Severe Thunderstorm Warning 59, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO IL
1215 PM CDT SUN JUL 6 2003

AVIATION DISCUSSION FOR 18Z TAFS...

CONVECTIVE TRENDS CONTINUE TO BE FOCUS OF FORECAST. THUNDERSTORMS
EXPECTED TO INCREASE IN COVERAGE AND INTENSITY THIS AFTERNOON AS MID
LEVEL VORTICITY MAX MOVES OUT OF NORTHERN IA AND ACROSS SRN WI.
UNSTABLE AIRMASS REMAINS ACROSS FORECAST AREA...WITH SB CAPES
ALREADY APPROACHING 4000 J/KG JUST SOUTH OF TERMINAL SITES. CONCERN
WITH APPROACHING VORTICITY CENTER AND ASSOCIATED MID LEVEL SPEED MAX
IS THAT 0-6 KM DEEP LAYER SHEAR WILL INCREASE INTO 40-45 KT
RANGE...ENOUGH FOR SUPERCELLULAR STORMS...AND SPC HAS JUST ISSUED
TOR WATCH FOR WHOLE FORECAST AREA VALID UNTIL 00Z.

TSTMS ALREADY FIRING ALONG OUTFLOW BOUNDARY FROM SE LASALLE CO INTO
SE IROQUOIS COUNTY. HAVE GONE WITH TEMPO TSTSMS 20-24 KRFD AND 21-01
ORD/MDW/DPA/GYY. SHOULD BE A LULL EARLY TONIGHT AS MID LEVEL WAVE
PASSES...THEN EXPECT ANOTHER ROUND AFTER MIDNIGHT AS DEVELOPMENT
OCCURS ON NOCTURNAL LLVL JET WEST OR NORTHWEST OF FORECAST AREA.

RATZER

DISCUSSION FOR MORNING UPDATE OF GRIDS AND ZONES...

ACTIVE WEATHER PATTERN TO CONTINUE FOR NEXT SEVERAL DAYS.  MAIN
ISSUE THIS MORNING IS POSSIBLE HEAVY RAINS EARLY WITH PERSISTENT
ECHO TRAINING FAR SOUTH CWA WHICH. AFTER THAT IS NEXT THREAT OF A
SEVERE WEATHER EPISODE OVERNIGHT SUNDAY NIGHT.

MODEL PROBLEMS, AS NOTED IN PMDHMD: ETA NOT HANDLING MCS IN WESTERN
IOWA WELL THIS MORNING WHILE GFS BLOWS UP A SURFACE/850 LOW OVER THE
WESTERN GREAT LAKES TONIGHT INTO MONDAY WHICH IS ALMOST CERTAINLY
SPURIOUS.  INITIAL PROBLEMS NOTWITHSTANDING WILL FOLLOW GENERAL
SCENARIO LAID OUT BY ETA.

FORECAST TIMING REMAINS INEXACT OF COURSE, BUT GENERAL SCENARIO LAYS
OUT ABOUT AS PREVIOUSLY FORECAST IN ZONES.  EXPECT PERSISTENT
OVERRUNNING OF SURFACE BOUNDARY TO DECREASE THIS MORNING. WARM FRONT
WILL ATTEMPT TO REFORM TO THE NORTH THIS AFTERNOON AND EVENING TO
PROVIDE NEW FOCUS WHILE HOPEFULLY THE OLD BOUNDARY IN OUR AREA IS
WASHED OUT.  BELIEVE WITH IA MCS HEADING MAINLY SOUTHEAST AND NOT A
THREAT IT SHOULD RUN OUT OF OF FUEL SOMEWHERE IN MO ANYWAY THIS
MORNING.  THUS A QUIET PERIOD EXPECTED BY THE AFTERNOON AND EARLY
NIGHTTIME HOURS. TEMPS SIMILAR TO YESTERDAY. NEXT MCS DEVELOPMENT
EARLY SUNDAY NIGHT MORE LIKELY IN MINNESOTA ON NOSE OF LOW LEVEL
JET, AND TRANSLATING SOUTHEASTWARD INTO OUR AREA AFTER
MIDNIGHT...WITH HIGHER LIKELY POPS NORTH AND EAST. ANOTHER ROUND OF
LATE NOCTURNAL SEVERE EXPECTED. SEE SWODY1. BY MID TO LATE MORNING
MONDAY THIS SHOULD BE OUT OF THE PICTURE. ETA PRECIP DURING MONDAY
AFTERNOON ON FIRST INSPECTION WOULD SEEM BOGUS WITH AREA STRONGLY
CAPPED. THINGS WILL BE EXTREMELY UNSTABLE HOWEVER WITH CAPES ABOVE
4000, SO SUPPOSE A 30 POP IS WORTH MAINTAINING AS PER MOS BLEND.
WILL KEEP GOING TEMPS GENERALLY 93-95 ACROSS THE AREA.

REMAINDER OF SCENARIO PLAYED OUT IN GRIDS STILL LOOKS GOOD: COLD
FRONT TO PUSH THRU THE AREA TUESDAY AND WEDNESDAY ACCOMPANIED BY
MORE STORMS AND THEN A BREAK.  LATEST GFS NOT SUPPORTED BY OTHER
MODELS IN DEVELOPING A RATHER INTENSE SURFACE LOW AND BRINGING
PRECIP BACK IN AFTER WEDNESDAY EVENING.  LOOKS LIKE MORE GRID SCALE
FEEDBACK.  WILL MAKE NO CHANGES AT THIS TIME.

.CHI...NONE

KAPLAN

ILZ003>006-008-010>014-019-020>023-032-033-039-INZ001-002-010-011-019
-071100-

HAZARDOUS WEATHER OUTLOOK/SUPPLEMENTAL
NATIONAL WEATHER SERVICE CHICAGO IL
1100 AM CDT SUN JUL 6 2003

THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTH CENTRAL AND NORTHEAST
ILLINOIS...NORTHWEST INDIANA...AND THE ADJACENT WATERS OF LAKE
MICHIGAN.

.DAY ONE... THIS AFTERNOON AND TONIGHT

A WATCH BOX WILL BE ISSUED FOR THE ENTIRE AREA AROUND NOON.

FOLLOWING A LULL IN ACTIVITY THIS EVENING...THERE IS A MODERATE RISK
OF SEVERE THUNDERSTORMS AFTER MIDNIGHT IN AREAS NORTH OF ABOUT I
80...AND A SLIGHT RISK OF SEVERE STORMS SOUTH OF I 80.

DAMAGING WINDS WILL BE THE MAIN THREAT THIS AFTERNOON WITH ISOLATED
SUPERCELLS CAPABLE OF PRODUCING LARGE HAIL AND TORNADOES.

THE PRIMARY THREAT TONIGHT WILL BE DAMAGING WINDS AND LARGE HAIL.

A MESOSCALE STORM SYSTEM OVER NORTHEAST IOWA LATE THIS MORNING
WILLBE TRACKING EASTWARD ACROSS SOUTHERN WISCONSIN THIS AFTERNOON.
AS IT DOES SO IT WILL BE MOVING INTO VERY UNSTABLE AIR OVER NORTHERN
ILLINOIS AND NORTHWEST INDIANA.  DUE TO THE DEGREE OF INSTABILITY IN
PLACE...WITH SURFACE TEMPERATURES AROUND 90 AND DEW POINTS IN THE
LOWER AND MIDDLE 70S...SOME SUPERCELLS ARE POSSIBLE.  WIND SHEAR
THROUGH THE MID LEVELS OF THE ATMOSPHERE MAY PROVIDE AN ENVIRONMENT
FAVORABLE FOR ROTATION.

ANOTHER MESOSCALE WEATHER SYSTEM IS EXPECTED TO BLOW UP THIS
AFTERNOON AND EVENING AHEAD OF A COLD FRONT IN MINNESOTA AND TRACK
SOUTHEAST INTO OUR AREA AFTER MIDNIGHT.

.DAYS TWO THROUGH SEVEN... MONDAY THROUGH SATURDAY

TEMPERATURES REACHING THE MID 90S...AND DEWPOINTS INTO THE 70S WILL
COMBINE TO PRODUCE HEAT INDICES APPROACHING 105 IN SOME AREAS.

PERIODS OF THUNDERSTORMS ARE LIKELY INTO WEDNESDAY MORNING.  SOME OF
THESE MAY BE SEVERE WITH ADDITIONAL HEAVY RAIN.  DRIER AND COOLER
AIR IS EXPECTED LATER WEDNESDAY THROUGH FRIDAY. MORE THUNDERSTORMS
ARE POSSIBLE SATURDAY.

SPOTTER ACTIVATION IS LIKELY THIS AFTERNOON AND MAY BE NEEDED AGAIN
LATE TONIGHT...AFTER MIDNIGHT.

&&

           NWS CHICAGO WEBPAGE

    HTTP://WWW.CRH.NOAA.GOV/LOT (LOWER CASE)
OR  HTTP://WEATHER.NOAA.GOV/CHICAGO (LOWER CASE)

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