Severe Thunderstorm Warning 9, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO IL
640 PM CDT SAT MAR 31 2007

.DISCUSSION...AFTERNOON FCST
CONCERN OVER EVENING THUNDERSTORMS AND ANOTHER POSSIBLE THUNDERSTORM
THREAT ON TUESDAY PRECEDING A MAJOR SHIFT TOWARD COLDER WEATHER ARE
THE FORECAST ISSUES THIS AFTERNOON.

VIGOROUS VORTEX OVER KS/NEB WILL BEGIN TO WEAKEN AND MOVE NE
TONIGHT. THIS WILL AID SFC COLD FRONT NOW OVER WRN MO TO SWING
RAPIDLY ENE THROUGH NRN IL AND NW IN AFTER MIDNIGHT. SFC WARM FRONT
HAS EASED NORTHWARD DURING THE MIDDAY WITH DEW POINTS RISING INTO
THE 50S OVER MOST OF FCST. WIDESPREAD CLOUDS PERSIST ALBEIT WITH
SOME HOLES FOR HEATING. WDLY SCTD SHOWERS AND FEW TSTMS HAVE BUBBLED
UP. BLOW OFF FROM CONVECTION OVER ARK HEADING RAPIDLY NW THOUGH AND
THIS WILL CUT OFF ANY FUTHER HEATING IN MANY AREAS REST OF THIS
AFTERNOON.

STILL..WILL WATCH BAND OF CONVECTION OVER WRN MO AS FOCUS FOR PUSH
OF WESTERN COLD FRONT LATER TONIGHT. MODEL SOUNDING SHOW VERY MARKED
VEERING SIGNATURE WITH NEAR SATURATION IN THE COLUMN AROUND 03Z.
THIS COINCIDES WITH EXPECTED PUSH OF MAIN BAND OF RW/TRW DURING THE
EVENING. MID LEVEL LAPSE RATES MAY STEEPEN SOME AND BULK SHEAR
VALUES WILL MAX OUT ABOUT THIS TIME SO A BRIEF PERIOD/THREAT OF
TSTMS WITH PERHAPS HAIL AND LOCAL HEAVY SHOWERS IS REASONABLE
BETWEEN 00-06Z.

TRAILING AIRMASS FROM THE CENTRAL AND SRN PLAINS IS MODESTLY MILD SO
TEMPS SUN MAY BE CLOSE TO THOSE OF TODAY. FEW SHOWERS CONTINUING IN
CYCLONIC BACKWASH FLOW. THE NEXT SHORT WAVE IN A SOMEWHAT ZONAL FLOW
APPROACHES TUESDAY. TIME OF DAY WILL BE KEY TO TSTM THREAT BUT
SYSTEM IS PROGRESSIVE AND HAS GOOD UPPER LEVEL SUPPORT.  THIS MAY
OFFSET MARGINAL MOISTURE INFLOW.

THIS MID WEEK SYSTEM USHERS IN A PATTERN CHANGE AS TUE TROUGH
DEEPENS OVER THE EASTERN STATES AND EASTERN CANADA.  THIS HAS BEEN
ADVERTIZE FOR MANY RUNS NOW.  INCREASING AMPLITUDE UPSTREAM FROM
THIS TROUGH WILL LIKELY MAINTAIN A COLD NW FLOW INTO EASTER WEEKEND.
TEMPS DURING THIS PERIOD WIL BE BELOW BELOW AVE WITH AT LEAST A
COUPLE NIGHTS OF FROST/FREEZG TEMPS.  NEAR TERM CONDITIONS WILL BE
MONITORED FOR CONVECTION THIS EVENING.  MOS TEMPS/POPS LOOK
REASONABLE AT THIS TIME. KML

&&

.AVIATION DISCUSSION FOR 00Z TAFS...
640 PM CDT

WARM FNTL BNDRY LAYING ACROSS NRN IL AND NWRN INDY FROM MLI TO VPZ.
THIS BNDRY PUSHING NWD AND SHUD BE MOVING THRU TERMINALS EARLY THIS
EVENING. SOME HAZE AND FOG NORTH OF THIS LINE SHUD IMPROVE WITH
PASSAGE AS WINDS SHIFT FROM ESE TO MORE SLY DIRECTION. WIND SPEEDS
FAIRLY BRISK OVERNIGHT FROM 15 TO 20 KTS IN RATHER TIGHT PRESS
GRADIENT OUT AHEAD OF LOW PRES IN WRN IA. BEST CAPE ARE ALONG AND
SOUTH OF THIS BNDRY BUT PERSISTENT CLOUD COVER OVER TERMINALS SO FAR
CAPPING ANY MAJOR OUTBREAK LIKE THAT ALG MS RVR WHERE MOST ACTIVE
AREA IS BREEDING AND SLOW EDGING EAST. RFD MAY SEE SOME SUN BFR IT
SETS BUT NOT LIKELY FOR DPA AND ORD ON EAST. WILL SEE HOW MUCH
STORMS STABLILIZE AT SUNSET...BUT FOR NOW AM CARRYING BEST CHANCE
BTWN 02Z AND 05Z. CIGS GENERALLY VFR BUT VSBY MVFR TO HIGH IFR IN
TSTMS BTWN THOSE HOURS. ASSOCIATED COLD FRONT WITH THIS SYSTEM
WRAPPING ACRS MO INTO WRN IL NOW AND SWINGING THRU ERN IL AS IT
OCCLUDES AROUND MIDNGT WITH WINDS SHIFTING TO SW. LARGE PRESS RISES
AND STRONG CAA SUNDAY INDICATING WINDS GUSTING TO 30 KTS ALL DAY
FROM THE SW WITH LOW VFR CIGS THRU 00Z SUN EVENING.

RLB
&&

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

&&

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
535 PM CDT SAT MAR 31 2007

ILZ003>006-008-010>014-019>022-023-032-033-039-INZ001-002-010-011-
019-010930-
WINNEBAGO-BOONE-MCHENRY-LAKE IL-OGLE-LEE-DE KALB-KANE-DU PAGE-COOK-
LA SALLE-KENDALL-GRUNDY-WILL-KANKAKEE-LIVINGSTON-IROQUOIS-FORD-
LAKE IN-PORTER-NEWTON-JASPER-BENTON-
535 PM CDT SAT MAR 31 2007

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

.DAY ONE... TONIGHT

A TORNADO WATCH IS IN EFFECT THROUGH MIDNIGHT ALONG AND WEST OF
ILLINOIS ROUTE 47. THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS
FOR THE REST OF THE OUTLOOK AREA THROUGH THIS EVENING. THUNDERSTORMS
OVER EASTERN MISSOURI AND EASTERN IOWA WILL PIVOT NORTHEAST ACROSS
NORTH CENTRAL ILLINOIS THIS EVENING.

.DAYS TWO THROUGH SEVEN... SUNDAY THROUGH FRIDAY

THERE WILL BE A CHANCE OF THUNDERSTORMS MONDAY NIGHT THROUGH
TUESDAY. THE NATURE OF ANY THREAT IS YET TO BE DETERMINED.

.SPOTTER INFORMATION STATEMENT...

SPOTTER REPORTS ARE REQUESTED THIS EVENING.

&&

COMPLETE FORECAST INFORMATION IS AVAILABLE ON THE
NWS CHICAGO WEBPAGE: WEATHER.GOV/CHICAGO (LOWER CASE)

LMZ740>745-766-010930-
WINTHROP HARBOR IL TO MICHIGAN CITY IN-
LAKE MICHIGAN-
535 PM CDT SAT MAR 31 2007

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

.DAY ONE... TONIGHT

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING.  THE
PRIMARY THREAT IS HAIL.

.DAYS TWO THROUGH SEVEN... SUNDAY THROUGH FRIDAY

THERE WILL BE A CHANCE OF THUNDERSTORMS MONDAY NIGHT THROUGH
TUESDAY.

&&

COMPLETE FORECAST INFORMATION IS AVAILABLE ON THE
NWS CHICAGO WEBPAGE: WEATHER.GOV/CHICAGO (LOWER CASE)

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