052 
fxus65 kmso 220941 
afdmso 


Area forecast discussion 
National Weather Service Missoula Montana 
341 am MDT Monday Mar 22 2010 


Discussion... 
today...a cold upper trough of low pressure will be moving over 
the northern rockies today in the wake of a surface cold front. 
The atmosphere will be quite moist and unstable...in fact there 
is a slight chance of thunder across southwest Montana and eastern 
Lemhi County this afternoon. Snow showers are likely in the 
mountains with some significant accumulations possible over lost 
trail and Bannock passes along the southern Bitterroot and 
Beaverhead Mountain ranges. A Winter Weather Advisory remains in 
effect for these areas until 12 noon MDT today. Otherwise...lighter 
accumulations expected over other area mountain passes. The 
combination of a westerly surface pressure gradient and winds 
aloft will promote breezy conditions today region-wide. 


Tonight and Tuesday...as the upper trough shifts to the east...a 
ridge of high pressure will be building into the northern rockies 
region from the west. Shower activity will be decreasing overnight 
with virtually all showers ending by Tuesday morning as the air 
mass stabilizes. The surface pressure gradient will be relaxing 
this evening with winds decreasing. 


Wednesday...a ridge of high pressure will reside over the region 
with dry conditions and a warming trend on tap. Some high level 
moisture may move through the ridge for some filtered sunshine... 
but otherwise a pleasant day. 


The ridge over the intermountain west will breakdown Wednesday 
night as a Pacific disturbance approaches the northern rockies 
Thursday morning. Model trends indicate slowing the onset of 
precipitation into north central Idaho by late Thursday morning 
and into western Montana by Thursday afternoon. With a fairly warm 
air mass in place...snow levels should remain high Thursday with 
only light snow accumulations over the highest terrain possible. A 
secondary trough is expected to dig into the Great Basin Friday 
and bring a reinforcing shot of precipitation and cooler air to 
the region. Snow levels are expected to drop to all but the lowest 
valley floors in western Montana by Friday morning...allowing the 
potential for brief snowfall with little to no accumulation in 
said valleys prior to temperatures warming to seasonal values 
during the day. 


A ridge of high pressure will build over the region Saturday and 
Sunday thus producing a drying and warming trend through the weekend. 
Given the precipitation anticipated prior to this time...the 
potential for fog development exists across portions of north 
central Idaho and western Montana each morning. 




&& 


Aviation...a moist unstable conditions are expected today and 
into the evening hours as a cold upper level trough of low 
pressure resides over northern Idaho and western Montana. Clouds 
and snow showers will be obscuring the mountains and their 
respective passes...while valley locations will see scattered rain 
showers or a rain/snow mix. Breezy westerly winds will also be on 
tap for most airfields during the day today. Conditions are 
expected to gradually improve overnight tonight and into Tuesday 
morning as a ridge of high pressure builds in from the west and 
the air mass begins to dry and stabilize. 




&& 


Mso watches/warnings/advisories... 
Montana...Winter Weather Advisory until noon MDT today Bitterroot/Sapphire 
Mountains. 


Idaho...Winter Weather Advisory until noon MDT today eastern Lemhi 
County. 


&& 


$$ 


Short term...msj 
long term....derosa 
aviation...msj