Freezing Rain Advisory 2, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
429 AM MST Thu Feb 27 2014

...Powerful Winter Storm with Near Blizzard Conditions Tonight
Through Friday Night in Western Montana...

.DISCUSSION...A powerful late winter storm will impact the region
later today through Saturday. This storm will bring about a
variety of major winter impacts to Western Montana. They are:

1. FLASH FREEZE - Impacting Travel this evening & Friday morning
                  commutes.

2. HEAVY SNOW - 4 to 12 inches Tonight and Friday.

   AREAS IMPACTED - West Central Montana 50 miles south and north
   I-90 corridor (Alberton to Deerlodge).

   SPECIFICALLY - Missoula Valley, Southern Mission Valley,
   Seeley/Swan Valley, Highway 93 Missoula to St. Ignatius.

3. Near BLIZZARD CONDITIONS - Tonight through Saturday with SUSTAINED
   EAST TO NORTHEAST WINDS 20 to 30 mph for 12-24 hours.

   MAIN AREAS IMPACTED - Flathead and Missoula Valleys, especially
   Badrock and Hellgate Canyons. US 12 Clearwater Junction to
   Macdonald Pass, Marias Pass to Essex.

4. DANGEROUS WIND CHILLS - FRIDAY AND SATURDAY

   Northwest Montana & Continental Divide 15-30 degrees below zero
   West Central Montana - 0 to 15 degrees below Zero

5. FRIGID COLD TEMPERATURES BELOW ZERO SATURDAY AND SUNDAY NIGHT

The following describes in more detail the upcoming storm:

Today through Sunday Morning...

The increasing southwest flow and associated moisture will bring a
potential for mixed precip and freezing rain this afternoon and
evening which could be both a positive and a negative.
The good: Freezing rain and ice have a harder time accumulating
on surfaces during the day, even when the sky is expected to be
cloudy. The BAD: This will shift a slight chance for freezing
rain into the evening hours where surfaces will have a chance to
quickly lose their heat and potentially become icy, and icy really
quick. Snow overnight on top of this ice will severely impact the
Friday morning commute. The area with a slight chance for freezing
rain will be narrow, but possibly quite significant. The greatest
areas of concern for freezing rain are along I-90 from St. Regis
to Butte, US-93 from Missoula to Hamilton, MT-200 from Ravalli to
Trout Creek and in NW Montana MT-56 (Bull Lake Road) and US-2 from
Libby to Troy.

An arctic air mass poised over central Canada will be invading
western Montana tonight into Friday, eventually moving across
north-central Idaho Friday night. This is an exceptionally cold
air mass which will be ushered into western Montana by very strong
northeast winds. The greatest wind and cold impacts will be
targeting western Montana all the way through the weekend, while
north-central Idaho impacts will mostly be centered around steady
light to occasionally moderate snow.

WIND AND SNOW: Northeast winds will be developing through the gaps of
the Continental Divide late this afternoon, eventually surging
into the northern Flathead valley this evening. Cold easterly
winds will punch through the Blackfoot region tonight, spilling
into the Missoula valley by early Friday morning. Wind speeds
associated with this influx of arctic air will be 25 to 35 mph
sustained with potential gusts in excess of 50 mph locally. The
combination of these winds, lots of light fluffy snow already on
the ground and several inches of new snow will create considerable
blowing and drifting. In fact, local blizzard conditions
(visibilities less an 1/4 mile and roads blocked by snow drifts)
are quite possible through the gaps of the Divide, the northern
Flathead and Missoula valleys.

Speaking of new snow: Models are generating a focused area with
the potential for heavy snow early Friday morning generally 50
miles north and south of the I-90 corridor where an additional 4 to
8, locally up to 12 inches plus are expected. As was the case last
weekend, the heaviest amounts will occur in a narrower region
which we hope to further define over the next 24 hours.

WIND AND COLD: The combination of strong winds and a flood of colder
and colder air into the western Montana will create dangerously low
wind chill values. Initially, nasty wind chills will be confined
along the Continental Divide Thursday night and Friday. However, as
arctic air advances through the rest of western Montana, wind chills
will become a real hazard Friday night through Saturday night. Wind
chill values of 20 below to 40 below zero are possible. Newborn
livestock would be highly vulnerable under these conditions.

NORTH-CENTRAL IDAHO IMPACT: Mountain snow and valley rain can be
expected today through Friday with mountain passes becoming snow-
covered and slick. However, as colder air moves into Idaho, snow
levels will lower to the valley floor Friday night/Saturday. A
westerly flow at the mid/high levels of the atmosphere will
maintain a steady influx of Pacific moisture falling mostly as
light to moderate snow. Accumulating snow is expected at all
elevations making travel difficult. There will also be a threat
for freezing rain during this same time in some of the narrow
valleys of north central Idaho, including portions of US-12
generally from Powell to Lolo Pass, ID-11 from Weippe to Pierce,
Grangemont and along ID-14 to Elk City. The chance for freezing
rain will gradually end Thursday night into Friday morning as
colder air begins its surge across the region.

Next week..

A more westerly flow is expected to develop early next week. This
should direct surges of moisture across north central Idaho and
western Montana. This milder westerly flow should cause
temperatures to gradually moderate. However, there is a lot of
uncertainty in timing of systems/surges of moisture and how quick the
valleys should moderate. Typically (and after every arctic air mass
we have seen this past winter) the forecast model guidance warms
up the valleys much too fast and see no reason it will not happen
again.

&&

.AVIATION...Patchy low stratus in the Bitterroot Valley, Mission
Valley and Camas Prairie will erode early this morning.
Precipitation will be a mix of rain/freezing rain after 27/2000z
and impact all terminals with lowering ceilings and mountain
obscurations. AVIATION OPERATIONS HAVE THE POTENTIAL TO BE
SEVERELY IMPACTED at KGPI, KMSO and KBTM this evening through all
day Friday with near Blizzard conditions at times. Wind gusts of
35-50 mph are expected at KGPI and KMSO. Visibilities less than
1/4 are likely from blowing snow at KGPI, KMSO and KBTM. Heavy
snow, 4-10 inches, is possible at KMSO from 28/0600z through
29/0000z.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER STORM WATCH from this evening through Friday morning
     Butte/Blackfoot Region.

     WINTER STORM WATCH from this evening through Friday morning
     Bitterroot/Sapphire Mountains...Lower Clark Fork Region.

     BLIZZARD WATCH from late tonight through Friday afternoon
     Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region.

     BLIZZARD WATCH from this evening through Friday morning
     Flathead/Mission Valleys...West Glacier Region.

ID...WINTER STORM WATCH from this evening through Friday morning
     Southern Clearwater Mountains.

&&

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