Weather

206 
fxus63 klbf 070749 
afdlbf 


Area forecast discussion 
National Weather Service North Platte NE 
249 am CDT sun Sep 7 2008 


Synopsis... 


A broad large scale trough remains positioned over the plains...with 
fast /100+ knots at h25/ upper level flow located on both sides of the 
trough axis. Middle level height rises and large scale subsidence are 
occurring behind a short wave trough that passed through the region Saturday 
afternoon. This has favored generally clear skies over the eastern 
half of the County Warning Area...yielding favorable radiational 
cooling...combined with recent ground wetting precipitation...resulting 
in patchy fog development during the last few hours. A pronounced 
thermal gradient is observed in the 850-500 mb layer per 00z upper 
air observation...positioned roughly from the central/southern High Plains 
northwest into the northern rockies. This temperature gradient 
will eventually play a role in precipitation development Sunday 
night as an upper level disturbance entering northern Montana digs 
south into the region. 


&& 


Discussion... 


Short term model guidance indicates that cloud cover will be 
persistent over the northern half of the County Warning Area this afternoon...as 
previously mentioned upper wave approaches the Central Plains. This 
will limit daytime heating...resulting in highs in the middle to upper 
50s. Strong isentropic ascent /and moisture transport/ will occur 
in response to the approach of the upper wave. In addition...intense 
low to middle level frontogenesis is forecast to become focused along 
meridional temperature gradient. Best combination of moisture and 
lift will initially develop over the northwestern portion of the 
County Warning Area between 00-06z Monday. Fgen zone then begins to shift south 
and east between 06-12z...finally exiting the County Warning Area shortly after 
18z. Isentropic ascent over the southwestern portion of the County Warning Area 
may result in elevated thunderstorms due to the presence of 
200-300 j/kg of MUCAPE rooted near 700 mb. Confidence in thunder 
occurrence further north is lower...but will follow recent Storm Prediction Center 
guidance and include a mention for the entire County Warning Area after 06z. 
Precipitation is expected to quickly come to an end by middle 
afternoon Monday...as dry isentropic downglide spreads across 
the region behind the departing wave. 


Clear skies Monday night...and high pressure over south central Nebraska 
will likely result in upper 30s/low 40s for overnight lows between 
klbf and kbbw. Low to middle 40s are more likely over far western and 
north central Nebraska where south/southwest surface winds increase on the 
western side of the high pressure center. The pattern will also 
favor fog...especially given the recent widespread rainfall. The surface 
high then migrates towards the middle MS valley region by Tuesday 
afternoon...with low pressure located over the northern High Plains. 
This will favor continued strengthening of southerly flow across the 
County Warning Area and temperatures ranging from the low 70s near kbbw to around 80 
in the eastern Panhandle. A very pronounced eml will move east 
across the region Tuesday night...associated with warm 700 mb air and 
steep middle level lapse rates. As this airmass gets swept across the 
County Warning Area...boundary layer moisture is forecast to rapidly surge northward as 
a 50 knots low level jet develops. This pattern will favor stratus Tuesday 
night...thus bumped up cloud cover. 


Moderate 700 mb warm air advection is forecast by the GFS to develop over the eastern 
portion of the County Warning Area by 18z Wednesday...which may favor a slight 
chance of elevated showers/thunderstorms. The slight chance of 
showers/storms persists towards 00z Thursday...perhaps becoming 
refocused along a surface trough/developing cold front over western Nebraska. 
As this front moves east across the County Warning Area Wednesday 
night...precipitation coverage will likely increase as well. The GFS 
and European model (ecmwf) then develop a closed middle level low over the central rockies 
by Thursday night...with moist isentropic ascent developing over the 
far western County Warning Area. Decided to add a slight chance of showers/storms 
over that region...though uncertainty associated with the upper level 
feature in The Rockies is reflected in the GFS ensemble. 


&& 


Aviation... 


Patchy fog with visibilities down to 3sm will be a concern at the 
klbf terminal through 14z this morning. Thereafter to 00z this 
afternoon mostly cloudy conds will be expected at both the kvtn and 
klbf terminals with ceilings at or above 9000 feet. Ceilings will begin to 
decrease after sunset this evening as a decent upper level 
disturbance approaches the klbf and kvtn terminals. This disturbance 
will spread rain showers...isolated thunderstorms and mfvr ceilings to the klbf 
and kvtn terminals through 06z. From 06z to 12z 
tonight...precipitation will continue across western and north 
central Nebraska...with ceilings possibly dropping below 1000 feet 
and fog development toward 12z Monday morning. Numerical guidance is 
not supportive of this just yet and will forgo mention of IFR 
ceilings late tonight in the 12z taf issuance as this is near the 
end of the forecast period. 


&& 


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


$$ 


Garner/clb