Severe Thunderstorm Warning 353, NWS Des Moines, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
304 PM CDT FRI SEP 21 2007

.SHORT TERM /THROUGH TONIGHT/...
COMPLICATED FORECAST TONIGHT.  COLD FRONT IS CURRENTLY FROM A LAKE
MILLS TO A SAC CITY LINE MOVING EAST AT 25 KTS OR SO.  18Z SOUNDINGS
SHOWED A TREMENDOUS CAP OVER THE CWA AND THE MESOANALYSIS PAGE
INDICATED SOME PRETTY STRONG CIN OVER THE CWA AS WELL.  LOCAL RADARS
SHOWING THINGS HAVING A TOUGH TIME GETTING STARTED BUT THINGS ARE
STARTING TO PERCOLATE NOW.  DEFINITELY BETTER DYNAMICS AND LIFT NE
OF THE CWA AND THE FURTHER SOUTH YOU GO THE LESS THE BOUNDARY HAS
GOING FOR IT.  CURRENTLY LOOKING AT THE FAR NE CORNER OF THE  CWA
FOR POTENTIAL...AROUND THE MCW/ALO AREA BUT THIS WOULD BE FOR A
LIMITED TIME THIS AFTERNOON BEFORE PUSHING INTO EASTERN IOWA WHERE
THE BETTER AIRMASS IS.  FOR THE GRIDS...I WILL DOWNPLAY THE THUNDER
OVER THE WESTERN TWO THIRDS OF THE CWA TONIGHT WITH JUST AN ISOLATED
MENTION AND KEEP THE MORE AGGRESSIVE WORDING FOR THE MCW/ALO AREA.
THE FRONT SHOULD EXIT THE FAR SERN CWA BY 02Z WITH RAPIDLY CLEARING
SKIES.  GENERALLY WENT BELOW GUIDANCE TONIGHT...ESPECIALLY NW WHERE
CAA HAS BEEN IN PLACE FOR A FEW HOURS.

.LONG TERM /SATURDAY-FRIDAY/...
REALLY ONLY ONE PRECIPITATION EVENT TO FOCUS ON DURING THIS PERIOD.
WHILE NOT QUITE YET A SLAM DUNK...IT DOES APPEAR THAT THE BEST
CHANCE OF RAIN AND THUNDERSTORMS WILL OCCUR MONDAY AND MONDAY NIGHT.
TIMING DOES HINGE GREATLY ON THE ABSORPTION OF THE LARGE CUT OFF
UPPER LEVEL LOW SPINNING OFF THE SRN CA COAST AT THIS TIME.
EXPERIENCE HAS SHOWN THAT THESE LARGE LOWS DO NOT GET UP AND GO AS
FAST AS MODELS WANT TO MAKE THEM. ON THE OTHER HAND IT IS HARD TO
IGNORE THE AMAZING CONSISTENCY BETWEEN THE GFS AND NAM REGARDING
THIS FEATURE. SINCE THE ECWMF/UK/ENSEMBLE PROGS ARE SHOWING A
SLIGHTLY SLOWER MOVEMENT...I TRIED TO LEAN THIS FORECAST PACKAGE IN
THEIR DIRECTION. THE OTHER WRINKLE WITH THIS EVENT IS THE ENTRAINING
AND ABSORPTION OF THE GULF COAST TROPICAL SYSTEM EXPECTED TO GET
PICKED UP BY LONG WAVE UPPER LEVEL TROUGH MOVING EAST ACROSS THE
ROCKIES. MODELS APPEAR TO BE IN DECENT AGREEMENT SHOWING THESE
FEATURES COMING TOGETHER IN OUR VICINITY MONDAY OR MONDAY NIGHT. SO
WITH BROAD SYNOPTIC SCALE LIFT IMPINGING ON THE AREA...ALONG WITH
FRONTAL FORCING AND PRECIPITABLE WATER VALUES 200 PERCENT OF
NORMAL...SHOWERS AND THUNDERSTORMS ARE A GOOD BET...WITH LOCALLY
HEAVY RAIN POSSIBLE. HAVE INCREASED POPS INTO LIKELY CATEGORY FOR
MONDAY...AND COULD VERY WELL HAVE GONE LIKELY MONDAY NIGHT TOO...BUT
LEFT IN HIGH CHANCE CATEGORY FOR BETTER COLLABORATION WITH
SURROUNDING OFFICES.

OTHERWISE...SATURDAY WILL BE A PERFECT DAY IF YOU LIKE SUNNY AND
HIGHS IN THE 70S WITH LIGHT WINDS. SOUTHERLY RETURN FLOW KICKS BACK
IN LATE SATURDAY NIGHT AND SUNDAY...BOOSTING TEMPS AND HUMIDITY
LEVELS. SUNDAY WILL LIKELY BE ANOTHER DAY WITH HIGHS WELL INTO THE
80S. CLOUD COVER INCREASES SUNDAY NIGHT WITH THE CHANCE OF RAIN ALSO
INCEASING...SO SMALLER DIURNAL RANGE IN STORE FOR SUN NIGHT AND
MONDAY. A BROAD TROUGH OR ZONAL FLOW SETS UP ACROSS THE NRN CONUS IN
THE WAKE OF MONDAYS SYSTEM. THIS WILL KEEP OUR AREA DRY AND
SEASONABLE THROUGH AT LEAST THURSDAY NIGHT. ANOTHER TROUGH IS
EXPECTED TO DIG THROUGH THE NRN ROCKIES BY FRIDAY AND THIS MAY BRING
US A CHANCE OF RAIN FRIDAY. STILL TOO EARLY TO TELL FOR SURE.

.AVIATION...21/18Z
TAFS WERE WRITTEN BEFORE THOSE DOING 18Z SOUNDINGS CAME OUT.
THOUGHT IS PERPLEXING THAT ONLY SHOWERS WERE DEVELOPING DESPITE THE
FAIR AMOUNT OF CIN...GIVEN THE SHEAR AND STRENGTH OF THE COLD FRONT.
18Z SOUNDINGS CAME OUT WITH A +16 DEGREE AREA OF WARM AIR ALOFT SO
IT MAY BE THAT THUNDER MAY NOT DEVELOP...ONLY SHOWERS.  TAFS HAVE
THUNDERSTORMS MENTIONED IN LINE WITH 12Z DATA BUT MAY NEED A
TWEAKING.  CONDITIONS WILL CONTINUE TO BE MONITORED W/R/T
THUNDERSTORM DEVELOPMENT.  WIND GUSTS WILL BE AN ISSUE FOR ALL TAF
SITES REGARDLESS.  FROPA INTO EASTERN IOWA IS FORECAST TO OCCUR
AROUND 00Z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

AVIATION/SHORT TERM...FAB
LONG TERM...MOYER

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
857 AM CDT FRI SEP 21 2007

IAZ005>007-016-017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-221400-
KOSSUTH-WINNEBAGO-WORTH-HANCOCK-CERRO GORDO-POCAHONTAS-HUMBOLDT-
WRIGHT-FRANKLIN-BUTLER-BREMER-SAC-CALHOUN-WEBSTER-HAMILTON-HARDIN-
GRUNDY-BLACK HAWK-CRAWFORD-CARROLL-GREENE-BOONE-STORY-MARSHALL-
TAMA-AUDUBON-GUTHRIE-DALLAS-POLK-JASPER-POWESHIEK-CASS-ADAIR-
MADISON-WARREN-MARION-MAHASKA-ADAMS-UNION-CLARKE-LUCAS-MONROE-
WAPELLO-TAYLOR-RINGGOLD-DECATUR-WAYNE-APPANOOSE-DAVIS-
857 AM CDT FRI SEP 21 2007

THIS HAZARDOUS WEATHER OUTLOOK IS FOR MUCH OF CENTRAL IOWA.

.DAY ONE...TODAY AND TONIGHT

SCATTERED THUNDERSTORMS ARE EXPECTED BY THIS AFTERNOON. THE STORMS
ARE EXPECTED TO FORM ALONG AN LINE THAT WILL SWEEP ACROSS CENTRAL
IOWA BY EARLY THIS EVENING. SOME STORMS WILL BE SEVERE WITH DAMAGING
WIND THE MAIN THREAT.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

ADDITIONAL THUNDERSTORMS ARE EXPECTED LATE SUNDAY NIGHT THROUGH
EARLY TUESDAY. SOME OF THESE STORMS MAY BE SEVERE AND MAY ALSO
PRODUCE LOCALLY HEAVY RAINFALL.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION IS LIKELY BETWEEN 1 PM AND 7 PM TODAY. SPOTTERS
MAY ALSO BE NEEDED NEXT MONDAY.

IAZ004-015-221400-
EMMET-PALO ALTO-
857 AM CDT FRI SEP 21 2007

THIS HAZARDOUS WEATHER OUTLOOK IS FOR A SMALL PORTION OF CENTRAL
IOWA.

.DAY ONE...TODAY AND TONIGHT

SCATTERED THUNDERSTORMS ARE EXPECTED BY NOON. AN ISOLATED SEVERE
STORM WILL BE POSSIBLE WITH DAMAGING WIND THE MAIN THREAT.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

ADDITIONAL THUNDERSTORMS ARE EXPECTED LATE SUNDAY NIGHT THROUGH
EARLY TUESDAY. SOME OF THESE STORMS MAY BE SEVERE AND MAY ALSO
PRODUCE LOCALLY HEAVY RAINFALL.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION IS NOT ANTICIPATED TODAY...HOWEVER SPOTTERS MAY
BE NEEDED NEXT MONDAY.

FAB

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
518 PM CDT FRI SEP 21 2007

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0515 PM     TSTM WND DMG     NORWALK                 41.50N 93.68W
09/21/2007                   WARREN             IA   TRAINED SPOTTER

            8INCH DIAMETER TREE LIMB DOWN IN THE CITY OF NORWALK.

&&

FBOKSA

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
519 PM CDT FRI SEP 21 2007

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0518 PM     DOWNBURST        2 S NORWALK             41.47N 93.68W
09/21/2007                   WARREN             IA   LAW ENFORCEMENT

            POWER POLE DOWNED IN RURAL WARREN COUNTY.

&&

FBOKSA

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
523 PM CDT FRI SEP 21 2007

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0508 PM     TSTM WND DMG     WAVERLY                 42.73N 92.47W
09/21/2007                   BREMER             IA   EMERGENCY MNGR

            TREES AND LARGE LIMBS DOWN.

&&

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