955 
fxus65 kmso 210950 
afdmso 


Area forecast discussion 
National Weather Service Missoula Montana 
350 am MDT sun Mar 21 2010 


Discussion...high pressure has begun to weaken overnight allowing 
high level clouds to push into the northern rockies ahead of an 
approaching weather system. Despite these high level clouds we 
anticipate a fair bit of heating this afternoon...especially 
over western and southwestern Montana where steep lapse rates will 
support good mixing of warm air aloft. 


The cold front of much discussion these past few days is coming 
onshore in Washington this morning. Over the past 12 hours 
forecast models have come into very good agreement with the timing 
of this system as well as the locations and amounts of 
precipitation forecast with the passage of the front. Various 
factors exist which support the likelihood of heavy precipitation 
near the front. The first is a very visible stream of subtropical 
moisture that can clearly be seen on infrared satellite imagery as 
well as through total precipitable water channels. Analysis of 
this moisture suggest precipitable water values of at least 150 
percent of normal. The second factor lies in the forecast 
positioning of the upper level jet as the cold front moves into 
central Idaho and western Montana. The front is expected to move 
into Idaho after 03z and into Montana shortly after 06z. During 
this time all models show the upper level jet shifting south as 
the front moves east. This movement will aid in strengthening the 
frontal and subsequently lead to a period of heavier 
precipitation. Locations which consistently appear in the best 
lift and precipitation include the Clearwater and Bitterroot 
Mountains...westerly favored up-slope locations such as 
Pierce...Headquarters and Lowell as well as the lower valleys of 
Orofino and Grangeville. 


Previously it was thought the front would be slower and thus allow 
snow levels to fall which would permit heavier accumulations at 
pass level. However given the faster timing now consistent in the 
models believe snow levels will fall after the front has passed 
and heavies precipitation ended. Thus have tapered back snowfall 
totals to fall more in the 1 to 4 inch range with the heaviest 
accumulations possible over Lolo and lost trail passes. The 
atmosphere during the day Monday will remain moist and unstable 
with widespread rain and snow showers expected through the early 
evening hours. But little additional accumulation is anticipated 
throughout the day. 


Tuesday...an upper level trough of low pressure will be exiting 
the northern rockies region early in the day. However...there 
will be enough residual moisture and instability for clouds and 
showers. Although...most showers should be confined to the 
mountainous regions along the Continental Divide. Behind this 
trough system we anticipate a ridge of high pressure building in 
from the west. 


Tuesday night through Wednesday night...the high pressure ridge 
will provide dry conditions with a warming trend. By Wednesday 
night...an approaching Pacific system will be increasing clouds 
and the southerly flow at the lower levels. Expect warmer night- 
time temperatures across the lower western valleys of 
Clearwater/Idaho counties in north-central Idaho where these 
southerly winds will mix warmer air to the surface. 


Thursday through Saturday...a progressive weather pattern will 
see a pair of Pacific systems move through the northern rockies 
region. This period will be highlighted by moist unstable air 
and a healthy chance of showers. Current timing of these systems 
will be one on Thursday...followed by a more robust system for 
Friday. Some lingering showers are expected for Saturday... 
otherwise medium range models agree with building a ridge back 
over the area later Saturday. 


&& 


Aviation...high clouds will be present throughout the morning. 
Thereafter ceiling heights across the area will slowly begin to 
lower as an approaching Pacific storm system spreads moisture 
eastward. Mountains and passes may become obscured in northwest 
Montana this afternoon. Widespread obscuration of terrain is 
anticipated after 00z. By 06z lowered ceilings and the possibility 
of reduced visibility in precipitation are possible at kgpi and 
kmso as a cold front passes through the region. The front will 
reach ksmn and kbtm by 12z. 


&& 


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


$$ 


Short term...Dickerson 
long term....msj 
aviation...Dickerson