Damaging Winds in Illinois, June 27, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
254 PM CDT SAT JUN 27 2009

.DISCUSSION...

247 PM CDT

SHORT TERM FOCUS IS WITH THUNDERSTORMS AND POTENTIAL SEVERE THREAT
THIS EVENING...SHIFTING TO CLOUDS/DIURNAL PRECIP CHANCES ASSOCIATED
WITH UPPER LOW WHICH LINGERS ACROSS THE GREAT LAKES REGION THROUGH
MEDIUM RANGE.

DEEP OCCLUDED LOW PRESSURE OVER FAR SOUTHWEST ONTARIO THIS
AFTERNOON...WITH OCCLUSION EXTENDING SOUTH INTO THE UPPER
MISSISSIPPI VALLEY. SOUTH OF TRIPLE POINT...WARM SECTOR ACROSS
EASTERN IA/MO/WESTERN IL HAS WARMED INTO LOWER 90S...WITH DEW POINT
TEMPS IN LOW/MID 70S YIELDING MLCAPES 2000-2500 J/KG PER SPC RUC
MESOANALYSIS DATA. CONVECTION INITIATION BEGINNING EAST OF ALO/CID
IN EASTERN IA AT THIS TIME...AND THIS ENVIRONMENT EXPECTED TO SPREAD
EAST INTO NORTHERN IL INTO THE EVENING HOURS...INCREASING THREAT OF
THUNDERSTORMS AND SEVERE WEATHER AHEAD OF COLD FRONT. BASED ON THIS
SPC HAS ISSUED SVR WATCH THROUGH 10 PM CDT FOR ABOUT WESTERN HALF OF
CWA. THREAT SHOULD DIMINISH FARTHER TO THE EAST AS STORMS WILL
EVENTUALLY ENCOUNTER DECREASING BOUNDARY LAYER INSTABILITY WITH
SUNSET...THOUGH INCREASING DEEP LAYER DEGREE OF LINEAR ORGANIZATION
MAY CONTINUE TO SUPPORT DAMAGING WIND THREAT FARTHER EAST LATER THIS
EVENING. STORMS SHOULD CONTINUE TO MOVE EAST AND THEN DIMINISH
OVERNIGHT AS COLD/OCCLUDED FRONT MOVE THROUGH...AND SUBSIDENCE
DEVELOPS IN WAKE OF MID LEVEL TROUGH AXIS EARLY SUNDAY MORNING.

SUNDAY APPEARS TO BE A RELATIVELY NICE DAY TO END THE WEEKEND...WITH
SUBSIDENCE AND DRIER AIR EXPECTED TO PRODUCE A GOOD DEAL OF
SUNSHINE...WITH LOWER HUMIDITY. MODEST WESTERLY FLOW SHOULD HOLD OFF
LAKE BREEZE...THUS COMFORTABLE TEMPS IN LOWER 80S SHOULD REACH ALL
THE WAY TO THE LAKE MICHIGAN SHORE.

MEANWHILE...UPPER TROUGH OVER ONTARIO/NORTHERN LAKES BECOMES CLOSED
OFF...AND MEDIUM RANGE GUIDANCE INDICATES THIS FEATURE TO LINGER
ACROSS THE GREAT LAKES THROUGH MID WEEK. AS USUAL IN THESE SETUPS IT
IS HARD TO ACCOUNT FOR TIMING/STRENGTH OF INDIVIDUAL VORT LOBES
ROTATING AROUND THE SOUTHWESTERN PERIPHERY OF THIS CIRCULATION AND
MODELS ALL HANDLE THIS A BIT DIFFERENTLY. CENTER OF MAIN MID LEVEL
LOW FAR ENOUGH TO THE NORTHEAST/EAST OF CWA TO MAINLY PRESENT AN
AFTERNOON DIURNAL SHOWER/ISOLATED THUNDERSTORM THREAT
MONDAY/TUESDAY. OTHERWISE...COOLER TEMPS EARLY IN THE WEEK WITH
CAA IN AMPLIFIED NORTHWEST FLOW SHOULD EVENTUALLY MODERATE BACK TO
NORMAL LATER IN THE WEEK AS UPPER LOW DRIFTS EAST.

RATZER

&&

.AVIATION...

18Z TAFS...MAIN FORECAST CONCERN/CHALLENGE REMAINS CONVECTION
LATER THIS EVENING.

THE SYSTEM RESPONSIBLE FOR THE EXPECTED THUNDERSTORM DEVELOPMENT
HAS BEEN RATHER WELL BEHAVED IN THE MODEL GUIDANCE OVER THE PAST
24-36 HOURS...SO HAVE ONLY MADE MINOR TWEAKS TO THE TIMING OF TS
IMPACT ON THE TERMINALS. STILL EXPECT TS DEVELOPMENT ALONG AND
AHEAD OF A STRONG COLD FRONT CURRENTLY MOVING SLOWLY THROUGH
CENTRAL IOWA. THE FRONTAL SPEED IS LOOKING A LITTLE SLOW THAN
PREVIOUS GUIDANCE...SO HAVE PUSHED BACK THE ONSET OF THUNDER BY AN
HOUR. HOWEVER...THERE IS SOME CONCERN THAT A LINE OF STORMS COULD
OUTRUN THE COLD FRONT...ACCELERATING THROUGH NRN IL THIS EVENING.
THIS WOULD MAKE THE DECISION TO DELAY THE THUNDER A MISTAKE...BUT
WITH THE UPPER LEVEL JET STREAM ORIENTED MORE SW-NE AND THE WIND
MAXIMA OF THE JET OVER THE WRN GREAT LAKES...PROGRESSION COULD BE
SLOWER. WILL HAVE TO MONITOR UPSTREAM CONVECTIVE TRENDS CLOSELY
FOR ANY POTENTIAL UPDATES.

AHEAD OF THE FRONT...WINDS WILL BECOME MORE SWLY AND INCREASE TO
10KT DURG THE AFTERNOON...AND THEN BECOME MORE INCREASINGLY GUSTY
FOR A PERIOD PRIOR TO THE FRONTAL PASSAGE. WITH THE FROPA...WINDS
WILL QUICKLY TURN NWLY. DURG THE LATE MORNING HOURS
TOMORROW...INCREASING COLD ADVECTION WILL ALLOW WINDS TO BECOME
GUSTY AGAIN...BUT AS SFC HIGH PRESSURE BUILDS ACROSS THE LOWER
MISSOURI VALLEY...WINDS SHOULD BACK MORE WLY.

KREIN

&&

.MARINE...
300 PM...

BROAD WEAK HIGH PRESSURE OVER THE GREAT LAKES WILL MOVE EAST
TONIGHT. LOW PRESSURE OVER SOUTHWEST ONTARIO WILL MOVE EAST ACROSS
SOUTHERN ONTARIO TONIGHT AS IT DEEPENS. A TRAILING COLD FRONT WILL
PUSH EAST ACROSS LAKE MICHIGAN SUNDAY MORNING. THE LOW WILL THEN
BEGIN TO WEAKEN AS IT SLOWLY DRIFTS SOUTHEAST OVER LAKE SUPERIOR
SUNDAY NIGHT...THEN ACROSS LAKE HURON MONDAY NIGHT. THE LOW IS
EXPECTED TO WEAKEN FURTHER TUESDAY BUT STILL ONLY REACH WESTERN
QUEBEC BY WEDNESDAY. AS THE LOW FOLLOWS THIS TRACK...A PROLONGED
PERIOD OF WEST/NORTHWEST WINDS IS EXPECTED OVER THE WESTERN LAKES
WITH SPEEDS AS HIGH AS 15-25KTS. WINDS DIMINISH TO 10-15KTS OR
LESS BY MIDWEEK AS THE GRADIENT BECOMES RATHER WEAK AGAIN.

&&

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

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
553 PM CDT SAT JUN 27 2009

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019-
282300-
WINNEBAGO-BOONE-MCHENRY-LAKE ILLINOIS-OGLE-LEE-DE KALB-KANE-
DUPAGE-COOK-LA SALLE-KENDALL-GRUNDY-WILL-KANKAKEE-LIVINGSTON-
IROQUOIS-FORD-LAKE INDIANA-PORTER-NEWTON-JASPER-BENTON-
553 PM CDT SAT JUN 27 2009 /653 PM EDT SAT JUN 27 2009/

THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTH CENTRAL ILLINOIS...
NORTHEAST ILLINOIS AND NORTHWEST INDIANA.

.DAY ONE...TONIGHT.

  WEATHER HAZARDS EXPECTED:

   THUNDERSTORMS...SOME STORMS MAY PRODUCE:
   DAMAGING WIND GUSTS IN EXCESS OF 60 MPH.
   HAIL TO THE SIZE OF QUARTERS.
   FREQUENT CLOUD TO GROUND LIGHTNING.
   LOCALLY HEAVY DOWNPOURS.

   AREAS AFFECTED AND TIMING:

THUNDERSTORMS DEVELOPING OVER EASTERN IOWA THIS AFTERNOON WILL
MOVE INTO NORTH CENTRAL ILLINOIS DURING THE LATE AFTERNOON AND
EARLY EVENING HOURS...AND INTO NORTHEAST ILLINOIS AND NORTHWEST
INDIANA LATER THIS EVENING. THE GREATEST POTENTIAL FOR SEVERE
THUNDERSTORMS WILL BE ACROSS NORTHWEST AND NORTH CENTRAL ILLINOIS
EARLY THIS EVENING.

  DISCUSSION:

LOW PRESSURE OVER SOUTHWEST ONTARIO WILL MOVE SLOWLY NORTHEAST
TONIGHT...WITH A COLD FRONT MOVING FROM EASTERN IOWA EAST INTO
ILLINOIS AND NORTHWEST INDIANA THROUGH THIS EVENING. WARM...MOIST
AND UNSTABLE AIR WILL SURGE NORTH AHEAD OF THIS FRONT INTO THE
OUTLOOK AREA LATE THIS AFTERNOON...ALLOWING THE DEVELOPMENT OF
STRONG THUNDERSTORMS ACROSS EASTERN IOWA AND SOUTHWEST WISCONSIN.
THESE THUNDERSTORMS WILL LIKELY DEVELOP INTO A SQUALL LINE AS THEY
MOVE INTO NORTHWEST AND NORTHCENTRAL ILLINOIS BY EARLY THIS
EVENING...WITH THE POTENTIAL FOR DAMAGING STRAIGHT LINE WINDS AND
HAIL. THE LINE OF THUNDERSTORMS WILL THEN MOVE EAST ACROSS THE
REMAINDER OF NORTHERN ILLINOIS DURING THE EVENING HOURS...AND INTO
NORTHWEST INDIANA BY MIDNIGHT. DECREASING INSTABILITY AFTER SUNSET
SUGGESTS THE GREATEST THREAT FOR SEVERE WEATHER WILL BE ACROSS
WESTERN PORTIONS OF THE OUTLOOK AREA...WITH STORMS EXPECTED TO
WEAKEN SOMEWHAT AS THEY MOVE EAST LATER THIS EVENING. THE FRONT
WILL THEN MOVE EAST OF THE AREA DURING THE PREDAWN HOURS OF
SUNDAY...WITH THE THREAT OF THUNDERSTORMS ENDING. THE PRIMARY
THREAT FROM THESE STORMS WILL BE THE POTENTIAL FOR DAMAGING
STRAIGHT LINE WINDS.

PORTIONS OF THE DES PLAINES AND FOX RIVERS WILL REMAIN IN FLOOD.

.DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY.

THERE IS AN INCREASED RISK OF RIP CURRENTS FOR THE INDIANA
NEARSHORE WATERS SUNDAY INTO SUNDAY EVENING.

THERE IS A CHANCE OF ISOLATED THUNDERSTORMS MONDAY AFTERNOON.

PORTIONS OF THE DES PLAINES AND FOX RIVERS WILL REMAIN IN FLOOD.

.SPOTTER INFORMATION STATEMENT...

SPOTTERS WILL BE NEEDED THIS EVENING.

&&

COMPLETE FORECAST INFORMATION IS AVAILABLE ON THE NWS CHICAGO
WEBPAGE: WEATHER.GOV/CHICAGO

LMZ740>745-282300-
WINTHROP HARBOR TO WILMETTE HARBOR-
WILMETTE HARBOR TO NORTHERLY ISLAND-
NORTHERLY ISLAND TO CALUMET HARBOR-CALUMET HARBOR TO GARY-
GARY TO BURNS HARBOR-BURNS HARBOR TO MICHIGAN CITY-
553 PM CDT SAT JUN 27 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE ILLINOIS NEARSHORE
WATERS OF LAKE MICHIGAN AND THE INDIANA NEARSHORE WATERS OF LAKE
MICHIGAN.

.DAY ONE...TONIGHT.

THERE IS A CHANCE OF THUNDERSTORMS THIS EVENING.

.DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY.

INCREASING WESTERLY WINDS ON SUNDAY MAY BECOME HAZARDOUS FOR
SMALL CRAFT.

THERE IS A CHANCE OF ISOLATED THUNDERSTORMS MONDAY AFTERNOON.

&&

COMPLETE FORECAST INFORMATION IS AVAILABLE ON THE NWS CHICAGO
WEBPAGE: WEATHER.GOV/CHICAGO

LMZ080-261-362-364-366-563-565-567-669-671-673-675-777-779-868-
870-872-874-876-878-282300-
LAKE MICHIGAN FROM MICHIGAN CITY IN TO ST. JOSEPH MI 5 NM
OFFSHORE TO MID LAKE-
LAKE MICHIGAN FROM SEUL CHOIX POINT TO ROCK ISLAND PASSAGE 5 NM
OFFSHORE TO MID LAKE-
LAKE MICHIGAN SOUTH OF A LINE FROM SEUL CHOIX POINT TO THE
MACKINAC BRIDGE AND NORTH OF A LINE FROM CHARLEVOIX MI TO SOUTH
FOX ISLAND 5 NM OFFSHORE-
LAKE MICHIGAN FROM CHARLEVOIX TO POINT BETSIE MI 5 NM OFFSHORE TO
MID LAKE-
LAKE MICHIGAN FROM POINT BETSIE TO MANISTEE MI 5 NM OFFSHORE TO
MID LAKE-
LAKE MICHIGAN FROM ROCK ISLAND PASSAGE TO STURGEON BAY WI-
LAKE MICHIGAN FROM STURGEON BAY TO TWO RIVERS WI 5 NM OFFSHORE TO
MID LAKE-
LAKE MICHIGAN FROM TOW RIVERS TO SHEBOYGAN WI 5 NM OFFSHORE TO
MID LAKE-
LAKE MICHIGAN FROM SHEBOYGAN TO PORT WASHINGTON WI 5 NM OFFSHORE
TO MID LAKE-
LAKE MICHIGAN FROM PORT WASHINGTON TO NORTH POINT LIGHT WI 5 NM
OFFSHORE TO MID LAKE-
LAKE MICHIGAN FROM NORTH POINT LIGHT TO WIND POINT WI 5 NM
OFFSHORE TO MID LAKE-
LAKE MICHIGAN FROM WIND POINT WI TO WINTHROP HARBOR IL 5 NM
OFFSHORE TO MID LAKE-
LAKE MICHIGAN FROM WINTHROP HARBOR TO WILMETTE HARBOR IL 5 NM
OFFSHORE TO MID LAKE-
LAKE MICHIGAN FROM WILMETTE HARBOR IL TO MICHIGAN CITY IN 5 NM
OFFSHORE TO MID LAKE-
LAKE MICHIGAN FROM PENTWATER TO MANISTEE MI 5 NM OFFSHORE TO MID
LAKE-
LAKE MICHIGAN FROM WHITEHALL TO PENTWATER MI 5 NM OFFSHORE TO MID
LAKE-
LAKE MICHIGAN FROM GRAND HAVEN TO WHITEHALL MI 5 NM OFFSHORE TO
MID LAKE-
LAKE MICHIGAN FROM HOLLAND TO GRAND HAVEN MI 5 NM OFFSHORE TO MID
LAKE-
LAKE MICHIGAN FROM SOUTH HAVEN TO HOLLAND MI 5 NM OFFSHORE TO MID
LAKE-
LAKE MICHIGAN FROM ST. JOSEPH TO SOUTH HAVEN MI 5 NM OFFSHORE TO
MID LAKE-
553 PM CDT SAT JUN 27 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE OPEN WATERS OF LAKE
MICHIGAN.

.DAY ONE...TONIGHT.

THUNDERSTORMS ARE LIKELY THIS EVENING.

.DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY.

ISOLATED AFTERNOON THUNDERSTORMS ARE POSSIBLE MONDAY THROUGH
WEDNESDAY.

&&

COMPLETE FORECAST INFORMATION IS AVAILABLE ON THE NWS CHICAGO
WEBPAGE: WEATHER.GOV/CHICAGO

MARSILI