711
FXUS65 KMSO 020855
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
255 AM MDT Sun Jun 2 2024

...HEAVY RAIN TONIGHT INTO MONDAY LEADING TO INCREASED RISK OF
SMALL STREAM FLOODING, ROCK AND DEBRIS FLOWS, ACROSS FLOOD PRONE
AREAS...

.DISCUSSION...An atmospheric river remains on track to impact the
Northern Rockies later this afternoon and evening through Monday
night. Pacific moisture and precipitation will begin moving into
the region this afternoon as a trough approaches the WA/OR coast.
Widespread moderate to locally heavy rain will develop across
central Idaho and along the ID/MT border this evening into Monday
morning as the core of the atmospheric river moves inland.
Ensemble model systems continue to indicate deep Pacific moisture,
with integrated water vapor values reaching 400-500 kg/ms, which
typically occurs once every 10 years during early June. Mountain
areas will see impressive precipitation totals given strong
orographic forcing associated with a strengthening pressure
gradient and upper level jet aloft. The latest precipitation
forecast generally has 1.00-2.50 inches across mountain ranges in
central Idaho and northwest Montana, with localized amounts of
3.00+ inches. The higher end totals continue to focus across the
Clearwater Mountains, where the HREF has >70% probability for 2.00
inches or more. Primary impacts include an increased risk of
small stream flooding and rock and/or debris flows in steep
terrain and flood prone areas. Flood Watches remain in effect for
the Clearwater Mountains and Orofino/Grangeville region to address
this concern. The risk for main stem river flooding is reduced
given antecedent river flows running below normal. The Clearwater
River at Stites continues to be an exception, with probabilities
for flows to reach minor flood stage at 20%.

Forecast winds have increased across central Idaho and northwest
Montana Monday afternoon and evening. Pressure rises associated
with Pacific cold front passage Monday afternoon/evening have
trended higher, with the risk for gusts of 40-50 mph increasing.
Furthermore, high resolution models suggest convective showers and
thunderstorms along and behind the front, which will produce
localized gusts of 40 mph or greater. The EPS EFI suggests
climatologically extreme winds for early June as Camas Prairie of
Idaho, the Flathead and Mission Valleys, the mountain ranges in
northwest Montana. NBM wind gust probabilities for gusts over 40
mph reach 60-80% across aforementioned valleys, with higher
probabilities in the terrain. Wind advisories may be needed if
this trend continues.

Beyond the atmospheric river, a ridge builds over the region by
mid-week through next weekend. Temperatures will warm to above
average by Wednesday and into the upper 80s and low 90s by
Thursday and Friday. /Lukinbeal

&&

.AVIATION...Widespread rain, associated with an atmospheric river
will reach the ID/MT border between 02/2200-2400Z, before pushing
eastward across the divide by 03/0300Z. Widespread terrain
obscurations are expected as ceilings lower below 5,000 ft AGL.
Turbulence, associated with mountain wave activity, will increase
after 03/300Z. /Lukinbeal


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...ID...Flood Watch from this evening through Monday evening for
     Northern Clearwater Mountains...Orofino/Grangeville
     Region...Southern Clearwater Mountains.

&&

$$