Severe Thunderstorm Warning 67, NWS Des Moines, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
330 PM CDT MON JUN 12 2000

MAIN CONCERN IS SEVERE WEATHER THREAT TUESDAY AFTERNOON AND EVENING.
GENERALLY FOLLOWING AN ETA/AVN BLEND FOR THIS FORECAST.

SOME BETTER ORGANIZED CONVECTION NOW FIRING ALONG THE FRONTAL
BOUNDARY BISECTING OUR CWA. MOST OF THIS ACTIVITY SHOULD BE CONFINED
TO THE CENTRAL PORTION OF OUR AREA THROUGH EARLY EVENING. WILL KEEP
SCATTERED PRECIPITATION WORDING THROUGH THE EVENING AND CARRY
ISOLATED WORDING SOUTH OF FRONTAL BOUNDARY.

ON TUESDAY AREA LOOKS TO BE PRIMED FOR A SEVERE WEATHER EVENT. VERY
UNSTABLE AIR MASS WILL BE PRESENT. MODELS INDICATE THAT SOME INITIAL
WARM CAPPING LAYER AIR WILL SHIFT TO THE EAST ALLOWING A CORRIDOR OF
UNCAPPED AIR IN ADVANCE OF THE COLD FRONT MOVING INTO NORTHWEST IA
LATE AFTERNOON INTO EARLY EVENING. AGREE WITH KARX IN TAKING THE
SLOWER MOVEMENT OF THE FRONT AS VERY STRONG UPPER WINDS ON THE BACK
SIDE OF THE APPROACHING UPPER TROUGH SHOULD YIELD A SLOWER MOVEMENT.
BIG QUESTION TOMORROW IS WEATHER CONVECTION OVER WESTERN IOWA WILL
REMAIN CELLULAR OR QUICKLY DEVELOP INTO A SQUALL LINE. WINDS WILL BE
VERY CONDUCIVE TO ROTATING SUPERCELLS. ALSO WILL CELLS DEVELOP IN
ADVANCE OF THE FRONT OR WILL THE ONLY ACTION BE RIGHT ALONG THE
FRONT. IF IT IS THE LATER...THEN ACTION WOULD PROBABLY BE CONFINED
TO THE WESTERN CWA DURING THE LATE AFTERNOON AND HOLD OFF IN THE
EAST TIL THE EVENING. EITHER WAY...WE WILL PROBABLY HAVE SEVERE
STORMS. SO WILL KEEP GOOD CHANCE TO LIKELY GOING IN ALL AREAS
TUESDAY AFTERNOON AND NIGHT. COLD POOL AND A LITTLE QG FORCING ON
WEDNESDAY WILL NECESSITATE MENTION OF POPS ON WEDNESDAY. MOS TEMPS
LOOK FAIRLY GOOD.

IN THE EXTENDED...MRF CONTINUES TO SHOW A STRONG SYSTEM COMING
THROUGH FRIDAY. UPPER TROUGH OVER AREA SATURDAY SO WILL KEEP MENTION
OF SHOWERS ON SATURDAY.

.DSM...SEVERE THUNDERSTORM WATCH UNTIL 02Z6.

JAW

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-131100-

CENTRAL IOWA THUNDERSTORM OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE DES MOINES IA
1230 PM CDT MON JUN 12 2000

...TODAY...
HAZARD--ISOLATED SEVERE THUNDERSTORMS WITH WIND GUSTS NEAR 60 MPH
AND HAIL UP TO ONE INCH IN DIAMETER.

AREA OF CONCERN--ALL OF THE CENTRAL THIRD OF IOWA...ESPECIALLY THOSE
AREAS NORTH OF HIGHWAY 34 AND SOUTH OF HIGHWAY 20.

TIMING--THIS AFTERNOON THROUGH 8 PM.

METEOROLOGICAL COMMENTS--A WARM FRONT CONTINUES TO EXTEND EAST TO
WEST ACROSS CENTRAL IOWA. THIS FRONT WILL SLOWLY LIFT NORTH
THROUGHOUT THE AFTERNOON AND EVENING. IN ADDITION...A WEAK UPPER
LEVEL DISTURBANCE ALONG THE MISSOURI RIVER WILL MOVE OVER THE STATE
TODAY. AN UNSTABLE AIRMASS PERSISTS OVER MUCH OF IOWA...ESPECIALLY
CENTRAL AND SOUTHERN PORTIONS OF THE STATE. A WEAK SMALL SCALE
SURFACE LOW PRESSURE SYSTEM OVER SOUTHEAST IOWA CONTINUES TO ENHANCE
CONVERGENCE ALONG THE FRONT...ESPECIALLY OVER CENTRAL IOWA. AS THE
UPPER DISTURBANCE TRAVERSES THE STATE...SCATTERED THUNDERSTORMS ARE
EXPECTED TO DEVELOP NEAR THE FRONTAL BOUNDARY. WIND SPEEDS OVER THE
LOWER TEN THOUSAND FEET OF THE ATMOSPHERE REMAIN VERY LIGHT. THIS
WILL LIMIT THE MAIN ISOLATED SEVERE THREAT TO LARGE HAIL AND SUDDEN
DOWNBURST WINDS. IN ADDITION...THE SLOWER MOVEMENT OF THE STORMS
COULD PRODUCE SOME LOCALLY HEAVY RAINFALL.

SPOTTER PLANNING--SPOTTER AND AMATEUR RADIO GROUPS MAY BE NEEDED
DURING THE AFTERNOON AND EARLY EVENING HOURS.

...OUTLOOK FOR TUESDAY...
HAZARD--A FAST MOVING SQUALL LINE OF THUNDERSTORMS WITH DAMAGING
WINDS AND LARGE HAIL AND THE POSSIBILITY OF TORNADOES.

AREA OF CONCERN--ALL OF CENTRAL IOWA.

TIMING--THIS LINE OF THUNDERSTORMS IS EXPECTED TO DEVELOP ON
TUESDAY AFTERNOON AHEAD OF A COLD FRONT AND THE LINE WILL MOVE
SOUTHEAST THROUGH THE EVENING.

METEOROLOGICAL COMMENTS--A STRONG UPPER LEVEL SYSTEM ACCOMPANYING A
COLD FRONT WILL MOVE INTO VERY MOIST AND VERY UNSTABLE AIR OVER
IOWA.  INSTABILITY AND WIND SHEAR FACTORS ARE FORECAST TO BE HIGH AND
FAVORABLE FOR SEVERE WEATHER DEVELOPMENT OVER MUCH OF CENTRAL IOWA.

SPOTTER PLANNING--EMERGENCY MANAGERS MAY WANT TO BEGIN TO TAKE THE
NECESSARY PLANNING STEPS IN PREPARATION FOR THIS EVENT. AMATEUR
RADIO AND SPOTTER GROUPS SHOULD ALSO KEEP ADVISED ON THE LATEST
INFORMATION CONCERNING THIS EVENT.

JW