Weather

792 
fxus65 kmso 052142 
afdmso 


Area forecast discussion 
National Weather Service Missoula Montana 
242 PM MST Monday Jan 5 2009 




Discussion...snowfall is expected to continue across western 
Montana and central Idaho for the remainder of today but will 
gradually lessen as the night progresses. Immediate concerns with 
snowfall amounts during the overnight period will be the potential 
for banding to occur later tonight...affecting locations of west- 
central Montana through early Tuesday morning. There is enough 
stability present along with strong winds necessary to generate 
snow bands...but uncertainty lies in whether the possible bands 
will remain stationary or move as the winds shift overnight. This 
will affect snow amounts depending on the location of these snow 
bands should they set up. That being said...the next concern is 
for during the day Tuesday with the onset of the next 
weather system. Warm air will begin to creep into south central 
Idaho...warming the valleys in the Camas Prairie and southern 
Clearwater region of Idaho by Tuesday evening. At this 
time...moisture will begin to enter from the west and fall in the 
form of snow. As upper levels begin to warm this snow may have a 
brief changeover to freezing rain late in the evening Tuesday. 


Tuesday night through Thursday...a low amplitude will not provide 
much protection as an onslaught of Pacific moisture barrels into 
the northern rockies. The jet stream arcs northward into Canada 
allowing for an influx of warmer low-level air from the south. A 
messy forecast on tap as precipitation starts out as snow or a mix 
of rain and snow...possibly with a brief period of freezing 
rain...before changing over to all rain in the valleys Tuesday 
night. Mountains will be getting hammered with snow Tuesday night 
and Wednesday as snow levels gradually rising from roughly 3500 
feet Tuesday evening to between 5000 and 6000 feet by Wednesday 
afternoon. A Winter Storm Warning has been issued to highlight 
this snow potential. In addition to precipitation...strong winds 
aloft combined with a tightening surface pressure gradient will 
promote strong southerly winds in the lower Hells Canyon and Camas 
Prairie areas of north-central Idaho. This southerly low level 
flow will also warm surface temperatures into the upper 30s to 
middle 40s in many valley location resulting in considerable snow 
melt and potential small stream and urban flooding problems. 


Friday through Monday...a cool down is expected by the end of the 
work week into the coming weekend. A weak frontal system will pass 
through the area early Friday bringing some light snow 
accumulations to the area with temperatures returning to 
seasonable norms. A northwest flow aloft will set up over the northern 
rockies over the weekend into early next week with the high 
pressure ridge aloft remaining along the West Coast. Periods of snow 
is possible...mainly focused over the mountains in this time frame. 


&& 


Hydrology...as snow transitions to rain in most valleys of north 
central Idaho and western Montana Tuesday night into Wednesday...ponding 
of water and flooding of Road ways will be a concern. In addition... 
warming temperatures and gusty southerly winds will also aide in 
melting snow at the lower elevations through Thursday. Snow loading on 
buildings with hefty snow will also be a concern...especially in 
northwest Montana. 


&& 


Aviation...current storm system will continue to bring snow 
through this evening in north central Idaho and western Montana. 
Snow will keep visibilities reduced through the overnight in most 
of central Idaho. Kmso will see low ceilings and visibilities 
continuing but with gradual improvement later this 
evening...though intermittent snow showers will reduce flight 
conditions from time to time overnight. MVFR will be the dominant 
flight category for all terminals through the overnight hours. 




&& 


Mso watches/warnings/advisories... 
Montana...Winter Storm Watch from Tuesday afternoon through Wednesday 
morning for Bitterroot/Sapphire Mountains. 


Winter Storm Warning until 11 PM MST this evening for 
Bitterroot/Sapphire Mountains. 


Winter Storm Warning until 11 PM MST this evening for 
Potomac/Seeley Lake region-West Glacier region. 


Winter Storm Watch from Tuesday afternoon through Wednesday 
afternoon for West Glacier region. 


Winter Storm Watch from Tuesday afternoon through Wednesday 
morning for Kootenai/Cabinet region-Lower Clark Fork region- 
Potomac/Seeley Lake region. 


Winter Storm Warning until 11 PM MST this evening for Lower 
Clark Fork region. 


Idaho...Winter Weather Advisory until 4 PM PST this afternoon for lower 
Hells Canyon/Salmon river region. 


Wind Advisory from 1 PM Tuesday to 4 PM PST Thursday for lower 
Hells Canyon/Salmon river region-Orofino/Grangeville region. 


Winter Storm Watch from Tuesday afternoon through Wednesday 
morning for northern Clearwater Mountains-southern 
Clearwater Mountains. 


Winter Storm Warning until 10 PM PST this evening for southern 
Clearwater Mountains. 


Winter Storm Warning until 4 PM PST this afternoon for 
Orofino/Grangeville region. 


Winter Storm Warning until 10 PM PST this evening for northern 
Clearwater Mountains. 


&& 


$$ 


Short term...allegretto 
long term....msj 
aviation...allegretto