Severe Thunderstorm Warning 224, NWS North Platte, NE

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTH PLATTE NE
235 PM CDT TUE AUG 11 1998

MODEL PERFORMANCE DISCUSSION...
    THE NGM AND ETA SOLUTIONS ARE SIMILAR IN THEIR DEALING WITH THE
UPPER LEVEL TROUGH MOVING SLOWLY THROUGH CENTRAL AND WESTERN NEBRASKA
TONIGHT. BOTH INDICATE SOME CYCLONIC VORTICITY ADVECTION OF 2-4/S IN
MOST OF THE NORTH PLATTE UPDATE AREA TONIGHT WITH THE BULLSEYE CENTERED
AROUND LBF. THIS PRETTY MUCH CORRESPONDS TO THE 850-500MB QG-FORCING
WHICH MOVES THROUGH SOUTH CENTRAL NEBRASKA BY WEDNESDAY EVENING. THE
NGM, HOWEVER, MOVES THE CVA OUT OF NEBRASKA ON WEDNESDAY MORNING WHILE
THE ETA KEEPS IT IN THE EASTERN HALF OF NEBRASKA INTO THE AFTERNOON. THE
OUTPUT FROM LAST NIGHTS AND YESTERDAY MORNINGS CYCLES OF BOTH MODELS
DID WELL WITH THE LOCATION OF THAT FEATURE THIS MORNING. HOWEVER,
CONSIDERING THE WEAK FLOW AND HEIGHT FIELD AT 500MB, I LEAN TOWARD THE
LESS PROGRESSIVE ETA SOLUTION FOR THE FIRST THREE PERIODS. SINCE THE
AVN/MRF AND THE ETA SOLUTIONS ARE SIMILAR AT 48 HOURS, HAVE CHOSEN THE
AVN/MRF FOR THE LAST PERIOD.

FIRST PERIOD DISCUSSION...
    WITH THE THETA-E RIDGE BUILDING INTO WESTERN NEBRASKA THIS EVENING
AND SURFACE DEWPOINTS IN THE 60S BEING ADVECTED TO THE NORTHWEST, IT
APPEARS THAT A FAIRLY HIGH PROBABILITY OF THUNDERSTORMS IS APPROPRIATE
IN ALL OF THE NORTH PLATTE UPDATE AREA EXCEPT MAYBE SHERIDAN AND CHERRY
COUNTIES, WITH THE BEST CHANCES SOUTH AND WEST OF A MRR-ANW-BBW LINE.
IN THAT AREA, BOUNDARY LAYER MIXING RATIOS OF 12-14 G/KG AND STRONG
MOISTURE FLUX CONVERGENCE AND CONVERGENT BOUNDARY LAYER WIND WILL
COMBINE TO PRODUCE THUNDERSTORMS WITH POSSIBLE HEAVY RAIN. WITH
THUNDERSTORMS IN SOUTH DAKOTA MOVING SOUTHEAST AND THE OUTFLOW BOUNDARY
ABOUT AINSWORTH-THEDFORD-MERRIMAN...WILL GO WITH SCATTERED EVENING
THUNDERSTORMS FOR VTN AND ANW.
    NORMALLY, THE CLOUDINESS WOULD HOLD TEMPERATURES UP TONIGHT BUT,
COOLING FROM THE STORMS SHOULD ALLOW TEMPERATURES TO FALL INTO THE
LOWER 60S IN THE SOUTHWEST WITH AROUND 60 IN THE NORTHWEST AND MID 60S
IN THE NORTH CENTRAL ZONES.

SECOND PERIOD AND BEYOND...
    THE ETA KEEPS THE CVA AND MOISTURE IN A LARGE PART OF THE NORTH
PLATTE FORECAST AREA THROUGH WEDNESDAY MORNING. SO, SUGGEST KEEPING 50
PERCENT POPS IN THE SOUTHWEST WEDNESDAY MORNING. IN THE CENTRAL ZONES,
THE BEST CHANCES APPEAR TO BE WEDNESDAY AFTERNOON AND NIGHT. BY
WEDNESDAY AFTERNOON, THE THETA-E RIDGE SHIFTS BACK INTO THE HIGH PLAINS
AND THE CVA AND QG-FORCING SLIPS INTO KANSAS. SO, IN THE NORTH CENTRAL
PART OF THE STATE, THUNDERSTORMS ARE UNLIKELY. HOWEVER, BECAUSE OF THE
AVAILABILITY OF MOISTURE, CONTINUED INSTABILITY, AND THE LIKELIHOOD OF
AN OUTFLOW BOUNDARY, THEY SHOULD NOT BE DISCOUNTED COMPLETELY. BY THE
END OF THE FOURTH PERIOD, HIGH PRESSURE IS BUILDING ON THE HIGH PLAINS
AND CVA HAS SHIFTED INTO EASTERN NEBRASKA.
    FOR TEMPERATURES, THE THICKNESS AND 850MB TEMPERATURE PROGS
INDICATE LITTLE OR NO ADVECTION. SO, THE AMOUNT OF SUNSHINE IS THE
CRITICAL FACTOR IN THE DAYTIME TEMPERATURE FORECASTS. THERE SHOULD BE
ENOUGH CLOUDINESS WEDNESDAY TO KEEP TEMPERATURES ABOUT THE SAME AS
TODAYS. ON THURSDAY, CLOUDS SHOULD BE OUT OF THE WESTERN HALF OF THE
STATE. IN THE EASTERN HALF, SLOWER CLEARING IS EXPECTED. SO WILL RAISE
THURSDAYS HIGHS ABOUT 5F FROM WEDNESDAY FOR THE WESTERN ZONES AND KEEP
THEM ABOUT THE SAME IN THE EASTERN ZONES.

SUGGESTED CCF

PRELIMINARY GUIDANCE FOR COORDINATION ONLY. OFFICIAL FORECAST IS
CONTAINED WITHIN THE PRODUCT OMAZFPLBF.

LBF EB 062/080 058/085 058 49653
VTN BB 060/082 058/085 058 49522

OGA EB 063/081 059/085 059 49653
IML EB 063/080 060/086 060 49653
BBW TB 063/078 062/080 062 49353
ANW BB 064/080 060/084 060 49533
ONL BB 064/080 063/082 063 49233

EASTERN PANHANDLE
    BB 059/084 056/087 056 49222

SPRINGER