Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 042359
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
659 PM CDT FRI SEP 4 2015

.SHORT TERM...
235 PM CDT

THROUGH SATURDAY NIGHT...

SHOWERS AND THUNDERSTORMS HAVE BEEN MOST CONCENTRATED...BUT STILL
LARGELY SCATTERED...IN NORTHWEST INDIANA...AND EAST INTO EASTERN
ILLINOIS BUT WELL SOUTH...CLOSER TO A REMNANT FRONTAL BOUNDARY FROM
LAST NIGHT THAT STALLED OUT SOMEWHERE IN THE ROCHELLE TO JOLIET TO
VALPARAISO LINE.  MOST OF THIS ACTIVITY HAS CONTINUED EAST. THIS
BOUNDARY STILL REMAINS...BUT WE ALSO NOW HAVE A LAKE BREEZE BOUNDARY
THAT IS NOW MAKING STEADY BUT NOW SLOWING INLAND PROGRESS THIS
AFTERNOON. THIS COMBINED WITH UPPER LEVEL LOW PRESSURE ATOP THE AREA
1500-2500 J/KG OF MIXED LAYER CAPE AWAY FROM THE LAKE IS RESULTING
IN ADDITIONAL ISOLATED DEVELOPMENT THIS AFTERNOON.  THE BETTER
INSTABILITY AND CONVERGENCE SLIGHTLY COOLER TEMPS ALOFT STILL RESIDE
FROM THE KANKAKEE RIVER EASTWARD WILL ADDITIONAL DEVELOPMENT WILL
LIKELY CONTINUE. THESE STORMS STILL POSE A HEAVY RAINFALL THREAT IF
THEY CAN CONGEAL AS THEY ARE SLOW MOVING...WITH A LOW CONCERN FOR
GUSTY WINDS IN FORD/IROQUOIS COUNTY ILLINOIS INTO
BENTON/NEWTON/JASPER COUNTIES OF INDIANA...WITH THE HIGHER
CONCERN EAST INTO CENTRAL INDIANA.

LIKE YESTERDAY...THE LOSS OF DAYTIME HEATING...WE SHOULD SEE
ACTIVITY DIMINISH QUICKLY. THE UPPER LOW LINGERS IN THE REGION
OVERNIGHT. ANOTHER WEAK WAVE WILL ATTEMPT TO MOVE INTO THE UPPER
LEVEL RIDGE THAT WILL BUILD IN FROM THE WEST OVERNIGHT. MANY
GUIDANCE MEMBERS SUGGEST THAT THIS COUPLED WITH SOME RAMPING UP OF
ISENTROPIC LIFT AHEAD OF THIS FEATURE MAY TRIGGER SOME ISOLATED
SHOWERS OR THUNDERSTORMS LATE TONIGHT INTO EARLY SATURDAY. HAVE
MOVED TOWARD THE DRIER NAM SOLUTION AT THIS POINT IN TIME WHICH IS
REFLECTED IN THE EXPERIMENTAL HRRR AND HI-RES NMM/ARW AS WE DO NOT
FEELING FORCING IS STRONG ENOUGH TO TAP INTO ANY ELEVATED
INSTABILITY BUT WILL BE A PERIOD TO WATCH.

THE UPPER RIDGE IS OVER WESTERN ILLINOIS...WITH THE UPPER LOW
INCHING EASTWARD. LOW LEVEL SOUTHWESTERLIES SHOULD ALLOW WARMING ALL
THE WAY TO THE LAKEFRONT WITH 90S INLAND AND UPPER 80S AT THE LAKE.
A WEAKENING OF THE PRESSURE GRADIENT NEAR THE LAKE WOULD SUPPORT A
SE WIND SHIFT NEAR THE LAKE LATER IN THE DAY TO SUPPORT SOME
COOLING. HEAT INDICES WILL GET INTO THE MID TO UPPER 90S IN MANY
SPOTS. HAVE HELD ONTO THE SLIGHT CHANCE POPS FOR TOMORROW
AFTERNOON GIVEN THE MOIST/UNSTABLE ENVIRONMENT...BUT AT THIS POINT
MOST AREAS LOOK TO REMAIN DRY AS THE MAIN TRIGGER IS LACKING.

KMD

&&

.LONG TERM...

SUNDAY THROUGH FRIDAY...

300 PM...MULTIPLE FORECAST CONCERNS INCLUDING HIGH TEMPS SUNDAY
AND MONDAY...PRECIP CHANCES/TIMING LATE SUNDAY NIGHT THRU
WEDNESDAY ALONG WITH HEAVY RAIN POTENTIAL MONDAY NIGHT/TUESDAY.

UPPER RIDGE WILL BE RIGHT OVER THE AREA SUNDAY WITH DRY WEATHER AND
MOSTLY SUNNY SKIES. THIS SHOULD ALLOW TEMPS TO EASILY REACH THE
LOWER 90S INCLUDING ALONG MUCH OF THE IL LAKESHORE AS SOUTHWESTERLY
WINDS APPEAR STRONG ENOUGH THRU MID AFTERNOON TO PREVENT A LAKE
BREEZE. AS THE WINDS DIMINISH IN THE LATER AFTERNOON...WINDS WILL
LIKELY TURN SOUTHEASTERLY ALONG THE IL SHORE COOLING TEMPS A BIT.
WITH DEWPOINTS IN THE UPPER 60S/LOWER 70S...AFTERNOON HEAT INDEX
VALUES COULD CLIMB INTO THE UPPER 90S.

A FRONTAL BOUNDARY WILL MOVE SOUTHEAST ACROSS THE MIDWEST AND
WESTERN LAKES REGION SUNDAY NIGHT REACHING THE AREA MONDAY MORNING.
THE SLOWER TIMING FROM 24 HOURS AGO HAS HELD WITH THE MOST RECENT
GUIDANCE WITH MUCH OF THE AREA STAYING DRY UNTIL EARLY MONDAY
MORNING AND THE BEST CHANCES FOR PRECIP ACROSS NORTHWEST IL TOWARD
DAYBREAK. IT STILL APPEARS THAT PRECIP WILL BE DISSIPATING MONDAY
MORNING BUT TO WHAT EXTENT IS UNCERTAIN. HAVE INTRODUCED MORE OF A
GRADIENT WITH POPS DROPPING OFF TO SLIGHT CHANCE ACROSS THE
SOUTHEAST. POSSIBLE ADDITIONAL DEVELOPMENT MAY OCCUR LATER IN THE
AFTERNOON/EARLY EVENING. PRECIP COVERAGE AND ASSOCIATED CLOUD COVER
MAKE FOR A TRICKY TEMP FORECAST. STILL POSSIBLE TEMPS MAY CLIMB TO
NEAR 90 ACROSS THE SOUTHEAST MONDAY AFTERNOON.

THIS FRONTAL BOUNDARY THEN SLOWS AND STALLS ACROSS THE REGION MONDAY
NIGHT INTO TUESDAY AS AN UPPER WAVE LIFTS NORTHEAST ACROSS THE AREA.
WITH MOISTURE POOLING ALONG THIS BOUNDARY...PERIODS OF HEAVY RAIN
WITH THUNDERSTORMS IS LIKELY SOMEWHERE ACROSS THE REGION...A BIT
EARLY FOR SPECIFICS AND LOCATIONS. AS A STRONGER COLD FRONT MOVES
ACROSS THE AREA TUESDAY INTO TUESDAY NIGHT...THIS SHOULD PUSH THE
PRECIP CHANCES SOUTH OF THE AREA BUT STILL SOME UNCERTAINTY AS TO
HOW FAST THIS OCCURS. ANOTHER WAVE MOVES ACROSS THE AREA LATE NEXT
WEEK AS A LARGE UPPER TROUGH IS PROGGED TO MOVE ACROSS THE LAKES
REGION. GFS/ECMWF HAVE BEEN SHOWING THIS FOR SOME TIME...BUT IT
CONTINUES TO BE DELAYED. IF/WHEN THIS FINALLY ARRIVES...MUCH COOLER
TEMPS AND MUCH LOWER HUMIDITY WITH SPREAD ACROSS THE REGION. CMS

&&

.AVIATION...

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

* OUTSIDE CHANCE OF A SHOWER LATE TONIGHT AND SATURDAY MORNING.

* POTENTIAL FOR IFR CIGS OVERNIGHT AND SATURDAY MORNING.

* CHANCE OF STORMS SATURDAY AFTERNOON.

* POSSIBILITY OF LAKE BREEZE WIND SHIFT LATE IN THE DAY SATURDAY.


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

PRIMARY SURFACE BOUNDARY IS MOVING SLOWLY SOUTHWARD JUST SOUTH OF
THE TRACON AREA AND TAKING THE CONVECTION WITH IT. HOWEVER...A
BROAD UPPER LEVEL DISTURBANCE CENTERED OVER LOWER MICHIGAN
CONTINUES TO PIVOT ACROSS THE REGION. A STRAY SHOWER IS POSSIBLE
ON ITS PERIPHERY OVERNIGHT AND SATURDAY MORNING.

OF GREATER LIKELIHOOD IS THE EXPANSION OF IFR CIGS ON THE WESTERN
PERIPHERY OF LAKE MICHIGAN...WITH SOME ALREADY PRESENT IN FAR
SOUTHEAST WISCONSIN AND AT WAUKEGAN EARLY THIS EVE. WITH THE LAKE
COOLING IN FAR NORTHEAST ILLINOIS THROUGHOUT TODAY...THE
ATMOSPHERE IS SOMEWHAT PRIMED FOR SATURATION IN THE LOW-LEVELS
AFTER DARK. WITH A SLIGHT WIND COMPONENT REMAINING...AT LEAST
THIS EVE...THE PROFILES SUPPORT STRATUS MORE THAN FOG. WILL WATCH
THIS CLOSELY BUT FAVORED TIME FOR CHICAGOLAND TAF SITES WOULD BE
WITHIN THE 07Z-15Z WINDOW.

ANOTHER UNSTABLE DAY IS FORECAST ACROSS THE REGION ON SATURDAY
WITH WEAK FORCING ALOFT...SO ISOLATED STORMS WILL BE
POSSIBLE...INCLUDING ON THE LAKE BREEZE BOUNDARY. THIS FEATURE IS
EXPECTED TO REMAIN EAST OF ORD AND MDW...AT LEAST FOR MOST OF THE
DAY. HOWEVER...IF ANY ISOLATED STORMS DEVELOP NEAR THIS SOURCE OF
CONVERGENCE...THEY COULD DRIFT WESTWARD AND SO HAVE CONTINUED THE
PROB30 FOR THIS CHANCE.


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

* LOW IN ANY RAIN DURING THE TAF PERIOD AND LOW-MEDIUM IN TIMING.

* HIGH THAT IFR CIGS WILL FORM IN PARTS OF NORTHEAST ILLINOIS AND
  SOUTHEAST WISCONSIN LATER TONIGHT...BUT LOW IF AT ORD AND MDW.
  MEDIUM-HIGH IN TIMING IF THEY OCCUR.

* MEDIUM IN WINDS REMAINING SOUTH-SOUTHWEST THROUGHOUT THE ENTIRE
  DAY SATURDAY.


//OUTLOOK FOR ORD/MDW FOR 12Z SUNDAY-12Z FRIDAY...UPDATED 00Z...

SUNDAY...VFR LIKELY. SOUTH WINDS.
MONDAY...CHANCE OF TSRA. SOUTHWEST WINDS.
TUESDAY...TSRA LIKELY. SOUTHWEST WINDS.
WEDNESDAY...CHANCE OF TSRA. NORTHEAST WINDS.
THURSDAY...SLIGHT CHANCE OF SHRA. NORTH WINDS.

MTF

&&

.MARINE...

315 PM...THE GRADIENT WILL SLOWLY TIGHTEN ACROSS LAKE MICHIGAN
THIS WEEKEND AS A LARGE TROUGH OF LOW PRESSURE OVER THE PLAINS
MOVES EAST WITH HIGH PRESSURE REMAINING OVER THE EASTERN U.S.
SOUTHERLY WINDS WILL LIKELY INCREASE INTO THE 15-25 KT RANGE ON
SUNDAY ESPECIALLY OVER THE NORTHERN PORTION OF THE LAKE. A COLD
FRONT WILL MOVE ACROSS THE LAKE MONDAY AFTERNOON INTO MONDAY
EVENING BUT THIS FRONT WILL SLOW AND STALL ACROSS THE SOUTHERN
LAKES REGION WITH WINDS TURNING BACK SOUTHERLY AHEAD OF ANOTHER
APPROACHING COLD FRONT THAT ARRIVES THURSDAY OR THURSDAY NIGHT
THOUGH CONFIDENCE REGARDING TIMING REMAINS LOW. CMS

&&

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

$$

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