Severe Thunderstorm Warning 177, NWS Des Moines, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
318 AM CDT SUN AUG 6 2000

TOUGH FCST AHD WITH MANY THINGS TO CONSIDER AS IA RMNS ON SRN EDGE
OF WLYS. WATER VAPOR IMAGERY SHOWS SVRL SHRT WVS FM NE PNHDL THRU
DAKOTAS...WITH OTHER UPSTREAM WVS THRU MT AND WY.  EVEN THOUGH LOW
LVL SUPPORT IS NIL AND OUTFLOW WELL AHD OF TSTMS...DYNAMIC EFFECTS
OF SHRT WVS THRU UPR MO VLY ARE APPARENTLY ENOUGH TO KEEP ELEVATED
TSTMS JUST ENTERING NWRN IA GOING.  THUS XPC THESE TO PROGRESS ACRS
NRN SXNS THIS MRNG.  07Z SFC ANLYS INDCS OUTFLOW HAS ENTERED
NWRN FA.  OTRW CNVGNC ALG WEAK CDFNT FM KS ACRS ERN NE INTO NRN MN
RELATIVELY MINIMAL.  WEAK CNVGNC ALSO NOTED ALG SUBTLE WRMFNT FM IA
TO IN/MI BORDER.  QUESTION FOR LATER TDA WL TURN TO WHETHER LEFT OVR
MESOSCALE BNDRYS AND APCHG FNT WL BE STG ENOUGH TO GENERATE MORE
TSTMS.  ETA LOW LVL CNVGNC MUCH WEAKER THAN AVN BUT EXTRAPOLATION OF
SHRT WV INVOF NE/SD SHOULD MAKE IT TO FNTL PSN BY LATE AFTN SO XPC
MORE DVLPMT CONSIDERING AVBL LOW LVL MSTR AND POTENTIAL
INSTABILITY.  PLACEMENT WL BE DIFFICULT BUT BEST GENERAL LOCATION
WOULD APPR TO BE OVR SERN SXNS OF FA.

FOR TNGT FNT SHOULD FINALLY PUSH S OF FA GIVING US A TEMPORARY
BREAK.  HWVR LOW LVL RIDGING IS SHRT LIVED WITH RTRN FLOW BGNG AGN
SUN NGT.  00Z AVN WEAK WITH THETA-E ADVCTN AS IS QUICK LOOK AT NEW
06Z ETA BUT CONFIDENCE NOT HIGH ENOUGH TO DROP...SO WL CONT GOING
LOW POPS.  SITUATION MON BCMS MORE DIFFICULT.  BOTH AVN AND ETA
GENERATE SGFNT MCS ACRS UPR MS VLY...BUT EXACT LOCATION UP IN THE
AIR WITH AVN FARTHER N.  00Z ETA SEEMS TO PICK UP ON MT/WY SHRT WV
BETTER WHICH MAY PUSH BACLIN ZN A BIT FARTHER S SO WL GO WITH ETA
QPF PLACEMENT THRU HEART OF FA MON INTO MON NGT. AVN ALSO APPRS TO
SUFFER FM CONVECTIVE FEEDBACK.  WL NOT GET TOO AGRESSIVE BUT
DEFINATELY RAISE LOW POPS AND MENTION HVY RAIN POSSIBILITIES.  00Z
ETA PWS ARND 2 INCHES...SHOWALTER INDICES AS LOW AS MINUS 9 AND LOW
LVL JET FEEDING RGT INTO IA.  GUID POPS ALL FAIRLY LOW THRU PD AND
ARE REALLY NO HELP.  NO SGFNT CHGS TO EXTENDED.

TEMP WISE FWCS AGN SEEM TOO WRM...SCPLLY TDA CONSIDERING WL START
OUT WITH FOG AND LGT WNDS.
,
.DSM...NONE

SMALL

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-061100-

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE DES MOINES IA
1259 AM CDT SUN AUG 6 2000

HAZARD--SCATTERED THUNDERSTORMS WITH THE POSSIBILITY OF SOME
SEVERE...WITH HAIL UP TO GOLF BALL SIZE AND DAMAGING WINDS TO AROUND
60 MPH.

AREA OF CONCERN--NORTH OF A LINE RUNNING FROM DENISON TO
WATERLOO.

TIMING DETAILS--2 AM TO 6 AM.

METEOROLOGICAL COMMENTS--MOIST AND VERY UNSTABLE AIR MASS IS IN PLACE
OVER IOWA WITH DEW POINTS FROM 65 TO 70 ACROSS NORTHERN
SECTIONS OF THE STATE. A UPPER LEVEL DISTURBANCE DROPPING SOUTHEAST
ACROSS EASTERN SECTIONS OF SOUTH DAKOTA IS FORECAST TO ENTER
NORTHWEST SECTIONS OF THE STATE DURING THE PRE-DAWN HOURS. A LINE OF
THUNDERSTORMS...SOME SEVERE ARE CURRENTLY DROPPING SOUTHEAST THROUGH
SOUTH DAKOTA. THIS LINE IS EXPECTED TO REACH EMMET COUNTY AFTER 2
AM. THE VERTICAL SHEAR WILL BE MARGINAL...HOWEVER INSTABILITY WILL BE
SUFFICIENT TO OFF SET THE WEAKER SHEAR TO PRODUCE SOME SEVERE
THUNDERSTORMS. THE POTENTIAL FOR TORNADOES IS SMALL OVER NIGHT.

PRECAUTIONS--SPOTTER ACTIVATION MAY BE NEEDED LATER THIS MORNING
AFTER 2 AM. PEOPLE INVOLVED IN OUTDOOR ACTIVITIES SHOULD LISTEN FOR
NATIONAL WEATHER SERVICE FORECASTS AND STATEMENTS FOR UPDATED
INFORMATION.