Severe Thunderstorm Warning 7, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
330 AM CDT THU MAR 22 2007

.DISCUSSION FOR AFTERNOON ZONES/GRIDS...
330 AM CDT

MAIN FORECAST CONCERN WILL BE ON CONVECTION MAINLY THIS
MORNING...AND THEN CONTINUED CHANCES OF SHOWERS/THUNDERSTORMS FROM
LATER TONIGHT THROUGH THE WEEKEND AND THE POTENTIAL FOR HEAVY
RAINFALL AND CONTINUED FLOOD POTENTIAL. APOLOGIES FOR AN ABBREVIATED
DISCUSSION DUE TO INCREASING CONVECTION ACROSS THE AREA THIS MORNING.

SURFACE COLD FRONT IS NOW APPROACHING THE MISSISSPPI RIVER WITH SPC
RUC MESOSCALE ANALYSIS GRAPHICS INDICATING BETWEEN 500-1000 J/KG OF
ELEVATED INSTABILITY IN CLOSE PROXMITY TO THE SFC FRONT WHERE LOW
LEVEL DEW POINT POOLING IS OCCURRING. EXPECTING INCREASE COVERAGE OF
SHOWERS AND THUNDERSTORMS THIS MORNING WITH APPROACH OF THIS
INSTABILITY AXIS AND ALSO INCREASING UPPER SUPPORT TOWARD 12Z AS
NORTHERN STREAM VORT MAX PUSHES INTO THE NORTHERN GREAT LAKES AND AS
UPPER JET STREAK CUTS THROUGH THE BASE OF THIS TROUGH INTO THE MID
MISSISSIPPI RIVER VALLEY. FRONT WILL RAPIDLY MOVE ACROSS THE CWA
TODAY...AND BY LATE MORNING ANY REMAINING SHOWERS/THUNDERSTORMS
SHOULD BE CONFINED TO FAR SOUTHEASTERN SECTIONS OF THE CWA. GIVEN
INSTABILITY AND INCREASING SHEAR NEAR THE FRONTAL BOUNDARY...WILL
HAVE TO MONITOR FOR WIND/HAIL THREAT FOR FIRST FEW HOURS THIS
MORNING. FAST MOVING NATURE OF STORMS SHOULD KEEP RAINFALL TOTALS
UNDER CONTROL BUT COMBINATION OF ADDITIONAL RAINFALL AND RAIN OF
PREVIOUS AFTERNOON MAY LEAD TO SOME MINOR FLOODING PROBLEMS THIS
MORNING.

HAVE MADE SOME REVISIONS TO THE FORECAST TONIGHT INTO FRIDAY...WITH
LATEST MODEL RUNS COMING INTO BETTER AGREEMENT IN A MORE NORTHERLY
POSITION OF SFC FRONTAL BOUNDARY AND BETTER CHANCES OF PERIODIC
EPISODES OF SHOWERS/THUNDERSTORMS...ESPECIALLY SOUTH OF INTERSTATE
80 WHERE ISENTROPIC LIFT WILL BE MORE PRONOUNCED. ALREADY HIGH RIVER
LEVELS WILL HAVE TO BE MONITOR FOR ADDITONAL FLOODING GIVEN. WILL
REINTRODUCE LIKELY POPS FOR TSRA ACROSS SOUTHERN AREAS LATE
TONIGHT...AND LIKELY SHOWERS AS FAR NORTH AS INTERSTATE
88...TAPERING TO CHANCE POPS FAR NORTH. ALSO INCREASED POPS FOR
FRIDAY WITH BOUNDARY STILL IN THE AREA.

LITTLE CHANGES MADE TO THE REMAINDER OF THE FORECAST.

MARSILI

&&

.AVIATION...

1254 AM CDT

VIGOROUS S/WV TROF PROPAGATING ACROSS NRN PLAINS AND UPPER MIDWEST
EARLY THIS MORNING...WITH ASSOCIATED SFC TROF/COLD FRONT FROM NRN WI
TO ERN IA AT 05Z. REGIONAL PROFILER DATA...AND 0445Z ACARS SOUNDING
OUT OF KRFD INDICATE 55 KT SOUTHWESTERLY LLJ IN 930-850 HPA LAYER
AHEAD OF LOW LEVEL TROF/FRONT. SOUNDING ALSO INDICATES STEEP LAPSE
RATES IN THIS LAYER...THOUGH A BIT OF A CAP NOTED AROUND 670 MB.
ISOLATED WEAK ELEVATED CONVECTIVE CELLS HAVE BEEN TRYING TO INITIATE
IN THIS REGION OF MOIST/WAA ACROSS FAR NRN IL...THOUGH APPEARS THEY
ARE FIGHTING CAP. NAM FCST SOUNDINGS DEPICT COOLING OF MID/UPR LVLS
WITH APPROACH OF S/WV EARLY THIS MORNING HOWEVER...WITH ELEVATED
CAPE INCREASING TO 550-650 J/KG FOR PARCELS BASED 875-900 HPA. THIS
ALONG WITH INCREASING LLVL CONVERGENCE AHEAD OF FRONT SHOULD BE
SUFFICIENT TO RESULT IN SCATTERED TRW AS BOUNDARY ADVANCES ACROSS
FCST IL. CURRENT REGIONAL RADAR TRENDS INDICATE STRONG CONVECTION IN
SWRN WI AND SRN IA...WITH SOME INCREASE IN CELL DEVELOPMENT ACROSS
ERN IA. GIVEN STRONG FAIRLY UNIDIRECTIONAL WIND FIELD MAY SEE SOME
GUSTS IN 40 KT RANGE WITH TSTMS...AND PERHAPS SOME SMALL HAIL GIVEN
MARGINAL CAPE PROFILES. THUS WILL MAINTAIN TS IN TERMINALS THIS
MORNING...MAINLY DURG 09-13Z PERIOD FROM WEST TO EAST AS FRONT MOVES
THRU. MAY SEE BRIEF PERIOD OF MVFR CIGS IN FRONTAL ZONE...THOUGH
STRONG LLVL DRYING BEHIND FRONT SHOULD DECREASE CLOUDS FAIRLY
RAPIDLY WITH VFR CONDITIONS PREVAILING WITHIN A FEW HOURS OF SUNRISE.

STRONG SW LLVL WIND FIELD IN TIGHT PRES GRADIENT AHEAD OF COLD FRONT
RESULTING IN GUSTY 20-30 KT WINDS ATTM. WILL SEE GRADIENT DECREASE
JUST A BIT WITHIN TROF AXIS TOWARD SUNRISE...THEN WSHFT TO NWLY
DIRECTION. SOME GUSTINESS AS SFC PRES RISE BUILDS IN EARLY
MORNING...THEN DECREASING WINDS THIS AFTN AS WEAK SFC HIGH BUILDS
IN. DVLPMNT OF NE WINDS OFF LAKE A GOOD PROBABILITY VERY LATE
AFTN/EARLY EVE AS GRADIENT WINDS DECREASE. NEXT S/WV AND ELEVATED
PCPN THREAT BEYOND 06Z TONIGHT...MORE ON THAT WITH NEXT FCST
ISSUANCE.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
.IL...NONE.
.IN...NONE.
.LM...SMALL CRAFT ADVISORY ILLINOIS NEARSHORE THROUGH 18Z THURS.
      SMALL CRAFT ADVISORY INDIANA NEARSHORE 18Z WED-18Z THURS.

&&

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
915 PM CDT WED MAR 21 2007

ILZ003>006-008-010>014-019>022-023-032-033-039-INZ001-002-010-011-
019-220930-
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-
915 PM CDT WED MAR 21 2007

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

.DAY ONE... OVERNIGHT

THUNDERSTORMS ARE EXPECTED TO DEVELOP ACROSS NORTHWEST ILLINOIS
AFTER MIDNIGHT...THEN MOVE EAST ACROSS NORTHERN ILLINOIS THROUGH
DAWN. SEVERE WEATHER IS NOT EXPECTED. HOWEVER...SMALL HAIL...AND
VERY HEAVY RAIN WILL BE POSSIBLE.

SOME AREAS HAVE ALREADY RECEIVED ONE TO TWO INCHES OF RAINFALL IN
THE PAST 24 HOURS. AN ADDITIONAL INCH OF RAINFALL IS POSSIBLE
THROUGH MIDDAY THURSDAY. MANY STREAMS...CREEKS...AND RIVERS ARE AT
OR NEAR BANKFULL WITH MINOR FLOODING FORECAST ON SOME RIVERS.
ADDITIONAL RAINFALL OVERNIGHT INTO THURSDAY MAY CAUSE FURTHER
RISES AND LOCALIZED FLOODING.

.DAYS TWO THROUGH SEVEN... THURSDAY THROUGH TUESDAY

THUNDERSTORMS WILL BE CONTINUING THURSDAY MORNING. BY LATE MORNING
THE THUNDERSTORMS ARE EXPECTED MAINLY SOUTH OF INTERSTATE 80.
SEVERE WEATHER IS NOT EXPECTED THURSDAY...BUT LOCALLY HEAVY RAINFALL
WILL REMAIN POSSIBLE.

HIGH WATER LEVELS ON AREA RIVERS COMBINED WITH THE NEW RAINFALL MAY
LEAD TO MINOR FLOODING PROBLEMS ON AREA STREAMS AND RIVERS.

.SPOTTER INFORMATION STATEMENT...

SPOTTER REPORTS OF RAINFALL AND FLOODING WILL BE APPRECIATED
OVERNIGHT AND THURSDAY.

&&

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

LMZ740>745-766-220930-
WINTHROP HARBOR IL TO MICHIGAN CITY IN-
LAKE MICHIGAN-
915 PM CDT WED MAR 21 2007

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

.DAY ONE... OVERNIGHT

A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE NEARSHORE WATERS.

THUNDERSTORMS WILL BE POSSIBLE ACROSS SOUTHERN LAKE MICHIGAN
TONIGHT. A FEW THUNDERSTORMS MAY BECOME SEVERE. THE MAIN THREATS
WILL BE LARGE HAIL AND DAMAGING WINDS.

.DAYS TWO THROUGH SEVEN... THURSDAY THROUGH TUESDAY

A SMALL CRAFT ADVISORY WILL REMAIN IN EFFECT FOR ALL OF THE
NEARSHORE WATERS THROUGH EARLY THURSDAY AFTERNOON.

SHOWERS AND THUNDERSTORMS ARE EXPECTED ACROSS SOUTHERN LAKE MICHIGAN
THURSDAY MORNING. WIND GUSTS IN EXCESS OF 35 KNOTS ARE POSSIBLE
EARLY THURSDAY MORNING WITH THE THUNDERSTORMS.

&&

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

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