258 
fxus65 kmso 210309 
afdmso 


Area forecast discussion 
National Weather Service Missoula Montana 
809 PM MST Friday Nov 20 2009 


Update...updated earlier tonight to lower the probability of precipitation across 
northwest Montana from 00z to 03z. Also lowered some min 
temperatures tonight. 


&& 


Discussion...models are slower at bringing the system into 
western Montana and north central Idaho tonight. In 
addition...satellite imagery and radars to our west has shown that 
the system is moving slower then earlier expected. So updated the 
probability of precipitation and weather for tonight to show the slower trend. Some 
locations were at the forecasted low temperature for tonight and 
others were within a couple of degrees. Most locations from 
Missoula north never warmer up today...with observed highs about 
6 degrees below numeric guidance. So lowered a number of locations 
mainly across northwest Montana. Rest of the forecast looks to be 
on track. Will continue the Winter Weather Advisory for idz005-006 


&& 


Aviation...an approaching Pacific system will continue to 
lower clouds decks and increase precipitation chances across the 
northern rockies overnight. Ahead of the advancing surface cold 
front...showers will be a mix of rain and snow before turning to 
all snow after 21/0600z. Expect the most intense periods of 
precipitation along and just behind the frontal passage between 
21/0600z-21/1200z. Ceilings are expected to lower overnight 
causing mountains and passes to be obscured. 


&& 


Previous discussion... /issued 301 PM MST Friday Nov 20 2009/ 


Discussion...complex weather pattern setting up over the 
intermountain region for this weekend. Been an interesting day as 
clouds ahead of approaching system were quite more intense than 
the models had advertised. As a result...the warming expected 
today was limited to the southern most valleys (higher valleys) 
that mixed out earlier this morning and achieved full mixing 
despite the clouds. Central and northern Montana valleys were 
inhibited by these clouds and kept the much cooler temperatures 
trapped in their valleys. Example...Missoula 37 degrees...Hamilton 
54 at 230 PM MST. 


The aforementioned weather system will move over the Pacific 
northwest this evening spreading precipitation from west to east 
this evening and overnight. Warm air ahead of the cold front will 
keep snow levels rather high...then dropping to most valley floors 
after the cold frontal passage after midnight. The higher valleys 
of the northern Clearwater Mountains look to be the most impacted 
by enough accumulating snow to warrant a winter headline. 
Therefore the Winter Weather Advisory for snow and blowing snow 
above 3000 feet will remain in place. Other valleys of north 
central Idaho and western Montana can also expect to see some 
light snow accumulations come Saturday morning. At this 
time...looks like the best time frame for central and southern 
valleys of western Montana to see snow accumulations will be 2 am 
through 9 am...but not enough accumulations to warrant headlines. 
However...this snow will impact any travelers headed over the 
Continental Divide passes. In addition...winds behind the front 
will blow the snow around and reduce visibilities at 
times...especially over the passes. 


A second weather system and associated surface low will impact the 
intermountain region Sunday. This system is looking more impressive 
with each run...and may bring another round of accumulating snow 
to the Clearwater Mountains and portions of the Camas 
Prairie...mainly Grangeville...Greer and whitebird grades. With 
current winter headlines out...have opted to hold off on a watch 
but if the models continue to advertise the strength and position 
of this second system the way they are now...winter weather 
watches may be necessary once again. In addition...the placement 
of this second wave looks good for portions of western and 
southwest Montana valleys to see accumulating snow to the valley 
floors. Again...amounts do not appear to be headline worthy...but 
may impact travelers. Another round of wind is expected with this 
second system...so more visibility issues from blowing and 
drifting snow are possible. 


A ridge of high pressure will begin to build over the region 
Monday with the ridge axis settling over the region by Wednesday. 
Initially...the ridge will be weak enough to allow a few weak 
disturbances to move through the region both Monday and Tuesday. 
The main focus of precipitation is expected to be across northwest 
Montana but should remain fairly light with little in the way of 
accumulating snow. As the ridge axis moves overhead Tuesday 
night...shower activity will dwindle. With strengthening 
inversions under this strong upper level Ridge...Valley fog could 
become an issue Wednesday and Thursday. Have gone ahead and added patchy 
fog to the forecast for the time being but the potential for dense 
fog...potentially impacting Holiday travel is not out of the 
question. 


Forecast models indicate another low pressure trough impacting the 
northern rockies Friday in to next weekend. The details and timing of 
this system remain low for the time being. 


Aviation...an approaching Pacific system will continue to 
lower clouds decks and increase precipitation chances across the 
northern rockies this evening and overnight. Ahead of the advancing 
surface cold front...showers will be a mix of rain and snow before 
turning to all snow after 21/0600z. Parts of northwest Montana and 
north central Idaho will see shower activity start earlier Friday 
evening before chances spread across the remainder of western 
Montana by 21/0600z. Expect the most intense periods of 
precipitation along and just behind the frontal passage between 
21/0600z-21/1200z. Ceilings are expected to lower overnight 
causing mountains and passes to be obscured. 


&& 


Mso watches/warnings/advisories... 
Montana...ID...Winter Weather Advisory until 10 am PST Saturday for northern 
Clearwater Mountains-southern Clearwater Mountains. 


&& 


$$ 


Short term...kolata 
long term....Mead/pallister 
aviation...kolata