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afdtfx 


Area forecast discussion 
National Weather Service Great Falls Montana 
1100 PM MST Friday Nov 20 2009 


Updated aviation section 


Update... 
progression of shortwave trough seems to have slowed a bit more so 
have lowered overnight probability of precipitation along the Continental Divide and 
portions of central Montana. Weak cold front still expected to 
push through the area overnight...with temperatures falling closer 
to seasonal averages on Saturday. Light snow will develop along 
the Continental Divide after midnight with light snow accumulation 
of 1 to 2 inches possible. Gusty winds and falling snow may cause 
times of reduced visibilities along the mountain passes. Mls 


&& 


Aviation... 
updated 0600z. 
An upper level trough of low pressure will move eastward towards 
central/southwest Montana late tonight. Expect generally overcast 
conditions until 00z Sunday...with the chance for snow developing 
after 09z over western and southwestern Montana. The best chance for 
snow on Saturday will be on Saturday from 13z to 20z in the Bozeman 
and West Yellowstone area. Thus IFR/LIFR conditions will be possible 
at times over this region. Mountains will be obscured at times 
throughout the region though the period. Additionally...expect 
surface winds to become very breezy over north central Montana after 
09z Saturday. Expect a generally clearing trend with lighter winds 
expected from 0z Sunday until 6z Sunday. Brusda 


&& 


Previous discussion... 
/issued 500 PM MST Friday Nov 20 2009/ 
an upper level trough of low pressure...currently moving onto the 
West Coast...will track east across the region late tonight 
through early Saturday afternoon. A brief period of ridging behind 
this initial system Saturday night will be quickly followed by 
another trough moving into the northern rockies on Sunday. A cold 
front associated with the initial trough will move east across the 
region late tonight. Moisture will increase along and behind the 
front with snow showers developing after midnight over the Rocky 
Mountain front and Glacier park area...spreading east over SW Montana late 
tonight through Saturday morning. Westerly downslope flow will 
limit precipitation chances over the plains...however occasional 
snow showers over the mountains may bring accumulations of 1 to 3 
inches to elevations above 6000 feet. Significant cooling will occur 
behind the front over the forecast area on Saturday...however this 
will only mean temperatures dropping back to seasonal averages. 
Models have been trending slower and deeper with the second system 
to arrive on Sunday and this system will likely bring another 
round of snow showers to the mountains with perhaps a little better 
chance of snow showers extending farther east onto the plains due 
to deeper moisture and marginal instability. Accumulating snow 
again looks to be confined mainly to the mountainous areas with 
another 1 to 3 inches possible above 6000 feet. Gusty southwest 
winds will shift to the west behind tonights cold cold front with 
western portions of north central Montana and mountain passes seeing the 
strongest winds with gusty winds continuing through the weekend. 
Combination of gusty winds and periods of light snow may have an 
impact to travelers over mountain passes this weekend and will 
highlight this in the hazardous weather outlook. Hoenisch 


Sunday night through Friday night...the medium range models are in 
decent agreement with moving the upper level trough out of the area 
Sunday night into Monday for a decreasing chance of precipitation. 
As the cold front associated with this system also moves through the 
area Sunday night...west winds will become breezy and gusty. These 
winds will decrease on Monday as the front exits the area to the 
east...as temperatures continue to hover around normal. The upper 
level high pressure ridge over the eastern Pacific will then begin 
to build back into the western United States Monday night through 
Tuesday night. However...a weak shortwave will move down the east 
side of the ridge and through Montana during this time. This will 
bring a chance of precipitation to the area once again...but it will 
mainly be in the mountains as westerly downslope winds will increase 
with the passage of the shortwave. These winds will also warm 
temperatures to about 5 to 10 degrees above normal on Tuesday. The 
upper level ridge will then build into Montana Tuesday night through 
Thursday (thanksgiving day)...which will keep only a slight chance 
of precipitation in the western Montana Mountains. Also...westerly 
downslope winds will increase once again through this period...which 
will keep temperatures about 10 degrees above normal. The models 
begin to diverge for the remainder of this forecast period. The GFS 
is much faster than the European model (ecmwf) with bringing the next Pacific weather 
system through the area. The GFS moves it through Montana on 
Friday...while the European model (ecmwf) is 12 to 24 hours slower in moving it 
through Friday night into Saturday. With this uncertainty...have 
gone with a widespread chance of precipitation for Friday and Friday 
night...with the chance being higher than climatological averages. 
Have also cooled temperatures closer to normal. The increases in wind 
Sunday night...Monday night and Tuesday...and Thanksgiving day 
into Friday will need to be monitored for possible highlights. 
Coulston 


&& 


Preliminary point temps/pops... 
gtf 36 42 25 41 / 0 10 0 30 
ctb 31 40 24 39 / 10 10 0 20 
hln 29 41 23 41 / 20 20 0 40 
bzn 23 37 13 37 / 20 50 0 40 
wey 22 29 8 30 / 50 90 30 90 
dln 27 32 14 34 / 40 50 0 50 
hvr 29 49 21 45 / 0 10 10 20 
lwt 32 42 20 38 / 0 10 0 20 


&& 


Tfx watches/warnings/advisories... 
none. 


&& 


$$ 


Weather.Gov/greatfalls