Area Forecast Discussion
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FXUS63 KLOT 191825
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
125 PM CDT TUE AUG 19 2014

.DISCUSSION...
1114 AM CDT

MAIN CONCERNS/CHALLENGES FOR MORNING UPDATE WERE MONITORING
SHOWER/THUNDERSTORM CHANCES THIS MORNING THROUGH THE AFTERNOON.

LATEST RADAR IMAGERY SHOWING THUNDERSTORM COMPLEX WHICH HAS MOVED
THROUGH NORTHERN MISSOURI AND WEST CENTRAL ILLINOIS THIS
MORNING...CONTINUING EAST AND CURRENTLY IMPACTING AREAS FROM LA
SALLE TO PONTIAC. EXPECT THIS COMPLEX TO CONTINUE EAST THROUGH THE
EARLY AFTERNOON...WITH THUNDERSTORMS LIKELY TO BUILD FURTHER TO
THE NORTHEAST TOWARDS THE CHICAGO METRO AREA OVER THE ONE TO TWO
HOURS...ALONG APPROACHING SURFACE TROUGH AND FRONT TO THE
NORTHWEST. FEEL FAIRLY CONFIDENT WITH THIS POSSIBILITY AS THIS
MID LEVEL WAVE AND SURFACE TROUGH/FRONT WILL MOVE THROUGH AN
ENVIRONMENT WITH LITTLE TO NO CIN AND MODERATE INSTABILITY.
CONFIDENCE IS LOWER WITH REGARD TO INTENSITY OF THESE STORMS AS
LARGE CLOUD SHIELD HAS LIMITED STEEPNESS OF LOW LEVEL LAPSE
RATES...AND WITH WESTERLIES/BULK SHEAR VALUES REMAINING ON THE
LOWER SIDE. COULD SEE THE STRONGEST DEVELOPMENT OCCURRING OVER
EAST CENTRAL ILLINOIS AND NORTHWEST INDIANA AS THIS BROKEN LINE OF
THUNDERSTORMS PUSHES EAST/SOUTHEAST THROUGH MID AFTERNOON...WITH
AN ISOLATED SEVERE THREAT STILL POSSIBLE. HIGH WIND GUSTS WOULD
BE THE MAIN THREAT...AS WELL AS HEAVY RAINFALL. SURFACE DEWPOINTS
RIGHT AROUND 70 DEG WITH AXIS OF HIGHEST PWATS RIGHT OVER THE
SOUTHEAST TWO THIRDS OF THE CWA SHOULD ALLOW FOR EFFICIENT RAIN
PRODUCERS TODAY...AND HAVE MADE MENTION OF THIS POTENTIAL FOR
LOCALLY HEAVY RAINFALL IN THE HWO.

RODRIGUEZ

//PREV DISCUSSION...
336 AM CDT

SHORT TERM...THROUGH TUESDAY NIGHT...

FOR THE REMAINDER OF THE OVERNIGHT HOURS...THE MAIN FOCUS WILL BE
THE PROGRESSION OF THUNDERSTORMS ALONG A WEAK COLD FRONT MOVING
THROUGH SRN WI/NERN IA AND PUSHING INTO NRN IL.  THE MAIN SHORTWAVE
ASSOCIATED WITH THE ACTIVITY IS MOVING QUICKLY OVER ERN WI AND CNTRL
LAKE MICHIGAN.  AS THE SHORTWAVE RACES RATHER QUICKLY TO THE
EAST...THE LINE OF THUNDERSTORMS IS TAKING ON A MORE EAST-WEST
ORIENTATION ALONG THE WISCONSIN BORDER...BUT IS SAGGING INTO
WINNEBAGO AND BOONE COUNTIES.  SOME STRONG THUNDERSTORMS ALONG THIS
PORTION OF THE FRONT HAVE THE POTENTIAL TO PRODUCE SOME SMALL HAIL
AND LOCALIZED STRONGER WIND GUSTS.  THERE IS STILL SOME CONCERN
ABOUT HOW FAR SOUTH THE ACTIVITY WILL REACH.  CURRENTLY ANTICIPATE
THAT THE GREATEST THUNDERSTORM COVERAGE SHOULD BE OVER THE NORTHERN
TIER COUNTIES ALONG THE WISCONSIN BORDER.  HOWEVER...THERE IS STILL
SOME SWLY FLOW ALOFT WITH WITH 850MB WINDS ARND 30KT...WHILE SFC
FLOW IS LIGHT AND GENERALLY SOUTHERLY.  THERE IS PLENTY OF LOW
LEVEL MOISTURE IN PLACE AND WITH AT LEAST MODEST ISENTROPIC LIFT IN
THE WARM SECTOR WHICH COULD AT LEAST GENERATE SCT SHRA/TSRA.  THE
LATEST HIGH RES GUIDANCE IS INDICATING A DIMINISHING TREND TO THE
CONVECTION ALONG THE SRN PORTION OF THE FRONT AS IT MOVES THROUGH
THE AREA THIS MORNING...SO HAVE BACKED OFF POPS TO SLIGHT
CHANCE...BUT WITH DAYTIME WARMING...THERE SHOULD BE AN UPTICK IN THE
CONVECTIVE ACTIVITY AD THE FRONT PUSHES THROUGH THE SRN PORTIONS OF
THE CHICAGO METRO AREA AND INTO ECNTRL IL AND NWRN INDIANA. PORTIONS
OF THE CWA.  WITH INCREASING SLWY DEVELOPING WITH THE APPROACH OF
THE COLD FRONT AND VEERING TO WLY FOLLOWING THE FROPA...NO LAKE
BREEZE DEVELOPMENT IS EXPECTED TODAY AND THE WARM AIR SHOULD HAVE NO
TROUBLE REACHING RIGHT UP TO THE LAKEFRONT...WITH TEMPS IN THE LOWER
TO MIDDLE 80S EXPECTED FOR THE ENTIRE CWA.

ANY RESIDUAL PCPN ASSOCIATED WITH THE PASSING COLD FRONT SHOULD PUSH
TO THE EAST OF THE CWA BY LATE THIS EVENING.  WEAK HIGH PRESSURE
SHOULD BUILD IN BEHIND THE FRONT WEDNESDAY EVENING AND OVERNIGHT.
WITH GENERALLY WLY FLOW BEHIND THE FRONT...AND NO SGFNT DRY AIR
UPSTREAM...THE LOW LEVELS SHOULD REMAIN VERY MOIST...SO FOG
DEVELOPMENT LATE TUESDAY NIGHT AND POSSIBLY INTO WEDNESDAY MORNING
IS A DISTINCT POSSIBILITY.

KREIN

WEDNESDAY THROUGH THURSDAY...
MAIN FEATURE IN THE MEDIUM TERM WILL BE THE DEPARTING MID-LVL VORT
FROM THE GREAT LAKES...STEADILY WEAKENING AND EJECTING EAST.
MEANWHILE A DEVELOPING 500MB RIDGE WILL TAKE SHAPE ACROSS THE
PLAINS...THEN LIFT NORTHEAST THUR. THE STALLED FRONTAL BOUNDARY WILL
REMAIN SOUTH OF THE FORECAST AREA THRU WED THEN AS THE RIDGE
STEADILY BUILDS AND EXPANDS NORTH WARM AIR ADVECTION WILL LIFT THE
BOUNDARY NORTH. THIS WILL LIKELY FEATURE MOISTENING LLVLS WITH
DECENT ISENTROPIC LIFT ALONG THE BOUNDARY...TO SUPPORT INCREASING
POPS LATE WED FROM SOUTH TO NORTH. WHILE IT APPEARS FAVORABLE FOR A
CHANNEL OF LIKELY POPS LATE WED-EARLY THUR...THERE IS SOME CONCERN
THAT WE COULD QUICKLY BECOME CAPPED WITH A STRONG PUSH OF VERY WARM
AIR ALOFT. SOME GUIDANCE IS SUGGESTING AFT 00Z 850MB THERMAL TEMPS
RISE TO ARND 19-20 DEG C...WHICH COULD BE ENOUGH TO LIMIT
COVERAGE/INTENSITY. HOWEVER IF THIS ENDS UP BEING SLIGHTLY COOLER
ANY PRECIP THAT DOES DEVELOP COULD EASILY PRODUCE LOCALLY HEAVY
RAINFALL...AS PWAT VALUES ARE EXPECTED TO RISE TO NEAR 1.7-2.0"
OVERNIGHT. TEMPS WED WILL PUSH INTO THE MID 80S...HOWEVER IT IS
PLAUSIBLE THAT A WEAK LK BREEZE COULD FORM EARLY IN THE AFTN AND
KEEP THE NEARSHORE AREAS COOLER.

WITH WARM AIR STEADILY ADVECTING NORTH INTO THE FORECAST
AREA...EXPECT TEMPS TO REMAIN MILD OVERNIGHT IN THE MID/UPR 60S. IF
CLOUDS AND PRECIP ARRIVE EARLIER WED EVE...THEN TEMPS MAY NOT DIP
BELOW 70.

THUR WILL FEATURE STEADILY RISING MID-LVL HEIGHTS AS THE ROBUST
RIDGE ESTABLISHES ITSELF ACROSS A BULK OF OF THE REGION. THIS POSES
CHALLENGES WITH PRECIP FOR THUR. GUIDANCE CONTINUES TO FAVOR A
SHORTWAVE LIFTING NORTHEAST INTO THE UPPER MIDWEST WITH BETTER
MID-LVL VORTICITY THEN SLIDING EAST ALONG THE NORTHERN PERIPHERY OF
THE RIDGE...BEFORE DIVING SOUTHEAST TOWARDS THE EASTERN GREAT LAKES.
THIS COUPLED WITH VERY WARM 850MB TEMPS NEARING 20 DEG C MAY RESULT
IN A CAPPED ENVIRONMENT. CONFIDENCE IS NOT YET STRONG ENOUGH TO
FURTHER REDUCE POPS THUR AFTN. DEPENDING ON CLOUDS/PRECIP THUR WILL
STEER HOW WARM SFC TEMPS BECOME. GUIDANCE CONTINUES TO POINT TOWARDS
MID/UPR 80S. DEW PTS WILL BE PUSHING INTO THE LOW 70S...WHICH THE
COMBINATION OF TEMP/DEW PT COULD BE SUPPORTIVE OF HEAT INDEX VALUES
NEARING THE MID 90S.

CONCERN REMAINS FOR THUR NGT ON POPS...BUT MORE IMPORTANTLY ON
POTENTIAL FOR HEAVY RAINFALL. PWAT VALUES CONTINUE TO HOVER ARND 2"
WITH VERY WARM DEW PTS BETWEEN 72-74 DEG. WITH THE LACK OF A STRONG
WIND FIELD...STORMS COULD BE SLOW MOVING AND EASILY PRODUCE LOCALLY
HEAVY RAINFALL. OVERNIGHT LOWS MAY NOT DIP BELOW THE MID/UPR
70S...WHICH SETS THE STAGE FOR A POTENTIALLY HOT/HUMID DAY FRI.

FRIDAY THROUGH EARLY NEXT WEEK...
MID-LVL RIDGING CONTINUES TO BE PROGGED FOR THE FRI-SUN
PERIODS...WHICH SETS THE STAGE FOR POTENTIALLY A VERY WARM STRETCH
ACROSS THE REGION WITH NUMEROUS AREAS REACHING 90 DEG IN THIS
TIMEFRAME. GUIDANCE CONTINUES TO FAVOR A HIGH PROBABILITY THAT DEW
PTS WILL EASILY REMAIN IN THE 72-74 DEG RANGE FRI/SAT...WHICH
COUPLED WITH TEMPS ARND 90 COULD EASILY PRODUCE 100 DEG HEAT INDEX
READINGS. THE CURRENT ECMWF SOLUTION PAINTS AN EVEN WARMER THERMAL
RIDGE AT 850MB FOR SUN AT 23-25 DEG C. CLOUDS/PRECIP WILL LIKELY
STEER HOW WARM SUN BECOMES.

THEN ENSEMBLES CONTINUE TO DEMONSTRATE SOME CONSISTENCY THAT THE
MID-LVL RIDGE WILL WEAKEN EARLY NEXT WEEK WITH A RETURN TO TROUGHING
ACROSS THE MIDDLE OF THE CONUS. CURRENT GUIDANCE SUGGESTS A FRONTAL
BOUNDARY WILL ARRIVE IN THE MON/TUE TIMEFRAME AND BRING TEMPS BACK
TO SEASONAL OR SLIGHTLY SUB-SEASONAL CONDS FOR THE MIDDLE OF NEXT
WEEK.

BEACHLER

&&

.AVIATION...

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

* ISOL SHRA/TSRA POSSIBLE THROUGH MID AFTERNOON.

* WESTERLY WINDS GUSTING TO AROUND 20 KT THROUGH EARLY EVENING.

* POTENTIAL FOR MVFR VIS IN BR LATE TONIGHT INTO WED MORNING.

RODRIGUEZ

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

LATEST RADAR IMAGERY SHOWING AREA OF THUNDERSTORMS EXITING TO THE
EAST OF THE TERMINALS...BUT WITH GYY THE ONLY TERMINAL BEING
IMPACTED AT THIS TIME. THIS WILL CONTINUE EAST OVER THE NEXT HOUR
WITH GENERALLY DRY CONDITIONS THEN EXPECTED OVER THE NEXT SEVERAL
HOURS. ISOL TO WIDELY SCATTERED SHOWERS ARE STILL DEVELOPING OVER
FAR NORTHERN ILLINOIS AND SOUTHERN WISCONSIN...OUT AHEAD OF FRONT
STRETCHED FROM SOUTHERN WISCONSIN TO EASTERN IOWA. THIS WILL
LIKELY CONTINUE INTO THE AFTERNOON AS THIS FRONT DROPS TO THE
SOUTHEAST...BUT EXPECT IT TO REMAIN LIMITED IN COVERAGE AS BETTER
LIFT ALOFT IS LOST. ONLY HAVE VCSH MENTIONED DUE TO ANTICIPATED
COVERAGE...BUT WILL NEED TO CONTINUE MONITORING THE POTENTIAL FOR
A STRAY SHOWER AND EVEN AN OUTSIDE POSSIBILITY FOR A BRIEF/ISOL
STORM TO MOVE OVER THE TERMINALS. CEILINGS STILL HOVERING AROUND
MVFR/VFR...BUT WITH VFR CEILINGS EXPECTED THIS AFTERNOON THROUGH
THE REST OF THE FORECAST PERIOD. FAIRLY MOIST CONDITIONS LINGERING
TONIGHT COULD ALLOW FOR FOG DEVELOPMENT...AND HAVE CONTINUED
MENTION OF THIS IN THE TAF.

RODRIGUEZ


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

* MEDIUM HIGH CONFIDENCE WITH ISOL SHRA/TSRA DEVELOPING AROUND THE
  TERMINALS THIS AFTERNOON...LOW CONFIDENCE WITH THIS IMPACTING
  THE TERMINALS.

* HIGH CONFIDENCE WITH WIND SPEED AND DIRECTION.

* MEDIUM CONFIDENCE WITH MVFR VIS OCCURRING LATE TONIGHT INTO
  WEDNESDAY MORNING.

RODRIGUEZ

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

WEDNESDAY NIGHT THROUGH SUNDAY...PERIODIC CHANCES FOR TSRA AND
OCNL REDUCED CIGS/VIS.

SUNDAY NIGHT THROUGH MONDAY...VFR. WX NIL.

&&

.MARINE...
316 AM CDT

AN AREA OF LOW PRESSURE OVER CENTRAL WISC WILL CONTINUE TO DRIFT
NORTHEAST ACROSS THE NORTHERN PORTION OF THE LAKE MIDDAY...THEN
CONTINUE TO SLIDE EAST AND WEAKEN WEDNESDAY AFTERNOON ACROSS
SOUTHEAST ONTARIO. THE GRADIENT IS EXPECTED TO TIGHTEN SLIGHTLY
FROM THE SOUTH...WITH WINDS INCREASING THIS MORNING TO ARND 15 TO
25KT. THEN AS THE LOW SLIDES EAST WINDS WILL BECOME WEST AND
DIMINISH TO ARND 10 TO 20 KT LATE TONIGHT. WAVES SHUD BE ABLE TO
BRIEFLY BUILD ACROSS THE LAKE TO ARND 2-4FT...THEN WITH A LIGHTER
GRADIENT ARRIVING WED WAVES WILL SUBSIDE TO ARND 2 FT OR LESS. IN
THE DEPARTING SFC LOW WILL BE A MUCH WEAKER GRADIENT...WITH WINDS
GENERALLY LIGHT FROM THE NORTH ARND 5 TO 10 KT ACROSS THE NORTHERN
HALF OF THE LAKE AND AROUND 10 KT FROM THE SOUTH FOR THE SOUTHERN
HALF OF THE LAKE. A WARM FRONTAL BOUNDARY WILL LIFT NORTH ACROSS
THE SOUTHERN PORTION OF THE LAKE EARLY THUR...WITH MUCH WARMER AIR
LIFTING OVER THE LAKE. AT THIS TIME FEEL THAT FOG MAY NOT
DEVELOP...BUT COULD SEE SOME HAZY CONDITIONS DEVELOP LATE IN THE
WEEK. SOUTH/SOUTHEAST WINDS FOR THE WEEKEND. THE UPPER FLOW
PATTERN DOES LOOK TO SHIFT SUNDAY OR MONDAY BRINGING A COLD FRONT
ACROSS THE LAKE WITH A SHIFT TO NORTHERLY WINDS WHICH MAY BE
RATHER BREEZY EARLY NEXT WEEK.

BEACHLER

&&

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

$$

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