Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 191745
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1245 PM 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. FOG
HAS ALSO DEVELOPED...WITH A FEW PATCHES OF DENSE FOG INDICATED
FROM A FEW OBS SITES. EXPECT THE DENSE FOG TO REMAIN CONFINED IN A
FEW OF THE PROTECTED AREAS SINCE THE WINDS ARE NOT COMPLETELY
CALM...THEN SHORTLY AFT SUNRISE FOG SHUD QUICKLY LIFT. 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 18Z...

* SOUTH WINDS GUSTING NEAR 20 KT THIS AFTERNOON.
* SOUTHWEST WINDS NEAR 40 KT AROUND 1000 FT AGL OVERNIGHT. SOME
  SURFACE GUSTINESS WILL NEGATE TRUE LLWS.
* SOUTHWEST WINDS GUSTING AROUND 20 KT AGAIN SATURDAY.
* POTENTIAL FOR SHRA/TSRA DURING THE DAY SAT...MAINLY AFTERNOON.

RATZER

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

SOUTH WINDS STRENGTHENING ACROSS THE REGION THIS AFTERNOON...IN
RESPONSE TO SURFACE PRESSURE FALLS ASSOCIATED WITH LOW DEVELOPING
NEAR THE CANADIAN BORDER. FORECAST SOUNDINGS DEPICT MIXING TO
AROUND 3500-4000 FT AGL AND TAPPING WINDS AROUND 20 KT FOR GUSTS.
SUNSET SHOULD BRING SOME WEAK DECOUPLING OF THE BOUNDARY LAYER
WHICH SHOULD ALLOW GUSTS TO LESSEN...THOUGH SPEEDS SHOULD REMAIN
12-15 KTS THROUGH THE NIGHT. WITH WEAK INVERSION...WINDS 22040KT
EXPECTED IN THE 1000-2000 FT AGL LAYER WHICH MAY PRODUCE LLWS AT
TIMES...THOUGH EXPECTATION IS THAT SOME MECHANICAL CONTINUED MIXING
AND SPORADIC GUSTINESS AT THE SURFACE WILL PREVENT PERSISTENT WIND
SHEAR LAYER. WINDS ARE EXPECTED TO BECOME MORE WEST-SOUTHWEST
DURING THE SATURDAY...AGAIN WITH GUSTS AROUND 20 KT AS BOUNDARY
LAYER MIXES AFTER SUNRISE.

SURFACE LOW PRESSURE WILL PAS NORTH OF LAKE SUPERIOR LATE TONIGHT
AND SATURDAY...TRAILING A COLD FRONT ACROSS THE MIDWEST WHICH WILL
APPROACH THE TERMINALS DURING THE AFTERNOON AND PASS SATURDAY
EVENING. THE COMBINATION OF LOW LEVEL MOISTURE POOLING AHEAD OF
THE FRONT AND A PAIR OF UPPER DISTURBANCES MOVING ACROSS THE AREA
WILL PROVIDE THE POTENTIAL FOR A PERIOD OF TWO OF SHRA/TSRA FROM
SATURDAY MORNING INTO SATURDAY EVENING. FORECAST SOUNDINGS FROM
SEVERAL MODELS DEPICT A CAPPING INVERSION IN PLACE DURING MUCH OF
THE DAY...WITH THE STRONGEST FORCING AND GREATEST POTENTIAL FOR
ERODING THE CAP AND PRODUCING TSRA JUST AHEAD OF THE COLD FRONT
SATURDAY AFTERNOON. SOME HIGH-RES GUIDANCE DEVELOPS SPOTTY SHOWERS
DURING THE MORNING AS WELL...WHILE OTHER RUNS SUCH AS THE WPC 4 KM
WRF SUGGEST LITTLE IN THE WAY OF COVERAGE THROUGH THE DAY. WITHIN
THIS SCENARIO THE GREATER FORCING AND DIURNAL INSTABILITY MAXIMA
DURING THE MID/LATE AFTERNOON HOURS FAVORS THAT PERIOD FOR
INCLUSION OF A PROB30 TOWARD THE END OF THE ORD 30 HOUR FORECAST.
OTHERWISE CONFIDENCE IS TOO LOW FOR INCLUSION OF SHRA/TSRA THROUGH
SATURDAY MORNING FOR THE OTHER TERMINALS.

RATZER


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

* HIGH IN WIND TRENDS.
* MEDIUM THAT CONTINUED NIGHTTIME SFC WIND GUSTS WILL PREVENT
  PROLONGED LLWS CONDITIONS.
* MEDIUM-LOW IN COVERAGE OF SHRA/TSRA ACROSS TERMINALS SATURDAY.

RATZER

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

SATURDAY NIGHT...THUNDERSTORMS LIKELY. POSSIBLY IFR CONDS WITH
STRONGER STORMS.

SUNDAY THROUGH THURSDAY...VFR.

BEACHLER

&&

.MARINE...
357 AM CDT

A POTENT STORM SYSTEM WILL SHIFT EASTWARD ACROSS SOUTHERN ONTARIO
AND GREAT LAKES REGION TODAY AND THROUGH SATURDAY. THIS WILL RESULT
IN A PERIOD OF SOUTHERLY GALES ACROSS THE NORTHERN 2/3RDS OF LAKE
MICHIGAN BY MID TO LATE THIS AFTERNOON THROUGH EARLY SATURDAY
MORNING. A COLD FRONT WILL SHIFT OVER THE LAKE LATE SATURDAY...WITH
WINDS SHIFTING WESTERLY IN ITS WAKE. THESE WESTERLY WINDS COULD
INCREASE SOME (TO 25 TO 30 KT) SATURDAY NIGHT INTO EARLY SUNDAY AS A
SECONDARY SURFACE LOW/TROUGH DEVELOPS OVER THE EASTERN LAKES REGION.
THIS SHOULD DROP A SECONDARY COLD FRONT...WITH MUCH COLDER
AIR...SOUTHWARD OVER LAKE MICHIGAN BY LATE SUNDAY.

CONDITIONS COULD BECOME FAVORABLE FOR WATERSPOUTS FOR A PERIOD
SUNDAY. HOWEVER...AT THIS TIME I WILL HOLD OFF ON INCLUDING THE
MENTION OF SUCH IN THE FORECAST DUE TO UNCERTAINTIES ON THE OVERALL
EXTENT. OTHERWISE...SURFACE HIGH PRESSURE SHOULD BUILD OVER SOUTHERN
SECTIONS OF THE LAKE BY MONDAY...WITH LIGHTER WESTERLY WINDS EXPECTED
DURING THIS PERIOD.

KJB

&&

.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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