879 
fxus65 kmso 212146 
afdmso 


Area forecast discussion 
National Weather Service Missoula Montana 
246 PM MST Sat Nov 21 2009 


Discussion...shower activity is expected to wane this evening as 
a shortwave ridge builds ahead of an approaching upper level 
trough. This will allow skies to clear enough to see a quick drop 
in temperatures after sunset and the potential for some valley fog 
formation...mainly in north central Idaho valleys. 


The upper level trough is expected to begin to move through the 
northern rockies during the day Sunday. This feature will be 
accompanied with a strong upper level jet and surface cold 
front...which will combine with abundant moisture to produce 
widespread precipitation over most all of western Montana and 
north central Idaho. Temperatures are expected to remain cool 
enough that all but the lowest valleys (below 2500 feet) should 
see precipitation fall as snow. The most favorable dynamics with this 
system will be focused over north central Idaho however...and thus 
the best snow production is expected over the Clearwater and 
Bitterroot Mountains. In addition to expected accumulating 
snow in these areas...gusty southwest to west winds will create lowered 
visibility and hazardous driving conditions over portions of 
northcentral Idaho and western Montana. A Winter Weather Advisory 
has been issued...please see www.Weather.Gov/Missoula for further 
details. The upper level trough will move eastward overnight 
Sunday...bringing a northwesterly flow over the region. While the 
heaviest precipitation should come to an end...there is the 
possibility of banded precipitation developing in to Monday. 


Tuesday through Sunday...a shortwave trough will pass to the north 
along the Montana Canadian border on Tuesday bringing a chance for 
precipitation in northwest Montana before a high pressure ridge 
begins to overtake the region. This high pressure has been showing 
similar timing and slightly different amplitudes between the European model (ecmwf) 
and the GFS. The amplifying ridge will pose a threat of widespread 
valley fog starting Wednesday morning. Model plan views and cross 
sections show energy passing through the ridge Wednesday morning... 
which could inhibit valley inversion development...however...model 
sounding for the same time frame do show inversions 
initiating...so will keep the mention of fog in the forecast. High 
pressure looks to linger into Thursday with drier...subsiding air 
and strong inversions forecast by model soundings. This is 
somewhat different from yesterday...but these features would 
promote fog on Thursday. Will continue to monitor fog potential 
especially for the Glacier Park International Airport and the 
Missoula Airport as Holiday airline travel may become adversely effected. 


&& 


Aviation...strong west-southwest surface winds will continue through 
early Saturday evening and clouds will be on the increase as the 
next Pacific weather system pushes into the northern rockies. Snow 
showers will start to spread across the Idaho Panhandle and 
northwest Montana Sunday morning...mostly after 22/1200z. The 
remainder of western Montana and north central Idaho will see 
showers spread areawide later midday Sunday. Significantly 
lowered ceilings and reductions in visibility will be possible 
with heavier snow showers at the terminals. Surface winds will 
remain breezy... but to a lesser degree than winds observed on 
Saturday. Obscurations of mountains and passes can be expected. 




&& 


Mso watches/warnings/advisories... 
Montana...Winter Weather Advisory from 8 am to 11 PM MST Sunday for 
Lower Clark Fork region. 


Winter Weather Advisory from 8 am to 11 PM MST Sunday for 
Bitterroot/Sapphire Mountains. 


Idaho...Winter Weather Advisory from 7 am to 10 PM PST Sunday for 
Orofino/Grangeville region. 


Winter Weather Advisory from 7 am to 10 PM PST Sunday for 
northern Clearwater Mountains-southern Clearwater Mountains. 


&& 


$$ 


Short term...pallister 
long term....kitsmiller 
aviation...Nolte