Severe Thunderstorm Warning 271, NWS North Platte, NE

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTH PLATTE NE
310 AM CDT SUN OCT 04 1998

MODEL PERFORMANCE DISCUSSION...
    LAST NIGHT'S INITIAL CONDITIONS INDICATE THAT THE NGM, ETA, AND
AVN/MRF FROM THE PREVIOUS TWO RUNS WERE TOO FAST WITH THE MOVEMENT OF
THE 500MB VORT MAX IN WESTERN NEVADA. ALL THREE SOLUTIONS FROM THE
PREVIOUS TWO RUNS HAD THE SYSTEM ALONG THE UTAH-NEVADA LINE BY 00Z LAST
NIGHT. ALL THREE DID ALRIGHT WITH THE VORT MAX IN WISCONSIN; AND THE
NGM HIT THE POSITION OF THE ONE IN CENTRAL WYOMING. SINCE THE MODELS
HAVE GENERALLY BEEN TOO PROGRESSIVE WITH THE SITUATION, I PREFER THE
SLOWEST SOLUTION, WHICH IS THE AVN/MRF. UNFORTUNATELY, WITH THE
PROBLEMS WITH THE OSO SERVER LAST NIGHT, AVN GRIDDED DATA IS AVAILABLE
ONLY TO 18 HOURS. SO, BEYOND THAT, I NEED TO EXTRAPOLATE INFORMATION
USING THE OLD FASHIONED WAY, I.E., THE AFOS CHARTS.

FIRST PERIOD DISCUSSION...
    THE 12 HOUR 500MB VORTICITY INCREASE OF 2-6/S FROM SUN/12Z TO
MON/00Z INDICATES THE POSSIBILITY SIGNIFICANT QG-FORCING. EVEN MORE
IMPRESSIVE IS THE OVERLAY OF THE 850MB HEIGHT AND 1000-500MB THICKNESS
WHICH INDICATES STRONG AND DEEP ISENTROPIC UPGLIDE. THE MOISTURE IS
CERTAINLY AVAILABLE WITH EXPECTED 850MB MIXING RATIOS OF 6-10 G/KG.
BASED ON THAT ASSESSMENT AND LOOKING AT RADAR AND SATELLITE, WOULD
SUGGEST RAISING PROBABILITIES TO SOMETHING SIGNIFICANTLY HIGHER THAN 50
PERCENT, MAYBE EVEN GOING CATEGORICAL IN THE WEST WITH OCCASIONAL RAIN
WORDING. IN THE NORTHWEST, WHERE THE COLDEST TOPS ARE, THUNDERSTORMS
ARE POSSIBLE AT LEAST INITIALLY.
    THICKNESS CHANGES OF ZERO TO -60M ARE INDICATED IN THE WESTERN HALF
OF THE STATE WITH INCREASES OF UP TO 30M IN THE EASTERN PART OF THE
NORTH PLATTE FORECAST AREA. EVEN THE NGM AND ETA ARE KEEPING THE MEAN
1000-500MB RH AT OR ABOVE 70 PERCENT. SO, WITH WIDESPREAD CLOUDINESS AND
PRECIPITATION IN MOST OF THE FORECAST AREA, I CAN'T REALLY SEE
TEMPERATURES CHANGING MUCH FROM YESTERDAY'S. IF ANYTHING, THEY SHOULD
BE COLDER. SO, WILL SUGGEST CUTTING THEM BACK SIGNIFICANTLY FROM THE
CURRENT FORECAST.

SECOND AND THIRD PERIOD...
    THE 12 HOUR VORTICITY CHANGES FROM MON/00Z-12Z ARE STILL
INCREASING. INDEED, CYCLONIC VORTICITY ADVECTION AND ISOTHERMAL
VORTICITY ADVECTION ARE EXPECTED TO CONTINUE BEYOND MON/00Z AND BECOME
NEUTRAL BY MON/12Z. ALSO, THE MEAN 1000-500MB RH IS EXPECTED TO BE 70
PERCENT OR HIGHER THROUGH MOST OF THE NIGHT. SO, WOULD SUGGEST RAISING
THE PROBABILITIES OF PRECIPITATION TO 50 PERCENT OR HIGHER NORTH CENTRAL
AND NORTHWEST WHERE THE HIGHEST MEAN RH IS, AND TO 30 PERCENT OR HIGHER
IN THE SOUTHERN PART OF THE LBF FORECAST AREA. ON MONDAY, ANTICYCLONIC
VORTICITY ADVECTION IN THE ENTIRE FORECAST AREA INDICATES AN END TO THE
PRECIPITATION. IN ADDITION, DECREASING MEAN RH IN THE EASTERN HALF OF
NEBRASKA WOULD INDICATE A BREAK IN THE CLOUDS EVEN. HOWEVER, SINCE THE
MODELS HAVE ESTABLISHED A RECORD OF MOVING THINGS TOO FAST FOR THIS
REGIME, MY CONFIDENCE ISN'T HIGH ENOUGH TO GO FOR A PARTLY CLOUDY
FORECAST.
    THICKNESS DECREASES OF 60-120M FROM THIS EVENING TO MONDAY EVENING
POINT TO HIGHS MONDAY SIGNIFICANTLY COOLER THAN TODAY'S EVEN WITH A
DECREASE IN CLOUDINESS. THE CLOUDINESS DURING THE NIGHT SHOULD KEEP LOWS
IN THE MID 40S. ON MONDAY, THOUGH, THE CURRENT FORECAST FOR HIGHS IN
THE 50S IS TOO HIGH IN ALL BUT THE SOUTH THREE COUNTIES WITHOUT A BREAK
IN THE CLOUDS. SINCE BREAKS ARE POSSIBLE, I WILL ONLY TRIM THEM A
CATEGORY. IN THE SOUTHERN THREE, A SHARP GRADIENT (POSSIBLY 15-20F) WILL
PROBABLY BE EXPERIENCED BETWEEN LBF-OGA AND MCK-IML DUE TO THE EDGE OF
CLOUDINESS. SO WILL RAISE THOSE TO UPPER 50S TO BETTER FIT IN GLD'S
FORECAST.

SUGGESTED CCF

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

LBF RE 047/043 053/040 052 49963
VTN RE 048/043 049/040 050 49953

OGA RE 048/043 052/041 053 49963
IML RE 050/043 057/042 054 49753
BBW RE 051/043 053/042 0(52 49863
ANW RE 049/043 052/040 051 49863
ONL RE 048/043 052/042 050 49863

SPRINGER

NEZ004>010-022>029-035>038-056>059-069>071-051200-

SEVERE WEATHER OUTLOOK FOR WESTERN AND NORTH CENTRAL NEBRASKA
NATIONAL WEATHER SERVICE NORTH PLATTE NE
650 AM CDT (550 AM MDT) SUN OCT 4 1998

THIS OUTLOOK PERTAINS TO THE FOLLOWING NEBRASKA COUNTIES...

ARTHUR     BLAINE     BOYD       BROWN      CHASE      CHERRY
CUSTER     DEUEL      FRONTIER   GARDEN     GARFIELD   GRANT
HAYES      HOLT       HOOKER     KEITH      KEYA PAHA  LINCOLN
LOGAN      LOUP       MCPHERSON  PERKINS    ROCK       SHERIDAN
THOMAS     WHEELER

...STORM SPOTTER ACTIVATION MAY BE REQUIRED TODAY IN CENTRAL AND
SOUTHWEST NEBRASKA...

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN CENTRAL...NORTH
CENTRAL...AND SOUTHWEST NEBRASKA SOUTH AND EAST OF A LINE FROM
SPRINGVIEW IN NORTH CENTRAL NEBRASKA THROUGH THEDFORD TO NORTH PLATTE.
A STRONG STORM SYSTEM MOVING OUT OF EASTERN COLORADO THROUGH KANSAS AND
INTO NEBRASKA WILL PRODUCE THUNDERSTORMS IN MUCH OF NEBRASKA WITH A FEW
SEVERE STORMS POSSIBLE. THE GREATEST RISK FOR SEVERE WEATHER IN THIS
OUTLOOK AREA IS THIS MORNING.

SPRINGER