Tornadoes and Damaging Winds in Illinois, May 10, 2004

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO IL
302 PM CDT MON MAY 10 2004

.PUBLIC DISCUSSION FOR AFTERNOON GRIDS/ZONES...

MAIN FORECAST CONCERNS INVOLVE CONVECTIVE TRENDS IN SHORTER TERM.
WEAK CONVECTIVELY INDUCED S/WV DRIFTING ACROSS EASTERN IA JUDGING BY
PROFILER TIME/HEIGHT SECTIONS...AND AFTN SFC ANALYSIS SHOWS NEARLY
STATIONARY FRONTAL BOUNDARY FROM JUST NORTH OF MILWAUKEE...SOUTHWEST
ACROSS EXTREME NORTHWEST IL AND INTO SOUTHEAST IOWA. UNSTABLE
AIRMASS IN WARM SECTOR ACROSS FORECAST AREA AHEAD OF THESE
FEATURES...WITH 18Z ILX SOUNDING SHOWING 1200 J/KG AND ESSENTIALLY
NO CAP. DEEP LAYER SHEAR LACKING HOWEVER...RESULTING IN RELATIVELY
UPRIGHT UPDRAFTS AND NON-SEVERE CELLS. STILL WILL HAVE TO MONITOR
CONVECTIVE TRENDS FOR ADDITIONAL ORGANIZATION IF OUTFLOW
BOUNDARIES/COLD POOL DEVELOPMENT BECOMES MORE IMPRESSIVE...THOUGH
ATTM THINK EXCESSIVE RAINFALL TO BE MORE OF A THREAT THAN WIDESPREAD
HAIL/WIND.

FRONT LAYS OUT ACROSS CWA SOUTH OF I-80 AND LAKE MICHIGAN TONIGHT.
S/WV CONTINUES TO DRIFT EAST FROM IA...THOUGH MODELS INDICATING THIS
WAVE WILL SHEAR/DAMP AS IT MOVES INTO CONFLUENT MID/UPR LVL FLOW
ENTERING UPPER RIDGE TO THE EAST. WILL CONTINUE LIKELY POPS THROUGH
MIDNIGHT...THEN CHANCE POPS LATE AS UPPER LEVEL FORCING AND LOW
LEVEL INSTABILITY FADE...AND LOW LEVEL JET FOCUS FARTHER WEST ACROSS
THE PLAINS. SOME POTENTIAL FOR TRAINING OF CELLS THROUGH THIS
EVENING AS FRONT SLOGS ACROSS FCST AREA...AND CELL PROPAGATION
VECTORS SUGGESTING SLOW MOVEMENT ALONG BOUNDARY. EVENING SHIFT WILL
HAVE TO MONITOR RADAR TRENDS AND PRECIP AMOUNTS FOR ANY LOCALIZED
HEAVY RAINFALL THREAT.

FRONT THEN SURGES BACK NORTH LATE TUES AND TUES NGT AS UPPER PATTERN
AMPLIFIES AND DEEPER LAYER FLOW BACKS MORE SOUTHERLY. MAIN UPPER
FORCING FOR PCPN SHIFTS OVER NRN PLNS...THOUGH FCST WILL CONT WITH
CHC POP TUES WITH BOUNDARY LIFTING ACROSS FCST AREA DURING THE DAY.
COOLER MAX TEMPS WITH LOW 70S NRN CWA...ACCOUNT ERLY WIND COMPONENT
AND CLOUDS NORTH OF WMFNT. DRY TUES NGT WITH FRONT TO NORTH...THOUGH
SURGE OF GULF MOISTURE LIFTS UP MS VLY INTO WED WITH SEVERAL SHORT
WAVES ROUNDING BASE OF WESTERN TROUGH AND INTO LWR OH/MID MS VLYS.
THUS THE PCPN CHCS CONTINUE WED THROUGH FRI NGT AS UPPER PATTERN
SLOWLY PROGRESSES AND FRONT FINALLY SHIFTS SOUTHEAST OF CWA BY SAT.

RATZER

&&

.AVIATION DISCUSSION FOR 18Z TAFS...

SCATTERED SHOWERS AND THUNDERSTORMS DEVELOPING ACROSS THE AREA AS
CONVECTIVELY INDUCED SHORT WAVE ACROSS EASTERN IOWA APPROACHES THE
AREA. WILL CARRY THUNDERSTORM MENTION IN TAFS THROUGH 03Z TIME
FRAME...WITH FOCUS FOR CONVECTION SHIFTING EAST OF TERMINALS AFTER
03Z AS UPPER WAVE GETS DAMPED. GRADUALLY WILL SEE WIND SHIFT TO THE
NORTHEAST AFTER 04Z-06Z AS SURFACE BOUNDARY SAGS SOUTHWARD ACROSS
THE TERMINALS. WITH BOUNDARY IN GENERAL VICINITY AND WEAK SHORT WAVE
PROGGED TO LIFT NORTHEAST IN SOUTHWEST UPPER FLOW TOMORROW
AFTERNOON...COULD SEE SOME SCATTERED SHOWERS ONCE AGAIN BUT NOT
ENOUGH CONFIDENCE TO INSERT AT THIS POINT. VFR CONDITIONS EXPECTED
TO PREVAIL THROUGH FORECAST PERIOD WITH EXCEPTION IN STRONGER
THUNDERSTORMS THIS AFTERNOON. NORTHEAST WINDS WILL CONTINUE INTO
TOMORROW WITH BOUNDARY NOT EXPECTED TO LIFT BACK NORTHWARD UNTIL
TOMORROW EVENING.

NDM

&&

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

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE CHICAGO IL
315 PM CDT MON MAY 10 2004

ILZ003>006-008-010>014-019>022-023-032-033-039-INZ001-002-010-011-019
-111100-
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-
315 PM CDT MON MAY 10 2004

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

.DAY ONE...LATE AFTERNOON AND TONIGHT

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THROUGH 800 PM CDT
ACROSS MAINLY EASTERN PORTIONS OF THE OUTLOOK AREA...GENERALLY TO
THE EAST OF INTERSTATE 39. LARGE HAIL AND DAMAGING THUNDERSTORM
WINDS ARE THE MAIN SEVERE WEATHER THREATS.

SCATTERED THUNDERSTORMS WILL CONTINUE ACROSS THE AREA LATE THIS
AFTERNOON INTO EARLY THIS EVENING. A FEW OF THE STORMS MAY PRODUCE
HAIL UP TO ONE INCH IN DIAMETER WITH THUNDERSTORM WIND GUSTS UP TO
60 MPH. THE THREAT OF SEVERE WEATHER WILL DIMINISH AFTER 800 PM
CDT...BUT SHOWERS AND THUNDERSTORMS ARE EXPECTED TO CONTINUE THROUGH
MIDNIGHT. ALTHOUGH THESE STORMS ARE NOT EXPECTED TO BE SEVERE LATER
TONIGHT...SOME LOCALLY HEAVY RAINFALL IS POSSIBLE ACROSS THE AREA AS
A SLOW MOVING COLD FRONT BECOMES STALLED ACROSS THE AREA.

.DAYS TWO THROUGH SEVEN... TUESDAY THROUGH SUNDAY

THUNDERSTORMS ARE AGAIN POSSIBLE TUESDAY THROUGH FRIDAY.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION MAY BE NECESSARY LATE THIS AFTERNOON AND EVENING.

&&

NWS CHICAGO WEBPAGE: WWW.CRH.NOAA.GOV/LOT (LOWER CASE)

LMZ740>746-766-111100-
WINTHROP HARBOR IL TO NEW BUFFALO MI-
LAKE MICHIGAN-
315 PM CDT MON MAY 10 2004

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

.DAY ONE...LATE AFTERNOON AND TONIGHT

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER SOUTHERN LAKE
MICHIGAN LATE THIS AFTERNOON INTO THIS EVENING. SCATTERED
THUNDERSTORMS WILL CONTINUE TO SPREAD OVER THE SOUTHERN HALF OF LAKE
MICHIGAN LATE THIS AFTERNOON INTO EARLY THIS EVENING. SOME OF THE
STORMS ARE CAPABLE OF PRODUCING HAIL UP TO ONE INCH IN DIAMETER
ALONG WITH THUNDERSTORM WIND GUSTS UP TO 60 MPH. THE THREAT OF
SEVERE WEATHER WILL DIMINISH AFTER 800 PM CDT.

A SLOW MOVING COLD FRONT WILL STALL ACROSS SOUTHERN LAKE MICHIGAN
LATER TONIGHT. SHOWERS AND THUNDERSTORMS ARE EXPECTED TO CONTINUE
AFTER 800 PM CDT...BUT SEVERE WEATHER IS NOT EXPECTED WITH THESE
STORMS.

.DAYS TWO THROUGH SEVEN... TUESDAY THROUGH SUNDAY

THUNDERSTORMS ARE AGAIN POSSIBLE TUESDAY THROUGH FRIDAY.

&&

NWS CHICAGO WEBPAGE: WWW.CRH.NOAA.GOV/LOT (LOWER CASE)

NDM