERS TO
INCREASE LATE MORNING TO AROUND MIDDAY. RECENT GUIDANCE IS BACKING
OFF A BIT ON THIS SOLUTION...BUT THE PATTERN STILL SUPPORTS
SCATTERED ACTIVITY LATER IN THE MORNING. THERE COULD BE SOME
ACTIVITY NOT LONG AFTER DAYBREAK...BUT IT SHOULD BE ISOLATED AT
BEST. THIS IS INITIAL MORNING/EARLY AFTERNOON ACTIVITY DOES NOT
POSE A SIGNIFICANT RISK FOR SEVERE WEATHER...THOUGH LOCALLY HEAVY
DOWNPOURS WOULD BE A CONCERN IN THE INCREASING HIGH PW AIRMASS.

CONCERN THEN INCREASES LATER IN THE DAY/EVENING AS A COLD FRONT
SLIDES SOUTHEAST THAT A MARGINAL SEVERE WEATHER THREAT EXISTS.
ADDITIONAL SCATTERED THUNDERSTORMS ARE EXPECTED AHEAD OF THIS
FRONT. WE WILL STILL BE UNSTABLE...THOUGH TO WHAT DEGREE WILL
DEPEND ON ANY EARLIER CONVECTION. THE FRONT WILL ALSO INTACT WITH
AN INCREASING MID LEVEL SPEED MAX AS A POSITIVELY TILTED UPPER
TROUGH APPROACHES. WITH THIS INCREASED WIND SHEAR AND MODEST
INSTABILITY... LOCALLY HEAVY RAINFALL AND SOME STRONGER WIND GUSTS
LOOK TO BE THE MAIN HAZARDS. MARGINALLY SEVERE HAIL IS ALSO
POSSIBLE IN DEEPER UPDRAFTS WITH MODEST 500-300 MB LAPSE RATES.

BROAD WESTERN UPPER TROUGH WILL BE CROSSING THE AREA TONIGHT ALONG
WITH THE SURFACE COLD FRONT WHICH IS BEING DRIVEN BY A STRONG UPPER
WAVE IN SOUTHERN CANADA. SOME DIFFERENCES EXISTS WITH HOW QUICKLY
THIS WAVE MOVES THROUGH...SPECIFICALLY WHEN COMPARING THE WRF-NAM TO
THE GLOBAL MODELS...AND ANY IMPACT OF SURFACE WAVES POTENTIALLY
SLOWING THE FRONTAL PROGRESSION OVER LAND.  IMPRESSIVE FGEN FIELD
WITH THE FRONT PER THE NAM MAY LEAD TO SOME FOCUSED MODERATE TO
HEAVY RAINFALL. THE GLOBAL MODELS PUSH THE CORE OF THE PRECIP
THROUGH TONIGHT INTO SATURDAY MORNING. THE IMPACTS OF THE TIMING
OF THIS FRONT AND ITS SOUTHWARD PROGRESS WILL DIRECTLY CORRELATE
TO WHETHER ANY ADDITIONAL THUNDERSTORMS WILL DEVELOP OVER THE
SOUTHEASTERN PORTIONS OF THE CWA SATURDAY AFTERNOON...AND THE THE
TEMPERATURE PROGRESSION AWAY FROM LAKE AREAS DURING THE DAY.

LONG TERM DISCUSSION FORTHCOMING...

KMD

&&

.LONG TERM...
314 PM CDT

FRIDAY THROUGH THURSDAY...

MAIN FORECAST CONCERNS/CHALLENGES DURING THE PERIOD ARE WITH
SHOWER/THUNDERSTORM CHANCES FRIDAY INTO SATURDAY...AND THEN WITH
MUCH COOLER AIR TO PUSH ACROSS THE AREA SATURDAY.

SOME UNCERTAINTY AS TO HOW THE PERIOD WILL BEGIN...DUE TO HOW
PRECIP DEVELOPMENT WILL EVOLVE LATE TONIGHT INTO FRIDAY MORNING.
THE CURRENT UPSTREAM MID LEVEL TROUGH OVER THE CENTRAL PLAINS WILL
CONTINUE MOVING NORTHEAST THROUGH THE CWA FRIDAY MORNING...WITH
LINGERING PRECIP FROM THE OVERNIGHT STILL LIKELY TO AFFECT
PRIMARILY THE WESTERN HALF OF THE CWA IN NORTH CENTRAL ILLINOIS.
LIKELY SCENARIO WILL BE FOR THIS DEVELOPMENT TO CONTINUE EAST
THROUGH THE MORNING. CONFIDENCE WITH OVERALL COVERAGE DURING
FRIDAY MORNING IS LOW...AND HAVE THUS CAPPED POPS AT LIKELYS.
MORNING PRECIP AND EVEN CLOUD COVER SHOULD EXIT THROUGH LATE
MORNING AND MIDDAY...WITH SKIES BECOMING MORE PARTLY CLOUDY/PARTLY
SUNNY. THIS WILL HELP WITH EARLY DESTABILIZATION WITH LITTLE TO NO
INHIBITION AND WITH MID LEVEL ENERGY RETURNING DURING THIS
TIME...THINK DEVELOPMENT WILL ONCE AGAIN BE POSSIBLE IN THE
AFTERNOON. WITH A LACK OF GOOD FOCUS AT THE SURFACE...THINK BEST
DEVELOPMENT WILL STILL BE ACROSS NORTH CENTRAL ILLINOIS WHERE
BETTER LARGE SCALE ASCENT WILL RESIDE. WEAK MID/UPPER LEVEL FLOW
WILL BE A LIMITING FACTOR WITH HOW ORGANIZED ANY STORM WILL BE AND
ALTHOUGH I DONT THINK THERE WILL BE ANY WIDESPREAD SEVERE
THREAT...AN ISOLATED STRONG TO SEVERE STORM WILL STILL BE POSSIBLE
WITH STRONG WINDS AND HEAVY RAINFALL THE MAIN HAZARDS DUE TO
INCREASING INSTABILITY AND MOISTURE. 80 DEGREE WEATHER WILL BE
LIKELY FRIDAY...BUT WITH TEMPS LIKELY BEING LIMITED IN ANY AREA
THAT OBSERVES MORE PRECIP/CLOUD COVER.

STILL THINK THAT THE BEST CHANCES FOR WIDESPREAD
SHOWER/THUNDERSTORM DEVELOPMENT WILL BE FRIDAY NIGHT INTO SATURDAY
MORNING. HOWEVER...GUIDANCE IS VARYING WITH REGARDS TO HOW THE
NIGHT WILL EVOLVE. THIS IS LIKELY DUE TO LARGER TROUGH OVER
SOUTHERN CANADA AND NORTHERN PLAINS MOVING EAST/SOUTHEAST...WHILE
A SEPARATE TROUGH AND SEVERAL IMPULSES WORKS EAST THROUGH THE
PLAINS. NONETHELESS...BEST LOW/MID LEVEL COVERAGE WILL PUSH
THROUGH THE CWA DURING THIS TIME WHILE LARGE SCALE FORCING ALSO
INCREASES. SO DO THINK PRECIP WILL PUSH THROUGH THE CWA...BUT WITH
COVERAGE AND DURATION POSSIBLY LOWER IN SOME LOCATIONS. THUNDER
SHOULD BE DECREASING IN COVERAGE THROUGHOUT THE NIGHT AND
ESPECIALLY INTO SATURDAY MORNING AS THE INSTABILITY AXIS SHIFTS
SOUTH...WITH HEAVY RAINFALL THE MAIN HAZARD DURING THIS TIME. ANY
LINGERING PRECIP WILL CONTINUE SOUTH OF THE CWA THROUGH MIDDAY
SATURDAY AS SURFACE TROUGH AND FRONT PUSH THROUGH THE REGION.
CLOUD COVER WILL LIKELY LINGER THROUGH MUCH OF SATURDAY WHILE
WINDS/GUSTS INCREASE THROUGHOUT THE DAY. HIGH TEMPS WILL LIKELY
OCCUR EARLY SATURDAY MORNING WITH TEMPS FALLING THROUGH THE
DAY...WITH 40 DEGREE WEATHER LIKELY FOR MOST LOCATIONS IN FAR
NORTHEAST ILLINOIS AND NORTHWEST INDIANA FOR MUCH OF THE DAY.

RODRIGUEZ

&&

.AVIATION...

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

* SCATTERED TSRA LATE MORNING THRU MID AFTERNOON.
* SCATTERED SHRA/TSRA LATE TONIGHT/SATURDAY MORNING.
* GUSTY SOUTHERLY WINDS MID MORNING THRU LATE AFTERNOON. CMS

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

A WARM FRONT WILL MOVE NORTH ACROSS THE AREA LATER THIS MORNING.
EAST/SOUTHEAST WINDS UNDER 10KT EARLY THIS MORNING WILL GRADUALLY
TURN MORE SOUTHERLY BY MID MORNING AND BEGIN TO INCREASE INTO THE
10-12KT RANGE WITH HIGHER GUSTS POSSIBLE.

WHILE AN ISOLATED SHOWER IS POSSIBLE TOWARD DAYBREAK...THE BEST
CHANCES APPEAR TO BE WEST OF THE CHICAGO AREA TERMINALS AND
INCLUDED VICINITY MENTION AT RFD...BUT OVERALL CONFIDENCE IS
FAIRLY LOW FOR ANY PRECIP THRU MID MORNING. GUIDANCE APPEARS MORE
FOCUSED ON A WAVE LIFTING NORTHEAST ACROSS THE AREA LATE THIS
MORNING THRU MID AFTERNOON AND THIS APPEARS THE BEST CHANCE FOR
SCATTERED THUNDERSTORMS TO AFFECT THE AREA THOUGH CONFIDENCE FOR
COVERAGE/DURATION IS STILL SOMEWHAT LOW. ASSUMING THIS ACTIVITY
DOES MATERIALIZE...WOULD EXPECT A BIT OF LULL FROM LATE AFTERNOON
INTO THIS EVENING AS THE WAVE DEPARTS.

A COLD FRONT WILL MOVE ACROSS THE AREA AFTER MIDNIGHT AND MORE
LIKELY CLOSER TO DAYBREAK SATURDAY. ADDITIONAL SHOWERS AND
THUNDERSTORMS WILL LIKELY OCCUR BUT CONFIDENCE ON TIMING IS STILL
LOW...AS IT APPEARS MUCH OF THE PRECIP COULD BE POST FRONTAL INTO
SATURDAY MORNING. ADDED VICINITY MENTION IN THE 30 HOUR ORD TAF
FOR NOW.

WINDS/GUSTS COULD INCREASE SOME MID/LATE AFTERNOON ONCE PRECIP
ENDS AND CLOUDS SCATTER BUT WINDS SHOULD QUICKLY DIMINISH WITH
SUNSET. CMS

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

* MEDIUM FOR TSRA LATE MORNING THRU MID AFTERNOON.
* MEDIUM FOR SHRA/TSRA LATE TONIGHT/SATURDAY MORNING.
* MEDIUM FOR WINDS THRU THE PERIOD. CMS

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

SATURDAY....CHANCE SHRA EARLY. MVFR POSSIBLE. STRONG NORTH-
NORTHEAST WINDS.

SUNDAY...VFR/WX NIL. NORTHEAST WINDS.

MONDAY...VFR/WX NIL. EAST WINDS.

TUESDAY...VFR/WX NIL. SOUTHEAST WINDS.

WEDNESDAY...SLIGHT CHANCE TSRA. SOUTH WINDS.

KJB

&&

.MARINE...

335 AM...A STRONG COLD FRONT WILL MOVE ACROSS NORTHERN LAKE
MICHIGAN EARLY SATURDAY MORNING...THEN PUSH SOUTH OF THE LAKE BY
EARLY SATURDAY AFTERNOON. STRONG HIGH PRESSURE WILL BUILD ACROSS
THE NORTHERN PLAINS AND UPPER MIDWEST SATURDAY INTO SATURDAY
NIGHT...WHICH WILL STRENGTHEN THE GRADIENT BEHIND THE FRONT. WINDS
WILL SHIFT NORTHERLY AND INCREASE TO 30 KT WITH SOME GALE GUSTS
POSSIBLE. COULD BE A PERIOD OF LOW END GALES OVER THE SOUTHERN
PORTION OF THE LAKE FROM LATE SATURDAY MORNING THRU MID AFTERNOON
BUT CONFIDENCE TOO LOW TO INCLUDE PREVAILING GALES. WINDS WILL
TURN MORE NORTHEASTERLY SATURDAY NIGHT INTO SUNDAY...BUT THE
GRADIENT WILL BE SLOW TO RELAX ACROSS THE SOUTHERN PORTION OF THE
LAKE...SO A PROLONGED PERIOD OF 15-25KT IS POSSIBLE INTO SUNDAY
NIGHT. THE HIGH WILL SHIFT EAST INTO QUEBEC MONDAY...LEAVING A
WEAKER RIDGE OF HIGH PRESSURE ACROSS THE WESTERN LAKES INTO
TUESDAY. CMS

&&

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

$$

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