Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 182340

440 PM MST Thu Dec 18 2014

Updated Aviation

Tonight through Saturday...The freezing fog advisory for northern
Blaine and northern Hill Counties has been cancelled as visibilities
have generally improved.A couple of weather disturbances aloft
will affect the forecast area during the period. The first one
will move through this evening. It looks like the only areas that
could see some light snow are the Little Belt Mountains..mountains
near the Idaho border..and mountains of the Rocky Mountain Front.
The next weather disturbance will move through Friday night. This
disturbance will have more moisture with it so mountains will
likely see a period of snow while there will be a chance of
precipitation for the lower elevations. This precipitation will
likely be a mix of types with rain and snow most prevalent but
there will also be a small threat of freezing rain over the
eastern plains. If freezing rain should occur do not think it will
last very long or be very widespread. Then Saturday westerly flow
aloft will develop and expect gusty southwest winds over the east
slopes of the Rocky Mountains. Blank

Saturday Night through Thursday...Through Monday, an upper level
high pressure ridge will strengthen over the eastern Pacific Ocean
and an upper level low pressure trough will deepen over central
Canada and the northern CONUS plains. The resulting strong
northwesterly flow aloft will bring increasing westerly surface
winds to the forecast area. Although forecast models are not quite
as strong with these winds as they have been the last couple of
nights, there is still a chance for High Wind criteria winds along
the Rocky Mountain Front and over some ridge tops. Will continue to
monitor this for potential High Wind highlights. This increasing
flow aloft will also bring warm and moist Pacific air and a weak
disturbance into the area. The main impact from this will be at
least a rain/snow mix in the mountains, if not mostly rain, while
the plains and valleys will have less of a chance of precipitation.
This could be a concern in the mountains with regard to rain
possibly falling on snowpack. Will monitor this situation for
potential hydrologic highlights. With the warm air aloft moving over
cooler air at the surface over the plains of north central Montana
Saturday night into Sunday morning, have included a slight chance of
freezing drizzle there. The westerly downslope winds will quickly
spread east across the area during the day on Sunday, warming
temperatures well into the 40s by afternoon. Temperatures will
remain similarly mild through Monday. As the disturbance exits the
area Monday night, the chance for precipitation will be limited to
the mountains into Tuesday. However, a weak cold front will move
south through the area on Tuesday, cooling temperatures to near
seasonal averages. As was mentioned yesterday, a cold and moist
weather system will move southeast from the Gulf of Alaska and into
the northern Rockies for Wednesday (Christmas Eve Day) into Thursday
(Christmas Day). There is still a good chance of precipitation with
this system across the area, but forecast models are trending a bit
farther south with the heaviest precipitation, focusing it more over
southwest Montana than central Montana. Have therefore made this
adjustment to the forecast. There is still a good indication that
temperatures will cool significantly below normal through this
period, for widespread light snow. Will continue to monitor the
progress of this system for potential winter weather highlights, as
this system will likely negatively impact holiday travel. Coulston


Even though an upper-level ridge of high pressure will build over
the Northern Rockies tonight, Pacific moisture moving over the ridge
will keep SCT to BKN cigs with mountain obscuration likely through
Friday. TAF locations will remain dry, however light snow showers
are expected over the mountains, thus have added VCSH to KBZN and
KHLN. Patchy fog is also possible overnight at KHLN, but confidence
is low given the expected mid-level cloud cover. Southwest winds
will decrease over the plains this evening as the ridge continues to
build. VFR conditions will prevail at all terminals through the
forecast period with the exception of KHLN if fog develops. MLV


GTF  27  41  28  40 /  10  10  20   0
CTB  24  39  27  37 /  10  10   0  10
HLN  20  36  22  36 /  10  20  40  10
BZN  17  33  19  34 /   0  10  50  10
WEY  20  33  21  32 /  40  50  60  50
DLN  20  38  21  36 /   0  20  40  10
HVR  21  39  27  40 /  10  10  20   0
LWT  25  45  26  42 /   0   0  20  10



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