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fxus63 klbf 212007 
afdlbf 


Area forecast discussion 
National Weather Service North Platte NE 
307 PM CDT Thursday Aug 21 2008 


..forecast challenges are precipitation chances for Friday 
night/Saturday and Wednesday...along with temperatures... 


Synopsis... 
surface analysis from 19z has low pressure over northwestern South Dakota with a trough 
extending north and south of this feature and a cold front extending 
westward into northern Wyoming. Surface pressure gradient not nearly as tight 
today as winds have only been gusting to around 30 miles per hour east of a Oga 
to Merriman line. 500 mb pattern remains active across the Continental U.S. Today with 
high pressure noted over the Southern California coast...the Carolinas...and in 
the western Caribbean. A couple of cutoff lows to note...the first 
over eastern Kansas and the second over southern British Columbia. Heat falls with 
the latter low carry this feature due east over the next 24 hours 
which agrees well with the latest GFS and NAM model forecasts. 


Discussion... 
near term...tonight through Sunday...main forecasting challenge 
deals with precipitation associated with the frontal passage Friday night and 
upper level forcing for Saturday and Sunday. 12z NAM and GFS model 
runs remain consistent with with the cold frontal passage clearing the County Warning Area by 
18z Friday afternoon. Not expecting any precipitation in the 
vicinity of the frontal passage as lift confined to the lowest layers which 
will be fairly dry during the afternoon hours. 


Better chances for precipitation will arrive in the evening and 
overnight hours as a low level jet streams north from Kansas. 
Enhanced isentropic lift noted in the middle layers after midnight 
coupled with decent middle mixed layer convective available potential energy of 1000 to 1500 j/kg will 
lead to at least scattered showers and thunderstorms and rain showers in southern areas. Heavy 
rain potential continues to be focused just off to the east of the 
lbf County Warning Area on the nose of the low level jet and location of highest k indexes 
(aon 40c). 


The front will remain stalled across Kansas for the next 12 to 36 
hours...which will allow Erly flow...weak isentropic lift and 
cloudiness to persist through Sunday. Will continue mention of low 
precipitation chances for the remainder of the weekend as weak disturbances 
cross the southwestern County Warning Area during this period. 


Long term...Sunday night through Thursday...precipitation chances are the 
main forecasting challenge for Monday and Wednesday. For 
Monday...stalled out frontal boundary will lift slowly to the east 
as a warm front Sunday night and Monday. Decent 700 mb warm air advection and strong 
isentropic lift (315k) will follow the front eastward Sunday night and 
Monday. With a surface boundary in place and good middle level 
forcing...hard to leave a dry forecast in place so will introduce probability of precipitation 
for Sunday night and Monday. Tuesday will be a dry day as ridging 
amplifies across the central Continental U.S....in response to a broad trough 
traversing the western states. The aforementioned feature will usher 
in a cold frontal passage on Wednesday. Some tweaks to the precipitation and temperature grids 
were made to accommodate a slower frontal passage Wednesday and cooler temperatures 
for Thursday. Overall...temperatures remain seasonal for days 4 to 7 
with highs in the upper 80s to lower 90s and lows in the upper 50s to 
lower 60s. 




&& 


Aviation... 
areas of MVFR ceilings remain across much of eastern Nebraska into far north 
central Nebraska this afternoon...while VFR ceilings to sky clear conditions are in 
place westward. These lower ceilings may build back westward some 
overnight but their progress looks to be hampered somewhat by the 
approach of a cold front through western Nebraska toward sunrise. 
Would expect the main area of concern to be mainly along and east of 
Highway 183. Winds may be light enough across the larger river 
valleys in the southwest to produce some light fog with visibilities only 
dropping into the MVFR category. 


Lbf watches/warnings/advisories...none 




$$ 


Aviation...Phillips 
short and long term...buttler