340
FXUS65 KMSO 311951
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
151 PM MDT Fri May 31 2024

...An Atmospheric River to bring moderate to heavy rainfall,
potentially leading to rises in small streams along with minor
impacts due to rock and mudslides...

.DISCUSSION...Zonal (westerly) flow is expected over western
Montana and north central Idaho through this weekend. This will
lead to near normal temperatures, afternoon breezes and a mix of
sun and clouds. A weak disturbance on Saturday will bring the
potential for a few thunderstorms, mainly along and south of the
Highway 12 corridor. The main threat with any storms that develop
will be lightning and gusty winds up to 40 mph.

Sunday night into Monday night, an atmospheric river of moisture
is expected to arrive over the region bringing widespread
rainfall. Snow levels during this event will generally be around
9000 feet and above, allowing precipitation to fall as rain at
virtually all elevations. The heaviest rainfall is still expected
across north central Idaho where the probability of 1.00" or more
of rainfall continues to increase (60-90 percent chance),
especially along the Highway 12 corridor, through the Clearwater
Mountains and into portions of the Camas Prairie. And in fact,
forecast models are now showing a roughly 20 percent chance of
precipitation amounts topping 2.00" in these same areas. Notable
rises on small streams and rivers are anticipated. It is not out
of the question to see some minor flooding on small streams if the
heavier precipitation amounts are achieved. There will also be an
increased threat for rock and debris onto roadways and in flood
prone areas.

Further east in western Montana, precipitation is largely
anticipated to be beneficial. Most locations have a 60-80 percent
chance of seeing at least 0.25" of rainfall, with the exception
across southwest Montana and Lemhi County where rainfall amounts
will likely be lighter. Heavier amounts will be possible across
far northwest Montana near the Idaho panhandle, where ponding of
water is possible in poor drainage areas.

Precipitation will largely taper off on Tuesday, however breezy
conditions will linger across the region, with gusts up to 35 mph
likely (50 to 70 percent chance) for western Montana.

The latter part of next week, most (85%) of ensemble clusters
show a strong ridge building over the western U.S. leading to an
extended period of warmer, summer-like temperatures. Highs are
expected to be the warmest we have seen so far this year with 80s
to lower 90s for valleys. There is still considerable uncertainty
with the orientation of the ridge, thus a day or two of
thunderstorms may be mixed in.


&&

.AVIATION...Other than patchy morning fog, VFR conditions will be
common into Saturday morning as a brief ridge passes through the
Northern Rockies. Winds will be light and diurnally driven with a
few gusts to 20 kts in this afternoon. By Saturday afternoon,
showers and thunderstorms are possible, mainly along and south of
Highway 12. Airfields KHRF, KBTM and KSMN have a roughly 20
percent chance of lightning in the vicinity, along with gusty
outflow winds.



&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&

$$