Tornado and Flash Flood in Illinois, May 24, 2006

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
255 PM CDT WED MAY 24 2006

.DISCUSSION FOR AFTERNOON ZONES/GRIDS...

FORECAST DISCUSSION WILL BE SOMEWHAT ABBREVIATED DUE TO SEVERE
WEATHER.

CONVECTIVE POTENTIAL THIS EVENING REMAINS THE MAIN FORECAST ISSUE TO
RESOLVE. SURFACE ANALYSIS FROM 17Z INDICATED WARM FRONT EXTENDING
ACROSS EASTERN IOWA INTO CENTRAL ILLINOIS. OCCLUDED SURFACE LOW IS
LOCATED ACROSS FAR SOUTHEAST NORTH DAKOTA WILL OCCLUDED BOUNDARY
EXTENDING INTO NORTH CENTRAL IOWA...AND A COLD FRONT FROM CENTRAL
IOWA INTO NORTHWEST MISSOURI. ACTIVITY HAS BEEN DEVELOPING ALONG THE
WARM FRONTAL BOUNDARY OVER THE PAST HOUR ACROSS FAR WESTERN PORTIONS
OF CWA...AT THE LEADING EDGE OF MUCH HIGHER SURFACE BASED
INSTABILITY AND NO CIN. AXIS OF GREATEST INSTABILITY IS JUST IN
ADVANCE OF BOUNDARY ACROSS CENTRAL IOWA WITH SB CAPES ON THE ORDER
OF 2-3K J/KG. MID/UPPER LEVEL JET STREAK OF 50 KTS NOTED IN REGIONAL
PROFILER DATA ACROSS MO/SWRN IL HAS AIDED IN DEVELOPMENT OF
THUNDERSTORMS NORTH OF STL. EXPECTING THIS ACTIVITY TO GRADUALLY
SHIFT ACROSS SOUTHERN PORTIONS OF THE CWA THIS EVENING. ANOTHER AREA
OF EXPECTED CONVECTIVE INITIATION LATE THIS AFTERNOON INTO THIS
EVENING IS ACROSS NORTHWEST/NORTH CENTRAL ILLINOIS WHICH HAS BEEN
PLACED IN A MODERATE RISK AREA BY SPC. AFOREMENTIONED INSTABILITY
AXIS ACROSS IOWA WILL BE SHIFTING EASTWARD INTO THIS AREA BY EARLY
THIS EVENING...AND FORCING FOR ASCENT SHOULD GRADUALLY INCREASE BY
THIS EVENING AS SHORT WAVE AND ANOTHER ASSOCIATED SPEED MAX
APPROACHES FROM NORTHEAST NEBRASKA.

UPPER LOW WILL MOVE ACROSS THE SOUTHERN GREAT LAKES THURSDAY KEEPING
A CHANCE OF SHOWERS AND THUNDERSTORMS THROUGH THE DAY THURSDAY.
UPPER RIDGING BUILDS BACK INTO THE REGION FOR THE WEEKEND...ALLOWING
FOR DRY WARM CONDITIONS. LATEST GFS HAS BACKED OFF ON DAMPENING
UPPER RIDGE EARLY NEXT WEEK AND WILL KEEP PRECIP OUT OF THE FORECAST
AT THIS TIME.

NDM

&&

.AVIATION DISCUSSION FOR 18Z TAFS...
100 PM CDT

THE MAIN FORECAST CHALLENGE FOR THIS AFTERNOON AND EVENING WILL BE
TIMING THE RETURN OF THUNDERSTORM ACTIVITY TO THE TAF SITES.  THERE
IS LITTLE QUESTION THAT THERE WILL BE WIDESPREAD THUNDERSTORM
ACTIVITY...HENCE THE PREVAILING TSRA IN THE TAFS TONIGHT...BUT AS IS
FREQUENTLY THE CASE...TIMING IS THE PRIMARY CONCERN.  THE MODEL
GUIDANCE IS QUITE CONSISTENT IN BRINGING A SIGNIFICANT LOW LEVEL
INSTABILITY TO THE REGION AND STRONG MID LEVEL FORCING FROM A SHORT
WAVE ROTATING AROUND THE CYCLONIC FLOW ALOFT...BUT THERE IS SOME
SIGNIFICANT DIFFERENCES BETWEEN THE NAM AND GFS ON HANDLING THE LOW
LEVEL TEMPERATURE PROFILE.  THE GFS KEEPS STRONGER CAPPING OVER THE
REGION SUPPRESSING CONVECTION...WHILE THE NAM HAS BETTER
MIXING...BREAKING THE CAP EARLY THIS EVENING.  PREFER THE NAM IDEA
HERE AND HAVE BROUGHT PREVAILING TS TO THE TAFS DURG THE EVENING
HOURS.  THERE WILL LIKELY BE SOME LINGERING SHRA/TSRA THROUGH THE
EARLY MORNING HOURS BEFORE THE MAIN FRONTAL BOUNDARY CLEARS THE
REGION...BUT ACTIVITY SHOULD BE MORE SCT AND THUS HAVE INDICATED
CNVTV POTENTIAL THROUGH VCTY AND CB CLOUD GROUPS.  CIGS HAVE BEEN
VFR THROUGH THE DAY AND BASES ARE EXPECTED TO REMAIN SO...GENERALLY
AROUND 4KFT-5KFT.  VSBY SHOULD ALSO REMAIN UNRESTRICTED EXCEPT INVOF
PCPN...AND THEN SHOULD REMAIN IN MVFR RANGES.

KREIN

&&

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

&&

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
945 AM CDT WED MAY 24 2006

ILZ003>006-008-010>014-019>022-023-032-033-039-INZ001-002-010-011-
019-250930-
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-
945 AM CDT WED MAY 24 2006

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

.DAY ONE... TODAY AND TONIGHT

THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON AND
EVENING FOR LOCATIONS GENERALLY WEST OF INTERSTATE 39. THERE IS A
SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE REMAINDER OF THE
FORECAST AREA THIS AFTERNOON AND EVENING. AN INITIAL BAND OF SHOWERS
AND THUNDERSTORMS WILL CONTINUE TO TRACK ACROSS FAR NORTHEAST
ILLINOIS AND INTO NORTHWEST INDIANA THIS MORNING. THESE
THUNDERSTORMS MAY PRODUCE SOME HEAVY RAINFALL BUT ARE NOT EXPECTED
TO BE SEVERE. A SECOND AND MORE POWERFUL ROUND OF THUNDERSTORMS ARE
FORECAST TO DEVELOP THIS AFTERNOON...MOST LIKELY OVER NORTH CENTRAL
ILLINOIS...AND MOVING INTO NORTHEASTERN ILLINOIS AND NORTHWEST
INDIANA THIS EVENING. SOME OF THESE STORMS MAY PRODUCE LARGE
HAIL...DAMAGING WINDS...AND PERHAPS AN ISOLATED TORNADO.

.DAYS TWO THROUGH SEVEN... THURSDAY THROUGH TUESDAY

THERE IS A CHANCE OF THUNDERSTORMS ON THURSDAY. NO ADDITIONAL SEVERE
WEATHER IS EXPECTED.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION MAY BE NEEDED THIS AFTERNOON AND EVENING.

&&

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

LMZ740>745-766-250930-
WINTHROP HARBOR IL TO MICHIGAN CITY IN-
LAKE MICHIGAN-
945 AM CDT WED MAY 24 2006

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

.DAY ONE... TODAY AND TONIGHT

SCATTERED THUNDERSTORMS WILL CONTINUE DURING THE REMAINDER OF THE
MORNING HOURS...BUT SEVERE WEATHER IS NOT EXPECTED. THUNDERSTORMS
ARE LIKELY THIS EVENING...SOME OF WHICH MAY BE SEVERE WITH LARGE
HAIL AND STRONG WINDS THIS EVENING.

.DAYS TWO THROUGH SEVEN... THURSDAY THROUGH TUESDAY

THERE IS A CHANCE OF THUNDERSTORMS THROUGH THURSDAY. NO ADDITIONAL
SEVERE WEATHER IS EXPECTED.

&&

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