881 
fxus63 koax 212130 
afdoax 


Area forecast discussion 
National Weather Service Omaha/valley NE 
330 PM CST Sat Nov 21 2009 


Short term...tonight through Tuesday 
pleasant weather continues today ahead of more active and cooler 
pattern developing for the week. Subjective 12z upper air analysis 
indicates digging upper trough in the western US...with 500 mb heat falls 
of 150 M centered in northern Montana. Upper ridge extended from eastern Kansas 
through Iowa into central ont...and upper low was also centered in northern 
Texas. 850mb low was developing in response to strengthening upper 
trough...with low centered in northern ab and cold front extending 
through The Rockies. Strong 850mb warm air advection was present 
from the High Plains into the Dakotas...with 850mb moisture tongue 
from eastern Texas into eastern Kansas/western MO. Surface pressure gradient was 
tightening over the Central Plains with surface low developing on the 
Saskatchewan/mb border...and secondary low in eastern Colorado. 


Main forecast concern lies in evolution of digging upper low and 
associated weather impacts Monday nt/tues...with pesky fog/drizzle/lt 
rain/stratus in the meantime. Low-level moisture advection that has 
allowed fog/stratus development the last several nights continues. 
With increased surface winds tonight...think dense fog will be less 
of a concern than the last couple nights. Ample low-level moisture 
should keep stratus around though...and moisture combined with 
some weak lift as q-vector convergence increases should allow 
a little drizzle. Surface dewpoints are much higher than 
yesterday...which will keep temperatures much higher tonight and provide 
a warmer start tmrw. With continued warm air advection 
tmrw...temperatures should easily reach into the 50s in all but perhaps 
the northwestern County Warning Area despite clouds. Lead wave is prognosticated to eject across 
the central US on sun...dragging a weak cold front into central to 
northestern Nebraska before it washes out as return flow develops in advance 
of main upper wave. With some upper lift and presence of a cold 
front...could see a little light rain near the front...and have 
tweaked probability of precipitation to adjust to possible frontal position. Should see a 
break in lt rain chances sun nt/Monday as initial front weakens and 
main upper low/cold front approaches...though with abundant low- 
level moisture...still expect quite a few clouds. Again...think 
winds should be just enough to keep dense fog at Bay. 


While there is still some disagreement among models regarding upper 
low position when it closes and depth as it dives out of the northern 
rockies into the Central Plains...agreement is improving and models 
are trending toward a common solution. GFS remains somewhat faster 
than other solutions...thus continue to lean toward slower 
ECMWF/Gem. Main difference at this point is in low- to middle-level 
temperature profile...which /of course/ significantly impacts the expected 
precipitation type. European model (ecmwf) has been warmer than GFS/Gem and appears warmer 
than the 18z NAM heading into Monday nt...wrapping in less cold air in 
wake of the upper low. For now...have trended toward a compromise 
solution...leaning slightly toward the colder solutions as lift 
associated with strong upper low should be able to induce some 
dynamical cooling. Thus...have introduced more snow into the 
grids...with some light accumulate from central to northestern Nebraska on Monday nt 
/current snow grids end there...but current forecast would indicate some 
accumulate on Tuesday daytime as well/. Have kept Monday nt temperatures close to 
wet-bulb values...and lowered maximum temperatures on Tuesday significantly as 
strong cold air advection /not to mention clouds and precipitation/ should 
hinder any warming. 




Long term...Tuesday night through Saturday 
medium range models showing varying degrees of difference with 
shortwave energy dropping out of scntrl Canada Tuesday night as well 
as amplifying western Continental U.S. Ridge heading into next weekend. As for the 
Canadian wave dropping into the lower 48 middle week...models similar 
advertising a cold front will penetrate the northern states but vary with respect to 
placement of cold airmass. The GFS is the most aggressive pushing it 
through the Dakotas and down the Upper Middle west/middle MS Cly region while 
the Gem takes a southward track just a bit father east of the region. 
Meanwhile the European model (ecmwf) holds off pushing the brunt of the cold airmass 
till about early Wednesday evening and with a track more toward the 
Great Lakes region. For now will opt to temper adjusting temperatures 
downward on Wednesday. Nevertheless...enough cause to to include 
small chance for rain or snow Tuesday night with strong dynamic forcing 
coming in close vicinity of the County Warning Area. Otherwise...dry conditions 
with above normal temperatures on tap the rest of the forecast period when a 
building upper ridge eventually envelops the plains. 


&& 


Aviation... 
taf sites kofk/koma/klnk 


18z tafs 


Generally VFR conditions can be expected through 00z...however 
occasional MVFR ceilings near fl025 will affect eastern Nebraska at 
times. MVFR to IFR ceilings are expected to spread north from eastern 
Kansas to klnk...koma and kofk between 00z and 06z...with 
deteriorating conditions expected after 06z. Ceilings below fl010 and 
occasional visibilities below 3sm in fog are likely between 08z and 
18z Sunday. 




&& 


Oax watches/warnings/advisories... 
NE...none. 
Iowa...none. 
&& 


$$ 


Short term...Mayes 
long term...Dee 
aviation...dergan