Severe Thunderstorm Warning 206, NWS Des Moines, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
316 PM CDT FRI SEP 22 2000

CONCERNS THIS SET WL BE POTENTIAL DVLPMT ALG INVOF TRIPLE POINT AND
PCPN TRENDS AFT FROPA.  WATER VAPOR IMAGERY SHOWS LTL UPSTREAM IN
THE WAY OF SHRT WVS TO AFFECT FA TNGT.  HWVR APCHG ELONGATED LG WV
TROF TAKING SHAPE WITH SVRL SHRT WVS EITHER WITHIN SHEAR AXIS OR
DROPPING INTO BACKSIDE.  MOST PRONOUNCED NOW IN BASE OF TROF AND
STILL WELL UPSTREAM INVOF 130W 50N.  19Z SFC ANLYS AND 88D
REFLECTIVELY IN CLEAR AIR MODE PLACES FNTL BNDRY FM CNTRL KS INTO
CNTRL IA WITH TRIPLE POINT BTWN KCSQ AND DSM METRO AREA.  LAPS
THETA-E CNVGNC PLACES WRMFNT FM TRIPLE POINT TO JUST N OF KIRK WITH
WRM SECTOR AND 60+ DWPNTS NOW INTO SRN SXNS.  LAPS FIELDS AND STLT
DERIVED DATA THAT IS AVBL INDC CAPES 1500-3500 ALG AND S OF WRMFNT
BUT ONLY MINIMAL AREA WHERE CAP IN NEARLY BROKEN AND CINS AOB 20
J/KG.  SOME HINT AT ELEVATED CONVECTION IN MSTR FETCH FM KS INTO SRN
IA WHICH MAY HELP GET THINGS STARTED.  IF ANYTHING SFC BASED FORMED
SHRTLY IT WOULD LIKELY BE INVOF KCSQ-KLWD WHERE CNVGNC AND LOWEST
CINS ARE MAXIMIZED AND RECENT RADAR TRENDS POINT THIS WAY.  IF
ANYTHING SGFNT DVLPS NR FNTL BNDRY...0-6KM SHEAR HIGH ENOUGH FOR
INITIAL SUPERCELL DVLPMT THEN EVOLVING INTO SQUALL LN...BUT IF
THINGS FIRE FARTHER S 0-3 KM SHEAR AND LINEAR FORCING WOULD LIKELY
FORM SQUALL LN WHICH REMAINS S OF FA.  WL KEEP LIKELY POPS ALG AND E
OF FNT AND ALSO HOLD ONTO HIGH END CHCS POPS FOR REST OF NGT PER
FOLLOWING DISC.

IN LATER PDS MDLS IN GENERAL AGREEMENT.  ALL HINT AT WV MOVG ALG
FNTL BNDRY PRODUCING MORE PCPN IN ZN OF WEAK/MOD THETA-E ADVCTN AND
FRONTOGENETICL FORCING OVER FNT LATE TNGT INTO SAT.  UPR LVL DVRG IN
RIGHT ENTRACE RGN OF NRN GTLKS JET STERAK SHOULD AID UVM AS WELL.
AFT THAT...LTL SGFNT FORCING EVIDENT BUT LOW CLDS SHOULD HOLD STEADY
FOR COOL...DREARY FALL WX.  WEAK/MOD QG AND FRONTOGENETICAL FORCING
EXISTS ALG BACLIN ZN AND UPR LVL TROF/SHEAR AXIS BUT NOTHING
IMPRESSIVE.  UPR LVL JET SEEMS TO BE FAR ENOUGH S TO KEEP DVRGNC S
OF FA.  THIS LASTS THRU SAT AND THEN DMSHS SAT NGT.  CLDS SHOULD MRN
PERSISTANT HWVR TIL SUN WITH NEXT CHC OF LGT PCPN SUN NGT AS WHATS
LEFT OF CLOSED UPR LOW APCHS.  UVM SEEMS MAINLY MID/HIGH LVL PER QG
FORCING AND INCRG DIFFERENTIAL PVA FIELD.  ATTM WL KEEP CHC POPS FOR
RAIN LATER TNGT INTO SUN WITH LOWEST N AND HIGH END S.  WL ALSO
MENTION A FEW TSTMS SRN SXNS AS SHOWALTERS RMN NEUTRAL TO SLGTLY
UNSTABLE.

TEMP WISE WITH PERSISTANT LOW CLDS WRMR GUID MINS AND COOLER HIGHS
SEEM BEST...LTL DIURNAL RANGE.

IN EXTENDED PD...MRF/12Z AVN/UK HAVE NOW COME TOGETHER WITH TIMING
AND PHASING OF WRN TROF EVOLUTION.  MRF ENSEMBLES IN CLOSE AGREEMENT
TOO.  ALSO SLGTLY FAVOR OLD ECMWF SFC SOLUTION WITH INVERTED TROF
INTO SERN IA BY 12Z MON.  DO NOT FEEL THAT MRF HAS ENOUGH SFC
REFLECTION CONSIDERING STRENGTH OF UPR LVL SYS.  WITH THIS IN MIND
WL REINTRODUCE LGT RAIN CHCS ON EARLY MON.  THREAT NOT GREAT WITH
MAINLY MID/UPR LVL FORCING...BUT DO NOT FEEL CONFIDENT ENOUGH TO GO
WITH DRY FCST CONSIDERING STRENGTH OF UPR LOW.  WITH COLD CORE
OVHD...WL GO WITH LOW END OF MRF GUID SET MON/TUE AND WRMR READINGS
WEDNESDAY.

WORK ZNS SENT ARND 300 AM.

.DSM...NONE.

SMALL

IAZ060>062-072>075-081>086-092>097-231100-

CENTRAL IOWA THUNDERSTORM OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
1052 AM CDT FRI SEP 22 2000

HAZARD--SCATTERED SHOWERS AND THUNDERSTORMS.  ISOLATED SEVERE
STORMS ARE POSSIBLE WITH WINDS TO 60 MPH AND LARGE HAIL.

AREA OF CONCERN--THUNDERSTORMS MAY OCCUR OVER ALL OF CENTRAL IOWA
BUT THE THREAT FOR SEVERE WEATHER WILL REMAIN SOUTH OF A LINE FROM
GRINNELL THROUGH DES MOINES TO CORNING.

TIMING--THUNDERSTORMS ARE EXPECTED TO REDEVELOP AFTER 2 PM WITH THE
SEVERE WEATHER THREAT CONTINUING THROUGH 9 PM.

METEOROLOGICAL COMMENTS--A WARM FRONT ALONG THE IOWA/MISSOURI BORDER
LATE THIS MORNING WILL MOVE NORTH INTO IOWA BY EARLY AFTERNOON.  LOW
PRESSURE WILL RIDE ALONG THIS FRONT INTO CENTRAL IOWA DURING THE
AFTERNOON.  SOUTH OF THE FRONT ACROSS THE SOUTHERN TWO OR THREE
TIERS OF COUNTIES...DEWPOINTS ARE EXPECTED TO CLIMB INTO THE 60S WITH
CAPES RISING TO AS HIGH AS 1500 J/KG. THUNDERSTORMS ARE EXPECTED TO
DEVELOP ALONG THIS FRONT BY MID AFTERNOON.  WITH MODERATE INSTABILITY
AND STRONG LOW LEVEL SHEAR...SOME OF THE THUNDERSTORMS MAY BECOME
SEVERE.  THESE STORMS SHOULD MOVE RAPIDLY NORTHEAST AT 40 TO 50
MPH...THEREFORE ISOLATED WIND GUSTS TO 60 MPH MAY OCCUR AT THE
SURFACE.  IN ADDITION...UPDRAFT STRENGTH SHOULD BE ENOUGH TO PRODUCE
SOME HAIL UP TO QUARTER SIZE.

PRECAUTIONS--SPOTTER ACTIVATION MAY BE REQUIRED LATER THIS AFTERNOON
AND EVENING ACROSS SOUTHERN IOWA.  STAY TUNED TO THE NOAA WEATHER
RADIO OR YOUR FAVORITE MEDIA SOURCE FOR UPDATES TO THE WEATHER.

COGIL