Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 281042

Area Forecast Discussion
National Weather Service Great Falls MT
342 AM MST Tue Feb 28 2017


Today through Thursday...Satellite imagery shows broad upper
level trough in place across western N America with axis extending
south through the Northern Rockies and embedded shortwave moving
into the ID panhandle early this morning. At the surface, a weak
low is associated with the shortwave entering northern ID and a
trough extends east across NW MT with frontal boundary continuing
east roughly along Hwy 2 early this morning. Expect a somewhat
organized band of light snow to develop over NW zones early this
morning as the shortwave approaches and moves east along the
US/Canadian border by late this morning with accumulations up to 2
inches across primarily Glacier and Toole counties, including the
Cut Bank area. Trough axis shifts east across the entire area
this afternoon with scattered to numerous snow showers over much
of the area. Could see some more organized banding of convective
snow showers over central/SM MT this afternoon with a few
locations picking up a quick inch or so of snow accumulation,
though widespread significant accumulations are not expected.
Brief period of drying moves across the region tonight behind the
exiting shortwave. Vigorous shortwave/jet energy moving out of
the Gulf of AK drops SE into southern BC tonight continues SE
across the forecast area on Wednesday. Lee-side trough at the
surface forms along the East slopes of the Rockies tonight then
shift east with the passage of the shortwave energy on Wednesday.
Winds increase along the east slopes of the Rockies tonight and
spread east across adjacent portions of north central MT Wednesday
morning. Potential for strong winds is somewhat marginal with
gusts to 60kts likely along the immediate front range and gusts of
40-50kts possible further east, including areas as far east as
the I15 corridor and Judith Basin county, particularly as the
shortwave energy and surface trough move across the region
Wednesday morning. Drifting/Blowing snow impacts also possible
during this period, particularly along the Northern Rocky Mtn
front. Will issue a High Wind Watch for areas most likely to
experience strong winds and/or blowing/drifting snow impacts.
Another round of snow showers is likely Wednesday afternoon
though early evening with moist NW flow aloft favoring the Mtns of
Central MT and areas along the continental divide for
accumulating snowfall with less organized snow shower activity
across the valleys/plains. Weak upper level ridge then moves
overhead behind the exiting system Wednesday night through
Thursday for decreasing precipitation chances with temperatures
warming back to near or slightly above seasonal averages by
Thursday afternoon. Hoenisch

Thursday night through Friday night`s main forecast challenge will
be the potential for a second wind event to end the work week. By
00z Friday, 00z GFS/ECMWF-HiRes/Canadian models depict a H500 ridge
axis extending from Northern Nevada/Utah, north through Western
Montana and into the Canadian Province of Alberta, with a strong
H500 jet streak rotating around the base of a digging trough off of
the coast of Washington state and the Province of British Columbia.
The aforementioned trough will continue to dig south and east along
the Pacific Northwest Coast through the afternoon hours on Friday,
with a 65+ knot exit region of the H500 jet extending as far east as
the Lewis Range and Rocky Mountain Front. Cross barrier flow will
increase throughout Thursday night and into the day on Friday along
the Lewis Range and Rocky Mountain Front, with sustained H700 wind
speeds of 45 to 55 knots. While 00z Bufkit soundings do not support
strong stability above mountain top levels (which is ideal for a
strong downslope wind event), a significant reduction in wind speeds
of ~20knots was observed above mountain top levels, which should
promote the formation of mountain waves along the Lewis Range and
Rocky Mountain Front. In addition to the potential for strong winds,
snow showers will be possible across the higher elevations of the
Rocky, Little Belt, and Southwestern Montana Mountains Thursday
night through the evening hours on Friday. The combination of
falling snow and strong winds may create areas of blowing snow,
which would reduce visibilities. Confidence in the degree of blowing
snow from the current snowpack that resides over the region and snow
that is yet to fall was not high enough to introduce blowing
snow/reduced visibility wording into the current forecast. -

Saturday through Tuesday...Overall, the latest 00Z model guidance
has come into better agreement over the upcoming weekend and into
early next week and confidence is closer to average. That said,
confidence in the evolution of the upper level pattern from western
North America into the eastern Pacific is somewhat lower. WPC noted
that predictability in the pattern in this area has been somewhat
lower of late once you get beyond DY3. To that point, the models
begin to differ regarding a potential trough over the Pacific and
the downstream ridge over the western US. These differences appear
to mainly impact the MON/TUE timeframe next week. The main
differences right now appear to center around the ridge and how
amplified it may be, if in fact the trough remains offshore. The
EC/CMC are more amplified while the GFS/GEFS are less so.

As for sensible weather, the period starts with windy conditions
potentially lingering along the Rocky Mtn Front early SAT AM. The
wind should begin to diminish, though, as the SFC pressure gradient
relaxes and low/mid level winds decrease. In general, downsloping
should limit precip at lower elevations, but a moist W/SW should
keep precip going in the mountains, especially the
western/southwestern mountain ranges.

The next item of note is a potent and seasonably strong s/w that the
models have been very consistent in bring through the area on
Sunday, accompanied by a strong Pacific cold front. While low level
moisture may be limited over valleys/plains, forcing along the front
combined with steep lapse rates and strong jet dynamics should be
enough to generate at least isolated to scattered rain/snow showers.
Given the amount of lift forecast, it likely wouldn`t take much
additional moisture for a greater coverage of showers and some
models hint at this possibility. The chance of precip may need to be
increased for parts of the area in later forecasts if a more moist
trend develops in the models. A period of stronger winds is likely
with the passage of the front as well. In the wake of this front,
temperatures will likely cool briefly, but then a quick moderation
is possible by early next week as ridging builds in. How mild it
gets early next week will likely be dependent on how
strong/amplified the ridge is and whether it is located over the
western US or further east. For now, I have temps early next week on
the milder side, but given the model differences next week,
confidence in the temp forecast is somewhat lower. Martin



A broad area of low pressure over the area will continue at least
mountain top-obscuring snow in the mountains. Otherwise, VFR
conditions with mainly mid level cloudiness and a few snow showers
are expected to continue at the terminals through the next 24 hours.
Some heavier showers could cause brief periods of MVFR conditions,
especially with the passage of a disturbance after 12Z. A frontal
boundary will also move east of the area with the disturbance after
18Z, shifting winds more westerly across the area.


GTF  28  15  37  23 /  30  20  20  40
CTB  20  11  34  17 /  70  30  30  30
HLN  32  17  36  23 /  30  20  20  30
BZN  31  13  34  21 /  30  20  30  30
WEY  21   1  22  13 /  60  40  40  50
DLN  35  13  34  18 /  20  20  10  20
HVR  25  12  35  15 /  40  20  30  20
LWT  29  13  34  17 /  30  30  40  70


High Wind Watch from late tonight through Wednesday afternoon
Cascade...Central and Southern Lewis and Clark...Eastern
Pondera...Eastern Teton...Judith Basin...Toole.

High Wind Watch from this evening through Wednesday afternoon
Eastern Glacier...Northern Rocky Mountain Front...Southern Rocky
Mountain Front.


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