731 
fxus63 klbf 300807 
afdlbf 


Area forecast discussion 
National Weather Service North Platte NE 
307 am CDT Sat Aug 30 2008 


Synopsis... 


Fast westerly upper level flow is present along and north of the U.S. 
And Canadian border. An upper level ridge is beginning to amplify 
over the northern High Plains...in response to strengthening low 
level warm air advection ahead of a strong short wave trough over British Columbia. This 
trough will dig toward the intermountain west over the weekend. 
Meanwhile...as the upper ridge begins to progress east of the 
region...Lee side troughing will begin to take place over the 
central/northern High Plains...which will favor an increase in 
temperatures toward the low to middle 90s. 


&& 


Discussion... 


Higher quality/deeper boundary layer moisture is located over 
southwest Kansas this evening...and has shown signs of moving north 
during the last few hours. This trend is expected to continue 
through Saturday morning...with forecast soundings over southwest Nebraska 
depicting marginally favorable thermodynamic profiles for low 
clouds/br/fg. Thus...made a slight upward adjustment in cloud cover 
over that region at around 12z today. Well mixed planetary boundary layer is expected to 
develop over the northern and western portions of the County warning forecast area during 
the afternoon. With flow strengthening over the lowest 6000 
feet...breezy conditions are likely /gusts approaching 30 knots over the 
northern half of the cwa/. Mav indicates that sustained winds will 
meet Wind Advisory criteria this afternoon...while the met is not as 
strong. Will advise day shift this morning to reevaluate issuing an 
advisory for the afternoon. A strong /50 knots/ low level jet is forecast to develop 
over the northern plains Saturday night. This will favor relatively 
warm low temperatures /mid 60s/ due to stronger mixing...with breezy 
conditions persisting through the night in north central Nebraska. 


A weak pv disturbance is also forecast to lift northeast across the 
southern and eastern half of the County Warning Area Saturday night. However...the 
atmosphere appears to be too dry to support precipitation 
development. A windy day is again expected on Sunday...as a tight 
surface pressure gradient remains in place...and favorable boundary 
layer mixing brings 25-35 knots flow /with locally higher gusts over 
north central Nebraska/ down to the surface. A Wind Advisory will likely be 
needed...as mav forecasts sustained wind speeds at or above the 26 knots 
criteria during the middle to late afternoon. Similar to Saturday 
night...an intense 50-60 knots low level jet redevelops during Sunday 
night...likely enhanced by ejecting lead short wave trough lifting 
northeast across the northern High Plains. An upper level trough then 
becomes positioned over the intermountain west on Monday...and 
then begins to move east/northeast towards the High Plains Monday 
night. The latest GFS output brings a lead short wave trough east into 
the northern plains Monday afternoon...which drags a surface front 
east across the Dakotas into Minnesota. The 00z European model (ecmwf) has trended 
slightly closer to the GFS solution...though the GFS still remains 
more progressive. At this point...will focus highest probability of precipitation on 
Monday afternoon over our western counties...with low probability of precipitation 
extending east along the northern tier of counties in anticipation 
of the northern plains front progressing east. 


Regarding the evolution of the western Continental U.S. Trough...the European model (ecmwf) 
depicts a more meridional ejection of the upper level short wave trough by 12z 
Tuesday...while the GFS is more aggressive in digging the trough east 
into the Central High plains...resulting in a faster frontal passage. The 
slower frontal passage suggested by the European model (ecmwf) will favor at least a 
chance of showers and storms into Tuesday afternoon...as plentiful 
moisture resides along the slow moving boundary. The European model (ecmwf) then 
brings the circulation of Gustav north into the arklatex region by 
Wed/12z...and then merges it with the southeastward progressing 
front by Wednesday afternoon/night. The deterministic GFS solution 
stalls Gustav over the Texas/la coast. However...the two solutions are 
starting to come into better agreement with the evolution of the 
passing front...and suggest that precipitation should come to an end over 
the western half of the County Warning Area by 12z Wednesday...and likely across the 
remainder of the County Warning Area soon after. At this time...will leave low probability of precipitation 
over the eastern half of the County Warning Area through the end of this forecast 
package...mainly for blending purposes. On the other 
hand...temperatures do appear to be substantially cooler with 
either model solution that is followed. 


&& 


Aviation... 


Short term models indicate that some marginal ceilings may make 
it into southwest Nebraska to near Ogallala and McCook. However...we 
do not expect them to reach the North Platte or Valentine areas. Of 
some concern today is wind which will probably be sustained at 15 to 
25kt with gusts to about 30kt possible in north central Nebraska. By 
early Sunday...a low level jet of 40-50kt may develop in north 
central Nebraska east of Valentine. This could mean fairly strong 
non-convective low level wind shear...and will need to be watched 
with subsequent forecast cycles. There are also hints of increased 
low level moisture from Valentine and North Platte and east by early 
Sunday...another element that may warrant some changes to the 
terminal forecasts. 


&& 


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


$$ 


Garner/Springer