Severe Thunderstorm Warning 151, NWS Des Moines, IA

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-301700-

CENTRAL IOWA THUNDERSTORM OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
600 AM CDT SAT MAY 30 1998

HAZARD--THUNDERSTORMS WITH LARGE HAIL...DAMAGING WINDS...INTENSE
RAINFALL AND ISOLATED TORNADOES.

AREA OF CONCERN AND TIMING--THE ENTIRE FORECAST AREA FROM 3 TO 9 PM.

METEOROLOGICAL COMMENTS--A NEARLY STATIONARY FRONT FROM CEDAR RAPIDS
TO FORT DODGE AND THEN THROUGH SIOUX FALLS WILL MOVE LITTLE TODAY.
AN INCREASING SOUTHERLY FLOW WILL DRAW 60 TO LOWER 70 DEGREE
DEWPOINTS INTO THE STATE AS A RELATIVELY STRONG UPPER LEVEL SYSTEM
HEADS FOR IOWA THIS EVENING FROM THE ROCKIES. LIFTED INDICES OF -8
TO -12 ARE FORECAST SOUTH OF THE FRONT OVER MOST OF IOWA WITH CAPES
OF 2000-4000 J/KG. WARMER AIR ARRIVING IN THE MID LEVELS WILL
MAINTAIN A CAP OVER SOUTHWEST IOWA THROUGH MOST OF THE DAY...AT LEAST
UNTIL THE UPPER SYSTEM RELEASES THE INSTABILITY THROUGH MID-LEVEL
COOLING AND LOW-LEVEL CONVERGENT FORCING ALONG A BOUNDARY. THIS
BOUNDARY WILL SWEEP EAST ACROSS THE STATE THROUGH THE EVENING.

CURRENT THINKING IS THAT THE DIVIDING LINE FOR A SQUALL LINE TO
DEVELOP WILL BE CLOSE TO CENTRAL IOWA...PROBABLY JUST EAST OF
INTERSTATE 35...ALTHOUGH IT COULD JUST AS WELL FIRE 60 MILES WEST OF
INTERSTATE 35. IT IS TOO CLOSE TO CALL.

IN NORTHERN IOWA...THUNDERSTORMS MAY OCCUR MUCH EARLIER...BUT THE
BEST INGREDIENTS FOR SEVERE WEATHER WILL STILL COME TOGETHER LATER
TODAY. SINCE THE WIND FIELDS WILL BE INCREASING THROUGH THE DAY...
THE LIKELIHOOD FOR STRONG THUNDERSTORMS WITH ROTATING UPDRAFTS WILL
BE MAXIMIZED. ANY WATCHES WILL LIKELY BE TORNADIC. EVENTUALLY THE
STORMS ARE EXPECTED TO FORM INTO A LARGE SQUALL LINE WITH DAMAGING
STRAIGHT LINE WINDS...I.E.,A DERECHO FROM MINNESOTA AND EXTREME
NORTHERN IOWA EASTWARD.

PRECAUTIONS--EMERGENCY MANAGERS SHOULD REMAIN ABREAST OF THE WEATHER
SITUATION THROUGH THE DAY. FAST ACTION MAYBE REQUIRED IF THE CAP
BREAKS EARLIER THAN EXPECTED.

MYERS

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-310130-

CENTRAL IOWA THUNDERSTORM OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
1032 AM CDT SAT MAY 30 1998

HAZARD--THUNDERSTORMS WITH LARGE HAIL...DAMAGING WINDS...VERY HEAVY
RAINFALL AND ISOLATED TORNADOES.

TIMING AND AREA OF CONCERN--THE PRIMARY THREAT FOR SEVERE WEATHER WILL
BE ALONG AND NORTH OF INTERSTATE 80 FROM NOON THROUGH EARLY EVENING.
A LESSER THREAT FOR SEVERE WEATHER WILL OCCUR SOUTH OF INTERSTATE 80
SOUTH TO THE IOWA/MISSOURI BORDER.

METEOROLOGICAL COMMENTS--A VERY UNSTABLE AIRMASS IS IN PLACE OVER IOWA
TODAY.  IN ADDITION...THERE IS A RICH SUPPLY OF MOISTURE ORIGINATING
FROM THE PACIFIC...KNOWN AS THE PINEAPPLE CONNECTION.  AS A SOUTHERLY
WIND INCREASES ACROSS THE STATE THEY WILL INTERACT WITH A NEARLY
STATIONARY FRONT EXTENDING ACROSS THE STATE FROM A DAVENPORT TO SIOUX
FALLS LINE.

LIFTED INDICES OF -8 TO -12 ARE FORECAST OVER THE STATE WITH CAPES
INCREASING TO ALMOST 5000 J/KG AND HELICITY VALUES IN THE 400 TO 500
RANGE.

PRECAUTIONS--EMERGENCY MANAGERS FOR COUNTIES ALONG AND NORTH OF
INTERSTATE 80 SHOULD PREPARE THEIR SPOTTER NETWORKS FOR DEPLOYMENT
AROUND OR JUST AFTER THE NOON HOUR.  EMERGENCY MANAGERS FOR COUNTIES
TO THE SOUTH OF INTERSTATE 80 SHOULD ALERT THEIR SPOTTERS FOR POSSIBLE
DEPLOYMENT LATER THIS AFTERNOON OR EVENING.

FAB