Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211815
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
115 PM CDT THU AUG 21 2014

.DISCUSSION...
1155 AM CDT

THE AFTERNOON FORECAST HAS BEEN UPDATED TO REFLECT COOLER
TEMPERATURES AND INCREASE POPS FOR THE NORTHERN FORECAST AREA. IT
REMAINS CHALLENGING TO GET VERY SPECIFIC WITH REGARD TO TIMING
THUNDERSTORM CHANCES.

EARLY THIS AFTERNOON...THE SYNOPTIC WARM FRONT EXTENDS FROM
NORTHERN IA THROUGH FAR NORTHERN IL AND TO THE MI/IN BORDER. THIS
FEATURE IS SOMEWHAT BAGGY OVER OUR AREA DUE TO OUTFLOW BOUNDARIES
AND CLOUDS FROM EARLIER ACTIVITY KEEPING THE MAIN BAROCLINIC ZONE
SOUTH OF THE WIND SHIFT AND MORE SO IN THE SOUTHERN FORECAST AREA.

PROFILERS INDICATE 15 TO 25 KT WESTERLY FLOW IN THE 850-925MB
LAYER WITH WEAK TO MODEST ISENTROPIC ASCENT ALONG THIS BOUNDARY.
AREAS OF MORE FOCUSED ASCENT ARE BASICALLY IN TANDEM WITH
MULTIPLE SHORT WAVES...WITH THE ONES OF INTEREST OVER SOUTHERN WI
AND WESTERN/CENTRAL IA. THESE FEATURES WILL CONTINUE TO SUPPORT
AREAS OF SHOWERS WORTHY OF LIKELY TO CATEGORICAL RAIN CHANCES FOR
THE NORTHERN FORECAST AREA. ELEVATED INSTABILITY IS ON THE ORDER
OF 400-700 J/KG PER THE 12Z DVN SOUNDING AND RAP ANALYSIS...SO
THUNDER COVERAGE IS LIKELY TO REMAIN LESS THAN RAIN COVERAGE
THROUGH EARLY AFTERNOON...AND AT TIMES HAS ONLY BEEN ISOLATED.

THE WESTERN WAVE OVER IA IS ONE OF A LITTLE MORE INTEREST AS THAT
COMES EASTWARD. HIGH RESOLUTION MODELS HAVE A SURPRISINGLY LARGE
SPREAD IN SOLUTIONS WITH THIS...WITH THE RAP BEING A FURTHER SOUTH
ONE. RADAR AND SATELLITE TRENDS ARE SUPPORTING THIS HOWEVER. AS
THIS MOVES EASTWARD THERE COULD BE A RESULTANT UPTICK IN THUNDER
COVERAGE DURING THE LATTER AFTERNOON INTO EARLY EVE...BUT LAPSE
RATES ARE POOR AND LIKELY WILL CONTINUE TO BE SO...COMMON WITH A
HIGH PWAT ATMOSPHERE AND NO SIGNIFICANT DIFFERENTIAL TEMPERATURE
ADVECTION. THE BETTER WARM AIR ADVECTION AND MOISTURE TRANSPORT
DEVELOP OVER THE MIDWEST LATER THIS EVENING...AND AN EVEN LARGER
SPREAD IN MODELS EXIST WITH THIS. AS SUCH...HAVE NOT MADE ANY
MAJOR CHANGES TO TONIGHTS POPS YET. THERE STILL LOOKS TO BE A
HEAVY RAINFALL THREAT WITHIN THE GENERAL REGION...BUT SPECIFICS
ARE QUITE DIFFICULT TO LOCK IN.

THE BOUNDARY SHOULD STRUGGLE TO MOVE MUCH NORTHWARD THIS AFTERNOON
DUE TO THE AREAS OF RAIN. THE SHOWERS AND WARM AIR ADVECTION
ALONG THE FRONTAL ZONE WILL HELP SUPPORT CLOUDS AND KEEP
TEMPERATURES COOLER. HIGHS LOOK TO STAY BELOW 80 FOR SOME
COMMUNITIES IN NORTHERN IL. UPPER 80S ARE STILL FAVORED ACROSS
THE FAR SOUTHERN FORECAST AREA WHERE SCATTERING OF CLOUDS AND
INCREASING SOUTHWEST SURFACE FLOW WILL FURTHER WARMING.

MTF

//PREV DISCUSSION...
330 AM CDT

SHORT TERM...THROUGH THURSDAY NIGHT...

FOR THE REMAINDER OF THE OVERNIGHT HOURS...AN AREA OF SHOWERS AND
THUNDERSTORMS HAS DEVELOPED ALONG AN ELEVATED BAROCLINIC
ZONE...NORTH OF A WARM FRONT THAT EXTENDS FROM LOW PRESSURE OVER
SOUTH DAKOTA THROUGH CNTRL IL.  THE THE CELL MOTION OF THESE STORMS
IS TO THE SOUTHEAST AS UPPER RIDGING BUILDS OVER THE UPPER
MISSISSIPPI VALLEY AND AN UPPER LOW SITE OVER GEORGIAN BAY.  THE
RIDGE AXIS SHOULD REMAIN IN PLACE ALONG THE MISSISSIPPI RIVER...SO
FLOW ALOFT SHOULD REMAIN GENERALLY NWLY.  EXPECT THAT PCPN COVERAGE
THROUGH THE REMAINDER OF THE NIGHT SHOULD REMAIN GENERALLY SCATTERED
OR IN SHORT LINES.  WITH DAYTIME WARMING...ANTICIPATE THAT THE TSRA
COVERAGE SHOULD INCREASE...BUT REMAIN FOCUSED ALONG THE ELEVATED
BAROCLINIC ZONE WHICH SHOULD REMAIN OVER NRN IL/NWRN IN.  WHILE THE
SHORT RANGE MODELS HAVE NOT REALLY HANDLED THE CONVECTIVE TRENDS ALL
THAT WELL OVER RECENT RUNS...THE LATEST HIGH RES GUIDANCE SUGGESTS
THAT THERE COULD BE A LULL IN THE ACTIVITY DURG THE AFTERNOON HOURS
WITH SOME WEAK SHORT WAVE RIDGING MOVING OVER THE REGION. HAVE MADE
SOME ADJUSTMENT TO THE MAX TEMPS FOR TODAY AS WELL.  EXPECT THAT
THERE SHOULD BE MORE EXTENSIVE DEBRIS CLOUD OVER MUCH OF THE
AREA...REMNANTS OF THE DECAYING CONVECTIVE ACTIVITY...SO HAVE BACKED
OFF ON MAX TEMPS BY A FEW DEGREES EAST OF A LINE FROM DIXON TO
GIBSON CITY...WHICH COINCIDES WITH THE ELEVATED BAROCLINIC ZONE.
SO...WILL GO WITH HIGHS ARND 83-85F.  THERE IS STILL THE POTENTIAL
FOR THE FAR SOUTHWESTERN PORTIONS OF THE CWA TO SEE TEMPS IN THE
UPPER 80S TO NEAR 90F AS THAT PORTION OF THE CWA LIES SOUTH OF THE
MAIN BAROCLINIC ZONE AND THERE SHOULD BE LESS CLOUD COVER AND MORE
OPPORTUNITY FOR TEMPS TO CLIMB A LITTLE HIGHER.

LATEST THINKING IS THAT THE BEST OPPORTUNITY FOR MORE ORGANIZED
CONVECTION...INCLUDING THE CHANCE FOR LOCALLY HEAVY RAINFALL...WILL
BE DURG THE LATE EVENING THROUGH THE OVERNIGHT HOURS.  THE MODELS
ARE KEYING ON A SHORTWAVE CURRENTLY OVER KANSAS AND NEBRASKA TO RIDE
UP THE NWRN SIDE OF THE RIDGE THURSDAY AFTERNOON...OVERTOP THE RIDGE
EARLY THURSDAY EVENING AND THEN DIVE SOUTHEAST ACROSS NRN IL/IN LATE
THURSDAY EVENING AND OVERNIGHT THURSDAY NIGHT.  FOLLOWING THIS
TREND...HAVE GONE WITH CATEGORICAL POPS OVER THE NRN PORTIONS OF THE
CWA THIS EVENING AND LIKELY POPS FOR MUCH OF THE AREA FOR THE PERIOD
FROM 00Z-12Z FRIDAY.  THE THREAT FOR LOCALLY HEAVY RAINFALL IS STILL
PRESENT AS PWATS WILL BE SLIGHTLY IN EXCESS OF 2 INCHES.
HOWEVER...DO NOT EXPECT A WIDESPREAD FLOOD THREAT AS THUNDERSTORMS
SHOULD BE PROGRESSIVE...BUT LOCALLY HEAVY RAINFALL COULD CAUSE SOME
URBAN AND SMALL STREAM TYPE FLOODING ISSUES.

KREIN

FRIDAY THROUGH SUNDAY...
MAIN FOCUS WILL BE ON CONVECTIVE CHANCES...WHICH WILL ALSO PLAY A
PIVOTAL ROLE ON TEMPS/HEAT INDEX VALUES FOR THE FRI-SUN STRETCH.

GUIDANCE CONTINUES TO FAVOR A SLIGHT FLATTENING IN THE MID-LVL RIDGE
FRI...WHICH COULD SETUP THE ATMOSPHERIC RIVER OVER THE CWFA ORIENTED
FROM IOWA EAST THROUGH SOUTHERN WISC/NORTHERN IL...STRETCHING EAST
INTO NORTHERN IN. THE LOCAL ARW8KM HAS CONTINUED TO BE A LITTLE LESS
EXCITED THAN SREF/NAM...HOWEVER THE THEME DOES APPEAR TO BE
CONSISTENT. CONFIDENCE HAS DIMINISHED A LITTLE FOR FRI...BUT HAVE
ONLY MADE MINOR ADJUSTMENTS TO THE GOING FORECAST. IF LESS
CONVECTION/CLOUDS ARE PRESENT FRI...THEN TEMPS SHUD EASILY WARM TO
THE UPR 80S/ARND 90 WITH HEAT INDEX VALUES ARND 100 FOR MANY
LOCATIONS. PERHAPS EVEN THE LOW 100S SOUTH OF I-80.

THE FIRST WAVE OF ACTIVITY SHUD BE MAINLY IN THE MORNING
HOURS...SLIDING EAST/SOUTHEAST. THEN THE THETA-E RIDGE LIFTS NORTH
INTO IOWA...WITH ANOTHER LOBE OF VORTICITY POISED TO RIDE ALONG THIS
FEATURE AND SLIDE SOUTHEAST ACROSS THE FORECAST AREA LATE FRI/EARLY
SAT.

OVER THE NEXT SEVERAL DAYS...SAT IS LIKELY THE MOST CHALLENGING. AT
THIS TIME GUIDANCE HAS COME INTO BETTER AGREEMENT THAT THE FRONTAL
BOUNDARY WILL SLIDE BACK SOUTH ACROSS THE FORECAST AREA...ALLOWING
LLVL FLOW TO TURN NORTHEAST/EASTERLY WITH CLOUDS LINGERING MUCH OF
THE DAY. WITH THIS SCENARIO...SAT WILL LIKELY SEE TEMPS GENERALLY IN
THE MID/UPR 80S BUT COULD BE SLIGHTLY LOWER DEPENDING ON HOW THICK
CLOUD COVER WILL BE. FORTUNATELY THIS WILL KEEP HEAT INDEX VALUES
BELOW 95 DEG FOR MUCH OF THE AREA. THEN PROGRESSING TOWARDS THE
SECOND HALF OF THE WEEKEND AND SUN STILL IS ON TRACK TO BE THE
WARMEST DAY OF THE STRETCH. THE MID-LVL RIDGE REMAINS POISED TO
BUILD BACK INTO THE REGION...LIFTING THE WARM FRONTAL BOUNDARY NORTH
OF THE FORECAST AREA SUN. THIS SHUD KEEP MOST OF THE CONVECTIVE
CHANCES DISPLACED NORTH. SOUNDINGS SUGGEST THE MID-LVLS WILL BE MUCH
WARMER AS WELL...WHICH COULD BE ENOUGH TO EITHER LIMIT OR CAP THE
ENVIRONMENT FROM SEEING CONVECTION DEVELOP SUN. HAVE HEDGED POPS IN
THIS DIRECTION...WITH A DRY FORECAST ACROSS MUCH OF THE FORECAST
AREA SUN. SFC TEMPS SHUD EASILY WARM...WITH HIGHS ARND 90 TO
POSSIBLY THE LOW 90S. HOWEVER...LLVL MOISTURE WILL REMAIN HIGH AS
WELL WITH DEW PTS IN THE 72-74 DEG RANGE. THE COMBINATION WILL
LIKELY PRODUCE HEAT INDEX VALUES IN THE UPR 90S TO PERHAPS AS HIGH
AS 105 SUN AFTN FOR AREAS SOUTH OF I-80.


MONDAY THROUGH WEDNESDAY...
GUIDANCE HAS CONTINUED TO BE LESS EXCITED ABOUT DEPARTING OR
WEAKENING THE MID-LVL RIDGE VERY EARLY NEXT WEEK. AS A RESULT MON
AND PERHAPS TUE LOOK TO MAINTAIN THE HEAT. FORTUNATELY LLVL MOISTURE
DOES APPEAR TO BE A LITTLE LESS THAN THE PRIOR WEEKEND...HOWEVER
HEAT INDEX VALUES ARE STILL PROGGED MON TO REACH THE MID TO PERHAPS
UPR 90S. LLVL MOISTURE BEGINS TO DECLINE FURTHER FOR TUE...WHICH
COULD KEEP TEMPS GENERALLY IN THE MID/UPR 80S. THROUGHOUT THE FIRST
HALF OF NEXT WEEK PERIODIC CHANCES FOR CONVECTION REMAIN. THE FINAL
FEW PERIODS ENSEMBLES HAVE THE IDEA THAT SOME WEAKENING TO THE
ROBUST RIDGE WILL OCCUR.

BEACHLER

&&

.AVIATION...

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

* TIMING ADDITIONAL THUNDERSTORM CHANCES TODAY...WITH A CHANCE
  LATE THIS AFTERNOON...AND AGAIN OVERNIGHT.

* LOWER CEILINGS OR VISIBILITY FRIDAY MORNING

* POTENTIAL EAST WINDS FRIDAY AFTERNOON BUT GENERALLY UNDER 10 KT


KMD

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

THUNDER HAS REMAINED WEST AND NORTH WITH THE CURRENT BAND OF
SHOWER ACTIVITY THAT WILL PUSH THROUGH NORTHEAST ILLINOIS IN THE
NEXT FEW HOURS. THERE IS ANOTHER AREA OF THUNDERSTORMS THAT
CONTINUES TO ORGANIZE IN EASTERN IOWA THAT WILL GET AN ADDITIONAL
SHOVE EASTWARD LATER THIS AFTERNOON AS A SHORT WAVE IN WESTERN
CENTRAL IOWA SPREADS INTO ILLINOIS AND THEN SOUTHEASTWARD INTO
INDIANA THIS EVENING. THIS MAY BRING A PERIOD OF SHOWERS OR
THUNDERSTORMS TO THE TERMINALS DURING THE LATE AFTERNOON INTO THE
EARLY EVENING PERIOD. HAVE GONE WITH SHOWERS FOR NOW GIVEN RECENT
TRENDS BUT THIS MAY BE A PERIOD DURING THE AFTERNOON-EARLY EVENING
RUSH WHERE THERE MAY BE THUNDER IN THE VICINITY AND IS A PERIOD WE
WILL MONITOR IN THE COMING HOURS.

THEN THERE APPEARS TO BE A BREAK IN THE EVENING TIME FRAME AS
UPPER LEVEL FORCING DIMINISHES. WARM AIR ADVECTION THEN RAMPS BACK
UP IN WEST CENTRAL IOWA. IF THIS ACTIVITY GETS MOVING INTO
ILLINOIS IT LOOKS TO COME THROUGH OVERNIGHT. THEREFORE PUSHED THE
GOING PROB30 BACK TO ACCOUNT FOR THIS TIME FRAME.

THERE MAY ALSO BE SOME LOWER CLOUDS OR REDUCED VISIBILITIES LATE
TONIGHT INTO EARLY FRIDAY AFTER ADDITIONAL CONVECTIVE ACTIVITY
WINDS DOWN AND WINDS SLACKEN.

KMD

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

* MEDIUM-HIGH IN PERIODS OF SHOWERS THIS AFTERNOON...HIGH IN NO
  THUNDER FOR THE NEXT FEW HOURS. LOW IN THUNDER CHANCES LATER
  THIS AFTERNOON. LOW- MEDIUM IN THUNDER CHANCES TONIGHT AND LOW
  IN TIMING.

* LOW-MEDIUM IN WIND SPEEDS THIS AFTERNOON...HIGH IN DIRECTION
  EXCEPT AROUND SHOWERS.

* LOW IN WIND DIRECTION FRIDAY AFTERNOON


KMD

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

* FRIDAY NIGHT THROUGH WEDNESDAY...PERIODIC CHANCES FOR TSRA/SHRA
  WITH OCNL MVFR CIGS/VIS.

KREIN

&&

.MARINE...
310 AM CDT

NOT MUCH IN THE WAY OF A GRADIENT FOR THE NEXT SEVERAL DAYS...WITH
WEAK SYSTEMS EXPECTED. HIGH PRESSURE OVER QUEBEC AND A SERIES OF
WEAK TROUGHS OVER THE NORTHERN PLAINS. THERE WILL BE A QUASI-
STATIONARY FRONTAL BOUNDARY THAT WILL MEANDER NORTH AND SOUTH
ACROSS LAKE MICHIGAN...ALLOWING WINDS TO CHANGE DIRECTION BUT
STILL NOT EXPECTING MUCH OF A WIND GRADIENT TO DEVELOP. THE
HIGHEST WINDS WILL LIKELY BE ASSOCIATED WITH ANY THUNDERSTORMS
THAT CAN PRODUCE BRIEF GUSTS.

BEACHLER

&&

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

$$

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