953
FXUS65 KMSO 011954
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
154 PM MDT Sat Jun 1 2024

...HEAVY RAIN STARTS SUNDAY NIGHT WITH SMALL STREAM FLOOD RISK IN
CENTRAL IDAHO THROUGH MONDAY...

.DISCUSSION...A tropical cyclone in the west Pacific last week has
transitioned into the mid-latitudes and injected quite a lot of
moisture into the jet stream, and an elongated jet stream across
the Pacific Ocean is bringing it to our region. It will begin
impacting our region tomorrow (Sunday) night. It will bring
widespread rain across the whole region, but ensembles have been
consistent in targeting central Idaho with the heaviest rain. The
terrain in that region will combine with the wind direction,
extremely moist air, and an approaching trough to produce heavy
rain. There is about a 20 percent chance for some mountain
locations to pick up 2" or more of rain in about 30 hours, and the
chances of picking up over an inch in that time is over 80 percent
for most of the higher elevations of Clearwater and Idaho counties
in central Idaho.

With such heavy rain, flooding is always a concern. Currently, the
mainstem rivers have a lot of capacity to hold more water, so
mainstem river flooding isn`t a concern for most areas. The main
exception is the Clearwater River at Stites, which has about a 20
percent chance of reaching flood stage. Smaller streams and creeks
however are much more vulnerable to the heavy rain, and we are
expecting to see a lot of problems with ponding of water, high
flows in culverts, and small stream flooding. We`ve issued a flood
advisory for these impacts.

Additionally, short-term high resolution models are showing the
potential for embedded thunderstorms with higher precipitation
rates. These are a concern because the ground will already be
saturated and localized, persistent heavy rain from these storms
could cause flash floods and debris flows. These events tend to be
very localized.

Beyond the atmospheric river, a ridge builds over the region by
mid-week with nearly 100 percent agreement by the ensembles.
Temperatures will warm to above average by Wednesday and into the
upper 80s and low 90s by Thursday and Friday.

&&

.AVIATION...A shortwave trough moving through the region today and
this evening is causing increased cloud coverage and some showers
and thunderstorms. Thunderstorms this evening will be typical for
this time of year with the main impact being lightning and wind
gusts 30-40 mph. Tomorrow night an atmospheric river will arrive
in the region and cause low ceilings and reduced visibility
through Tuesday morning.

&&

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

&&

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