Severe Thunderstorm Warning 209, NWS Des Moines, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
200 PM CDT FRI OCT 13 2000

OVERALL NO SIGNIFICANT CHANGES MADE TO PREVIOUS SET OF ZONES. MODELS
STILL FAIRLY CONSISTENT. PREFER THE AVN/ETA SOLUTION.

H2O VAPOR LOOP SHOWING SHORT WAVE ENERGY NOW ROTATING UP THROUGH
BASE OF UPPER TROUGH. MODELS CONSISTENT IN SHOWING MAIN WAVE WILL
LIFT OVER THE NW PORTION OF THE CWA OVERNIGHT. MUCH MORE SUNSHINE
TODAY THAN WHAT I PREVIOUSLY EXPECTED. THIS HAS ALLOWED A PRETTY GOOD
SURFACE BASED INSTABILITY AXIS TO DEVELOP QUITE FAR NORTH OVER ABOUT
THE WESTERN QUARTER OR THIRD OF IA. THIS SHOULD HELP THUNDERSTORM
CHANCES THIS EVENING AS MOST FORCING FOR THIS EVENT APPEARS TO BE
LOW LEVEL BASED. AS PREVIOUSLY MENTIONED OVERNIGHT...AIR MASS
REMAINS FAIRLY HOMOGENEOUS WITH LITTLE FORCING DUE TO ADVECTIVE
PROCESSES. MODELS MAINTAIN ONLY WEAK QG FORCING OVERNIGHT WITH
STRONG LOW LEVEL CONVERGENCE ALONG THE FRONT OVER WESTERN IA.
APPEARS CONVERGENCE WEAKENS A LITTLE AS SYSTEM MOVES EAST THROUGH
SATURDAY MORNING...ESPECIALLY OVER THE NORTHERN HALF OF IOWA. AS
SHORT WAVE TROUGH ROTATES FURTHER NORTHEAST LATE THIS AFTERNOON AND
EVENING...SURFACE PRESSURE RISES WILL DEVELOP OVER THE CENTRAL
PLAINS ALLOWING FOR AN ACCELERATION OF THE FRONT EAST AND
INTENSIFICATION OF THE LOW LEVEL CONVERGENCE THIS EVENING. BAND OF
MID LEVEL DRYING OVERSPREADING THE AREA THIS EVENING... COUPLED WITH
ENOUGH INSTABILITY AND LOW LEVEL H9-H7 WINDS FROM 35 TO 55
KTS...SHOULD ALLOW FOR A FEW STRONG STORMS. PREVIOUS CITO LOOKS GOOD
WITH AN ISOLATED HAIL OR WIND EVENT POSSIBLE TONIGHT.

BY SATURDAY...STRONG Q-VECTOR DIVERGENCE AND SUBSTANTIAL DEEP
LAYERED DRYING WILL BRING PRECIPITATION TO AN END WITH A RETURN TO
AT LEAST PARTLY SUNNY CONDITIONS. H8 WINDS REMAIN MORE WESTERLY WITH
MUCH LESS COOLING INDICATED. SO WARMER MOS TEMPS LOOK GOOD.

THEN THINGS REALLY GET QUESTIONABLE SATURDAY NIGHT AND SUNDAY. WITH
SURFACE AND H8 FRONTAL BOUNDARIES RESIDING JUST TO THE SOUTH OF IOWA
AND ANOTHER SHORT WAVE TROUGH MOVING EAST OVER THE AREA LATE
SATURDAY NIGHT AND SUNDAY...THERE LOOKS TO BE SOME ISENTROPIC AND
DEF ZONE PRECIPITATION POSSIBLE OVER SOUTHERN IA. MODELS ARE QUITE
VARIED AS TO HOW FAR NORTH THIS PRECIPITATION WILL MAKE IT WITH NAVY
NOGAPS FURTHEST SOUTH OVER CENTRAL MO AND ETA/AVN BRINGING IT ALONG
THE S IA BORDER. WILL MAINTAIN HIGHEST CHANCE POPS OVER THE SOUTHERN
TWO TIER OF ZONES. WILL ALSO KEEP LOW POPS FURTHER NORTH INTO SUNDAY
AS MID LEVEL COLD POOL SKIRTS EXTREME NORTHERN IA. CENTRAL PORTION
OF THE STATE MOST LIKELY AREA TO REMAIN DRY. WITH NORTHEAST LOW
LEVEL FLOW AND PLENTY OF CLOUD COVER SUNDAY...TEMPS SHOULD STRUGGLE
TO REACH 60.

ON THE EXTENDED...HIGHEST CONFIDENCE REMAINS IN THE MONDAY/TUESDAY
PERIOD. BY WEDNESDAY...MRF BRINGS A WEAK CUTOFF LOW THROUGH THE MEAN
FLAT UPPER RIDGE. WILL NOT HOLD MY BREATH ON THAT ONE3. FOR NOW WILL
KEEP THINGS DRY WITH A GRADUAL WARMING TREND. POSITIONS OF SURFACE
RIDGE AXIS ALSO CRITICAL AS TO HOW MUCH WARMING WILL ACTUALLY TAKE
PLACE.

.DSM...NONE

JAW

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-141100-

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE DES MOINES IA
329 PM CDT FRI OCT 13 2000

HAZARD--SCATTERED TO NUMEROUS THUNDERSTORMS...A FEW OF WHICH MAY BE
BRIEFLY SEVERE WITH DAMAGING WINDS NEAR 65 MPH AND HAIL NEAR ONE
INCH IN DIAMETER.

AREA OF CONCERN--ALL OF THE DES MOINES NATIONAL WEATHER SERVICE
FORECAST AREA.

TIMING DETAILS--THUNDERSTORMS SHOULD DEVELOP AND MOVE INTO AREAS WEST
OF INTERSTATE 35 BETWEEN 7 PM AND 11 PM...AND EAST OF INTERSTATE
35 FROM 11 PM TO 3 AM.

METEOROLOGICAL COMMENTS--A SLOW MOVING COLD FRONT OVER EASTERN
NEBRASKA WILL ACCELERATE EAST TONIGHT AS A MID LEVEL STORM SYSTEM
LIFTS NORTHEAST ACROSS THE REGION. A LINE OF THUNDERSTORMS IS
EXPECTED TO DEVELOP BY EARLY EVENING ALONG A LINE FROM FAR NORTHWEST
IOWA INTO SOUTHEAST NEBRASKA. MID LEVEL DRYING WILL ENHANCE BOTH HAIL
AND WIND GUST POTENTIAL WITH SOME THESE STORMS. LOW AND MID LEVEL
WINDS ARE FORECAST TO BE QUITE STRONG...FROM 35 TO 55 KNOTS. IT IS
POSSIBLE THAT SOME STORMS MAY PRODUCE AN ISOLATED STRAIGHT LINE WIND
GUST NEAR 65 MPH. IN ADDITION...ENOUGH COOL AND DRY AIR WILL RESIDE
IN THE MID LEVELS OF THE ATMOSPHERE THAT SOME ISOLATED LARGE HAIL MAY
OCCUR IN THE STORMS. FAIRLY LOW INSTABILITY SHOULD LIMIT THE AREAL
EXTENT OF SEVERE THUNDERSTORMS.

PRECAUTIONS--PEOPLE PLANNING OR PARTICIPATING IN OUTDOOR ACTIVITIES=
TONIGHT SHOULD MONITOR THE WEATHER SITUATION AND BE READY TO TAKE
ACTION IF THREATENING WEATHER DEVELOPS.  ALTHOUGH THE LIKELIHOOD IS
NOT GREAT...SPOTTER ACTIVATION MAY ALSO BE NEEDED TONIGHT...
ESPECIALLY OVER WESTERN AND CENTRAL IOWA.