109 
fxus65 kbou 222218 
afdbou 


Area forecast discussion 
National Weather Service Denver Colorado 
317 PM MST sun Nov 22 2009 


Short term...upper level trough shown by satellite over the Pacific 
northwest. Surface data indicating front just now entering northwest 
Colorado...with current radar indicating the precipitation still in 
Utah. Models still indicate trough moving into northwest Colorado 
overnight...with surface front reaching Front Range around 11z. 
Moisture to be on the increase across mountains this evening...with 
cross sections holding off on the deeper moisture until after 06z 
with passage of trough. Orographics become favorable as southwest 
flow aloft becomes west-northwest. With 700-500 mb lapse rates 
around 8 c/km...7h winds of 20kts...and 7h temperatures around -7c... local 
orographic snow model through around 15z Monday yields roughly 3 to 
7 inches for zones 31..33 and 34 with the higher amounts in zone 31. 
Though dynamics could add a few more inches...amounts still look 
below advisory numbers...will not issue any hilites. Will increase 
probability of precipitation to categorical across mountains in the 06z to 12z time frame. 
Moisture begins to decrease Monday afternoon as qg descent moves 
over area. Despite continued orographics...descent will limit 
snowfall. Current trend of lower probability of precipitation for afternoon still seems 
reasonable. As for plains...qg ascent increases after 06z ahead of 
the trough. But models show lack of any low level moisture...and 
combined with northwest surface winds providing downslope flow... 
snow chances look limited. Will keep the current low probability of precipitation for plains 
going for most of area late tonight Monday. Exception will be along 
the northern Colorado border into the northeast corner in vicinity 
of the better dynamics. Trough moves onto plains during the 
afternoon... with qg ascent weakening and downward motion taking 
hold. Latest NAM still indicating surface winds trying to go upslope 
for a couple of hours along the Front Range during the afternoon.... 
with some light snow amounts. However...by that time...the downward 
motion prevails which should limit snow chances. Still that small 
threat for snow...feel the 20 percent probability of precipitation should suffice. 
Temperature and thickness forecasts as well as guidance suggest highs 
roughly around 40 degrees across plains. This looks reasonable and 
is in line with current temperature grids. 


Long term...models show an upper closed low just northeast of the 
forecast area Monday evening at 00z. Strong north-northwesterly 
flow aloft is prognosticated over the forecast area Monday night. The flow 
aloft decreases significantly on Tuesday and becomes northwesterly 
with an upper ridge over the Great Basin. Late Tuesday night 
through Wednesday...the flow aloft increases and becomes more 
north-northwesterly again as a wave moves down the back-side of a 
mean trough over the eastern United States. Pretty strong downward 
motion is indicated on the qg vertical velocity fields Monday night 
and Tuesday. After that...weak downward motion is prognosticated through 
Wednesday. The low level pressure and wind fields are showing 
downsloping in the boundary layer through the period. For moisture 
..there is some around Monday evening. Then it is quite dry 
through Tuesday evening. After midnight Tuesday night...continuing 
into Wednesday afternoon...there is some middle level moisture over the 
eastern third of the forecast area. The only measurable 
precipitation on the quantitative precipitation forecast fields is on the GFS Monday evening...and 
it is a tiny amount. Will go with limited probability of precipitation in the high 
mountains and northeast corner Monday evening...otherwise dry. 
Concerning high winds...there is a bit of a mountain top stable 
layer prognosticated Tuesday. The low level pressure gradient is 
fair...but the directional and speed shear are not great...nor is 
the Cross Mountain component. No wind event expected. For 
temperatures...tuesday's highs are 3-4 c warmer than monday's. 
Wednesday's warm up 1.0-2.5 c from tuesday's. For the later days 
..Thursday through Sunday...Models have an upper ridge moving 
slowly eastward Thursday and Friday. The GFS has a weak upper 
trough push across Friday night into Saturday. The European model (ecmwf) closes off 
an upper low and drops down into across southern Arizona late 
Saturday into early Monday. The GFS keeps US in northwesterly flow 
Saturday and Sunday. Will keep it dry until Sunday...with only 
minimal probability of precipitation in the mountains. Temperatures are a bit above normals 
Thursday and Friday...then a bit below Saturday and Sunday. 




&& 


Aviation...an upper level trough will move across the area late 
tonight...with lowering cloud ceilings along with a slight chance 
for snow showers. No accumulations expected. VFR ceilings and 
visibility should prevail...although 5000-6000 feet above ground level ceilings 
could affect ils approaches. Surface front to move across area 
around 11z with surface winds becoming west-northwest. Trough to 
exit area after 18z Monday...with ceilings improving. Still a slight 
chance for snow showers...no accumulations anticipated. There is a 
slight chance that surface winds could go northeast during the 
afternoon and provide upslope flow. But downward motion behind 
exiting trough should limit snow potential. 


&& 


Bou watches/warnings/advisories...none. 


$$ 


D-l/rjk