851 
fxus63 klbf 181938 
afdlbf 


Area forecast discussion 
National Weather Service North Platte NE 
238 PM CDT Friday Jul 18 2008 


Discussion... 
the forecast for tonight and Saturday is getting interesting. Water vapor 
clearly shows a significant subtropical disturbance over the central 
rockies moving northeast toward Nebraska. Timing tools bring this feature 
across the forecast area late tonight or Saturday morning. None of the 
models acknowledge this feature and this has been a problem of late. 
The nam12 12z run for example was oblivious to the mesoscale convective system which worked 
over the ncntl zones today. This system has reinforced the weak cold 
front currently over cntl zones this afternoon. If this front is able to 
maintain position tonight...it would provide excellent focus for 
thunderstorms tonight and Saturday morning. Grids have been manually 
adjusted for storms tonight through Saturday afternoon until we are sure 
this disturbance has exited the area. 


Sunday and Monday...blended solutions of the GFS and nam12 are dry 
with high temperatures in the 90s. Probability of precipitation have been manually adjusted 
upward as a weak cold front is expected to waiver north and south 
across S Dakota and Nebraska providing focus for storm development. The 
atmosphere will remain heavily influenced by subtropical moisture. 


During this time...the threat of severe thunderstorms appear 
relatively low given winds aloft are fairly light. Storm Prediction Center 
nonetheless has outlooked parts of the forecast area tonight through 
Sunday for severe weather. 


Tuesday through Friday...the tropics remain very active as a 
series of tropical disturbances will move east through the 
Caribbean...Gulf of Mexico and eastern Pacific and become partially 
entrained in a subtropical ridge covering the cntl and western U.S.. a 
persistent trough over the northern Pacific is expected to facilitate 
moisture entrainment in the ridge over the U.S. This is a text-book 
late Summer rainfall pattern which appears to have started weeks 
early. 


As currently indicated by satellite and forecast by the GFS and European model (ecmwf)... 
deep tropical moisture and embedded subtropical disturbances will 
rotate around the western U.S. High pressure system eventually crossing 
the the cntl and northern plains. Presumably...a weak surface front will 
wobble north and south through the central and northern plains providing 
focus for isolated to scattered thunderstorms across Nebraska. The HPC 
provided grids did an excellent job lacing the forecast area with 20 to 
40 probability of precipitation almost daily and the maximum temperature grids with lower to middle 
90s looked very reasonable for the moist environment. Probability of precipitation have been 
limited to around 30 percent as 40 percent probability of precipitation in the extended 
periods generally implies likely chances for storm development. 


Thus the chances of rain during this critical period of the growing 
season look quite favorable as the tropics are active and a 
persistent trough across the northern Pacific lifts moisture from the deep 
south north across the plains. 


&& 


Aviation... 
morning thunderstorm activity over north central Nebraska will 
continue to exit the region during the early afternoon hours. A cool 
and stable low level airmass has been left behind in the wake of the 
morning thunderstorm complex...which may delay renewed thunderstorm 
development until the late afternoon or evening hours. Given the 
amount of uncertainty regarding destabilization and focus for storm 
development...have opted to only mention cumulonimbus in the kvtn taf. The 
most likely scenario for evening/overnight storms in north central 
Nebraska is for activity to develop over Wyoming/SD/neb Panhandle...and then 
move east/southeast toward the kvtn terminal after 00z Saturday. 
Otherwise...surface winds will begin to veer towards southeasterly 
Friday afternoon...then south/southwest during the overnight. A surface 
cold front is forecast to move south across western Nebraska Saturday 
morning...causing winds at kvtn to become northwesterly by 12-15z. 
In addition to the wind shift...a very humid surface airmass combined 
with a strong boundary layer capping inversion may yield a scattered cloud 
deck at 1000 feet after 09z Saturday at klbf. Future taf forecasts may 
need to further lower this ceiling and increase the coverage to broken 
or overcast. 
&& 


Lbf watches/warnings/advisories...none. 


&& 


$$ 


Cdc/jg