Severe Thunderstorm Warning 252, NWS Des Moines, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
205 PM CDT MON JUN 18 2001

SHORT TERM...
FORECAST CONVECTIVE INITIATION AREA SEEMS WELL AT HAND ALONG WARM
FRONT BOUNDARY EAST OF SURFACE LOW ACROSS EC SD TO NEAR TWIN CITIES,
AS OUTFLOW BOUNDARY NEARLY COMPLETELY MIXED OUT IN NORTHERN IA.
WHEN? IS THE BIGGER QUESTION GIVEN LARGE CIN STILL EVIDENT ON OAX
SOUNDING.  CORN CROP IS IN CANOPY OVER MANY AREAS OF STATE, DOING
IT'S MAGIC TO ADD SOME SHALLOW LOW LEVEL MOISTURE.  19Z DEW POINTS
IN THE UPPER 60S TO NEAR 70 IN A FEW AREAS, ESPECIALLY ALONG AND
JUST S OF WARM FRONT.  WEAKER CAP IN MN/SD, SO 21-00Z TIME FRAME LOOKS
GOOD THERE, THEN BACK BUILDING DOWN DEEPENING MOISTURE CONVERGENCE
ALONG AN EST-SUX LINE OR THEREABOUTS AFTER 00Z.  310K ISENTROPIC
MOISTURE CONVERGENCE IS RATHER STRONG FOCUSING BEST DEVELOPMENT IN
NW INTO WC AND SW THROUGH THIS EVENING AS 50KT IMPINGES BOUNDARY
WHERE WINDS SLOW TO 20KT.  THERE IS A RISK THAT CONVECTION FORMS A
BREAK SOMEWHERE IN CENTRAL IA AS MAIN SHORTWAVE PUSHES E INTO WI
WHILE WESTERN PORTION OF COLD FRONT ALIGNS MORE ENE-WSW ACROSS W AND
SW IA INTO SE NEBRASKA.  CAPES ARE AMPLE FOR LARGE HAIL, WITH RATHER
HIGH DCAPES FOR STRONG DOWNDRAFTS VIA SPC MESOANALYSIS WEB PAGE.
LFCS RATHER HIGH, SO WOULD SUSPECT TORNADO THREAT MUCH HIGHER ALONG
MN/WI BOUNDARY THAN IN IA.  SUPERCELLS WILL NEED TO HAVE TIME TO
MOISTEN LOW LEVELS AND MODIFY NEAR STORM ENVIRONMENT FOR HIGHER
TORNADO THREAT IN OUR CWA.  HEAVY RAIN THREAT STILL COULD BE
LOCALIZED WHERE TRAINING CAN OCCUR IN SW IA LATER TONIGHT.  IN THE
ZONE FORECAST, WILL GO WITH SCT N HALF THIS EVENING, WITH 50-60 POP
SW AND 30 POP SE WHERE "SPLIT" MAY INDEED OCCUR.

LONGER TERM...
TUESDAY...SURFACE OUTFLOW AND SYNOPTIC BOUNDARY SHOULD BE ALONG
IA/MO BORDER, OR QUITE POSSIBLY WELL INTO MO.  WILL CARRY A DAY LONG
POP SE, WITH MAINLY MORNING TO EARLY AFTERNOON SHRA WITHIN GENERAL
ISENTROPIC ASCENT IN MID LEVELS INTO CENTRAL IA.  DON'T FORESEE
PRECIP N SECTIONS.  ALL MODELS PUSH LOW LEVEL BAROCLINIC ZONE S OF
IA ON WEDNESDAY.  WILL TREND TO AT OR SLIGHTLY ABOVE GUIDANCE TEMPS.
NW FLOW SHORTWAVE IS WORRISOME ON WED NIGHT AND THURSDAY, WHICH
COULD BRING BOUNDARY BACK N A BIT FURTHER THAN PROGGED. A SECONDARY
COOL FRONT DROPS INTO THE STATE, WITH SOME Q-VECTOR CONVERGENCE,
SOME WEAK TO MODERATE INSTABILITY IF HEATING CAN OCCUR.  THEREFORE,
AM INCLINED TO PUT A LOW POP IN FORECAST IN LINE WITH MAV GUIDANCE.

.DSM...NONE

KULA

CENTRAL IOWA THUNDERSTORM OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
330 CDT MON JUN 18 2001

A LOW PRESSURE CENTER NEAR SIOUX FALLS SOUTH DAKOTA WILL MOVE TO
DULUTH BY MIDNIGHT...WHILE A COLD FRONT MOVES FROM NORTHWEST TO
SOUTHEAST ACROSS IOWA BY TUESDAY MORNING. LOW-LEVEL MOISTURE HAS
INCREASED ACROSS IOWA PRODUCING MODERATE INSTABILITY FOR
THUNDERSTORM DEVELOPMENT. HOWEVER...DESPITE TEMPERATURES AROUND 90
DEGREES...A CAPPING LAYER OF WARM AIR ALOFT MAY SLOW THUNDERSTORM
DEVELOPMENT AND DECREASE THE AREAL COVERAGE. IF THUNDERSTORMS
DEVELOP...MODERATE WIND SHEAR FAVORS SOME SEVERE WEATHER...
ESPECIALLY FROM HIGHWAY 30 NORTH.

THUNDERSTORMS ARE EXPECTED TO DEVELOP FROM SOUTHERN MINNESOTA INTO
NORTHWEST IOWA AROUND 6 PM AND THEN TRACK SOUTHEASTWARD. THE THREAT
FOR DAMAGING WINDS AND HAIL IS GREATEST IN THE WEBSTER
CITY...WATERLOO AND MASON CITY AREAS. STORMS WILL BE SLOWER MOVING
TO THE WEST AND SOUTHWEST OF DES MOINES...SO HEAVY RAINS AND FLASH
FLOODING MAY ALSO OCCUR.

SEVERE THUNDERSTORM OR TORNADO WATCHES MAY BE ISSUED FOR PORTIONS OF
IOWA THIS EVENING INTO THE OVERNIGHT HOURS. LISTEN FOR LATER
NATIONAL WEATHER SERVICE FORECASTS AND STATEMENTS FOR UPDATED
INFORMATION AS THE EVENT NEARS.

SEE SITE SPECIFIC DETAILS BELOW FOR MORE INFORMATION.

IAZ004>007-015-016-023-024-033-034-044-045-191100-
CALHOUN-CARROLL-CRAWFORD-EMMET-HANCOCK-HUMBOLDT-KOSSUTH-PALO
ALTO-POCAHONTAS-SAC-WINNEBAGO-WORTH-
INCLUDING THE CITIES OF...ALGONA...CARROLL...DENISON AND ESTHERVILLE
606 AM CDT MON JUN 18 2001

* EVENT TIMING... FROM 7 PM THROUGH MIDNIGHT

* THREAT CATEGORIES...
     TORNADO.......... LOW
     DAMAGING WINDS... MODERATE
     LARGE HAIL....... MODERATE
     FLASH FLOODING... LOW

* SPOTTER ACTIVATION OUTLOOK... MODERATE

IAZ017-025>028-035>039-046>050-057>062-070>073-081-191100-
ADAIR-ADAMS-AUDUBON-BLACK HAWK-BOONE-BREMER-BUTLER-CASS-CERRO
GORDO-DALLAS-FRANKLIN-GREENE-GRUNDY-GUTHRIE-HAMILTON-HARDIN-JASPER-
MADISON-MARSHALL-POLK-POWESHIEK-STORY-TAMA-WARREN-WEBSTER-WRIGHT-
INCLUDING THE CITIES OF...AMES...ATLANTIC...BOONE...DES MOINES...FORT
DODGE...GRINNELL...INDIANOLA...IOWA FALLS...MARSHALLTOWN...MASON
CITY...NEWTON...PERRY...WATERLOO...WAVERLY AND WEBSTER CITY
606 AM CDT MON JUN 18 2001

* EVENT TIMING... FROM 9 PM INTO THE NIGHTTIME HOURS

* THREAT CATEGORIES...
     TORNADO.......... LOW
     DAMAGING WINDS... MODERATE
     LARGE HAIL....... MODERATE
     FLASH FLOODING... MODERATE

* SPOTTER ACTIVATION OUTLOOK... MODERATE

IAZ074-075-082>086-092>097-191100-
APPANOOSE-CLARKE-DAVIS-DECATUR-LUCAS-MAHASKA-MARION-MONROE-RINGGOLD-
TAYLOR-UNION-WAPELLO-WAYNE-
INCLUDING THE CITIES OF...CENTERVILLE...CHARITON...CRESTON...
KNOXVILLE...LAMONI...OSKALOOSA...OTTUMWA AND PELLA
606 AM CDT MON JUN 18 2001

* EVENT TIMING... MIDNIGHT INTO THE EARLY MORNING HOURS

* THREAT CATEGORIES...
     TORNADO.......... LOW
     DAMAGING WINDS... LOW
     LARGE HAIL....... LOW
     FLASH FLOODING... MODERATE

* SPOTTER ACTIVATION OUTLOOK... MODERATE

JUNGBLUTH

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THREAT KEY:
    TORNADO.......... ANY TORNADO OCCURRENCE
    DAMAGING WINDS... WINDS >= 58 MPH
    LARGE HAIL....... HAIL >= 3/4 INCH DIAMETER
    FLASH FLOODING... FLOODING CAUSED BY HEAVY RAINS WITHIN 6 HOURS
                      OF RAIN EVENT. POSES THREAT TO LIFE OR
                      PROPERTY.

THREAT CATEGORY KEY:  PROBABILITY OF THREAT OCCURRING WITHIN 25
                      MILES OF POINT WITHIN FORECAST AREA
    NONE....... NOT EXPECTED.  LESS THAN 5% ( 2% FOR TORNADOES)
    LOW........ 5-15% (2-15% FOR TORNADOES)
    MODERATE... 25-35%
    HIGH....... 45% OR GREATER

SPOTTER ACTIVATION OUTLOOK KEY:
    NOT EXPECTED
    LOW........ ACTIVATION POSSIBLE BUT NOT LIKELY
    MODERATE... POSSIBILITIES ABOUT 50-50
    HIGH....... ACTIVATION LIKELY

FOR MORE INFORMATION ON PROBABILISTIC SEVERE WEATHER OUTLOOKS...GO
TO THE STORM PREDICTION CENTER WEBSITE AT
WWW.SPC.NOAA.GOV/PRODUCTS/OUTLOOK/PROBIN&FO.HTML .