276 
fxus63 klbf 210740 
afdlbf 


Area forecast discussion 
National Weather Service North Platte NE 
240 am CDT Thursday Aug 21 2008 


Synopsis... 


Upper air pattern continues to be characterized by a weak closed middle 
level low over Oklahoma...with a broad area of anticyclonic upper level 
flow over Canada...and troughs over the northwest and northeastern 
coasts. Several smaller scale upper level waves are rotating east 
away from the northwest trough...one of which was crossing the 
High Plains Wednesday evening. Showers and thunderstorms formed 
with the approach of this disturbance...focused along a weak surface 
trough extending south from the northern High Plains. 
However...limited low level moisture and a capping inversion have 
prevented this activity from spreading east into the County Warning Area. Short 
term forecast problem is fog advancing north into the County Warning Area. At this 
point...do not expect fog to develop north of Highway 2 and west 
of Highway 7/183 due to stronger mixing. 


&& 


Discussion... 


A surface low is forecast to consolidate over the northern High Plains 
Thursday afternoon ahead of a strong upper level trough progressing east 
across the northern rockies. This will maintain a strong surface 
pressure gradient...which will favor breezy conditions similar to 
Wednesday afternoon. Over the western half of the County Warning Area...boundary 
layer mixing will be stronger...with winds veering slightly to the 
southwest. This area will favor daytime highs in the upper 80s to 
middle 90s...with lower 80s further to the east. Some stratus are 
possible over the eastern half of the County Warning Area Thursday night...but 
latest forecast soundings indicate that boundary layer moisture depth 
will be rather shallow...which may offset that potential somewhat 
due to strong mixing from nocturnal low level jet. The upper trough is 
expected to move east across the northern plains on Friday...and 
will be accompanied by a surface frontal passage over north 
central/western Nebraska during the middle day hours. Fairly cool 
airmass is currently positioned beneath the upper trough over 
southwest Canada...and latest objective guidance suggests that 
temperatures will not warm substantially after the frontal passage. Therefore 
have cut daytime highs on Friday back a few degrees. On the other 
hand...Post frontal cloudiness over the southern half of the County Warning Area 
will likely keep overnight lows a bit warmer compared to north 
central Nebraska. Airmass behind the advancing frontal boundary is 
forecast to remain stable during Friday afternoon...and this continues 
to suggest that daytime probability of precipitation should remain low. 


As the main upper disturbance passes well to the northeast of the 
region...and zonal flow develops...front will likely stall just 
south of the County Warning Area Friday night. The NAM and GFS continue to indicate 
low level warm air advection and isentropic lift developing north of the front during 
Friday night over southwest Nebraska. This...combined with MUCAPE of 
1000-1500 j/kg...should be sufficient for elevated thunderstorms 
between 00-12z Saturday. Low level warm air advection/isentropic lift continue across 
the eastern and southern portions of the County Warning Area through Saturday 
afternoon and night...with showers/storms remaining in the forecast. Middle 
level anticyclone then becomes established over The Four Corners 
region by Sunday afternoon...placing the County Warning Area within northwest flow 
aloft. Focus for thunderstorms at the surface appears to be lacking in 
latest model forecasts during the afternoon...thus have trimmed probability of precipitation 
down a bit...but remaining focused over southwest Nebraska. Low level jet then 
strengthens during the night...which may favor nocturnal convection 
over western/southwest Nebraska. Lee trough becomes positioned over the 
western portion of the County Warning Area on Monday. Atmosphere remains potentially 
unstable with low 60s dewpoints and steep middle level lapse rates in 
place. However...surface convg is not particularly strong...and large 
scale destabilization processes are not in place...which continues 
to suggest leaving the forecast dry for now. 


&& 


Aviation... 


Main aviation forecast focus the extent of fog and stratus this 
morning. 


Strong surface gradient across western Nebraska kept surface winds 
strong and well mixed through the overnight. This mixing held fog 
and stratus at Bay through the early morning hours and will need to 
be monitored. RUC model continues to show strong gradient and 
surface winds holding across the west through morning. Based on 
this removed mention of IFR ceilings and visibilities at kvtn. Choose to keep 
mention of IFR conditions in at klbf but shortened the duration for 
now. It will be a close call as best gradient is across the 
northwest and as a result lighter winds at klxn have already allowed 
for lower visibilities and approaching stratus seen on satellite for last 
few hours. If IFR conditions do not reach klbf they will be near 
and certainly across portions of central and eastern Nebraska...see 
those tafs and area forecast discussions for additional 
information. 


After this morning/S clouds clear scattered skies expected after 15z and 
the primary concern becomes strong and gusty winds. 


&& 


Lbf watches/warnings/advisories...none. 
&& 


$$ 


Aviation...Hirsch 
synopsis/discussion...Garner