525 
fxus65 kmso 231319 cca 
afdmso 


Area forecast discussion...corrected 
National Weather Service Missoula Montana 
617 am MST Monday Nov 23 2009 


..fog to potentially impact Holiday travel on Wednesday and 
Thanksgiving day... 


Discussion...snow showers are expected to continue across west 
central Montana and the higher elevations of central Idaho this 
morning. These showers will begin to taper off gradually by middle 
morning as this system weakens further. Unstable northwest flow 
will help mountain showers to last through the afternoon but only 
light accumulations are anticipated. 


A developing ridge of high pressure will be the next feature to 
set up across the northern rockies...but this ridge will be 
interrupted briefly by a quick moving system that will graze 
northwest Montana Tuesday. Latest forecast models have been going 
bolder with precipitation expected out of this wave of energy. 
The bulk of moisture is still expected to remain far enough north 
of the Canadian border...though quite a bit of lift associated with 
this disturbance will help generate snow...particularly in northwest 
and west central Montana through Tuesday afternoon. Conditions 
will quickly improve as the high pressure ridge rebuilds and 
strengthens Tuesday evening into Wednesday morning. Depending on 
how much moisture results from tuesdays round of precipitation...this 
could create even more available low level moisture which would 
further aid in fog development expected for middle week. 


The biggest story for the extended forecast period from Wednesday 
through the weekend will be the potential for freezing fog both 
Wednesday and Thanksgiving day. Model moisture fields in the upper 
atmosphere begin to dry Wednesday and atmospheric soundings show 
modest stability developing near the surface. This should equate 
to at least some patchy fog developing Wednesday morning...depending 
upon local factors such as channeling...drainage winds...cloud 
cover and the amount of low level moisture available. The 
longevity of the potential fog Wednesday is still somewhat in 
question. But at this time it does appear to dissipate somewhat 
late Wednesday as the atmosphere appears to show some modification 
due mainly to variable high level cloud cover. 


However by evening the strong upper level ridge will amplify over 
the region with the apex becoming situated just east of the 
Continental Divide by early Thursday morning. This will help to 
enhance high pressure at the surface and lead to impressive 
stability in the valleys as indicated by atmospheric soundings 
during this time frame. Fog will most likely develop in the valleys 
sometime after sunset Wednesday evening and possibly remain foggy 
through the better part of Thanksgiving day. This scenario could 
change seeing as though it is only Monday. However the models have 
been showing the development of a strong ridge for about the past 
five days or so...giving this scenario a higher degree of confidence 
3 days in advance. The potential for dense fog is better than 
average and those traveling will want to keep monitor the forecast 
for future updates. 


The GFS...European model (ecmwf) and GFS ensemble mean all suggest delaying the 
next weather system until late Friday and keep the weather pattern 
fairly active through the weekend. On Friday a tightening southerly 
surface pressure gradient should induce winds sufficient to gradually 
mix the atmosphere on Friday thus moderating the intense valley 
inversions established earlier. Temperatures appear to warm ahead of an 
approaching cold front which is due into the northern rockies region 
late Friday at this time. Although different in their details...the 
model trend appears to be heading towards quickly digging these 
passing weather systems through the region and into the Desert 
Southwest and central Rocky Mountain region. This will not Bode 
well for heavy snowfall across Montana and Idaho as the energy 
appears to quickly exit the region. And yes...yet another high 
amplitude ridge is potentially waiting in the wings for early next 
week. All things considered...this is a very atypical weather pattern 
for an El Nino year. 


&& 


Aviation...periodically lowered ceilings and visibilities will exist 
across west central Montana and central Idaho through middle morning 
as lingering snow showers continue. These showers are expected to 
weaken throughout the morning and diminish by this afternoon with 
conditions improving as a result...though mountains will remain 
obscured for most of today. 


&& 


Air stagnation...forecast models are in very good agreement that 
very strong valley inversions will begin to build Wednesday and 
grow even stronger Wednesday night into Thanksgiving morning as a 
high amplitude ridge of high pressure strengthens aloft. Such strong 
valley inversions will likely inhibit appreciable mixing which could 
potentially lead to stagnant air and potential health issues due to 
trapped pollution. 


&& 


Mso watches/warnings/advisories... 
Montana...none. 
Idaho...none. 
&& 


$$ 


Short term...allegretto 
long term....Dickerson 
aviation...allegretto 
air stagnation...Dickerson