Severe Thunderstorm Warning 270, NWS North Platte, NE

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTH PLATTE NE
311 PM CDT WED SEP 10 2003

...TIMING SYSTEM EAST AND IMPACTS FROM CHALLENGE TONIGHT...

.DISCUSSION...
THIS EVENING MORE ACTIVE WEATHER ANTICIPATED AS COLD FRONT SWINGS
SOUTHEAST AND ENCOUNTERS UNSTABLE AIRMASS...AND INCREASING LIFT
OCCURS AS WAVE LIFTS NORTHEAST OUT OF CO. SURFACE HEATING TODAY WAS
BETTER AS TEMPERATURES TOPPED OUT AROUND 80 WITH THE GREATEST
INSTABILITY AT 19Z JUST NORTHWEST OF OGA WHERE MSAS DATA INDICATES
LIS TO -6 C. SURFACE MOISTURE CONVERGENCE REMAINS FOCUSED ACROSS THE
CENTRAL BUT WITH SURFACE LOW OVER NORTHEAST CO TO BACK WINDS AROUND
TO THE SOUTHEAST BRIEFLY. THEN AS WAVE LIFTS NORTHEAST OUT OF CO THE
PRIMARY FOCUS TO BE ACROSS THE CENTRAL FORECAST AREA THIS EVENING.

AT 18Z SURFACE LOW ACROSS NORTHWEST SOUTH DAKOTA WITH MODELS
PROJECTING THE SYSTEM KICKING NORTHEAST AFT 21Z. SURFACE CONVERGENCE
ACROSS CENTRAL NEBRASKA SHIFTS EAST WITH WINDS BACKING TO THE WEST...
WEST OF A VTN TO LBF LINE. VISIBLE SATELLITE SHOWS CU REFORMING OVER
THE SOUTHWEST WHILE WATER VAPOR IMAGERY INDICATE JETSTREAK LIFTING
NORTHEAST FROM NEAR LIC TO IML.

MODELS ARE IN AGREEMENT THROUGH 18 HOURS WITH FOCUS FOR
CONVECTION AND SEVERE THUNDERSTORM DEVELOPMENT TO BE ACROSS THE
CENTRAL THIS EVENING. THE COLD FRONT ENTERS FROM THE NORTHWEST AND
MOVES SOUTHEAST RAPIDLY AND WILL BE EAST OF CWA BY BETWEEN 9 PM
AND MIDNIGHT. OVERNIGHT THE H5 TROUGH AXIS WILL MOVE EAST INTO THE
PANHANDLE THEN RAPID DRYING FROM THE NORTHWEST TO SOUTHEAST ON
THURSDAY AS SUBSIDENCE INCREASES.

FRIDAY INTO SATURDAY FORECAST IS QUESTIONABLE IN REGARDS TO
POTENTIAL FOR PRECIPITATION AND TEMPERATURES. SPLIT FLOW PATTERN
MAY EVOLVE WITH FORECAST AREA POSSIBLY BETWEEN SYSTEMS INITIALLY.
THEN AS FLOW ALOFT BECOMES MORE ZONAL FLOW LITTLE CHANCE FOR SHOWERS
FRIDAY OR SATURDAY WITH COOLER TEMPERATURES AS SURFACE HIGH BUILDS
SOUTHEAST.

NO SIGNIFICANT CHANGES MADE TO THE EXTENDED PERIOD AS MODELS REMAIN
OUT OF PHASE AND USED A BLEND.

.LBF WATCHES/WARNINGS/ADVISORIES...TORNADO WATCH ZONES 007-009>010-
027>029-038 UNTIL 8 PM CDT.

&&

TLK

NEZ004>010-022>029-035>038-056>059-069>071-094-111000-

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE NORTH PLATTE NE
334 PM CDT WED SEP 10 2003

THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND NORTH CENTRAL
NEBRASKA.

.DAY ONE...TONIGHT

THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS THIS EVENING AND
TONIGHT ACROSS PORTIONS OF NORTH CENTRAL NEBRASKA...EAST OF A LINE
FROM BASSETT TO BROKEN BOW. A SLIGHT RISK OF SEVERE WEATHER EXISTS
AS WELL...EAST OF A LINE FROM VALENTINE TO OGALLALA.

SEVERAL WEATHER FEATURES WILL DEVELOP ACROSS THE OUTLOOK AREA
THROUGH THE EVENING. FIRST...STRONG SOUTHERLY WINDS WILL CONTINUE
TO SUPPLY THE AREA EAST OF A LINE FROM BASSETT TO BROKEN BOW WITH
PLENTY OF LOW LEVEL MOISTURE. SECONDLY...AN AREA OF LOW PRESSURE
WILL MOVE EASTWARD INTO THIS SAME AREA...AND WILL PROVIDE A LIFTING
MECHANISM TO GENERATE THUNDERSTORMS INTO THE EVENING HOURS. THE
MOISTURE ACROSS THE AREA...COMBINED WITH RELATIVELY COOL AIR IN THE
MID AND UPPER LEVELS OF THE ATMOSPHERE...WILL CAUSE UNSTABLE
CONDITIONS...AND WILL ALLOW THE POTENTIAL FOR ANY STORMS THAT
DEVELOP TO BECOME SEVERE RAPIDLY. THE MAIN SEVERE WEATHER THREAT
WILL BE LARGE HAIL AND DAMAGING WINDS...HOWEVER THE POTENTIAL ALSO
EXISTS FOR ISOLATED TORNADOES IN THE MODERATE RISK AREA.

FURTHER WEST...A STRONG COLD FRONT WILL ENTER THE WESTERN PORTION OF
THE OUTLOOK AREA THIS EVENING AS WELL...AND WILL ALSO GENERATE
THUNDERSTORMS ACROSS THE WESTERN PORTION OF THE OUTLOOK AREA THROUGH
THE EVENING. ANY STORMS THAT DEVELOP ALONG THE COLD FRONT WILL ALSO
HAVE THE POTENTIAL TO BECOME SEVERE AS THEY MOVE INTO AREAS WITH
BETTER MOISTURE...MAINLY EAST OF A LINE FROM VALENTINE TO OGALLALA.
THE MAIN SEVERE WEATHER THREAT WITH THESE STORMS WILL BE PRIMARILY
LARGE HAIL AND DAMAGING WINDS.

ONCE THE COLD FRONT PASSES...COOLER AND DRIER AIR WILL FILTER INTO
THE AREA WHICH WILL SERVE TO END THE SEVERE WEATHER THREAT BY LATE
TONIGHT.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

DRIER AND COOLER AIR WILL MOVE IN BEHIND THE FRONTAL
SYSTEM...INHIBITING THUNDERSTORM DEVELOPMENT THURSDAY.

ANOTHER COLD FRONT WILL MOVE INTO THE AREA FRIDAY NIGHT INTO EARLY
SATURDAY MORNING. HOWEVER...INSTABILITY WILL BE LIMITED WITH THIS
SYSTEM...AND THEREFORE MAINLY ONLY RAIN SHOWERS ARE EXPECTED LATE
FRIDAY AND SATURDAY.

.SPOTTER INFORMATION STATEMENT...

STORM SPOTTER ACTIVATION MAY BE REQUIRED TONIGHT...ESPECIALLY EAST
OF A LINE FROM AINSWORTH TO BROKEN BOW.

JWS