Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 042020

Area Forecast Discussion
National Weather Service Missoula MT
220 PM MDT Sun Jun 4 2023


A marginally unstable atmosphere will remain over the Northern
Rockies through this evening, with terrain enhanced showers and
thunderstorms present (most notable south of Montana Highway 200
corridor). Adding to this scenario, high-resolution models suggest
that a closed low pressure circulation over the northern Great
Basin/Snake River Plain will meander north and reach Lemhi
County/southwest Montana by this evening. This low pressure will
enhance some of the convection, however the high-resolution models
are struggling to reach consensus on storm initiation. There is a
5 percent probability of excessive rainfall (flash flood threat)
across Lemhi County/southwest Montana through this evening. Once
sunset nears this evening, the loss of daytime heating will likely
inhibit further shower and thunderstorm development for western
Montana. However, most near-term models depict showers and
isolated nocturnal thunderstorms spreading north/west over north
central Idaho throughout tonight.

By Monday afternoon, shower and isolated thunderstorm activity
associated with a weak closed low pressure circulation is expected
to spread eastward from north central Idaho to most of western
Montana. Daytime heating will intensify convection, however the lack
of strong flow aloft will prevent much storm organization.
Nevertheless, 5 percent probability of excessive rainfall remains
for Lemhi County/southwest Montana throughout Monday daytime. By
Monday evening, models suggest that the closed low pressure will
become an open wave (low pressure trough) and move northeast into

Weak high pressure is anticipated to be present over the Northern
Rockies Tuesday to Wednesday, however this will not necessarily
translate to dry conditions across the region. National Blend of
Models components point toward daytime temperatures to peak on
Wednesday, with low elevations in the 80s (western Montana) to low
90s (north central Idaho). The increase in surface temperatures will
allow rapid afternoon thunderstorm development (particularly near
the Continental Divide across Lemhi County/southwest Montana). Much
of this thunderstorm activity will be phased with daytime heating,
translating to convection dissipating shortly after sunset each

Thursday through next weekend, models suggest that another closed
low pressure circulation will develop and meander around the Great
Basin. The movement of atmospheric moisture counterclockwise around
the low pressure will provide daily showers and thunderstorms to
most of the Northern Rockies. There is generally a 60-80 percent
probability of 0.25" or greater rainfall for most locations Thursday-
Saturday. For Thursday specifically, there is 5-15 percent
probability of excessive rainfall for much of the Northern Rockies
(with emphasis for southwest Montana along the Continental Divide).
The increased moisture and cloud cover will suppress daytime
temperatures, while simultaneously maintaining relatively warm
overnight temperatures. In fact, National Blend of Models suggest
near-record overnight warmth Thursday-Saturday.



Showers and thunderstorms will continue to develop across Northern
Rockies air space through 05/0000Z, with most activity occurring
near KMSO and south. Watch for isolated occurrences of brief, heavy
rain to reduce visibility. Also, erratic wind gusts to 30 knots will
be common near any developing thunderstorm. After 05/0000Z, a weak
closed low pressure circulation will approach north central Idaho
from the south, sparking nocturnal showers and isolated thunderstorm
activity (mainly west of KMSO and south of KHRF) through 05/1500Z.
Patchy fog will develop Monday morning for valley locations that
receive rainfall late Sunday and experience clearing overnight. Past
Monday morning, showers and thunderstorms are expected to
proliferate across the remainder of northern Rockies air space, as
the closed low pressure eventually continues east of the Continental
Divide after 06/0600Z. Terrain obscurations will be closely phased
with the evolution of showers and thunderstorms, as well as Monday
morning patchy valley fog.



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