Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 190836
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
336 AM CDT FRI SEP 19 2014

.DISCUSSION...
257 AM CDT

TODAY...
EARLY THIS MORNING SKIES ARE GENERALLY CLEAR ACROSS MUCH OF THE
AREA...WITH EXCEPTION OF FAR NORTHERN IL WHERE SOME THIN CIRRUS
CLOUDS CONTINUE TO SLIDE EAST. THERE WAS ALSO A NARROW AREA OF
STRATUS ALONG THE WESTERN SHORE OF LAKE MICHIGAN FROM WAUKEGAN
NORTH...BUT WITH THE WINDS TURNING EAST/SOUTHEAST THIS SHUD PREVENT
MUCH OF THIS STRATUS FROM SPREADING BACK INTO NORTHEAST IL. TEMPS
EARLY THIS MORNING HAVE BEEN ABLE TO RADIATE INTO THE LOW 50S...AND
JUST BEFORE DAYBREAK EXPECT SOME UPR 40S WEST OF A CRYSTAL LAKE TO
KANKAKEE LINE ALONG WITH PORTIONS OF NORTHWEST IN. THEN AS THE DAY
PROGRESSES THE SFC RIDGE AXIS WILL SLIDE EAST...ALLOWING A
TIGHTENING PRESSURE GRADIENT TO ARRIVE FROM THE WEST. GOOD MIXING
WILL EASILY BRING TO THE SFC UPPER TEENS TO ARND 20KT GUSTS BY
MIDDAY.

MID-LVL HEIGHTS ARE PROGGED TO INCREASE THROUGHOUT THE DAY...WHICH
SHOULD LOCK-IN ANOTHER DRY DAY WITH ABUNDANT SUNSHINE AND TEMPS
RISING TO ARND 70 TO THE LOW 70S. THE FAR WEST/SOUTH COULD SEE TEMPS
TOP OUT IN THE 73-76 DEG RANGE.

TONIGHT...GUIDANCE SUGGESTS THAT MIXING WILL PERSIST BEYOND SUNSET
THIS EVENING. SO HAVE HELD ONTO THE HIGHER GUSTS THRU THE OVERNIGHT
HOURS. WINDS WILL REMAIN SOUTHERLY AS WELL...BUT BEGIN TO TURN
SOUTHWESTERLY CLOSER TO DAYBREAK SAT. A CHANNEL OF HIGHER PWAT AIR
WILL PIVOT EAST AND BE ADVECTING WARMER AIR ACROSS THE FORECAST AREA
LATE TONIGHT. TEMPS SHUD BE ABLE TO COOL INITIALLY...BUT COULD SEE
TEMPS BEGIN TO RISE WITH INCREASING CLOUDS OVERNIGHT. MOST GUIDANCE
HAS CONTINUED TO HOLD PRECIP OUT OF THE FORECAST AREA OR JUST NORTH
INTO WISC THRU THE OVERNIGHT HOURS.

BEACHLER


LONG TERM SATURDAY THROUGH THURSDAY...

THE MAIN FORECAST CONCERNS DURING THE LONG TERM PERIOD WILL BE THE
CHANCES AND TIMING OF THUNDERSTORMS SATURDAY...WITH THE POTENTIAL
FOR SOME STRONG TO SEVERE STORMS.

THE LARGER SCALE WEATHER PATTERN ACROSS THE COUNTRY IS EXPECTED TO
AMPLIFY AS AN UPPER RIDGE BUILDS ACROSS THE WESTERN CONUS. THIS
PROCESS WILL DRIVE A SIGNIFICANT DISTURBANCE/UPPER LEVEL SPEED MAX
SOUTHEASTWARD ACROSS THE UPPER MIDWEST AND GREAT LAKES BY SATURDAY
THROUGH SUNDAY. THIS SYSTEM WILL BE THE MAIN FOCUS OF INTEREST FOR
SATURDAYS WEATHER. THE UPPER LEVEL SYSTEM WILL DRIVE A SURFACE COLD
FRONT OVER THE AREA SATURDAY AFTERNOON AND EARLY EVENING. A WARM AND
MOIST AIRMASS IS EXPECTED WITHIN THE PREFRONTAL AIRMASS OVER
NORTHERN ILLINOIS SATURDAY. A CORRIDOR OF MID TO POSSIBLY UPPER 60S
DEWPOINTS SHOULD BE ADVECTED OVER THE REGION AS THE LOWER LEVEL FLOW
VEERS SOUTHWESTERLY DURING THE DAY SATURDAY. THIS COMBINED WITH
UPPER LEVEL HEIGHT FALLS...ASSOCIATED WITH THE APPROACHING
DISTURBANCE...LOOKS TO SET THE STAGE FOR THUNDERSTORMS ACROSS THE
AREA.

THE CHANCES FOR THUNDERSTORMS COULD COME AS EARLY AS LATE FRIDAY
NIGHT/EARLY SATURDAY MORNING ACROSS PORTIONS OF NORTHERN ILLINOIS.
ALTHOUGH THE MAIN FOCUS FOR CONVECTION FRIDAY NIGHT LOOKS TO REMAIN
NORTH ACROSS WI...SCATTERED THUNDERSTORM DEVELOPMENT COULD OCCUR
VERY LATE AS THE LOW LEVEL JET VEERS WESTERLY AND INDUCES BETTER LOWER
LEVEL THETA-E ADVECTION OVER THE AREA. THESE EARLY MORNING STORMS
COULD STILL POSS A MARGINALLY SEVERE THREAT...MAINLY DUE TO A SMALL
THREAT FOR HAIL...GIVEN THE DEVELOPMENT OF SOME STEEPER MID LEVEL
LAPSE RATES AROUND 7 DEGREES C/KM.

THIS EARLY MORNING CONVECTION COULD BECOME A COMPLICATING FACTOR FOR
THE SATURDAY AFTERNOON CONVECTION...AND THE OVERALL AFTERNOON SEVERE
THREAT...ESPECIALLY ACROSS FAR NORTHERN ILLINOIS...DEPENDING ON HOW
QUICKLY THINGS BREAK UP. NEVERTHELESS...IT DOES APPEAR THAT
ADDITIONAL THUNDERSTORMS WILL LIKELY DEVELOP ACROSS AT LEAST OVER THE
SOUTHERN HALF OF MY AREA IN THE AFTERNOON AS CONTINUED UPPER
LEVEL HEIGHT FALLS AND MID LEVEL COOLING ACT TO ERODE THE CAP. DEEP
LAYER UNIDIRECTIONAL SHEAR OF 40 TO 45 KT IS FORECAST TO COINCIDE
WITH THIS CONVECTIVE ACTIVITY...WHICH CERTAINLY POINTS TO POTENTIAL
FOR SOME MORE ORGANIZED STORM STRUCTURES. AS SUCH...THE SPC IS
FORECASTING A SLIGHT RISK FOR SEVERE STORMS OVER MUCH OF THE AREA ON
SATURDAY. BRIEF HEAVY DOWNPOURS WILL ALSO LIKELY ACCOMPANY THESE
STORMS...THOUGH SIGNIFICANT FLASH FLOODING APPEARS
UNLIKELY.

TEMPERATURES ON SATURDAY AHEAD OF THE COLD FRONT STILL LOOK TO REACH
THE LOWER 80S. HOWEVER...COOLER WEATHER WILL RETURN TO THE AREA BY
SUNDAY...WITH HIGHS BACK INTO THE UPPER 60S TO NEAR 70 NORTH AND THE
LOWER TO MID 70S SOUTH. A SECONDARY COLD FRONT LOOKS TO PUSH OVER
THE AREA DURING THE DAY SUNDAY AS THE MAIN MID LEVEL VORT MAX SHIFTS
SOUTHEASTWARD OVER SOUTHERN LAKE MICHIGAN. THE COLDER AIR ALOFT
UNDER THIS UPPER TROUGH SHOULD SET UP SOME GOOD LOWER LEVEL LAPSE
RATES...WHICH COULD ALLOW FOR SOME AFTERNOON SHOWERS ACROSS PORTIONS
OF NORTHEASTERN ILLINOIS...ESPECIALLY IF THE FLOW VEERS MORE
NORTHEASTERLY OFF LAKE MICHIGAN.

SURFACE HIGH PRESSURE SETS UP OVER THE AREA FOR MONDAY THEN SHIFTS
EAST OF THE AREA ON TUESDAY. COOLER TEMPERATURES...MAINLY IN THE
UPPER 60S TO LOW 70S ARE EXPECTED BOTH DAYS...WITH CHILLY OVERNIGHT
LOWS. TEMPERATURES BEGIN TO MODERATE BACK INTO THE 70S MID TO LATE
NEXT WEEK AS THE FLOW TURNS MORE SOUTHERLY. DRY CONDITIONS ARE
EXPECTED THROUGH THIS PERIOD.

KJB

&&

.AVIATION...

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

* PATCHY FOG AFT 08Z...BUT SHUD REMAIN WEST OF ORD/MDW.

* LIGHT SE WINDS STEADILY INCREASING LATER IN THE MORNING...AND
  WINDS BECOMING SOUTHERLY. GUSTS DEVELOPING BY MIDDAY UP TO 20
  KT.

BEACHLER

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

HIGH PRESSURE CONTINUES TO PROVIDE THE MAIN INFLUENCE ON THE
WEATHER ACROSS THE AREA THIS MORNING...ALTHOUGH THIS RIDGE OF HIGH
PRESSURE WAS STEADILY DRIFTING EAST. THIS WILL ALLOW THE LIGHT
EAST/SOUTHEAST WINDS TO INCREASE AS A TIGHTENING GRADIENT
DEVELOPS. OVERNIGHT SOME PATCHY FOG WILL DEVELOP BUT SHOULD ONLY
IMPACT DPA/RFD. A FEW SMALLER AIRFIELDS IN NORTHERN IL/NORTHWEST
IN COULD SEE PATCHY DENSE FOG...BUT NOT EXPECTED TO BE A PROLONGED
OR WIDESPREAD EVENT.

WINDS WILL BECOME SOUTHERLY SHORTLY AFTER DAYBREAK...WITH SPEEDS
INCREASING TO ARND 10-12KT AND GUSTS APPROACHING 17-20KT. IT
APPEARS GOOD MIXING WILL CONTINUE THRU THE EVENING HOURS THAT
GUSTS MAY PERSIST WELL INTO THE EVENING AND PERHAPS INTO EARLY SAT
MORNING.

BEACHLER

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

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS.

BEACHLER

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

SATURDAY...THUNDERSTORMS LIKELY.

SUNDAY...VFR.

MONDAY...VFR.

TUESDAY...VFR.

WEDNESDAY...VFR.

RODRIGUEZ

&&

.MARINE...
148 PM CDT

THE COLD FRONT HAS PUSHED SOUTH OF THE LAKE WITH HIGH PRESSURE
EXTENDING ALONG THE WEST SHORE OF THE LAKE AND OVER WISCONSIN AND
INTO NRN IL...WHILE AN INVERTED TROUGH HAS DEVELOPED OVER THE EAST
SIDE OF THE LAKE AND WESTERN LOWER MICHIGAN. THIS HAS SET UP A
MESOLOW OVER THE SOUTH END OF THE LAKE WITH A LINE OF CONVERGENCE
CLOUD OVER THE SRN PORTION OF THE LAKE. THERE COULD BE SOME ISOLD
SHRA ALONG THE BOUNDARY...BUT OTHERWISE...CONDITIONS SHOULD REMAIN
QUIET THROUGH THE EVENING AND OVERNIGHT. AS HIGH PRESSURE PUSHES
OFF TO THE EAST AND A TROUGH OF LOW PRESSURE AND AN ASSOCIATED
COLD FRONT DEVELOP OVER THE CENTRAL PLAINS...WINDS WILL BECOME
SOUTHERLY BY FRIDAY MORNING AND STRENGTHEN AS THE PARENT LOW OF
THE COLD FRONT DEEPENS OVER HUDSON BAY. SINCE THE SOUTHERLY WINDS
WILL BRING MUCH WARMER AIR ACROSS LAKE MICHIGAN...THERE REMAINS A
QUESTION OF HOW SOON THE AIRMASS WILL MODIFY OVER THE LAKE TO THE
POINT WHERE THE STRONGER WINDS ALOFT WILL BE ABLE TO MIX DOWN TO
THE SURFACE. THE FORECAST MODELS ARE ADVERTISING WINDS IN EXCESS
OF 45KT AT ARND 1500 FT ABOVE THE SURFACE...BUT THE MARINE LAYER
SHOULD PROHIBIT THE STRONGEST WINDS FROM MIXING ALL THE WAY DOWN
TO THE SURFACE...BUT AT THIS POINT...FEEL THAT GALE FORCE WINDS
ARE POSSIBLE FOR THE NORTHERN 2/3 OF THE LAKE...WHILE THE SURFACE
LAYER OF THE SOUTHERN PORTIONS OF THE LAKE SHOULD REMAIN TOO
STABLE FOR GALE FORCE WINDS TO MIX DOWN TO THE SURFACE. SO...WILL
HOIST A GALE WATCH FOR THE PERIOD FROM FRIDAY AFTERNOON THROUGH
LATE SATURDAY MORNING AS THE PERIOD MOST LIKELY TO SEE GALES OVER
THE LAKE. WHILE GALES ARE CURRENTLY NOT ANTICIPATED FOR THE
SOUTHERN 1/3 OF THE LAKE...WINDS WILL LIKELY REACH SMALL CRAFT
ADVISORY LEVELS. WITH SOUTHERLY FLOW...WAVES SHOULD REMAIN UNDER
CRITERIA...BUT A SMALL CRAFT ADVISORY MAY BE NEEDED FOR BOTH THE
ILLINOIS AND INDIANA NEARSHORE WATERS. FOLLOWING THE FRONTAL
PASSAGE SATURDAY...WINDS SHOULD TURN NWLY FOR SATURDAY AFTERNOON
AND INTO SUNDAY...BUT THERE IS NOT A STRONG PUSH OF COLD AIR
BEHIND THE FRONT...SO ONLY EXPECT A SHORT PERIOD OF BRISK NWLY
WINDS. ANTICIPATE THAT CONDITIONS COULD BE CLOSE TO SMALL CRAFT
ADVISORY LEVELS FOR A SHORT PERIOD...ESPECIALLY FOR THE INDIANA
NEARSHORE WATERS SATURDAY NIGHT. HIGH PRESSURE WILL THEN
OVERSPREAD THE MIDWEST AND LOWER GREAT LAKES REGION WHILE LOW
PRESSURE DROPS OUT OF NORTHERN CANADA AND SOUTH OF JAMES BAY BY
MONDAY NIGHT AND THEN HIGH PRESSURE WILL QUICKLY BUILD IN BEHIND
THIS SYSTEM...SO...WINDS ACROSS THE LAKE WILL REMAIN RELATIVELY
LIGHT FROM SUNDAY THROUGH MIDWEEK.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ669-LMZ868-LMZ870...4 PM FRIDAY TO 10 AM SATURDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...4 PM FRIDAY TO 4
     PM SATURDAY.

     SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...10 PM FRIDAY TO 4
     PM SATURDAY.

&&

$$

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