403
FXUS65 KMSO 010816
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
216 AM MDT Sat Jun 1 2024

...Widespread moderate to heavy rain, with rises to small streams
and increased risk for rock and mudslides, Sunday night into
Monday...

.DISCUSSION...GOES Satellite and regional radar imagery early this
morning shows a trough of low pressure moving onshore in
Washington. This trough will move across the Northern Rockies by
mid-afternoon, with scattered showers developing through the
evening hours. The best axis of mid-level moisture and instability
will set up across Idaho and Lemhi Counties and in southwest
Montana, where isolated thunderstorms will be capable of
producing outflow winds of 30-40 mph.

An atmospheric river remains on track to bring widespread moderate
to locally heavy rain Sunday night into Monday night. Ensemble
forecast systems suggest Pacific moisture will reach into the
99-99.5 percentile for this time of year, with integrated water
vapor transport values seeing return intervals of a once in 10
year event for early June! As with previous forecasts, the
heaviest rainfall is expected across Idaho and Clearwater
Counties, with probabilities of 1.00" or more of rainfall
reaching 80-90 percent along the Highway 12 corridor, Clearwater
Mountains, and portions of the Camas Prairie near Grangeville.
Probabilities for 2.00" of more reach 20 percent south and east
Grangeville and US Highway 12. Notable rises on small streams and
rivers are anticipated. There will also be an increased threat for
rock and debris onto roadways and in flood prone areas. Embedded
isolated thunderstorms within the atmospheric river precipitation
shield Sunday afternoon may lead to locally heavy rain and
enhanced precipitation totals. If high end precipitation totals
(2.00-3.00"+) are achieved, the risk for minor flooding will
increase, particularly for small streams.

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 eastern Lemhi County where rainfall
amounts will be lighter. Heavier amounts (0.50-1.00") are forecast
across far northwest Montana near the Idaho panhandle, where
ponding of water may occur in poor drainage areas.

A strong ridge will build over the western U.S. Wednesday into
next weekend, leading to an extended period of warmer, summer-
like temperatures. Highs are expected to reach into the 80s to
lower 90s for valleys. Ensembles continue to show uncertainty with
the orientation of the ridge, thus a day or two of thunderstorms
may be mixed in. /Lukinbeal


&&

.AVIATION...GOES Satellite and regional radar imagery as of
01/0830Z shows a Pacific trough moving onshore in Washington.
This trough will move across the Northern Rockies by 01/2000-2200Z,
with scattered showers developing thereafter. Activity will push
east of the divide by 02/0600Z. The best axis of mid-level
moisture and instability will set up across Idaho and Lemhi
Counties and in southwest Montana, where isolated thunderstorms
will be capable of producing outflow winds of 25-35kt. /Lukinbeal

&&

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

$$