Winter Weather Advisory 41, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
236 PM MDT Sat May 22 2021

.DISCUSSION...Moderate strong easterly winds have persisted today
thanks to a deep area of low pressure over eastern Oregon and a
ridge of high pressure situated over north-central Montana.
Water vapor satellite imagery also depicts an upper level
shortwave that is located south of Elk City, Idaho. This wave will
track northwestward this afternoon and will initiate showers
underneath for portions of north-central Idaho, especially from
Riggins to Orofino. There is even a slight chance for a weak
thunderstorm this afternoon.

The next weather maker will arrive later tonight over the region.
A mid-level circulation will form under the large-scale upper
trough over the western US, somewhere between West Yellowstone and
Lemhi County by daybreak Sunday. Even within 24 hours from this
event, models still disagree on exactly where this feature will
materialize. The location of this feature is important as areas of
heavier precipitation will form at points north and west. And in
the heaviest precipitation, the snow levels can be driven ground-
ward significantly. Confidence has increased that snow levels will
be as low as 5,000 feet from Lemhi County to southwest Montana
and along the Divide Sunday morning. These types of systems are
difficult to exact pinpoint snow levels as it is very dependent on
the precipitation intensity. So don't be surprised if snow
levels are higher or lower than forecast at times on Sunday. Our
confidence is pretty good for elevations above 6,000 feet like
Lost Trail Pass, Leadore, and William's Creek Summit to pick up
measurable snowfall, enough to cause slushy conditions. This
similar thinking is expected for southwest Montana north to
Marias Pass on US-2. Butte could start out as snow, but mix with
rain at times during the morning. Homestake and MacDonald Passes
should be slushy for a period as well.

As the mid-level circulation tracks northward on Sunday, the
area of steady precipitation enlarges to include the remainder of
western Montana and north-central Idaho. Besides a good soaking
rain, a rock slide potential is there in the prone regions. High
temperatures will struggle to get out of the 40s in places that
experience persistent rain. Marias Pass may mix with rain or stay
all wet snow into the afternoon, then change to mainly snow Sunday
night. The mid-level low will track into the Saskatchewan
Province by Monday morning and by then the winds will have shifted
to more of a westerly direction. This should allow the
precipitation to linger across northwest Montana. The higher
elevation mountain locations should do fairly well and pick up a
decent amount of wet snow...totaling up to 2 feet total by the
time its all said and done Monday.

On Monday, expect scattered showers across the region with much
cloudiness and cooler than normal temperatures. Over the Glacier
Park region steady rain will last through the day and
temperatures may really struggle to warm above 50 degrees!

Yet another cut-off low is expected to move into the region later
Tuesday into Wednesday. At this point, it appears ensemble data
would suggest a Oregon to the southwest Montana track of the
trough during this timeframe. This would bring another dose of
precipitation to the area with the higher amounts concentrated
from north- central Idaho, west-central to southwest Montana. Snow
levels range between 6,000 and 7,000 feet at this time.

Phew...after experiencing such a cool and wet period, there are
indications that next Thursday could end up being a nice day with
temperatures rebounding well above normal..60s and 70s for many
locations. There is a trough that will be nearing the Pacific
Northwest, so there could be another front that brings showers
and breezy conditions by the end of the week.

&&

.AVIATION...An easterly pressure gradient is causing increased
winds in western Montana, and model plan views show the pressure
gradient remaining through this evening. Model guidance agree
that a low will organize and approach western Montana tonight.
Rain and snow will be common with snow levels around 5000 feet.
Widespread low ceilings and lowered visibility are expected for
much of Sunday as the low slowly passes northeast-ward. There are
still subtle differences in the track of the low, which will
impact the areas with heaviest precipitation and lowest snow
levels.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 4 AM to noon MDT Sunday ABOVE 5000
     FEET for the Butte/Blackfoot Region.

     Winter Weather Advisory from 4 AM to noon MDT Sunday
     Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 4 AM to noon MDT Sunday ABOVE 5000
     FEET for the West Glacier Region.

ID...Winter Weather Advisory from 4 AM to noon MDT Sunday ABOVE 5000
     FEET for the Western Lemhi County.

     Winter Weather Advisory from 4 AM to noon MDT Sunday ABOVE 5000
     FEET for the Eastern Lemhi County.

&&