Tornado in Illinois, August 18, 2001

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PADUCAH KY
120 PM CDT SAT AUG 18 2001

MAIN FORECAST CONCERN IS POTENTIAL FOR SEVERE THUNDERSTORMS THIS
EVENING. THEREAFTER...A DRY WEEK IS SHAPING UP FOR THE FORECAST
AREA. LATEST 18Z SURFACE ANALYSIS HAD LOW PRESSURE OVER SOUTHERN
WISCONSIN WITH ANOTHER LOW DEVELOPING IN NORTHWEST ILLINOIS. FROM
THE SECOND LOW...A WARM FRONT WAS EVIDENT ACROSS CENTRAL ILLINOIS
AND INDIANA...WITH A COLD FRONT TRAILING SOUTHWEST ACROSS MISSOURI
AND SOUTHEAST KANSAS. 12Z UPPER AIR ANALYSIS HAD A 564MB LOW OVER
WESTERN WISCONSIN AND ASSOCIATED 50 KNOT JET DIVING SOUTHEAST ACROSS
THE UPPER MIDWEST.

AT MIDDAY...MOST OF THE FORECAST AREA WAS BASKING IN PLENTY OF
SUNSHINE. THIS HAS HELPED TO BOOST TEMPERATURES INTO THE LOWER AND
MIDDLE 80S...WITH MSAS LI/S OF -7 TO -9 OVER MUCH OF SOUTHERN
ILLINOIS AND SOUTHEAST MISSOURI. THROUGH THE REST OF THE
AFTERNOON...THE AIR MASS WILL CONTINUE TO DESTABILIZE. THIS COMBINED
WITH INCREASING SHEAR AHEAD OF THE APPROACHING SHORTWAVE SHOULD GIVE
THE REGION A ROUND OF STRONG AND SEVERE THUNDERSTORMS...NAMELY LATE
THIS AFTERNOON AND THIS EVENING. BY MIDNIGHT...THE SURFACE COLD
FRONT IS FORECAST TO STRETCH ACROSS SOUTHWEST INDIANA AND WESTERN
KENTUCKY...WITH ANY LINGERING CONVECTION FROM THERE ON EASTWARD.

SUNDAY LOOKS PARTLY SUNNY AS LOW-LEVEL MOISTURE LINGERS BEHIND THE
DEPARTING LOW. FOR THE REST OF THE WEEK...MRF STILL TAKES THE RIDGE
CURRENTLY OVER THE WESTERN U.S. AND SHIFTS IT EAST. SO A DRY WEEK IS
EXPECTED WITH A MODERATION TO NEAR OR JUST ABOVE SEASONAL NORMS BY
THE MIDDLE PART OF THE WEEK. DESPITE THE LATEST MRF RUN...WILL KEEP
THE LATE WEEK PERIOD PARTLY CLOUDY BUT DRY SINCE THE EVENTUAL TRACK
OF CHANTAL REMAINS UNCERTAIN AND IS SEVERAL DAYS OUT.

MOS OUTPUT FROM BOTH NGM/AVN LOOK REASONABLE AND COMPARE FAVORABLY
WELL...SO WILL FOLLOW CLOSELY. THE ONLY EXCEPTION TO THIS WOULD BE
POPS FOR THIS EVENING...WHICH APPEAR TO BE UNDERDONE BY BOTH MODELS.

FIRST-PERIOD ZONE BREAKOUT AND WORDING WILL LIKELY BE DICTATED BY
WATCH BOXES IN EFFECT AT PRESS TIME. RIGHT NOW...PLAN TO EMPHASIZE
SEVERE THREAT INCLUDING BOTH LARGE HAIL AND DAMAGING WINDS OVER THE
ENTIRE AREA FOR THE EVENING HOURS.

.PAH...NONE.

RJP

ILZ075>078-080>094-INZ081-082-085>088-KYZ001>022-MOZ076-086-087-100-
107>112-114-181930-

THUNDERSTORM OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE PADUCAH KY
NOON CDT SAT AUG 18 2001

THIS OUTLOOK COVERS SOUTHEAST MISSOURI...WESTERN KENTUCKY...SOUTHERN
ILLINOIS AND SOUTHWEST INDIANA...AND IS VALID UNTIL 600 AM SUNDAY.

THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER MOST OF
SOUTHERN ILLINOIS AND SOUTHWEST INDIANA THIS AFTERNOON AND
EVENING...GENERALLY NORTH OF A LINE FROM CARBONDALE ILLINOIS TO
EVANSVILLE INDIANA.

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER THE REMAINDER
OF EXTREME SOUTHERN ILLINOIS...SOUTHEAST MISSOURI...AND WESTERN
KENTUCKY.

A VIGOROUS UPPER LEVEL STORM SYSTEM OVER THE UPPER MIDWEST WILL MOVE
SOUTHEAST TOWARD THE GREAT LAKES REGION TODAY AND TONIGHT.
MEANWHILE...A COLD FRONT WILL MOVE SOUTHEAST ACROSS MISSOURI AND
ILLINOIS TOWARD OUR REGION LATE TODAY. AHEAD OF THE COLD FRONT...THE
AIRMASS WILL BECOME VERY UNSTABLE. SATELLITE SHOWS LITTLE IN THE WAY
OF CLOUD COVER...WHICH WILL ALLOW FOR SIGNIFICANT DAYTIME HEATING AND
DESTABILIZATION. IN ADDITION...STRONG UPPER LEVEL WINDS IN
ASSOCIATION WITH THE STORM SYSTEM WILL SPREAD SOUTHEAST...INCREASING
THE WIND SHEAR OVER THE AREA.

SCATTERED THUNDERSTORMS WILL CONTINUE TO DEVELOP AHEAD OF THE COLD
FRONT THIS AFTERNOON. THE ACTIVITY IS EXPECTED TO EVOLVE INTO A
SQUALL LINE AND MOVE SOUTHEAST ACROSS THE FOUR STATE REGION DURING
THE LATE DAY AND EVENING HOURS. DAMAGING WIND AND HAIL APPEAR TO BE
THE MAIN THREATS. HOWEVER...THE DEGREE OF POTENTIAL INSTABILITY AND
WIND SHEAR SUGGESTS THAT SUPERCELL THUNDERSTORMS ARE POSSIBLE. THIS
COULD IN FACT INCREASE THE THREAT FOR ISOLATED TORNADOES.

STORM MOTION IS EXPECTED TO BE EAST TO SOUTHEAST AT 35 TO 40 MPH.

SPOTTER ACTIVATION WILL LIKELY BE REQUESTED LATER THIS AFTERNOON AND
EVENING.

CTN