Severe Thunderstorm Warning 97, NWS Des Moines, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
324 AM CDT FRI JUL 2 1999

FORECAST CHALLENGES WILL FOCUS ON THUNDERSTORM CHANCES AND TIMING.
CONVECTION OVER NORTHERN NEBRASKA AND SOUTHERN SOUTH DAKOTA FIRING ON
NOSE OF LLJ AND HELPED BY STRONG MID AND UPPER LEVEL WINDS.  VERY
STRONG WAA IS OCCURRING IN THIS AREA AS PER PROFILER DATA SHOWING
DECENT VEERING IN A DEEP LAYER.  THIS ACTIVITY SHOULD SPILL OVER INTO
NORTHWESTERN ZONES THIS MORNING AND CONTINUE TO SLIDE RAPIDLY EAST
WITH QUICK FLOW ALOFT.

PATTERN CHANGE IN THE PROCESS OF OCCURRING WITH HEIGHTS TO RISE OVER
THE EASTERN US AS WESTERN TROUGH DEEPENS.  MODELS SEEM TO HAVE A GOOD
HANDLE ON THE OVERALL TRANSITION WITH THE USUAL DIFFERENCES IN THE
DETAILS.  ETA AND AVN SIMILAR IN PLACEMENT OF SYNOPTIC FEATURES
ALTHOUGH AVN TIMING IS BETTER.  NGM/S EASTERN RIDGE NOT QUITE AS
STRONG ALLOWING SFC FNT TO NEAR STATE BY 48 HOURS.  FEEL STRONGER
EASTERN RIDGE DEPICTED BY AVN/ETA BETTER AS WESTERN TROUGH DEEPENS
AND THEREFORE WILL BLEND THE ETA/AVN FOR THIS PACKAGE.

TODAY WILL SEE CONVECTION SLIDE ACROSS NORTHERN CWA WITH FAIRLY
RAPID PROGRESSION...ALTHOUGH ETA AND MESO ETA HINT AT SOUTHWARD
PROPAGATION IN WESTERN IA.  FOR NOW WILL GO WITH SCT WORDING NW ZONES
AND LEAVE POPS IN REMAINDER OF ZONES AS EXACT MOVEMENT IFFY.  WAA
WILL BE STRONG DURING THE DAY WITH INCREASING MOISTURE HELPING TO
DESTABILIZE THE ATMOSPHERE.  AS PER MANY NATIONAL DISCUSSION...FEEL
MCS TO DEVELOP OVER SOUTHERN SD AND NORTHERN NE THIS EVENING.  THIS
SHOULD MOVE EAST SOUTHEAST INTO THE STATE OVERNIGHT ON THE NOSE OF A
50+ KT LLJ.  BY SATURDAY MORNING...THE MCS IS EXPECTED TO DECAY OVER
AREA AND MOVE EAST LEAVING BEHIND WARM AND HUMID CONDITIONS FOR THE
REMAINDER OF THE HOLIDAY WEEKEND.  CAP SHOULD BE IN PLACE BY LATER
SATURDAY BUT ALWAYS THREAT OF SOMETHING DEVELOPING OUTSIDE THE CAPPED
AREA AND MOVING INTO THE CWA...FOR NOW WILL LEAVE THAT PART OF
FORECAST ALONE.  FWC A TAD WARM TODAY WITH FAN GUIDANCE LOOKING
BETTER AND MORE CONSISTENT.

.DSM...NONE

COGIL

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-031100-

CENTRAL IOWA THUNDERSTORM OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
1110 AM CDT FRI JUL 2 1999

HAZARD--SCATTERED THUNDERSTORMS WITH A FEW SEVERE THUNDERSTORMS
CAPABLE OF PRODUCING UP TO TWO INCH HAIL AND WIND GUSTS TO 65 MPH.
SOME OF THE SEVERE THUNDERSTORMS COULD PRODUCE A TORNADO.

AREA OF CONCERN AND TIMING DETAILS--ALONG AND SOUTH OF HIGHWAY 20
AND WEST OF INTERSTATE 35 REMAINDER OF THE MORNING AND THIS
AFTERNOON.  ALL OF FORECAST AREA FROM 900 PM TONIGHT TO 600 AM
SATURDAY MORNING.

METEOROLOGICAL COMMENTS--A THUNDERSTORM COMPLEX WILL MOVE ACROSS
SOUTHWEST IOWA.  STORMS WITHIN THIS SYSTEM WILL CONTINUE TO MAINTAIN
THEIR STRENGTH AS THEY MOVE INTO A MOIST AND UNSTABLE AIRMASS IN
PLACE OVER SOUTHWEST IOWA. BY THIS AFTERNOON...A WARM FRONT WILL BE
MOVING NORTH ACROSS THE STATE.  WARMER AIR AND AN INCREASE IN
MOISTURE WILL INCREASE INSTABILITY ACROSS THE FORECAST AREA.
ANOTHER COMPLEX OF THUNDERSTORMS IS EXPECTED TO DEVELOP ACROSS SOUTH
DAKOTA AND NEBRASKA OVERNIGHT AND MOVE EAST INTO IOWA.  WITH
MODERATE INSTABILITY IN PLACE...A FEW STORMS OVERNIGHT MAY PRODUCE
HAIL UP TO ONE AND A HALF INCHES IN DIAMETER AS WELL AS WIND GUSTS
APPROACHING 65 MPH.  A SEVERE THUNDERSTORM WATCH IS IN EFFECT UNTIL
300 PM THIS AFTERNOON.

PRECAUTION--SPOTTER ACTIVATION WILL BE NEEDED FOR THE REMAINDER OF
THE MORNING AND THIS AFTERNOON SOUTH OF HIGHWAY 20 AND WEST OF
INTERSTATE 35. ACTIVATION MAY ONCE AGAIN BE NEEDED BY THIS EVENING
AND INTO THE OVERNIGHT.  CONTINUE TO MONITOR THE SITUATION AS
CONDITIONS MAY CHANGE AS THIS SYSTEM EVOLVES.

TC

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-031100-

CENTRAL IOWA THUNDERSTORM OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
1110 AM CDT FRI JUL 2 1999

HAZARD--SCATTERED THUNDERSTORMS WITH A FEW SEVERE THUNDERSTORMS
CAPABLE OF PRODUCING UP TO TWO INCH HAIL AND WIND GUSTS TO 65 MPH.
SOME OF THE SEVERE THUNDERSTORMS COULD PRODUCE A TORNADO.

AREA OF CONCERN AND TIMING DETAILS--ALONG AND SOUTH OF HIGHWAY 20
AND WEST OF INTERSTATE 35 REMAINDER OF THE MORNING AND THIS
AFTERNOON.  ALL OF FORECAST AREA FROM 900 PM TONIGHT TO 600 AM
SATURDAY MORNING.

METEOROLOGICAL COMMENTS--A THUNDERSTORM COMPLEX WILL MOVE ACROSS
SOUTHWEST IOWA.  STORMS WITHIN THIS SYSTEM WILL CONTINUE TO MAINTAIN
THEIR STRENGTH AS THEY MOVE INTO A MOIST AND UNSTABLE AIRMASS IN
PLACE OVER SOUTHWEST IOWA. BY THIS AFTERNOON...A WARM FRONT WILL BE
MOVING NORTH ACROSS THE STATE.  WARMER AIR AND AN INCREASE IN
MOISTURE WILL INCREASE INSTABILITY ACROSS THE FORECAST AREA.
ANOTHER COMPLEX OF THUNDERSTORMS IS EXPECTED TO DEVELOP ACROSS SOUTH
DAKOTA AND NEBRASKA OVERNIGHT AND MOVE EAST INTO IOWA.  WITH
MODERATE INSTABILITY IN PLACE...A FEW STORMS OVERNIGHT MAY PRODUCE
HAIL UP TO ONE AND A HALF INCHES IN DIAMETER AS WELL AS WIND GUSTS
APPROACHING 65 MPH.  A SEVERE THUNDERSTORM WATCH IS IN EFFECT UNTIL
300 PM THIS AFTERNOON.

PRECAUTION--SPOTTER ACTIVATION WILL BE NEEDED FOR THE REMAINDER OF
THE MORNING AND THIS AFTERNOON SOUTH OF HIGHWAY 20 AND WEST OF
INTERSTATE 35. ACTIVATION MAY ONCE AGAIN BE NEEDED BY THIS EVENING
AND INTO THE OVERNIGHT.  CONTINUE TO MONITOR THE SITUATION AS
CONDITIONS MAY CHANGE AS THIS SYSTEM EVOLVES.

TC