829
FXUS65 KMSO 030204
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
804 PM 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...

.UPDATE...Added a flood advisory for Clearwater and Idaho counties in
north central Idaho. Moderate to heavy rain is on track to arrive
over the next couple of hours, continuing through Monday. Rainfall
amounts from 0.10 to 0.20" per hour are possible. Minor flooding
to low land areas, rises on small streams and creeks, ponding of
water in urban and poor drainage areas, and rock and debris on
prone roadways is anticipated. The Flood Watch has been
cancelled.

In addition, a wind advisory has been added for the Missoula and
Bitterroot valleys for tomorrow afternoon and evening. Gusts 40 to
45 mph are expected, and could even exceed 45 mph with
shower/thunderstorm activity in the afternoon.

&&

.AVIATION...An atmospheric river is poised to move through the
region later this evening through Monday. General showers are
possible this afternoon before steady rain arrives into north-
central Idaho by 1400 PDT and into northwest Montana by 1700 MDT.
The highest probability for periods of heavy rain, and lower
ceilings/visibility will be over north-central Idaho between 0100
and 1100 PDT Monday, including Grangeville. There is a 30 percent
chance for periods of moderate rainfall rates at GPI, MSO, and HRF
around daybreak Monday through early afternoon. There is a
moderate icing potential above 9,000 feet MSL tonight. Mountain
wave activity will ramp up over the Divide by Glacier National
Park tonight. A cold front is expected to arrive in north-central
Idaho by 11 am PDT Monday, and in western Montana by 2 pm MDT.
Gusty winds to 35 knots are possible with the frontal passage, and
gusts to 45 knots are possible with intense showers along and
behind the front. Showers and winds decrease by Monday night.
Mountain wave activity with mid-level westerly winds around 50 kts
are expected over the Cabinets, Bitterroot, Mission, Rattlesnake
mountains, and over the Divide Monday afternoon after the cold
front moves through.

&&

.PREV DISCUSSION... /ISSUED 147 PM 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...The main change from previous forecasts has been to
issue wind advisories for much of north-central Idaho and western
Montana for Monday afternoon. As can be typical with atmospheric
river events, gusty winds generally to 45 mph are possible in
these areas. The recent HREF ensemble depicted multiple solutions
of wind gusts greater than 50 mph in the Camas Prairie and Teakean
areas in Idaho, and also in the vicinity of Ashley Lake, west of
Kalispell. It will be hazardous for small-craft on Flathead Lake
as waves to 2 feet are expected. Opted to not issue a wind
advisory for the Missoula/Bitterroot valleys as probability
thresholds for gusts greater than 40 mph were not very high. A few
high resolution models depict a small section of the western
Bitterroot Valley experiencing mix-down winds over 40 mph late
Monday afternoon. Successive shifts will continue to monitor if
further advisories may be needed.

The rain part of this system continues to be similar as previous
updates. There remains a flood watch out for Clearwater and Idaho
counties as there continues to be an elevated risk for small
stream flooding and water issues with clogged drains and ditches.
The Clearwater Mountains of Idaho County continue to have up to 30
percent probability for greater than 2 inches of total rainfall
through Monday night. The Cabinet Mountains in northwest Montana
have around a 85 percent chance for greater than 1 inch of
rainfall.

Mountain showers continue with breezy winds on Tuesday. The higher
probability for gusts greater than 30 mph includes Flathead Lake,
along the Divide and portions of southwest Montana including Deer
Lodge, Anaconda and the Butte area.

A warm-up begins Wednesday as a broad upper level ridge builds
over the western U.S. in response to a digging upper level low
over the northeastern Pacific Ocean. There is a 90% probability
for the lower elevations of north-central Idaho and Lemhi County
to exceed 80 degrees on Wednesday. The remainder of the region has
a 75 to 90 percent chance of getting into the 80s by Thursday,
except around Flathead Lake where water temperatures are around 50
degrees. While the lower elevations of Idaho have a high chance
for getting into the 90s by Friday and Saturday, probabilities in
western Montana are lower, ranging from 28 percent in Kalispell
and 50 percent in Missoula. The reason-being, there have been
discrepancies in how the upper ridge evolves next weekend. ECMWF
ensembles have depicted the ridge axis further west...while it has
less amount of thunderstorm activity, this could open the door
for a somewhat cooler airmass from Canada to keep Montana from
getting to the 90s. Both the Canadian and GFS ensembles have the
ridge centered more over us with an increased chance for
thunderstorms.


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Wind Advisory from 1 PM to 8 PM MDT Monday for Kootenai/Cabinet
     Region.

     Wind Advisory from 1 PM to 9 PM MDT Monday for Potomac/Seeley
     Lake Region...West Glacier Region.

     Wind Advisory from noon to 9 PM MDT Monday for Flathead/Mission
     Valleys...Lower Clark Fork Region...Missoula/Bitterroot
     Valleys.

     Wind Advisory from 2 PM to 9 PM MDT Monday for
     Bitterroot/Sapphire Mountains.

     Wind Advisory from 3 PM to 9 PM MDT Monday for Butte/Blackfoot
     Region.

ID...Wind Advisory from 11 AM to 4 PM PDT Monday for Northern
     Clearwater Mountains...Orofino/Grangeville Region.

&&

$$