Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290612
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
112 AM CDT FRI AUG 29 2014

.DISCUSSION...
714 PM CDT

PAIR OF MCVS NOTED ON RADAR IMAGERY CONTINUE TO MOVE EASTWARD
DRIVING SLOWLY DECAYING CONVECTIVE COMPLEX EASTWARD INTO OUR CWA
EARLY THIS EVENING DESPITE THE MUCH MORE STABLE AND SOMEWHAT DRIER
AIR MASS OVER THE REGION. HAVE ALREADY UPDATED PRECIP TRENDS A
COUPLE TIME TO ACCOUNT FOR THE RAIN THIS EVENING. ANTICIPATE
CONTINUED WEAKENING TREND...PARTICULARLY FROM THE NORTH...IN THIS
AREA OF LEFT OVER RAIN AND EVEN SOME LINGERING THUNDER. THE THREAT
OF THUNDER WITH THE NORTHERN MCV TRACKING CLOSE TO I-88 LOOKS TO END
VERY SOON...WHILE THE CONVECTION WITH THE SOUTHERN MOST MCV LIES
CLOSE ENOUGH TO MORE UNSTABLE AIR TO POTENTIALLY MAINTAIN
THUNDERSTORMS ACROSS THE SOUTHERN TIER OF OUR CWA THE NEXT FEW
HOURS. IN THE WAKE OF THESE MCVS...WHICH REALLY HAVENT BEEN HANDLED
WELL BY NUMERICAL GUIDANCE...SHOULD SEE A PERIOD OF DRY CONDITIONS
HOWEVER MODELS DO BREAK OUT ADDITIONAL CONVECTION OVERNIGHT WHICH
ISN`T THAT UNREASONABLE GIVEN THE NORTHWARD ADVANCING WARM FRONT.

IZZI

//PREV DISCUSSION...
230 PM CDT

VISIBLE IMAGERY THIS AFTERNOON SHOWS A MID/UPR LEVEL CLOUD DECK
LIFTING NORTHEAST ACROSS NORTHERN IL. REMNANTS FROM A MORNING
COMPLEX OF THUNDERSTORMS RESIDES OVER IOWA...WITH SOME CONVECTIVE
LOOK TO THE CLOUDS LINGERING INTO THIS AFTN. SFC TEMPS ACROSS THE
FORECAST AREA HAVE BEEN ABLE TO WARM INTO THE MID/UPR 70S. A FEW
POINTS SOUTH OF PONTIAC TO KANKAKEE HAVE PUSHED TO ARND 80 DEG...BUT
WITH THE LIGHT EASTERLY FLOW AT THE SFC SOME WEAK SUBSIDENCE HAS
BEEN NOTED WITH DEW POINTS BEING PUSHED INTO THE LOW 50S ALONG THE
LAKESHORE AND MID/UPR 50S INLAND.

THE MAIN CHALLENGE FOR THIS EVENING WILL FOCUS ON CONVECTIVE
CHANCES/COVERAGE...WHICH WILL CONTINUE THROUGH DAYBREAK FRI.
GUIDANCE HAS CONTINUED TO BACK OFF ON THE ARRIVAL OF PRECIP INTO THE
FORECAST AREA...WHICH SEEMS TO LINE UP RELATIVELY WELL WITH THE
CURRENT TRENDS. THE FRONTAL BOUNDARY WILL SLOWLY LIFT NORTH THIS
EVENING...BUT NOT EXPECTING MUCH IN THE WAY OF PRECIP UNTIL PERHAPS
AFT 04Z WITH A GRADUAL UPGLIDE OF PARCELS ALLOWS FOR MOISTENING.
SOME ELEVATED CONVECTION COULD BEGIN TO DEVELOP...ALTHOUGH NOT
EXPECTING MUCH COVERAGE OVERNIGHT. THE FOCUS FOR SLIGHTLY BETTER
PRECIP COVERAGE WILL REMAIN ALONG THE NORTHERN ROW OF COUNTIES IN
NORTHERN IL...HOWEVER CLOSER TO DAYBREAK FRI IT APPEARS SOME
PRECIP/CONVECTION COULD SAG SOUTHEAST AND CLIP NORTHEAST
IL/NORTHWEST IN BEFORE DISSIPATING.

WITH INCREASING CLOUD COVER TONIGHT...IT SHUD LIMIT HOW COOL TEMPS
BECOME. SO HAVE MADE ONLY MINOR ADJUSTMENTS TO THE TEMPS WITH
GENERALLY MID/UPR 60S FOR LOWS.

BEACHLER

FRIDAY AND FRIDAY NIGHT...
CLOSED UPPER LOW CURRENTLY NEAR THE COLORADO FRONT RANGE WILL SLOWLY
TRAVERSE EAST TOWARDS THE MID-MISSOURI VALLEY FRIDAY BECOMING AN
OPEN WAVE. AT THE SURFACE...BROAD AND WEAK SURFACE LOW WILL LIFT
INTO THE UPPER MIDWEST WHILE SOUTHWEST FLOW RAMPS UP THROUGH THE
COLUMN AS THE WARM FRONT LIFTS INTO THE CENTRAL GREAT LAKES. HEAT
AND HUMIDITY WILL MAKE A RETURN AS THIS OCCURS. H85 TEMPS WARMING TO
AROUND 18C WILL HELP SUPPORT MID/UPPER 80S DURING THE DAY
FRIDAY...AND SURFACE DEWPOINTS ARE PROGGED TO PUSH BACK INTO THE LOW
POSSIBLY MID 70S. THIS WILL RESULT IN MODERATE INSTABILITY
DEVELOPING DURING THE DAY...AND ONLY WEAKLY CAPPED DURING THE
AFTERNOON AS BROAD HEIGHT FALLS ASSOCIATED WITH THE UPPER WAVE START
TO OVERSPREAD THE AREA. THERE DOESNT SEEM TO BE MUCH FOCUS IN THE
LOWER LEVELS SO WOULD EXPECT ONLY SCATTERED THUNDERSTORM DEVELOPMENT
ACROSS THE REGION FRIDAY AFTERNOON/EVENING. UPPER WAVE BEGINS TO
MOVE INTO THE UPPER MIDWEST TOMORROW NIGHT AND BROAD WARM ADVECTION
OUT AHEAD DURING THE EVENING AND OVERNIGHT HOURS WILL HELP SUSTAIN
SOME CONVECTION LATE BUT WOULD EXPECT SOME NOCTURNAL DOWNTICK IN
ACTIVITY OVERNIGHT WITH THE LOW LEVEL JET FOCUSED WELL TO OUR NORTH.
GFS/NAM SEEM TO BE OVERDOING POPS MUCH OF THE DAY AND NIGHT
FRIDAY...AND OPTED TO LEAN CLOSER TO ECMWF THESE PERIODS.

THIS WEEKEND...
COLD FRONT WILL SWEEP ACROSS THE FORECAST AREA STARTING VERY EARLY
ON SATURDAY. WHILE NOT A FAVORABLE DIURNAL TIME...WE SHOULD STILL
HAVE SOME VERY MODEST INSTABILITY IN PLACE THAT WILL SUPPORT SOME
EARLY DAY THUNDERSTORMS ALONG THE FRONTAL BOUNDARY...WHICH WOULD BE
ABLE TO EXPAND IN COVERAGE AS THE DAY PROGRESSES AND THE BOUNDARY
LAYER WARMS. IN ADDITION...GUIDANCE SUGGESTS A SECOND SHORTWAVE
WILL WRAP INTO THE AREA THAT WILL PROVIDE ADDITIONAL BROAD SCALE
ASCENT. THE ECMWF IS ABOUT SIX HOURS SLOWER WITH THIS FEATURE
COMPARED TO THE NAM/GFS...WITH THE GEM A COMPROMISE...SO THERE
REMAINS SOME UNCERTAINTY IN TIMING. HIGH PRESSURE WILL BUILD IN
BEHIND THE FRONT SATURDAY NIGHT INTO SUNDAY AND WILL PROVIDE A
BRIEF RESPITE FROM THE ACTIVE WEATHER. THE ATMOSPHERE LOOKS TO
QUICKLY RELOAD HOWEVER WITH THERMAL RIDGE ONCE AGAIN STARTING TO
TILT BACK OVER THE AREA WITH H85 TEMPS AROUND 20C BY LATER IN THE
DAY SUNDAY.

NEXT WEEK...
AN ACTIVE PATTERN LOOKS TO PERSIST THROUGH AT LEAST THE MIDDLE OF
NEXT WEEK WITH A SERIES OF WAVES PUSHING ACROSS THE REGION BRINGING
CONTINUED PERIODIC CHANCES FOR SHOWERS AND THUNDERSTORMS. ANOTHER
COLD FRONT WILL PUSH ACROSS THE AREA LATER ON MONDAY WITH THE
FRONTAL BOUNDARY STALLING JUST TO OUR SOUTH. THEREAFTER...THERE IS
CONSIDERABLE UNCERTAINTY ON HOW FAR NORTH THE BOUNDARY MAY LIFT
WITH THE BEST PRECIP CHANCES FOCUSED IN THE VICINITY OF THE
FRONT AS A SERIES OF UPPER LEVEL DISTURBANCES PUSH ACROSS THE
AREA. PWATS REMAIN JUICY ALONG THE FRONT...AROUND TWO INCHES...AND
GUIDANCE SUGGESTS THAT THERE COULD BE SEVERAL CONVECTIVE COMPLEXES
NEXT WEEK WITH A CONCERN FOR HEAVY RAIN AND FLOODING SHOULD THIS
PAN OUT.

DEUBELBEISS

&&

.AVIATION...

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

* ISOLATED SHRA/TSRA LATE TONIGHT INTO EARLY FRIDAY MORNING.

* SCATTERED TSRA FRIDAY AFTERNOON/EARLY EVENING.

* SOUTH WIND WITH GUSTS DEVELOPING TO 20KT LATE MORNING FRIDAY.

KREIN

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

FORECAST CONCERNS WILL FOCUS ON SHRA/TSRA POTENTIAL THROUGH THE
PERIOD AS WELL AS GUSTY SOUTH WINDS DEVELOPING LATE FRIDAY MORNING
AND PERSISTING INTO EARLY FRIDAY EVENING.

FOR THE REMAINDER OF THE OVERNIGHT HOURS...THE REGION WILL REMAIN
UNDER SELY FLOW...NORTH OF A WARM FRONT CURRENTLY OVER CNTRL MO TO
SRN IL. SOME REMNANT SHOWERS FROM EARLIER CONVECTIVE ACTIVITY ARE
STILL SEEN ON THE LATEST RADAR IMAGERY...BUT THIS ACTIVITY IS
DIMINISHING WITH TIME FOR MUCH OF THE REGION. THE EXCEPTION IS A
SMALL AREA OF RAIN WHICH IS MORE PERSISTENT OVER ECNTRL IL. WITH
MID LEVEL FLOW MORE SOUTHWESTERLY...THERE HAS BEEN A MORE
PERSISTENT FETCH OF LIGHT PCPN INTO NWRN IL...SO KGYY COULD LIKELY
SEE SOME LIGHT RAIN THROUGH THE NIGHT THOUGH NO ADDITION THUNDER
IS ANTICIPATED WITH THIS PCPN. WITH A VERY MOIST AIRMASS IN PLACE
FROM RECENT PCPN ALONG WITH LIGHT WINDS...THERE WILL BE THE
POTENTIAL FOR SOME PATCHY BR WITH KDPA/KRFD BEING THE MORE LIKELY
TAF SITES TO SEE VIS DROP AT LEAST INTO MVFR LEVELS. EXPECT THAT
ORD/MDW SHOULD REMAIN VFR AS WINDS WILL REMAIN A LITTLE STRONGER
AND TEMP/DEWPOINT SPREADS A LITTLE HIGHER.

THE WARM FRONT WILL SLOWLY LIFT NWD THROUGH THE NIGHT AND PUSH
NORTH OF THE TERMINALS BY LATE FRIDAY MORNING. AT THIS
POINT...WINDS WILL SHIFT TO MORE SOUTHERLY. SOME BREAKS IN THE
CLOUD COVER SHOULD ALLOW FOR DEEPER LAYER MIXING...TAPPING
STRONGER WINDS ALOFT AND EXPECT THAT WINDS WILL BEGIN TO GUST TO
20KT FOLLOWING THE WARM FROPA.

INSTABILITY WILL INCREASES VERY QUICKLY AS THE WARM FRONT PASSES
TO THE NORTH AND WARM SECTOR AIR OVERSPREADS THE REGION BRINGING
INCREASING TEMPERATURES AND DEWPOINTS. MID LEVEL IMPULSES SHOULD
TRACK ACROSS THE AREA FRIDAY AFTERNOON AND EVENING IN UPPER LEVEL
SWLY FLOW AS THE UPPER RIDGE PUSHES TO THE EAST AND A LARGER SCALE
UPPER TROUGH MOVES ACROSS THE CENTRAL PLAINS. THESE IMPULSES WILL
BE THE TRIGGERING MECHANISM FOR ADDITIONAL SHRA/TSRA...BUT IN THE
ABSENCE OF ANY SFC FORCING...ACTIVITY WILL GENERALLY BE SCATTERED
AND DIFFICULT TO TIME AT THE TERMINALS. SO...WILL MAINTAIN THE
GOING PROB30 GROUP FOR TSRA WITH THE MOST FAVORABLE TIMING DURG
THE AFTERNOON HOURS. HOWEVER...WITH SCATTERED COVERAGE...ANY OF
THE TERMINALS COULD SEE SOME SHRA/TSRA A LITTLE EARLIER OR A
LITTLE LATER THAN THE GOING PROB30 TIMING.

KREIN

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

* LOW CONFIDENCE IN THUNDER CHANCES/COVERAGE AND SPECIFIC TIMING
  FRIDAY AFTERNOON.

* HIGH CONFIDENCE IN WIND TRENDS THROUGH THE PERIOD.

* MEDIUM CONFIDENCE THAT ORD/MDW WILL REMAIN VFR THROUGH THE
  REMAINDER OF THE NIGHT AND INTO EARLY FRIDAY MORNING.

KREIN

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

* SATURDAY...TSRA LIKELY.
* SUNDAY...SLIGHT CHANCE OF TSRA.
* MONDAY AND TUESDAY...CHANCE OF TSRA.
* WEDNESDAY AND THURSDAY...SLIGHT CHANCE OF TSRA

KMD

&&

.MARINE...
123 PM CDT

SURFACE HIGH PRESSURE OVER THE GREAT LAKES REGION THIS AFTERNOON
WILL DRIFT TO THE EAST TONIGHT IN THE WAKE OF A DEPARTING UPPER
TROUGH OVER NEW ENGLAND/QUEBEC...AND AHEAD OF ANOTHER SURFACE TROUGH
AND WEAK COLD FRONT STRETCHING FROM FAR NORTHWEST ONTARIO TO THE
CENTRAL PLAINS. WINDS ON LAKE MICHIGAN WILL SHIFT TO THE SOUTHEAST
AND EVENTUALLY SOUTH HEADING INTO FRIDAY...AS AN AREA OF SURFACE LOW
PRESSURE DEVELOPS ACROSS THE PLAINS AND LIFTS ACROSS THE UPPER
MISSISSIPPI RIVER VALLEY. TIGHTENING OF THE PRESSURE GRADIENT
SOUTHEAST OF THE LOW TRACK WILL RESULT IN S-SE WINDS INCREASING INTO
THE 15-25 KT RANGE TONIGHT AND FRIDAY...GRADUALLY WEAKENING SATURDAY
AS THE LOW MOVES OFF TO THE NORTHEAST AND A WEAK COLD FRONT TRAILS
ACROSS THE LAKE FOLLOWED BY WEAK HIGH PRESSURE.

THIS COLD FRONT APPEARS TO DISSIPATE OR STALL ACROSS FAR SOUTHERN
LAKE MICHIGAN OR NORTHERN IL/IND LATE SATURDAY NIGHT OR EARLY SUNDAY
BEFORE DEVELOPING BACK TO THE NORTH AS A WARM FRONT SUNDAY NIGHT AS
ANOTHER AREA OF LOW PRESSURE DEVELOPS OVER THE PLAINS AND LIFTS
NORTHEAST INTO THE WESTERN/NORTHERN LAKES LATE SUNDAY NIGHT INTO
MONDAY. AGAIN...A TIGHTENING OF THE PRESSURE GRADIENT ALONG THE
SOUTHEAST PERIPHERY OF THE LOW TRACK IS EXPECTED WITH WINDS AGAIN
BECOMING SOUTHERLY INTO THE 15-25 KT RANGE ON MONDAY. ANOTHER
TRAILING COLD FRONT WILL PASS ACROSS THE LAKE MONDAY NIGHT...WITH
WEAK HIGH PRESSURE AND LIGHTER WINDS DEVELOPING TUESDAY.

RATZER

&&

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

$$

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