498
FXUS65 KMSO 211836
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
1236 PM MDT Tue May 21 2024

.DISCUSSION...A broad trough across the western U.S. will support
a couple of embedded cutoff low pressure systems over the next
week. So overall, the weather will remain cooler than average with
clouds and showers, but with a couple stronger systems embedded in
the overall pattern. A change to warmer, drier weather is in the
works for next week.

The first cutoff low pressure system arrives tomorrow. During the
day, this will just bring widespread rain across western Montana
and central Idaho. But tomorrow night some cooler air from the
north arrives and causes snow levels to drop. Currently, we have
winter weather watches out for areas south of I-90 in western
Montana for Wednesday night as a result. Forecasts have a wide
range from the low end (10th percentile) to the high end (90th
percentile). Lost Trail Pass ranges from around 3 inches to 1
foot, with Georgetown Lake and Phillipsburg ranging from 0.75 to 8
inches of snow by Thursday morning. In Lemhi County, Idaho
Gilmore Summit ranges from about 0.2 to 9 inches. And some of the
most extreme impacts will be in the back country of the Anaconda-
Pintlar Mountains where forecasts range from 5 to 25 inches of new
snow.

There are two main reasons for the high ranges of snow amounts in
the forecast. The first is that this is a very wet system, so IF
it turns to snow at any point, it will produce a lot of snow. But
the crux of the uncertainty lies in how low and when the snow
levels will drop. The bottom line is that the risk is high enough
for heavy snow that you need to be prepared for it in the back
country and over the passes of southwestern Montana and Lemhi
county Idaho.

The second cutoff low looks to arrive just in time for Memorial
Day weekend. It looks very similar to the system forecast for
tomorrow, though the uncertainty remains higher due to the longer
lead time. The bottom line there is if you are planning on heading
to the outdoors for Memorial Day weekend, be prepared for cool,
wet conditions and snow if you`re headed to higher elevations.

Ensembles and long range models agree well on a change to warmer
weather and above average temperatures around next Monday. So
while Memorial Day weekend starts out cold and wet, it looks to
end with some very pleasant weather.

&&

.AVIATION...Expect VFR conditions through this evening with clouds
increasing tonight. A cutoff low is moving into the region tonight
and tomorrow morning. Showers will start in western Montana
Wednesday morning with snow levels above 6,000 feet, so area
terminals won`t be affected by the snow. Snow levels start to fall
Wednesday night, so by Thursday morning snow will very likely be
impacting terminal KBTM.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Watch from Wednesday evening through Thursday
     morning for Bitterroot/Sapphire Mountains...Butte/Blackfoot
     Region.

ID...Winter Storm Watch from Wednesday evening through Thursday
     morning for Eastern Lemhi County...Western Lemhi County.

&&

$$