201 
fxus65 kbou 211104 
afdbou 


Area forecast discussion 
National Weather Service Denver Colorado 
350 am MST Sat Nov 21 2009 


Short term...still counting on another mild day even with periods 
of middle and high-level cloudiness. Cloud band approaching western 
Colorado at the moment largely above 15 thsnd feet mean sea level according to 
upstream radiosonde observation and acars soundings. Some of this moisture already 
reaching the northern Front Range and showing up as a standing mountain wave 
cloud. Model cross sections show the mountain wave in the Lee of the 
Front Range expanding southward through the morning as westerly wind spds 
aloft increase. Believe we/ll see some degree of mountain wave cloudiness 
over and near the I-25 urban corridor for much of the day. Could 
also get a little rough for general aviation over and east the 
foothills this morning through early this afternoon. Cross sections 
indicate potential gusts of 35 to 45kt on the Lee slope of the Front 
Range down to about 8 thsnd feet mean sea level. Have already seen gusts in the 
30-38kt range atop Berthoud Pass and in the vicinity of Estes Park 
this a.M.. on the plains will likely see the south-southwesterly surface 
flow shifting to an easterly component this afternoon as 
anticyclonic circulation drives a weak frontal boundary wwrd across the 
plains and up against the foothills sometime between 21z/Sat and 
00z/sun according to the NAM and GFS. 


Tonight...the passage of a shortwave trough will result in increased 
cloud cover but little in the way of precipitation as it looks now. Soundings 
and cross sections indicate cloud bases down to around 9000-10 thsnd 
feet mean sea level...low enough for widespread mountain top obscuration. Weak 
orographics and neutral lapse rates will also be sufficient to allow 
scattered snow showers in the high country most of the night. Any 
accumulation should be very light and confined mainly to west facing 
slopes. On the plains...passage of the 700-500 mb trough axis around 
06z/sun may be accompanied by a few snow showers. However with dry 
boundary layer conditions...see little if any of this moisture reaching 
the ground. Temperatures tonight about the same as those observed 
Friday night. 


Long term...several weak disturbances will move across Colorado in 
the Sunday and Monday timeframe. The first one will rapidly move 
across Colorado Sunday morning. Moisture is scarce with this 
wave...with moisture mainly in the middle and upper levels. Will 
leave the isolated snow showers in the am as the trough axis passes by 
in the am then a more subsident airmass by afternoon. A bit cooler 
across the entire area on Sunday. The next system is a bit stronger 
and moves across northern Colorado late Sunday night and Monday. A bit 
more moisture and ascent with this wave so a bit better chance for 
snow across the mountains. This open wave generally deepens into 
Nebraska but low level flow increases from the northwest and results in a 
downslope flow with precipitation chances very low across the northeast 
plains. Again will maintain very low probability of precipitation on the plains. 


Ridging aloft builds across the Rocky Mountain region Wednesday-Friday with 
dry conditions and a slow warming trend with temperatures around seasonal 
normals. Good travel conditions for Thanksgiving day across the 
region. 


&& 


Aviation...visual Landing conditions will prevail through Sunday 
with scattered to broken cloud cover generally above 14000 feet mean sea level 
today. With the passage of a weak upper level trough tonight... 
cloud bases in the Denver area should lower to around 9000-10000 feet 
towards midnight. May possibly see a few snow flurries late this 
evening and after midnight...but no visibility restrictions 
anticipated. Also anticipate typical diurnal wind patterns and 
speeds overnight. 


&& 


Bou watches/warnings/advisories...none. 


$$ 


Baker/entrekin