Tornado and Flash Flood in Nebraska, August 26, 2002

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTH PLATTE NE
250 AM CDT MON AUG 26 2002

...FORECAST CONCERN WILL BE POPS OVER THE NEXT SEVERAL DAYS...

S/W AND ASSOC SPEED MAX SEEN IN H2O MOVED ACROSS HIGH PLANS INTO
NORTH CENTRAL NE SPARKING TSRA ACROSS MAINLY CHERRY COUNTY. S/W IS
MOVING IN FAST WESTERLY FLOW BETWEEN H5 RIDGE OVER THE DESERT
SOUTHWEST AND H5 TROF OVER NW CONUS. MRRN PROF SHOWING THIS SPEED
MAX WITH GOOD VEERING/WAA THROUGH 500 MB...IN ADDITION H25 JET AXIS
ARCING OVER ROCKIES INTO WESTERN NE/NE KANSAS. OTHER CONERN IS MCS
OVER WESTERN SOUTH DAKOTA WHICH NEW ETA SHOWS BUILDING SOUTHWARD
INTO CWA ALONG THETA-E AXIS. SURFACE MAP SHOWING TROF ALONG FRONT
RANGE OF ROCKIES...BUT CAN NOT REALLY PICK OUT THE BOUNDARY ON WHICH
THE TSRA IS FORMING ALONG...EXECPT THIS IS AREA OF TIGHTER DPNT
GRADIENT.

FIRST BIG CHALLENGE TODAY IS WHERE WILL DAKOTA MCS MIGRATE TO THIS
MORNING...WILL KEEP AN EYE ON THROUGH FORECAST ISSUANCE. LOOKS LIKE
NORTHEAST CWA IS UNDER THE GUN. CORFIDI VECTORS SUPPORT THE
SOUTHEAST TO SOUTHWARD MOVEMENT. AVN DOESNT HAVE A GOOD HANDLE ON
IT A 06Z AND BY 12Z-18Z MOVES AREA OF PRECIP EASTWARD THROUGH SOUTH
DAKOTA. CURRENT THINKING IS TO PLACE CHC POPS NORTH AND WEST WITH
SLGHT CHC SOUTHEAST TODAY. HOWEVER IT WORKS OUT...BEST INSTABILITY
AS IN THE PAST FEW DAYS WILL BE IN THE WEST WHERE
SOUTH/SOUTHEASTERLY UPSLOPE FLOW WILL CONTINUE TO KEEP BEST
SHEAR...AND THETA E RIDGE IN PLACE. TEMPS TODAY WILL ALSO DEPEND ON
WHERE SYSTEM TRACK...IF SOUTHWARD WOULD EXPECT MORE CLOUD COVER AND
COOLER TEMPS.

H5 TROF OFF THE WESTERN COAST WILL MOVE INLAND TODAY AND INTO GREAT
BASIN AREA TONIGHT AND PHASE WITH NORTHERN LOW OVER MT. THIS WILL
PUSH THE UPPER JET FARTHER SOUTH AND MID LEVEL FLOW WEAKENS SOMEWHAT
OVER THE WESTERN CWA TONIGHT...BUT UPSLOPE FLOW WILL CONTINUE TO
KEEP GOOD SHEAR ACROSS WESTERN HIGH PLAINS. LVL JET EVOLVES AGAIN
TONIGHT WITH THE STRONGEST ELEVATED INSTABILITY OVER THE WEST WILL
KEEP CHC POPS WEST OF HIGHWAY 83 WITH SLIGHT CHC EAST.

WILL KEEP POPS IN FOR TUESDAY AFTERNOON OVERNIGHT WITH CHC
WEST...SLGHT CHC EAST. PATTERN REMAINS SIMILAR WITH UPPER LOW OVER
THE GREAT BASIN THROWING S/WAVES INTO PLAINS IN SOUTHWEST FLOW.
UPSLOPE SOUTHEASTERLY FLOW CONTINUES WITH WAA ADVECTION REGIME
REMAINING PLANTED OVER HIGH PLAINS.

CONFIDENCE LOW ABOUT PATTERN CHANGE WEDNESDAY THROUGH WEEKEND.
H5 LOW IN GREAT BASIN EJECTS IN RESPONSE TO JET ROUNDING THE
BASE OF LOW ON LATE WEDNESDAY INTO THURS MORN...AS STRONGER WAVE
APPROACHES THE WEST COAST. WILL LEAVE REMAINDER OF FORECAST MOSTLY
AS IS WITH A FEW SLIGHT CHANGES.

.LBF...NONE.

DP