Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 161137

Area Forecast Discussion
National Weather Service Missoula MT
437 AM MST Sat Feb 16 2019

.DISCUSSION...The convergence boundary responsible for yesterday`s
impressive morning snowfall has shifted east and is presently
gifting Great Falls with a round of heavy snow this morning. And
while clearing developed overnight across much of Central Idaho,
stubborn wrap around moisture from has kept clouds and steady
light to occasionally moderate snow falling across the
Flathead/Mission Valley, Seeley/Swan Region and Butte/Blackfoot
Region eastward to the Continental Divide. This is where Winter
Weather Advisories remain in place through this morning.

Today will be the proverbial `calm before the storm` with the area
positioned between the slowly exiting system to our east and broad
troughing deepening to the west. Today`s atmosphere remains
generally unstable resulting in scattered snow showers that will
overspread much of Central Idaho and Western Montana by this
afternoon. However this evening an arctic boundary, which is
already in place along the Highline of North Central Montana, will
spill over the Continental Divide. Its affects will be first felt
throughout Glacier National Park and along US-2 with the gusty
east winds ushering bitterly cold air into the Flathead Valley
via Bad Rock Canyon between 5 PM and 6 PM this evening. Gusty
northeast winds when combined with recent snowfall and additional
light snow along the arctic boundary will produce periods of
blowing and drifting snow that may prompt the issuance of another
Winter Weather Advisory. In fact, these Advisories are likely to
cover similar areas as the ones in place this morning and
therefore we`ll leave the final call up to the next shift.

Bitterly cold modified arctic air will eventually work its way
over Rogers Pass late this evening (thinking 8 PM) and then into
the Missoula Valley via Hellgate Canyon between 9 PM and 11 PM.
Persistent light snowfall is likely to precede the arctic cold
front which will likely result in some blowing and drifting of
snow in downtown Missoula, along I-90, US-12 and MT-200 all
leading up to the Continental Divide. The arctic boundary will
initially become hung up on the Bitterroot Mountains along the
Idaho/Montana border with the threat in Montana quickly shifting
to extreme wind chill concerns. However the depth of the cold air
filtering into Montana will ultimately overcome the unfavorable
geography and by Monday morning the modified arctic air will be
well into North Central Idaho.

Speaking of North Central Idaho, forecast models are depicting a
pattern which local studies have shown to be quite favorable for
persistent light snow in and around Grangeville. The northerly
flow through a deeply moist atmosphere starts to look very
favorable for light snow starting Sunday afternoon and actually
becomes even better during the evening/overnight hours due to the
added convergence from the approaching arctic cold front. On top
of it all, forecast models are showing the addition of upper level
support (closed low) descending from Canada Monday morning which
could not only maintain the snowfall, but perhaps even strengthen
it for a time. All together we`re expecting 2 to 4 inches in and
around Grangeville and perhaps up to 6 inches of snow on the
hills south of town by Monday morning. Meanwhile in Montana
extremely cold temperatures along with localized dangerous wind
chill will persist into Monday morning.

Tuesday/Wednesday: The trend in the ensemble model guidance for
this period has the northeastern Pacific upper level ridge more
amplified while forcing a deeper trough over the Pacific
Northwest. This pattern would be conducive for light snowfall
across our area towards Wednesday. Also another push of cold
Canadian air will certainly be possible along with its impacts of
cold wind chill and blowing and drifting snow. Expect colder than
normal temperatures with highs only reaching into the 20s across
western Montana and 30s over north-central Idaho.

Beyond Wednesday, ensemble guidance and the Climate Prediction
Center`s six to ten day forecast would suggest a continuation of
colder than normal temperatures and above normal chance for


.AVIATION...At KGPI light snow will persist, influencing
visibility and ceiling heights through much of the day. Strong
east winds gusting to 20 knots are expected to kick in by
17/0100z with gusts to around 40 knots possible shortly
thereafter. KMSO may see a few showers this afternoon and perhaps
some light snow affecting visibility and ceiling heights after
17/0000z. But the gusty east winds to around 20 knots will likely
draw or more substantial impact to airfield conditions by 17/0600z
with peak gusts to around 30 knots kicking in overnight. Visibility
is likely to remain reduced at both of these airfields into early
Sunday morning. KBTM and KSMN may see occasional snow showers
today and this evening. However impacts to airfield operations
should be limited.


MT...Winter Weather Advisory until 9 AM MST this morning
     Butte/Blackfoot Region.

     Winter Weather Advisory until 9 AM MST this morning
     Flathead/Mission Valleys.

     Winter Weather Advisory until 9 AM MST this morning West Glacier

     Winter Weather Advisory until 9 AM MST this morning
     Potomac/Seeley Lake Region.


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