Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 231459 AAA
AFDLOT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
959 AM CDT SAT AUG 23 2014

.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 14Z...

* MVFR VISIBILITY AND CIG IMPROVING BY 15Z-16Z.

* EAST TO NORTHEAST WINDS THROUGH THE DAY WITH SPEEDS POSSIBLY
  APPROACHING 10 KT.

* LOW CHANCE FOR SHRA/TSRA...ESPECIALLY LATE THIS AFTERNOON AND
  EARLY THIS EVE.

* CHANCE FOR REDUCED CIGS/VSBY OVERNIGHT.

MTF/MDB

//DISCUSSION...UPDATED 14Z...

CIGS AND VISBYS HAVE RAPIDLY IMPROVED OVER ORD AND MDW...WITH THE
GREATER RESTRICTIONS UNDER MORE STABILITY JUST TO THE NORTH OF
ORD. REGARDING WIND SPEED THIS AFTERNOON...VAD WIND PROFILERS ARE
INDICATING 10 KT OF EASTERLY FLOW AROUND 2000 FT WHICH WILL BE IN
THE MIXED LAYER THIS AFTERNOON BASED ON TEMPERATURE FORECASTS. SO
MAY SEE AT LEAST SPORADIC 10 KT WINDS 18Z AND AFTER. AS FOR
STORMS NEAR THE MISSISSIPPI RIVER THIS SHOULD CONTINUE TO VERY
SLOWLY FADE AS IT MOVES EAST-NORTHEAST. HAVE ADDED VCSH MENTION TO
RFD AT THIS TIME AND MAY HAVE TO MENTION SHOWERS WITH TEMPORARY
THUNDER DEPENDING ON OBSERVATIONAL AND LIGHTNING TRENDS.

MTF

PREV DISCUSSION...UPDATED 12Z...

WEAK LOW LEVEL FLOW FROM LAKE MICHIGAN COMBINED WITH THE VERY
MOIST AIRMASS HAS ALLOWED STRATUS/FOG TO SLOWLY PUSH INLAND THIS
MORNING. WITH ABUNDANT CIRRUS OVERHEAD WITH MORE UPSTREAM FROM
CONVECTION ACROSS IOWA IMPROVEMENT MAY BE VERY SLOW THIS MORNING
SO ADJUSTMENTS MAY BE NEEDED OVER THE NEXT FEW HOURS. WITH
GYY/MDW/RFD BEING ON THE FRINGES OF THE IFR IMPROVEMENT MAY BE
STEADIER THERE WITH ORD/DPA LIKELY BEING THE SLOWEST. DO EXPECT
THAT VFR WILL RETURN HOWEVER. EASTERLY WINDS WILL BE IN PLACE
THROUGH THE DAY WITH SPEEDS EXPECTED TO BE UNDER 10 KT.

SPOTTY CONVECTION HAS BEEN FESTERING OVER FAR EASTERN IOWA AHEAD
OF A MUCH LARGER COMPLEX ACROSS CENTRAL IOWA. THIS ACTIVITY IS
MOVING NORTHEASTWARD BUT STEERING FLOW WOULD TAKE IT MORE EASTERLY
THOUGH AT LEAST SOME DECAY IS POSSIBLE IN THE NEAR TERM. HAVE LOW
CONFIDENCE IN HOW CONVECTION PLAYS OUT THE REST OF THE DAY BUT AT
LEAST SCATTERED TSRA IS POSSIBLE AS THE PARENT FORCING DRIFTS
TOWARD THE AREA LATER TODAY. THIS POINTS TO THE AFTERNOON/EVENING
BEING THE MOST LIKELY PERIOD FOR STORMS BUT THE BEST CHANCES MAY
BE FOCUSED WEST OF THE TERMINALS. GIVEN THE UNCERTAINTY WILL KEEP
THE TAFS DRY FOR NOW BUT EXPECT THAT RFD MAY NEED A MENTION OF
TSRA ADDED BEFORE TOO LONG. WITH CONTINUED WEAK FLOW AND HIGH
HUMIDITY MORE FOG AND STRATUS WILL BE POSSIBLE TONIGHT.

MDB

//ORD AND MDW CONFIDENCE...UPDATED 14Z...

* HIGH IN CIGS AND VISIBILITY IMPROVING THROUGH MID-MORNING.

* HIGH IN WIND DIRECTION AND MEDIUM IN WIND SPEED FORECAST THROUGH
  TODAY.

* MEDIUM THAT POTENTIAL FOR SHRA/TSRA IS LOW THROUGH THE DAY.

* MEDIUM THAT THERE WILL BE SOME SORT OF CIG/VSBY REDUCTION
  TONIGHT.

MTF/MDB

//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.

MDB

&&

.MARINE...
340 AM CDT

BROAD LOW PRESSURE REMAINS ACROSS THE MID MISSISSIPPI RIVER VALLEY
THIS MORNING WHILE STRONGER LOW PRESSURE ORGANIZES OVER THE CENTRAL
HIGH PLAINS. AN UPPER LOW SPINNING OVER THE NORTHWEST U.S. WILL HELP
KICK THE HIGH PLAINS LOW NORTHWARD INTO SUNDAY WHILE HIGH PRESSURE
MOVES FROM QUEBEC TO NEW ENGLAND AND THE MID-ATLANTIC STATES. EAST
TO NORTHEAST WINDS WILL CONTINUE ACROSS LAKE MICHIGAN TODAY BUT WILL
SHIFT TO THE SOUTHEAST INTO SUNDAY...WITH SPEEDS INCREASING
MODESTLY. THE MAIN TRACK OF LOW PRESSURE WILL REMAIN OFF TO THE
NORTHWEST OF THE LAKE INTO THE MIDDLE OF NEXT WEEK WHICH WILL KEEP A
SOUTHERLY WIND GOING...THOUGH SOUTHERN AREAS MAY SEE WINDS SHIFT
ONSHORE EACH AFTERNOON WHERE THE PRESSURE GRADIENT IS WEAKER. A
FRONTAL BOUNDARY IS EXPECTED TO DROP OVER NORTHERN PARTS OF THE LAKE
TOWARD MIDWEEK WHICH WOULD LEAD TO EAST OR NORTHEAST WINDS ACROSS
THE NORTH WITH SOUTHERLY WINDS CONTINUING SOUTH. MEDIUM RANGE
GUIDANCE CONFLICTS WITH HOW QUICKLY THIS FRONT MOVES ACROSS THE LAKE
AND THE STRENGTH OF COLD AIR BEHIND IT DURING THE MIDDLE OF NEXT WEEK
LEADING TO LOW FORECAST CONFIDENCE BY THAT TIME.

MDB

&&

.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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