168 
fxus63 klbf 212105 
afdlbf 


Area forecast discussion 
National Weather Service North Platte NE 
305 PM CST Sat Nov 21 2009 


Discussion... 
a Pacific cold front is approaching the forecast area this afternoon. The 
front is forecast to drop through the forecast area overnight and lift north 
on Sunday. Earlier forecasts for stratus and fog are on hold as this 
front should push east of the forecast area and appears to be drying 
out the forecast area. The 18z NAM is pulling moisture west this evening 
to near klbf so it will be close. 


The models have come into somewhat better agreement on the storm 
system forecast to affect the cntl plains Monday and Tuesday. The 
European model (ecmwf)...NOGAPS and UKMET are tracking the 700 mb low east across northern Kansas 
while the nam12...GFS...GFS ensemble...sref and Gem track the 
disturbance through the upper Missouri Basin. The European model (ecmwf) deepens the 
storm across eastern and central Nebraska while some of the other models 
deepen the system over Iowa. Some models dont deepen the system at 
all. The 250 mb jet is less than 100kt as it crosses the cntl plains. 


The disturbance is still across the North Pacific but is forecast to be 
come ashore over the Pacific northwest coast Sunday morning. Thus in 24 hours 
the disturbance should be picked up by the upper air network and the 
models should be in better agreement. Overall there has been a net 
model solution convergence and the consensus is for the storm to affect 
northeast Nebraska with heavy snow and strong winds Monday night through 
Tuesday morning. HPC suggested 4 to 8 inches from northestern Nebraska across 
southeastern South Dakota and into southwestern Minnesota. 2 to 4 inches of snow could fall along 
Highway 281 across ncntl Nebraska. This official forecast uses a 50 percent 
blend of the 12z European model (ecmwf) and 12z nam12 which is what HPC used for their 
snowfall forecast. 


The models are wrapping a strong Pacific cold front into the storm 
producing an occluded low near koma Tuesday morning. 850mb winds are 
forecast to increase to 40 to 55kt Monday night across cntl and eastern Nebraska 
with 500m above ground level winds increasing to 35 to 40kt. The combination of wind 
and snow falling at night could produce a winter storm condition. 


As the Pacific front moves east Monday...the NAM and GFS indicate 
rain should mix with or change to wet snow west of Highway 83. 
Temperatures behind the front Monday night and Tuesday should be cold 
enough for snow across all of the forecast area. 


The storm should be well east of the forecast by early Tuesday evening. 
Winds should drop off quickly around sunset as Pacific high pressure 
builds across the Central Plains. Very quickly...a shot of Arctic air 
drops south through the western High Plains on Wednesday and 
Thursday. Low pressure developing across southern Canada Friday should 
produce southwest winds and warmer temperatures. Another Pacific cold 
front is forecast to move through Friday with slightly cooler weather 
Saturday. 


&& 


Aviation... 
visual flight rules will continue across western Nebraska through 
22/03z. High clouds at or below 15kft will continue to spread east into the 
evening while lower clouds to the east are a concern. A spread 
developed between the 12z models as to how far west the stratus over 
south central and southeast Nebraska will expand. The RUC and now 18z NAM 
models point to higher boundary layer moisture available this 
evening and possibly impacting the klbf terminal. Further north the 
cold front pushes east sufficiently that keeps low ceilings out for 
visual flight rules through 18z Sunday. Today at 20z the surface 
analysis showed the cold front into central Dakotas extending 
southwest into northeast Wyoming. This evening the surface ridge 
that extends west across central Nebraska at this time weakens with the cold 
front moving east into western Nebraska. During that time frame the 
RUC/NAM solutions increase low level moisture west with the 
potential for low ceilings to develop westward and possibly 
impacting the klbf terminal through 12z Sunday. Current tafs follow 
the 12z NAM solution that would move the front east that limited the 
westward extend to the stratus tonight. The 12z NAM was faster but 
given the agreement in short term models ceilings will need to be 
monitored with the potential for an update this evening for the klbf 
terminal based on the advection of stratus into the southwest. 
&& 


Lbf watches/warnings/advisories...none. 


&& 


$$ 


Cdc/tlk