Area Forecast Discussion
Issued by NWS Great Falls, MT

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581
FXUS65 KTFX 292329
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
530 PM MDT Wed Mar 29 2017

Updated Aviation Discussion

...Winter Weather Returns to Parts of Southwest Montana
Thursday...

.DISCUSSION...

Rest of today through Friday...An upper level trough over the
northern Pacific Ocean will reach the Pacific NW tonight, then dive
SE through the Great Basin/central Rockies, before closing
off/deepening over the Four Corners region by Friday. At the same
time, a cold front will drop south out of Canada on Thursday and
move through Montana. The crux of the forecast will probably lie
with the timing/location of this front. Along and south of this
front is where the best lift/moisture is expected and is where the
heaviest precip will be. For SW MT, this doesn`t pose as much of
a forecast challenge. For southern sections of central MT,
however, the front will likely be the dividing line between decent
QPF vs little to no QPF. Determining where, exactly, that line
will be is tricky. However, 12Z model guidance has come in
decidedly drier from Helena/Great Falls to Lewistown. For this
reason, I lowered pops/QPF to match the latest trends. It should
be noted, though, that if the front slows down and/or stalls a bit
further north, precip amounts could be higher.

Across SW MT, this setup favors the potential of some banded (ie.
heavier) precip rates/amounts. Wherever this band sets up, a period
of moderate to perhaps even heavy snow may develop. Confidence is
high that any banded precip would occur across far SW MT and is
where we decided to upgrade the Winter Storm Watch to a Winter
Weather Advisory. NOTE: this advisory does NOT include
Bozeman/Dillon as it is only for elevations of 6500 feet and
above. Despite the potential of higher amounts, we didn`t go with
a warning for now as it appears the mild antecedent conditions may
tend to limit how much snow actually accumulates on roads over
mountain passes. Impacts are likely, but temperatures near/just
above freezing may allow road conditions to be more slushy,
especially with the heaviest snow coming during the daylight
hours. We`ll continue to monitor impacts during the event, though.
Aside from travel impacts, there will be impacts for outdoor
enthusiasts in the mountains. At lower elevations, precip type
will likely be rate- driven during the day (ie. heavier rates at
times may cause enough cooling for a transition to snow). However,
as cooler air moves in late Thursday night/early Friday morning,
any lingering precip may be snow for any place that hasn`t already
switched over. Right now, impacts at lower elevations look to be
minor, if any at all. Best chance for any minor impacts would be
across SW MT. Precip moves out quickly on Friday as the system
pulls away. Martin

Friday night through Wednesday...No major changes to the extended
period this afternoon, other than confidence is increasing in the
storm system for early next week. Until then, expect showers to
decrease on Friday night with slightly above normal temperatures
expected over the region for Saturday and Sunday. By Sunday evening,
the next weather disturbance will be approaching the region, with a
surface area of low pressure developing over northern Wyoming Sunday
night/Monday. As a result, showers will be increasing towards Sunday
evening over Southwest MT, then become mostly a steady snow late
Sunday night through Monday over the Southwest. Some rain could mix
in during the day on Monday at lower elevations. The heaviest
precipitation is progged to fall south of a line from Helena to
Lewistown. The GFS still progs over 1 inch of precipitation, while
the EC has about 0.50 inches...thus they are getting closer and
leaning towards a wetter solution for Southwest MT. Over North
Central MT, much of the region will miss this event, other than a
passing rain/snow shower. Winter highlights might be needed for this
event. Otherwise, this system exits the region on Tuesday, with
showers ending. An upper level ridge builds over the area for
Wednesday, allowing for a brief warm up before the next system
arrives on Wednesday night. Brusda

&&

.AVIATION...
Updated 2330Z.

Upper level trough of low pressure moving inland into the Pacific NW
tonight will spread increasing cloud-cover across the region this
evening. Precipitation over western MT will spread SE overnight
across portions of SW MT with a period of widespread and at times
moderate precipitation for areas from KDLN to KBZN and KLWT and pts
south late tonight through Thursday morning. Snow levels initially
near 7000 ft msl will fall to near 5000 ft Thursday morning,
bringing the potential for change over to snow and associated lower
Visibilities for a period Thursday morning at KBZN and KEKS. Surface
winds from the SW will diminish some overnight then shift to NW
Thursday morning. Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  41  53  35  53 /  30  40  40  40
CTB  36  51  33  51 /  20  10  20  20
HLN  39  46  33  53 /  40  60  60  50
BZN  37  45  33  51 /  60  80  70  50
WEY  31  36  26  42 /  80  80  60  30
DLN  36  42  30  50 /  70  90  70  40
HVR  39  57  37  54 /  10  20  20  30
LWT  38  48  35  47 /  30  60  70  60

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 4 AM Thursday to 6 AM MDT Friday
above 6500 feet for Beaverhead...Gallatin...Madison.

&&

$$

weather.gov/greatfalls



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