Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 231854
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
154 PM CDT SAT AUG 23 2014

.MESOSCALE DISCUSSION...
115 PM CDT

LINE OF STORMS IS MOVING EAST INTO THE CHICAGOLAND AREA EARLY THIS
AFTERNOON. WHILE THIS HAS SHOWN A GRADUAL WEAKENING
TREND WITH LESS LIGHTNING...DESTABILIZATION ACROSS NORTHWEST
IN...EASTERN IL...AND INTO THE SOUTH CHICAGO METRO THE PAST FEW
HOURS SHOULD AID IN THE ACTIVITY TO AT LEAST PERSIST WITH HEAVY
RAINFALL RATES. IN ADDITION...CONVERGENCE FROM THE SYSTEM OUTFLOW
AND THE LAKE BREEZE HAS ALREADY FIRED A FEW ADDITIONAL CELLS AND
THAT WILL LIKELY CONTINUE TO OCCUR IN SUBURBAN AREAS THROUGH 2-3
PM. THE AREA OF STORMS ALSO SHOULD FURTHER SLOW AS IT RUNS INTO
THE MARINE LAYER. THIS MAY CAUSE HEAVY RAINFALL RATES TO PERSIST
OVER THE SAME URBAN AREAS FOR A WHILE...INCLUDING THOSE THAT WERE
HIT HARD LATE THURSDAY NIGHT. WILL BE MONITORING CLOSELY. SEVERE
THREAT APPEARS MINIMAL...WITH MAINLY THE FAR SOUTHEAST CWA BEING
THE ONE AREA THAT COULD SEE AN ISOLATED FLARE UP TO THAT LEVEL.
NOW THAT AREAS HAVE BEEN RAIN COOLED...WILL ADJUST TEMPERATURES
CONSIDERABLY DOWNWARD THE REST OF THE AFTERNOON.

MTF

&&

.DISCUSSION...
959 AM CDT

THE FORECAST HAS BEEN UPDATED TO REFLECT GREATER CLOUD COVER
AREAWIDE AND HIGHER POPS ACROSS MAINLY NORTH CENTRAL ILLINOIS
DURING THE EARLY-MID AFTERNOON.

A BROAD MESOSCALE CONVECTIVE SYSTEM WITH A LINE OF STORMS ON ITS
FRONT EDGE IS CURRENTLY ADVANCING EAST OVER THE MISSISSIPPI
RIVER AND FAR WESTERN ILLINOIS. THIS CONTINUES TO HOLD TOGETHER
WELL WITH ONLY SUBTLE CLOUD TOP WARMING AND NO FADE IN LIGHTNING
ACTIVITY OVER THE PAST COUPLE HOURS. THIS COMPLEX HAS BEEN ONGOING
FOR OVER NINE HOURS SO NOT SURPRISED TO SEE THAT THE RADAR LOOP
INDICATES THE LIKELY PRESENCE OF A CONVECTIVELY-INDUCED VORTICITY
MAXIMUM /MCV/ IN FAR EASTERN IA. THIS WITH A COLD POOL IS HELPING
TO USHER PUSH THE STORMS MORE EASTWARD THAN THE NORTHEAST MOVEMENT
OF THE PARENT SHORTWAVE WOULD INDICATE. DESPITE WHAT SHOULD BE A
GRADUAL FACE INTO THE EARLY AFTERNOON WITH THE UPPER SUPPORT
MOVING NORTHEAST AND A DECAYING LOW-LEVEL JET PER PROFILERS...2.2
INCH PWAT AIR /99TH PERCENTILE/ AHEAD OF THE MCV WILL LIKELY
SUPPORT SHOWERS AND SOME STORMS INTO NORTH CENTRAL ILLINOIS
BETWEEN NOON AND 3 PM. STILL SEEING SOME EMBEDDED HOURLY PRECIP
REPORTS OF A HALF TO ONE INCH WITH THIS...AND SOME OF THIS
EFFICIENCY COULD CERTAINLY OCCUR ALONG AND WEST OF I-39 DURING THE
EARLY AFTERNOON.

UNDER THE INFLUENCE OF WEAK UPPER LEVEL FLOW...THE MCV IS
FORECAST BY THE RAP AND NAM TO CONTINUE TO SLOW AS IT MOVES INTO
NORTH CENTRAL IL. DESTABILIZATION AHEAD OF IT MAY SUPPORT
ISOLATED OR SCATTERED SHOWERS/STORMS LATER THIS AFTERNOON FURTHER
EAST TOWARD CHICAGO AND THE I-57 CORRIDOR INTO NORTHWEST
INDIANA...BUT CONFIDENCE ON THIS IS LOW AS ANVIL SHIELD WILL
INHIBIT THE RATE OF WARMTH...WITH MOST GUIDANCE INDICATING THE
PRESENCE OF A CAP AND LOW-LEVEL FLOW OFF THE LAKE IN FAR NORTHEAST
IL.

MINOR TWEAKS MADE TO HOURLY TEMPERATURES AND DEW POINTS.

MTF

//PREV DISCUSSION...

327 AM...FORECAST CONCERNS INCLUDE THUNDERSTORM CHANCES/TIMING
THROUGH THE PERIOD ALONG WITH HIGH TEMPERATURES/HEAT INDICES
THROUGH MONDAY.

AREAS OF FOG CONTINUE TO DEVELOP WITH VISIBILITIES DROPPING UNDER
1SM IN A FEW LOCATIONS. NOT EXPECTING WIDESPREAD DENSE FOG THIS
MORNING BUT WILL NEED TO MONITOR OBS OVER THE NEXT FEW HOURS. THE
FOG WILL LIFT THIS MORNING BUT WITH LITTLE CHANGE TO THE AIRMASS
FOG WILL BE POSSIBLE AGAIN TONIGHT INTO SUNDAY MORNING.

A FEW SHOWERS DEVELOPED ACROSS NORTHERN LAKE COUNTY IN NORTHEAST
IL LATE FRIDAY EVENING AND PERSISTED FOR A FEW HOURS BEFORE
DISSIPATING. OTHERWISE...CONVECTION HAS BEEN FOCUSED FURTHER WEST
EARLY THIS MORNING IN THE VICINITY OF THE LOW LEVEL JET. SOME OF
THIS ACTIVITY OVER EASTERN IA MAY ARRIVE ACROSS THE WESTERN CWA
LATER THIS MORNING...LIKELY IN A DECAYING FORM. BUT OTHER THAN AN
ISOLATED SHOWER...EXPECT THE NEXT SEVERAL HOURS TO BE MAINLY DRY.

CONFIDENCE REGARDING THUNDERSTORM CHANCES THIS AFTERNOON INTO
THIS EVENING IS LOW THOUGH THERE IS SOME CONSENSUS FOR DEVELOPMENT
BY EARLY AFTERNOON ACROSS WESTERN/NORTHWEST IL FROM REMNANTS OF
CONVECTION OVER EASTERN NEBRASKA/WESTERN IA. HOW FAR IT CONTINUES
EAST OR NORTHEAST BEFORE WEAKENING IS ANOTHER UNCERTAINTY. MADE NO
SIGNIFICANT CHANGES FOR TODAY MAINTAINING 30-40 POPS. TRENDS WILL
NEED TO BE MONITORED THRU THE MORNING. DEWPOINTS IN THE 70S AND
PRECIPITABLE WATER VALUES APPROACHING 2 INCHES WILL LEAD TO VERY
HEAVY RAIN WITH ANY THUNDERSTORMS THAT DO DEVELOP AND POSSIBLY
FLASH FLOODING...ESPECIALLY IF THE RAIN FALLS OVER HARDER HIT
AREAS FROM FRIDAY MORNING.

THE UPPER RIDGE THEN BUILDS ACROSS THE AREA TONIGHT AND SUNDAY AND
PRECIP CHANCES LOOK BELOW MENTIONABLE FROM LATE TONIGHT THROUGH
MONDAY MORNING. CAN/T COMPLETELY RULE OUT SOME ISOLATED ACTIVITY...
IN PARTICULAR ON A LAKE BREEZE SUNDAY AFTERNOON. A FRONTAL
BOUNDARY WILL MOVE ACROSS THE UPPER MIDWEST SUNDAY NIGHT INTO
MONDAY WITH SOME DIFFERENCES AS TO HOW FAR SOUTH THIS BOUNDARY
WILL SETTLE MONDAY AFTERNOON INTO MONDAY NIGHT. THIS WOULD LIKELY
BE THE NEXT CHANCE OF THUNDERSTORMS WITH THE GEM/ECMWF BOTH
PUSHING THE FRONT THROUGH MUCH OF THE CWA BY TUESDAY MORNING. THIS
BOUNDARY THEN MEANDERS NEAR THE IL/WI BORDER INTO THURSDAY WITH
THE POSSIBILITY OF A PROLONGED UNSETTLED PATTERN ACROSS THE AREA.

CLOUD COVER/CONVECTION TODAY WILL LIKELY KEEP TEMPS IN THE 80S...
THOUGH WITH THE URBAN AREAS STARTING FROM LOWER/MID 70S...ANY
LONGER PERIOD OF SUNSHINE TODAY COULD ALLOW A FEW PLACES TO TAG
90. TEMPS WILL WARM A FEW DEGREES SUNDAY BUT SOUTHEASTERLY WINDS
WILL KEEP THE LAKESHORE COOLER. MONDAY LOOKS TO BE THE BEST
CHANCE FOR TEMPS INTO THE LOWER 90S CWA WIDE BUT WILL BE DEPENDENT
ON CLOUD COVER AND TIMING/ARRIVAL OF ANY PRECIP. DEWPOINTS MAY
LOWER A FEW DEGREES...BACK INTO THE MID/UPPER 60S ACROSS PARTS OF
THE AREA SUNDAY. HEAT INDICES COULD RANGE FROM LOWER/MID 90S TO
100-105 FAR SOUTHWEST. DEWPOINTS THEN BACK INTO THE 70S EVERYWHERE
MONDAY WITH HEAT INDICES CLOSER TO 100-105 MOST AREAS WITH STILL
THE POTENTIAL TO REACH HEAT ADVISORY CRITERIA ACROSS THE FAR
SOUTHERN CWA MONDAY AFTERNOON. CMS

&&

.AVIATION...

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

* RAIN WITH A PERIOD OF THUNDER AND POSSIBLE IFR VISIBILITY
  BETWEEN 19Z-21Z...WITH THE POTENTIAL FOR LINGERING SCATTERED
  SHRA/TSRA INTO THE EVE.

* EAST-NORTHEAST WINDS SHIFTING SOUTHWEST WITH SHOWERS AND STORMS
  THEN LIKELY BECOMING SOUTHEAST OR EAST AFTERWARD /NEAR 20Z/ AND
  POTENTIALLY BEING AROUND 10 KT.

* POTENTIAL FOR IFR CIGS AND/OR VISIBILITY TONIGHT INTO SUNDAY
  MORNING.

* LAKE BREEZE TURNING WINDS EASTERLY AROUND OR POSSIBLY ABOVE 10
  KT SUNDAY.

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

AREA OF SHOWERS AND STORMS CONTINUES TO MOVE EASTWARD WITH A SLOW
WANING TREND...ESPECIALLY IN PART OF THE SEGMENT JUST UPSTREAM OF
ORD AND MDW. WHILE THAT IS THE CASE...INCREASED CONVERGENCE
BETWEEN THE OUTFLOW AND THE LAKE BREEZE IN UNCAPPED AIR WILL
LIKELY SPARK MORE STORMS. THIS ALSO COULD EXTEND THE TSRA TIMING.
SO FEEL COMFORTABLE WITH THE TSRA STILL INDICATED IN THE
CHICAGOLAND TAFS. STRATIFORM RAIN BEHIND THIS WILL LIKELY BE
ACCOMPANIED WITH MVFR CONDITIONS. SCATTERED SHOWERS AND STORMS ARE
POSSIBLE BEHIND THIS DURING THE LATTER HALF OF THE AFTERNOON INTO
THE EVENING IN THE CONTINUED MOISTURE RICH AIR MASS.

THE WIND DIRECTION WILL LIKELY TURN SOUTHWEST AS THE STORMS
PASS...AS ALL OBSERVATIONS SITES HAVE INDICATED THAT UPSTREAM AND
RADAR SHOW WELL-DEFINED OUTFLOW. THE SPEED OF THE OUTFLOW HAS
SLOWED SOME...SO THE DEGREE OF GUSTS AT CHICAGOLAND TAF SITES IS
EXPECTED TO BE LESS THAN 30 KT. THE WIND DIRECTION BEHIND THE
INITIAL PUSH OF STORMS MAY BECOME VARIABLE...BUT IS ANTICIPATED TO
TURN EAST OR SOUTHEAST BASED ON THE SYNOPTIC FLOW AND CLIMATOLOGY
FOR POST-THUNDERSTORM WINDS AT ORD.

THE MOISTURE IN THE LOW-LEVELS AIDED BY RAIN EARLY THIS AFTERNOON
WILL SET THE STAGE ONCE AGAIN FOR FOG AND LOW CIGS
TONIGHT...HOWEVER THE COVERAGE OF THIS IS DIFFICULT TO DETERMINE
AT THIS TIME.

A LAKE BREEZE LOOKS LIKELY ON SUNDAY AFTERNOON INTO NORTHEAST
ILLINOIS THAT WILL PROBABLY REACH BOTH ORD AND MDW.

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

* HIGH IN THUNDER OCCURRING. MEDIUM IN DURATION. LOW IN SHRA/TSRA
  CHANCES THIS EVE.

* LOW IN WIND DIRECTION BETWEEN 19Z AND 21Z...MEDIUM IN WIND
  CONDITIONS OTHERWISE...INCLUDING LAKE BREEZE TIMING ON SUNDAY.

* MEDIUM IN SOME FORM OF IFR RESTRICTION LATE TONIGHT INTO EARLY
  SUNDAY MORNING. LOW ON WHETHER FOG AND/OR CIG.

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

SUNDAY NIGHT...MVFR POSSIBLE.

MONDAY THROUGH THURSDAY...PERIODIC CHANCES FOR SHRA/TSRA WITH
OCCASIONAL MVFR.

FRIDAY...VFR WITH SLIGHT CHANCE TSRA.

MTF

&&

.MARINE...
153 PM CDT

RELATIVELY LIGHT WINDS ARE IN PLACE THIS AFTERNOON AS A WEAK
PRESSURE GRADIENT IS IN PLACE ACROSS THE REGION. WINDS WILL BEGIN TO
FRESHEN UP SLIGHTLY TONIGHT AND VEER TO THE SOUTHEAST AS BROAD AREA
OF LOW PRESSURE OVER THE PLAINS BEGINS TO CONSOLIDATE AND DEEPEN AS
IT LIFTS TOWARDS THE CANADIAN PRAIRIES. THIS LOW WILL DRAG A WEAK
COLD FRONT SOUTH ACROSS LAKE MICHIGAN LATE TUESDAY INTO WEDNESDAY
WITH WINDS TURNING TO THE NORTH BEHIND THROUGH THE REMAINDER OF THE
WEEK.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...DENSE FOG ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 3 PM
     SATURDAY.

&&

$$

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