Severe Thunderstorm Warning 28, NWS Des Moines, IA

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-182220-

CENTRAL IOWA THUNDERSTORM OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
1118 AM CDT SUN MAY 18 1997

HAZARD--SCATTERED THUNDERSTORMS CAPABLE OF PRODUCING HAIL OF FROM 1 TO
2 INCHES IN DIAMETER AND WIND GUSTS TO 70 MPH.

AREA OF CONCERN--EASTERN TWO-THIRDS OF THE STATE...ALONG AND EAST OF
A MASON CITY TO CRESTON LINE.

TIMING DETAILS--200 PM THROUGH 800 PM.

METEOROLOGICAL COMMENTS--A COLD FRONT OVER WESTERN IOWA IS FORECAST TO
PUSH EAST INTO CENTRAL IOWA BY EARLY THIS AFTERNOON AND BE LOCATED OVER
FAR EASTERN IOWA THIS EVENING. THE AIR MASS AHEAD OF THE COLD FRONT IS
FORECAST TO BECOME QUITE UNSTABLE WITH LIFTED INDICES OF FROM MINUS 4
TO MINUS 8 AND CAPES OF 2000-3000 J/KG. FORECAST WIND PROFILES SUGGEST
THE GREATEST CONCERN WILL BE WITH STRAIGHT LINE WIND DAMAGE INITIALLY
AND THEN EVOLVE INTO MORE OF BOW ECHO AS THE CONVECTION INTERACTS WITH
AN OLD OUTFLOW BOUNDARY. FREEZING LEVELS ARE FORECAST TO LOWER SOMEWHAT
THIS AFTERNOON AS THE MAIN UPPER LEVEL SYSTEM ENTERS WESTERN
IOWA...WHICH WILL ENHANCE THE POTENTIAL FOR LARGE HAIL AS WELL. IT
APPEARS THE REAL THREAT FOR ANY PERSISTANT ROTATING UPDRAFTS AND
POSSIBLE TORNADOES/DOWNBURSTS WILL BE NEAR THE OUTFLOW BOUNDARY OVER
EASTERN IOWA...WHICH WAS LOCATED JUST EAST OF WATERLOO AND TOLEDO.

PRECAUTIONS--SPOTTER ACTIVATION WILL BE REQUIRED THIS AFTERNOON AS THE
THUNDERSTORMS DEVELOP OVER PARTS OF CENTRAL AND SOUTHWEST IOWA WITH THE
CONVECTIVE SYSTEM QUICKLY TRACKING INTO EASTERN IOWA BY LATE AFTERNOON.
KEEP ABREAST OF THE SHORT TERM FORECASTS AND LATER STATEMENTS ON THIS
DEVELOPING SEVERE WEATHER SITUATION.

SMITH

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-182220-

CENTRAL IOWA THUNDERSTORM OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
1118 AM CDT SUN MAY 18 1997

HAZARD--SCATTERED THUNDERSTORMS CAPABLE OF PRODUCING HAIL OF FROM 1 TO
2 INCHES IN DIAMETER AND WIND GUSTS TO 70 MPH.

AREA OF CONCERN--EASTERN TWO-THIRDS OF THE STATE...ALONG AND EAST OF
A MASON CITY TO CRESTON LINE.

TIMING DETAILS--200 PM THROUGH 800 PM.

METEOROLOGICAL COMMENTS--A COLD FRONT OVER WESTERN IOWA IS FORECAST TO
PUSH EAST INTO CENTRAL IOWA BY EARLY THIS AFTERNOON AND BE LOCATED OVER
FAR EASTERN IOWA THIS EVENING. THE AIR MASS AHEAD OF THE COLD FRONT IS
FORECAST TO BECOME QUITE UNSTABLE WITH LIFTED INDICES OF FROM MINUS 4
TO MINUS 8 AND CAPES OF 2000-3000 J/KG. FORECAST WIND PROFILES SUGGEST
THE GREATEST CONCERN WILL BE WITH STRAIGHT LINE WIND DAMAGE INITIALLY
AND THEN EVOLVE INTO MORE OF BOW ECHO AS THE CONVECTION INTERACTS WITH
AN OLD OUTFLOW BOUNDARY. FREEZING LEVELS ARE FORECAST TO LOWER SOMEWHAT
THIS AFTERNOON AS THE MAIN UPPER LEVEL SYSTEM ENTERS WESTERN
IOWA...WHICH WILL ENHANCE THE POTENTIAL FOR LARGE HAIL AS WELL. IT
APPEARS THE REAL THREAT FOR ANY PERSISTANT ROTATING UPDRAFTS AND
POSSIBLE TORNADOES/DOWNBURSTS WILL BE NEAR THE OUTFLOW BOUNDARY OVER
EASTERN IOWA...WHICH WAS LOCATED JUST EAST OF WATERLOO AND TOLEDO.

PRECAUTIONS--SPOTTER ACTIVATION WILL BE REQUIRED THIS AFTERNOON AS THE
THUNDERSTORMS DEVELOP OVER PARTS OF CENTRAL AND SOUTHWEST IOWA WITH THE
CONVECTIVE SYSTEM QUICKLY TRACKING INTO EASTERN IOWA BY LATE AFTERNOON.
KEEP ABREAST OF THE SHORT TERM FORECASTS AND LATER STATEMENTS ON THIS
DEVELOPING SEVERE WEATHER SITUATION.

SMITH

b

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-182240-

CENTRAL IOWA THUNDERSTORM OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
136 PM CDT SUN MAY 18 1997

HAZARD--SCATTERED THUNDERSTORMS CAPABLE OF PRODUCING WIND GUSTS TO 70
MPH AND HAIL TO 2 INCHES IN DIAMETER...WITH ISOLATED TORNADOES POSSIBLE
OVER PARTS OF NORTH CENTRAL AND EASTERN IOWA.

AREA OF CONCERN--EASTERN TWO THIRDS OF IOWA...ALONG AND EAST OF A LINE
EXTENDING FROM MASON CITY TO CRESTON

TIMING DETAILS--INITIAL DEVELOPMENT FROM 200 TO 400 PM WITH
THUNDERSTORM ACTIVITY EXPECTED TO BE ONGOING THIS EVENING.

METEOROLOGICAL COMMENTS--A COLD FRONT OVER WESTERN IOWA EARLY THIS
AFTERNOON IS FORECAST TO PUSH INTO CENTRAL IOWA BY LATE AFTERNOON AND
TO THE EAST OF THE STATE BY LATE EVENING. ATMOSPHERIC CONDITIONS AHEAD
OF THE FRONTAL SYSTEM ARE EXTREMELY UNSTABLE WITH CAPES OF FROM
4000-5000 J/KG AND LIFTED INDICES RANGING FROM MINUS 8 TO MINUS 12.
INITIALLY...A STRONG CAP WAS IN PLACE ACROSS THE STATE...KEEPING
CONVECTION FROM FIRING ALONG AND TO THE EAST OF THE BOUNDARY.
HOWEVER...AS TEMPERATURES ALOFT COOL THIS AFTERNOON THE CAP WILL BE
BROKEN ALLOWING CONVECTION TO DEVELOP RAPIDLY.

WIND PROFILES ACROSS THE CENTRAL PORTION OF THE STATE SUPPORT MORE
STRAIGHT LINE WIND DAMAGE AND LARGE HAIL WHILE OVER EAST AND NORTHEAST
IOWA...INDICATIONS ARE THAT SOME SUPERCELL DEVELOPMENT IS POSSIBLE IN
THE INITIAL STAGES OF THIS EVENT. A WARM FRONT EXTENDING ACROSS NORTH
CENTRAL THROUGH EASTERN IOWA WILL INTERACT WITH AN OUTFLOW BOUNDARY
FROM THIS MORNINGS CONVECTION WHICH WILL ENHANCE THE POTENTIAL FOR
SUPERCELL DEVELOPMENT IN THIS AREA THIS AFTERNOON.

PRECAUTIONS--SPOTTER ACTIVATION IS ALREADY UNDERWAY ACROSS CENTRAL IOWA
WITH ACTIVATION EXPECTED OVER EASTERN IOWA LATER IN THE AFTERNOON.
KEEP ABREAST OF THE SHORT TERM FORECASTS THIS AFTERNOON AS WELL AS ANY
WATCH OR WARNING INFORMATION.

SMITH