Severe Thunderstorm Warning 158, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO IL
649 PM CDT TUE AUG 3 2004

.DISCUSSION...
FCST ISSUANCE HOVERED AROUND THE FIRST THREE PERIODS. MAIN
CHANGES FROM PREVIOUS FCST WERE TO 1) PUSH BACK THE TIMING OF
TSRA TO THIS EVENING WESTERN ZONES THEN BECOMING LIKELY CWA WIDE
AFTER MIDNIGHT AND 2) INCREASE TSRA POPS TO LIKELY FOR TOMORROW
AND 3) TWEEK TOMORROWS HIGH TEMPS...LOWER NORTH.

CURRENT EASTWARD MOVING MCS IN SRN MN IS CONTINUING ITS UPSCALE
DEVELOPMENT AND EXPANDING INTO NCNRTL IA AHEAD OF AN AMPLIFYING
S/WV TROUGH. THE MCS IS EXPECTED TO MOVE INTO THE CWA LATER THIS
EVENING. HAIL AND WINDS WILL BE THE CONCERN. WE WILL LIKELY
BE UPDATING THE ZONES EARLY THIS EVENING TO REFINE TSRA TIMING
FOR TONIGHT. LARGE CONCERN AS TO HOW THE MCS WILL PLAY INTO
TOMORROWS SVR CHANCES AND TEMPS. THINK THE SRN IN AND SE IL
ZONES STAND THE BEST CHANCE OF SVR TOMORROW AS THE SFC BAROCLINIC
ZONE AND CLOUD DEBRIS FROM TONIGHTS MCS SHOULD INHIBIT SVR
CHANCES ELSEWHERE ON WEDNESDAY. MAIN THREATS WILL BE WIND AND
HAIL TOMORROW GIVEN THE VERTICAL SHEAR AND MOISTURE PROFILES.
VERY LARGE HAIL AND TOR THREATS APPEAR TO RESIDE SOUTH AND WELL
EAST OF THE CWA TOMORROW AFTERNOON ASSOCIATED WITH A SQUALL/MCS.
THE ACTIVITY SHOULD END WEST TO EAST BY LATE TOMORROW AFTERNOON
AS THE SFC LOW AND MID-LVL LOW PULL AWAY INTO MI/IN. HOWEVER
MAINTAIN LOW CHANCE EVENING POPS DUE TO POSSIBLE DEFORMATION
PRECIP.

LEFT EXTENDED PERIODS ALONE AS IN GOOD AGREEMENT WITH ISC AND NWP
TRENDS.

&&

.AVIATION...

COMPLEX AND DIFFICULT FORECAST NEXT 18 HRS AS SFC LOW DEEPENS ACROSS
MIDWEST OVERNIGHT INTO WEDNESDAY MORNING. LINE OF SVR STORMS EXTENDS
FROM SOUTHWEST WI INTO EASTERN IA. THIS LINE OF STORMS WILL CONTINUE
TO MOVE EAST SOUTHEAST AND IS EXPECTED TO MAINTAIN CURRENT
INTENSITY. HOWEVER...THE MOST INTENSE PORTION OF THE LINE MAY SAG
SOUTHEAST INTO CENTRAL IL WHERE BEST INSTABILITY AXIS REMAINS. IN
ADDITION TO BEST INSTABILITY...LAKE BREEZE CURRENTLY MOVING WEST
ACROSS NORTHERN IL COULD ENHANCE STORM DEVELOPMENT OR MAY HELP TO
FURTHER STABILIZE ATMOSPHERE. EVEN IF MOST INTENSE STORMS DO SLIDE
SOUTH OF THE TERMINALS...STILL EXPECT AT LEAST A BROKEN LINE OF TS
TO MOVE ACROSS NORTHERN IL THIS EVENING REACHING RFD AROUND 02Z...
DPA 03Z-04Z AND ORD/MDW 04Z-06Z.

BEHIND THIS LINE OF TS EXPECT A 2-4 HR PERIOD OF QUIET WEATHER
BEFORE SFC LOW APPROACHES. LOCATION OF SFC LOW...WHEN IT DEEPENS AND
LOCATION OF STATIONARY FRONT...WILL SIGNIFICANTLY IMPACT FORECAST
AFTER 06Z. HAVE OPTED TO TAKE SFC LOW ALONG I-80 CORRIDOR WHICH
WOULD LEAVE TERMINALS ON THE NORTH SIDE OF THE LOW RESULTING IN
LOWERING CIGS...LIGHT RAIN WITH SOME EMBEDDED TS AND HAVE TRENDED
TAFS THIS DIRECTION...TAKING CIGS TO IFR BETWEEN 08Z-10Z. AS SFC LOW
PULLS EAST MID/LATE WED MORNING...CIGS IMPROVE TO MVFR BUT SFC WINDS
INCREASE TO WITH GUSTS TO 25KTS LIKELY BY LATE MORNING. FORECAST HAS
MANY VARIABLES AND WILL LIKELY NEED REFINEMENT AS THE NIGHT WEARS
ON.

&&

.LOT...WATCHES/WARNINGS/ADVISORIES...
.IL...NONE.
.IN...NONE.
.LK MI...NONE.

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