Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 232144

Area Forecast Discussion
National Weather Service Great Falls MT
244 PM MST Mon Jan 23 2017


This afternoon through Wednesday...The upper level low that has been
sitting off the WA/OR coast over the past couple of days has begun
to move south and will eventually open up/weaken, leaving a broad
trough across the west. The trough axis will begin to progress east
over the next couple of days as well which will push the 160kt upper
level jet over the central Rockies S/E as well.

As the upper level low moves ashore, a new mid/upper level low will
begin to develop over southern ID and progress east into the central
Plains by Tuesday. The models have really had a harder time handling
this low over the past few days. That said, it still appears that as
the low deepens through this evening, there will be a window for
steadier snow to develop across far SW MT along the MT/ID border,
possibly aided by some mesoscale banding north of the low track.
However, the quick movement of the low should limit snowfall amounts
in this area. Thinking a general 2-4" can be expected here, mainly
impacting Monida, Raynolds, and Targhee Passes as well as the West
Yellowstone area. If any mesoscale banding can develop, then there
may be a brief window of heavier snowfall rates. However, confidence
remains somewhat lower regarding this potential. In addition,
several inches of snow shouldn`t cause significant impacts over the
passes and for these reasons, we`ll continue to hold off on any
winter weather products for those areas.

Further north, the warm front that moved through last night/this
morning has pretty much lost its upper level support which is
allowing a moist, northerly low/mid level flow to take over. Short
term guidance and latest radar imagery suggests there will be one
more push of decent moisture across southern sections of central
Montana (ie. from Great Falls to Lewistown) through tonight. Weak
upslope flow should allow light snow to continue in this area
through tonight before drier air begins to work in from the north on
Tuesday. Snowfall rates/amounts should remain light through tonight
due to a lack of better forcing, but given the somewhat persistent
light snow through tonight and in an attempt to avoid confusion,
we`ll keep the Winter Weather Advisory for going for our central
counties through 6am Tuesday. However, later shifts may need to
cancel the Advisory early if the snow ends sooner than forecast.

For Tuesday/Wednesday, northerly flow will persist across the
region. With some lingering low/mid level moisture, north-facing
slopes may continue to see some light snow during this time, with
generally drier conditions for the valleys/plains. Temperatures will
remain a bit below normal through Wednesday. Martin

Wednesday night through Monday...Overall a generally quiet weather
pattern...with a slight warming expected for the extended
forecast. Northwest flow and a broad scale trough will be found
Wednesday night into Thursday across the region. This combined with
lingering moisture could bring a few flurries to light snow showers
across the region. Best chances look to be across the
southwest...however...anywhere that receives snow will see light
accumulations at best. A large ridge then builds into the region
Thursday night...diminishing chances for any snow. This mentioned
ridge then takes over for Friday through Saturday. Dry air under
this ridge will provide for mostly sunny skies and dry conditions.
Ridge starts to flatten on Sunday...possibly bringing some breezy
winds at times to the region...but continued dry conditons. A weak
disturbance and cold front look to push through on
Monday...however...moisture appears limited with this feature...and
mainly dry conditions...with some gusty winds are expected at this
time. And even though an isolated snow shower could be
possible...left out mention for now. Temperatures are expected to
start off below normal on Thursday into Friday. The mentioned ridge
then begins a warming trend on Saturday...and continues through
Monday...with highs in the 30s to low 40s possible for the lower
elevations. The one hindering factor for the warming trend will be
found across the valleys as light flow under the ridge could bring
back an inversion set up. This would mainly affect the southwest and
the Helena Valley for Friday and Saturday. Anglin


Updated 1752Z.

With low-level upslope flow increasing as indicated by increased
radar returns, opted to place KGTF in IFR cigs with -SN through the
entire TAF period, though with less confidence toward the end of the
period. KCTB and KHVR should see gradually improving cigs through
the afternoon into the overnight, though fog could redevelop again
later tonight at both locations. Continued dense FZFG for KBZN until
20z with gradual vis and cig improvement thereafter until 00z, when
-SN is expected to develop. With surface high pressure to the north
and low pressure to the south, used the HRRR to predict development
of northerly channeled winds at KEKS by mid-afternoon and continuing
through the remainder of the period, with the potential for -SN
developing toward mid-evening.   Cassell


GTF  13  23  12  28 /  60  40  10  10
CTB  10  23  11  29 /  30  20  10   0
HLN   6  19   6  22 /  40  30  20  10
BZN   5  19   2  20 /  60  40  20  20
WEY   1  18  -6  15 /  70  60  20  30
DLN   9  22   6  22 /  50  40  20  10
HVR  12  22   8  24 /  30  20  20  10
LWT  11  21   9  26 /  60  40  20  10


Winter Weather Advisory until 6 AM MST Tuesday Cascade...
Chouteau...Eastern Teton...Fergus...Judith Basin...Southern
Rocky Mountain Front.


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