Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 260030
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
730 PM CDT MON AUG 25 2014

.DISCUSSION...
710 PM CDT

THE FORECAST UPDATED TO REFLECT CURRENT STORM TRENDS AS WELL AS
TO ADD AREAS OF FOG TONIGHT. DO ENVISION CANCELLING THE SEVERE
THUNDERSTORM WATCH FOR THE REMAINDER OF THE CWA BY 8 PM.

AN AREA OF LOOSELY ORGANIZED STORMS CONTINUES TO PROPAGATE SLOWLY
SOUTHEAST WITH OCCASIONAL FINGERS OF SLIGHTLY BETTER WIND
SIGNATURES ON RADAR. VERY EFFICIENT RAINFALL WILL REMAIN THE
PRIMARY HAZARD WITH THESE AS THEY CROSS THROUGH SOUTHERN IROQUOIS
AND BENTON COUNTIES. 00Z RAOB AND VAD WIND PROFILE FROM ILX
INDICATE SOUTHWEST FLOW AT 15-20 KT BETWEEN 3000-6000 FT...ENOUGH
TO HOLD UP THE OUTFLOW BOUNDARY NEAR THE I-74 CORRIDOR FROM NEAR
PEORIA TO BLOOMINGTON NORMAL BASED ON RADAR TRENDS. THIS FLOW DOES
NOT LOOK STRONG ENOUGH TO DEVELOP MUCH CONVECTION NORTH OF
THAT...SO OVERALL EXPECTING A QUIET EVENING FOR MUCH OF THE
FORECAST AREA.

WATER VAPOR AND LATE DAY VISIBLE SATELLITE INDICATE A SHEARED
SHORT WAVE MOVING EAST-NORTHEAST NEAR DES MOINES. WITH MINIMAL
CONVERGENCE IN THE 800-900MB LAYER /BASICALLY AT THE BASE OF THE
ELEVATED INSTABILITY/...IT IS DIFFICULT TO SAY WHAT THIS FEATURE
WILL DO IN TERMS OF SHOWERS OR CONVECTION LATE TONIGHT. HIGH
RESOLUTION GUIDANCE DOES VARY GREATLY...HOWEVER USING A SHORT
RANGE ENSEMBLE APPROACH INDICATES OVERLAP ACROSS NORTHWEST INTO
NORTH CENTRAL IL LATE. FOR NOW DO CONTINUE THE CHANCE OF THUNDER
OVERNIGHT...WITH SOMEWHAT HIGHER POPS IN THAT AREA.

BECAUSE OF THE RAIN-COOLED MOIST AIR GOING INTO DARK...AREAS OF
FOG SEEM MORE PROBABLE BY AFTER MIDNIGHT...SOME OF WHICH MAY BE
DENSE GIVEN SUCH LOW T/TD SPREADS AT PRESENT.

MTF

//PREV DISCUSSION...
325 PM CDT

SHORT TERM...THROUGH EARLY TUESDAY MORNING...

FOR THE REMAINDER OF THE AFTERNOON...MAIN FOCUS WILL CONTINUE TO BE
TWO LINES OF THUNDERSTORMS...ONE MOVING THROUGH NWRN IL AND ANOTHER
MOVING THROUGH NWRN IN AND ECNTRL IL.  THESE LINES ARE PRODUCING
STRONG TO OCNLY SVR TS...WITH THE PRIMARY THREAT BEING WIND GUSTS
TO 60 MPH AND TORRENTIAL DOWNPOURS.  THE TREND WITH TIME SHOULD BE
FOR THE  LINE OVER NWRN IL TO BEGIN TO WEAKEN AS IT MOVES INTO AN
AIRMASS OVER NERN IL WHICH HAS BEEN WORKED OVER BY EARLIER
CONVECTION.  THE LINE ACROSS NWRN IN INTO ECNTRL IL SHOULD MAINTAIN
SOME STRENGTH AS IT CONTINUES TO PUSH SEWD.  LATEST RADAR IMAGERY
SHOWS A DISTINCT OUTFLOW BOUNDARY EXTENDING WEST FROM KANKAKEE TO
PERU WHICH COULD YET BE THE FOCUS FOR SOME ADDITIONAL THUNDERSTORM
DEVELOPMENT INTO THE EARLY EVENING HOURS.  THUNDERSTORMS HAVE HAD A
HISTORY OF PRODUCING IN EXCESS OF 1 INCH IN 30 MINUTES...BUT SINCE
THEY HAVE BEEN PROGRESSIVE...SIGNIFICANT WIDESPREAD FLOODING IS NOT
ANTICIPATED...THOUGH LOCALLY HEAVY RAINFALL COULD PRODUCE URBAN ANS
SMALL STREAM TYPE FLOODING.  EXPECT THAT THE POTENTIAL FOR ORGANIZED
THUNDERSTORMS MAY CONTINUE INTO EARLY THIS EVENING AS WELL...WITH
SOME POTENTIAL FOR A FORWARD-PROPAGATING MCS ACROSS MAINLY THE
SOUTHERN HALF OF THE FORECAST AREA. THIS WOULD BE MAINLY CAPABLE
OF WIND. AS THE THUNDERSTORM ACTIVITY DIMINISHES LATER THIS
EVENING...PATCHY FOG AND WIDESPREAD MISTY/MURKY CONDITIONS WILL
SET UP. WHILE WIDESPREAD DENSE FOG IS NOT LIKELY DUE TO PERSISTENT
DEBRIS CLOUD FROM EARLIER CONVECTION...A VERY MOIST ENVIRONMENT
WILL REMAIN IN PLACE...AND COMBINED WITH LIGHT WINDS CONDITIONS
WILL REMAIN MUGGY OVERNIGHT.

THE TEMPERATURE FORECAST FOR TODAY WAS DISRUPTED BY THE THUNDERSTORMS
AND EXTENSIVE CLOUD COVER OVER AREAS IMPACTED BY THE THUNDERSTORMS.
HOWEVER...LOCATIONS SOUTH OF A LINE FROM MIDWAY AIRPORT TO PERU
STILL SAW MAX HEAT INDICES IN EXCESS OF 100F WHILE LOCATIONS SOUTH
OF THE ILLINOIS/KANKAKEE RIVERS SAW HEAT INDICES APPROACH OR SLIGHTLY
EXCEED 110F.  TEMPERATURES AND DEWPOINTS SHOULD REMAIN IN THE LOWER
TO MIDDLE 70S OVERNIGHT AS THERE WILL BE NO SGFNT CHANGE IN
AIRMASS.  WITH OLD OUTFLOW BOUNDARIES HANGING AROUND THE AREA
OVERNIGHT...THERE WILL STILL BE SOME LOW CHANCES FOR ADDITIONAL
SHOWERS OR THUNDERSTORMS AS A WEAK COLD FRONT MOVES DOWN THE LAKE
AND INTO THE WARM...HUMID ENVIRONMENT.

KREIN

LONG TERM...TUESDAY THROUGH MONDAY...

TUESDAY AND WEDNESDAY...
THE MAIN CHANGE TO THE FORECAST DURING THIS PERIOD WAS TO SHIFT
THE HIGHER CHANCES FOR STORMS SOUTH AS THE FRONTAL BOUNDARY
ITSELF LOOKS TO BE FURTHER SOUTH THAN EARLIER FORECAST...LIKELY
HELPED BY EPISODES OF CONVECTION SUCH AS THIS AFTERNOON. BECAUSE
OF THIS...CHANCES OF STORMS IN THIS PERIOD LOOK TO BE GREATER IN
THE SOUTHERN FORECAST AREA THAN THE NORTH. LAKE ADJACENT AREAS
ALSO LOOK TO BE INFLUENCED BY LAKE AIR ON TUESDAY...POSSIBLY EVEN
BY MORNING. BECAUSE OF THAT HAVE ADJUSTED TEMPERATURES DOWNWARD ON
TUESDAY...WITH THE POSSIBILITY OF 90 NOW BEING HARBORED ONLY TO THE
FAR SOUTH...ALTHOUGH LIKE TODAY IF CONVECTION OCCURS THAT CAN
GREATLY DISRUPT THINGS. LOWER 80S LOOK MORE LIKELY NORTH WITH
UPPER 70S NEAR THE LAKE SHORE.

THERE COULD BE ONGOING ELEVATED STORM ACTIVITY FROM IA INTO
NORTHWESTERN/WESTERN IL AT DAYBREAK TUESDAY MOVING EASTWARD...THOUGH
THE DEGREE OF COVERAGE IS UNCERTAIN. MUCH LIKE TODAY...IF THERE IS
ENOUGH ACTIVITY IT COULD JUST CONTINUE TO FESTER EAST-SOUTHEAST INTO
THE ILLINOIS FORECAST AREA AND THEN FEED ON THE INSTABILITY IN THE
FAR SOUTHEAST PART OF THE AREA. EVEN IF THAT WERE NOT TO PAN
OUT...SCATTERED DEVELOPMENT OF STORMS LOOKS FAVORED IN THE AFTERNOON
ALONG THE BOUNDARY WHICH SHOULD BE DRAPED FROM WEST-TO-EAST NEAR
INTERSTATE 80. ANY THUNDERSTORM ACTIVITY WILL BE SIMILAR TO MUCH IN
THE PAST WEEK...POSSESSING HEAVY RAIN POTENTIAL AND ISOLATED GUSTY
WIND CONCERNS.

CHANCES FOR STORMS CONTINUE WEDNESDAY...MAINLY SOUTH...BEFORE 850MB
FLOW TURNS BRIEFLY MORE NORTHWESTERLY WEDNESDAY NIGHT.

THURSDAY INTO THE WEEKEND...
THE BOUNDARY REMAINS FORECAST TO RETURN NORTHWARD AS A WARM
FRONT...POSSIBLY ON THURSDAY...IN RESPONSE TO AN UPPER LOW MOVING
EAST FROM THE CENTRAL ROCKIES. A SURFACE LOW AHEAD OF THIS IS
FORECAST TO LIFT NORTHEAST...BUT OVERALL THIS LOOKS TO BE SLOW GIVEN
THE UPPER LEVEL FEATURE IS CUTOFF. TAPPING BACK INTO THE MOIST AIR
MASS THAT WE ARE PRESENTLY IN...IT WOULD APPEAR SHOWER AND STORM
CHANCES WOULD SPREAD BACK NORTHWARD ACROSS THE AREA AND LAST THROUGH
MUCH OF THIS TIME...ALTHOUGH MOST FAVORED AS WE GET INTO THE
STRONGER LOW LEVEL JET/MOISTURE TRANSPORT...WHICH IS DIFFICULT TO
TIME AT THIS POINT. THE TEMPERATURE FORECAST IS LOW
CONFIDENCE...WITH HIGHS SYNOPTICALLY FAVORED IN THE 80S...ALTHOUGH
CONVECTIVE AND/OR LAKE COOLING POSSIBLE EVERY DAY.

MTF

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* LINGERING OUTFLOW BOUNDARIES PROVIDING VARIABLE WIND DIRECTIONS
  IN THE NEAR TERM...BUT WITH SPEEDS EXPECTED TO REMAIN UNDER 10KT
  DURING THIS TIME.

* SOUTHERLY WINDS BECOMING WESTERLY OVERNIGHT AND THEN NORTH
  NORTHEAST TUESDAY...WITH FROPA.

* POSSIBLE MVFR CEILINGS/VIS LATE TONIGHT/EARLY TUESDAY MORNING.

* POSSIBLE SHRA/TSRA LATE TONIGHT/EARLY TUESDAY MORNING.

RODRIGUEZ

//DISCUSSION...UPDATED 00Z...

SHOWERS AND THUNDERSTORMS CONTINUE TO SLOWLY DRIFT SOUTH OF THE
TERMINALS EARLY THIS EVENING BUT IN THE WAKE...SEVERAL OUTFLOW
BOUNDARIES REMAIN. THIS IS PROVIDING VARIABLE WIND DIRECTIONS AND
WILL CONTINUE TO DO SO OVER THE NEXT COUPLE OF HOURS...BUT WITH
SPEEDS LIKELY REMAINING UNDER 10 KT. A TREND BACK TOWARDS A LIGHT
SOUTHERLY WIND WILL OCCUR BEFORE FRONT APPROACHES AND WINDS SHIFT
TOWARDS THE WEST LATER TONIGHT. VEERING WILL CONTINUE WITH FROPA
EARLY TUESDAY MORNING AS WINDS TURN NORTH AND THEN NORTHEAST LATER
IN THE MORNING...REMAINING THIS DIRECTION THROUGH THE REST OF THE
FORECAST PERIOD. FOG AND STRATUS OUT AHEAD OF THIS FRONT COULD
DEVELOP ONCE AGAIN TONIGHT...WITH SHOWERS AND THUNDERSTORMS
DEVELOPING ALONG AND AHEAD OF THIS FRONT. DEVELOPMENT INITIALLY
APPEARS TO BE BEST MORE TOWARDS RFD...BUT DO EXPECT ANY PRECIP TO
SHIFT EAST SOUTHEAST AS THIS FRONT DROPS SOUTH THROUGH MID MORNING
TUESDAY AND THEN PUSHING SOUTH OF THE TERMINALS INTO MID DAY.

RODRIGUEZ


//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* LOW MEDIUM CONFIDENCE WITH WIND DIRECTION AND HIGH CONFIDENCE
  WITH SPEEDS STAYING UNDER 10KT IN THE NEAR TERM.

* MEDIUM CONFIDENCE WITH WIND SPEED/DIRECTION WITH FROPA.

* LOW MEDIUM CONFIDENCE WITH MVFR TRENDS LATE TONIGHT/EARLY
  TUESDAY MORNING.

* MEDIUM CONFIDENCE WITH SHRA/TSRA TRENDS LATE TONIGHT/EARLY
  TUESDAY MORNING.

RODRIGUEZ

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

WEDNESDAY...SLIGHT CHANCE OF TSRA.

THURSDAY...CHANCE OF TSRA.

FRIDAY...CHANCE OF TSRA.

SATURDAY...CHANCE OF TSRA.

SUNDAY...CHANCE OF TSRA.

RODRIGUEZ

&&

.MARINE...
315 PM CDT

SCATTERED STORMS THIS AFTERNOON HAVE DISRUPTED THE WINDS SOME
ACROSS THE FAR SOUTHERN PART OF LAKE MICHIGAN.
OTHERWISE...SOUTHERLY FLOW IS ANTICIPATED MUCH OF TONIGHT ACROSS
THE LAKE. DEW POINTS ARE NOT AS HIGH AS THEY HAVE BEEN BUT STILL
NEAR OR AT THE WATER TEMPERATURE...THUS POTENTIALLY LEADING TO
SOME PATCHY FOG. A COLD FRONT WILL MOVE ACROSS THE LAKE LATE
TONIGHT INTO TUESDAY MORNING...TURNING WINDS NORTHERLY. EVENTUALLY
THIS BOUNDARY WILL SHIFT BACK NORTH ACROSS THE LAKE MOST LIKELY ON
THURSDAY OR THURSDAY NIGHT. SOUTHERLY WINDS WILL THEN BE SEEN OVER
THE LAKE BEFORE LOW PRESSURE PASSES NEAR THE NORTHERN PART OF THE
LAKE SOMETIME EARLY THIS WEEKEND.

MTF

&&

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

$$

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