Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260746
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
246 AM CDT TUE AUG 26 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 06Z...

* SHRA/TSRA PREDAWN THRU MID MORNING.
* POSSIBLE MVFR CIGS THIS MORNING.
* WIND SHIFT TO NORTH MID MORNING...THEN NORTHEAST TO 10KT EARLY
  AFTERNOON. CMS

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

ISOLATED CONVECTION FIRING OVER NORTH CENTRAL IOWA IS EXPECTED TO
CONTINUE AND SPREAD EAST THROUGH THE OVERNIGHT HOURS. ADDITIONAL
CONVECTION COULD DEVELOP RIGHT OVER THE TERMINALS IN THE 09Z-11Z
TIME FRAME. THUS NO CHANGES TO CURRENT SHRA/VCTS MENTION OR TIMING
THOUGH TWEAKS MAY BE NEEDED AS TRENDS EMERGE INCLUDING THE
POTENTIAL FOR MORE THUNDER MENTION. THIS ACTIVITY IS THEN EXPECTED
TO MOVE SOUTHEAST OF THE AREA BY MID/LATE MORNING WITH ISOLATED
ACTIVITY POSSIBLE THIS AFTERNOON. ADDITIONAL SHRA/TSRA WILL BE
POSSIBLE LATE TUESDAY NIGHT INTO WEDNESDAY MORNING BUT IF IT WERE
TO OCCUR...PROBABLY SIMILAR TIMING TO THIS MORNING 09Z OR LATER
AND WILL TAKE A CLOSER LOOK FOR POSSIBLE MENTION WITH THE 12Z TAFS
LATER THIS MORNING.

MVFR CIGS/VIS OR BRIEF IFR WILL BE POSSIBLE WITH ANY SHRA/TSRA.
OTHERWISE...A LOW VFR DECK 3-5KFT CONTINUES TO EXPAND ACROSS THE
AREA. THIS MAY LOWER INTO MVFR BY MORNING BUT CONFIDENCE TO
INCLUDE ANY PREVAILING MVFR CIGS IS LOW.

A COLD FRONT WILL CONTINUE TO MOVE SOUTH ACROSS THE AREA THIS
MORNING. LIGHT SOUTHERLY WINDS WILL TURN WESTERLY BY DAYBREAK AND
THEN SHIFT TO THE NORTH AS THE FRONT PUSHES THROUGH. WIND
DIRECTION AFTER FROPA IS A BIT UNCERTAIN AND MAY SHIFT BACK TO
THE NORTHWEST BEFORE A LAKE BREEZE MOVES ACROSS NORTHEAST IL
SHIFTING WINDS TO THE NORTHEAST. SPEEDS OUTSIDE OF ANY
CONVECTIVE/OUTFLOW ACTIVITY SHOULD REMAIN UNDER 10KTS. HOWEVER
SPEEDS WITH THE FRONT COULD BE IN THE 10-15KT RANGE WITH
PREVAILING 10KTS POSSIBLE WITH AND BEHIND THE LAKE BREEZE. CMS

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

* MEDIUM FOR SHRA/TSRA THIS MORNING.
* MEDIUM FOR WINDS THRU THE PERIOD.
* LOW FOR MVFR CIGS THIS MORNING. CMS

//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...

246 AM...LOW PRESSURE NEAR JAMES BAY WILL CONTINUE EAST ACROSS
NORTHERN QUEBEC TODAY AS A TRAILING COLD FRONT MOVES SOUTH ACROSS
LAKE MICHIGAN THIS MORNING. WEST/SOUTHWEST WINDS WILL SHIFT TO THE
NORTH BUT SPEEDS SHOULD REMAIN IN THE 10-15KT RANGE. HIGH PRESSURE
OVER THE DAKOTAS WILL MOVE EAST ACROSS THE LAKES REGION WEDNESDAY
INTO WEDNESDAY NIGHT. AS IT DOES...THE GRADIENT WILL TIGHTEN SOME
ON THE FAR SOUTH END OF THE LAKE...ALLOWING NORTH TO NORTHEAST
WINDS TO INCREASE INTO THE 10-20KT RANGE. THIS HIGH WILL MOVE EAST
THURSDAY WITH A WARM FRONT LIFTING BACK NORTH ACROSS THE LAKE
THURSDAY NIGHT...SHIFTING WINDS BACK SOUTHERLY. CMS

&&

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

$$

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