Severe Thunderstorm Warning 388, NWS Omaha, NE

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
250 AM CDT SUN AUG 12 2001

FORECAST CHALLENGES -- PRECIPITATION CHANCES

SYNOPSIS -- LATEST KOAX REFLECTIVITY LOOP SHOWS A NARROW BAND OF
CONVECTION OVER NERN NEB, SLOWLY MOVING SEWD.  06Z RUCII DATA
SUGGESTS THIS CONVECTION IS OCCURRING ALONG A WEAK 850MB CONVERGENCE
ZONE IN AN AREA OF WEAK LOW-LEVEL WARM ADVECTION.  MEANWHILE, EARLY
MORNING MOISTURE CHANNEL IMAGERY DEPICTS A RATHER ACTIVE PATTERN
FROM THE GREAT LAKES WWD INTO THE CNTRL ROCKIES.  THE FEATURE OF
INTEREST IS A SHORTWAVE MOVING OUT OF WY INTO WRN PARTS OF NEB AND
SD.  AT THE SURFACE, 07Z MSAS OBJECTIVE ANALYSIS SHOWS A WEAK LOW
OVER NERN SD WITH AN ATTENDANT SURFACE TROUGH EXTENDING SWD INTO
NERN NEB, THEN WWD TO A SECONDARY LOW W OF BFF.

FORECAST -- WE WILL FOLLOW THE AVN THIS FORECAST, DUE TO:  1) ITS
BETTER VERIFICATION ON THE 6-HR QPF FIELDS, AND 2) ITS MAGNITUDE
AND HANDLING OF THE WY SHORTWAVE.  IN THE SHORT TERM, WE WILL
MONITOR RADAR AND SATELLITE TRENDS IN REGARD TO THE ONGOING
CONVECTION OVER NERN NEB AND WORD THIS MORNINGS ZONES ACCORDINGLY.
BY THIS AFTERNOON, THE AVN TRACKS THE WY SHORTWAVE ACROSS N-CNTRL
NEB INTO CNTRL IA.  ALTHOUGH FORECAST SOUNDINGS AND PLAN VIEW CAPE
FIELDS SUGGEST THE PRESENCE OF SOME AFTERNOON, SURFACE-BASED
INSTABILITY ACROSS THE CWA, IT APPEARS THAT INCREASING AMOUNTS OF
MID-LEVEL MOISTURE, STEEPER LAPSE RATES (OBSERVED ON UPSTREAM
SOUNDINGS AT LBF AND UNR) AND LARGE-SCALE ASCENT (SEEN IN Q-VECTOR
FIELDS) WILL BE THE KEYS TO CONVECTIVE DEVELOPMENT.  WE WILL CARRY
THE HIGHEST POPS ACROSS THE NWRN ZONES AND TAPER THEM SEWD.
TEMPERATURES MAY BE HINDERED BY INCREASING MID CLOUDS WITH HIGHS
CLOSE TO GUIDANCE VALUES.

TONIGHT, SOME CONVECTION WILL LIKELY BE ONGOING DURING THE EVENING,
WITH ADDITIONAL DEVELOPMENT POSSIBLE LATER IN THE PERIOD ALONG THE
SURFACE AND 850MB FRONTS WHICH ARE FORECAST TO STALL ALONG THE
KS/NEB BORDER.  WE WILL CARRY CHANCE POPS ACROSS ALL ZONES, BUT JUST
USE EVENING WORDING ACROSS THE N.

MONDAY, FRONTAL BOUNDARY TO THE S OF THE CWA WILL MOVE LITTLE
THROUGH THE DAY WITH A SLIGHT CHANCE OF AFTERNOON THUNDERSTORMS
ACROSS THE FAR SRN ZONES (ESPECIALLY SWRN).  OTHERWISE, TEMPERATURES
SHOULD BE CLOSE TO SUNDAYS HIGHS, IF NOT A LITTLE WARMER.

MONDAY NIGHT, FALLING PRESSURES OVER THE NRN HIGH PLAINS WILL AID IN
THE DEVELOPMENT OF A 30 TO 40KT LOW-LEVEL JET.  INITIALLY, THE JET
AXIS WILL REMAIN TO OUR W, BUT BY MORNING MODEL PROGS SUGGEST A
VEERING TREND WITH INCREASING ISENTROPIC ASCENT (305K SURFACE) OVER
THE CWA.  THE CURRENT FORECAST HAS 20 TO 30 PERCENT POPS GOING AND
THIS LOOKS GOOD.

IN THE EXTENDED, THE 12/00Z RUN OF THE AVN CONTINUES TO ADVERTISE A
SERIES OF SHORTWAVES TRANSLATING SEWD ACROSS THE NRN AND CNTRL
PLAINS THROUGH WEDNESDAY NIGHT, SO THE GOING CHANCE OF PRECIPITATION
LOOKS GOOD.  WE  WILL TAKE A LOOK A THE NEW MRF WHEN IT ARRIVES AND
MAKE ANY CHANGES TO THE REMAINDER OF THE FORECAST IF NECESSARY.

.OMA...NONE

MEAD

IAZ043-055-056-069-079-080-090-091-NEZ011-012-015>018-030>034-
042>045-050>053-065>068-078-088>093-122245-

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
1200 PM CDT SUN AUG 12 2001

THERE IS SOME CHANCE STORM SPOTTERS MAY BE NEEDED THIS AFTERNOON AND
EVENING IN EASTERN NEBRASKA AND SOUTHWEST IOWA.

A DISTURBANCE IN THE UPPER ATMOSPHERE MOVING ACROSS THE PLAINS
ALONG WITH A WEAK SURFACE FRONT MOVING SOUTH ACROSS EASTERN NEBRASKA
AND WESTERN IOWA...WILL GENERATE SHOWERS AND THUNDERSTORMS ACROSS
THE AREA THIS AFTERNOON INTO THIS EVENING.

BASED ON EXPECTED ATMOSPHERIC INSTABILITY AND WIND SHEAR...MOST OF
THE THUNDERSTORMS ARE EXPECTED TO REMAIN BELOW SEVERE CRITERIA.
HOWEVER A FEW OF THE STRONGER STORMS COULD PRODUCE LOCALLY STRONG
WIND GUSTS AND SMALL HAIL.

GW