Flash Flood in Idaho, September 16, 2024

Area Forecast Discussion
National Weather Service Missoula MT
106 PM MDT Mon Sep 16 2024

.DISCUSSION...GOES Satellite imagery this afternoon shows a closed
low spinning over the Great Basin, with southerly flow sending mid
and upper level moisture northward, as evident by widespread cloud
cover. Moisture and instability will increase across north-central
Idaho and southwest Montana this afternoon, with high resolution
models suggesting convective initiation ramping up between
200-500pm MDT across area mountain ranges. As mentioned in
previous discussions, locally heavy rain is a concern this
afternoon given atmospheric moisture values of 150-200% of
normal and weak storm motions of 10-20 mph. Idaho County in
particular will be monitored closely as the latest run of the
HREF has a 20-40 percent chance of 1-hour rainfall rates exceeding
0.50".

A closed low remains on track to bring widespread precipitation
across the Northern Rockies Tuesday into Wednesday. The general
trend over the last few days has been for the low to track moreso
from southeast Idaho through Yellowstone National Park into
north-central Montana. Shower activity, with embedded
thunderstorms, will become widespread Tuesday afternoon and
evening as the low tracks over Yellowstone. As the low moves into
north-central Montana late Tuesday night into Wednesday morning,
modeling systems are in strong agreement for a band of moderate-
heavy rainfall to set up along the north-northwest side of the
low. Forecast uncertainty on where this band sets up is higher
than normal, with models showing the band anywhere from the US-93
corridor eastward into central portions of the state. This
uncertainty can be visualized by comparing the low end (10th
percentile), most likely, and high end (90th percentile) rain
forecast amounts projected by the NBM during a 24-hr period ending
6pm Wednesday. On the low end, the NBM has 0.05-0.10" across
western MT, with mostly like rain amounts of 0.25-0.50" and 1.00"
along the divide. High end rain totals are in the neighborhood of
1.00-2.00 inches, with 2.5 inches along the divide in Glacier
Park. These closed lows are notoriously difficult to predict. The
higher end rain solution would increase the risk for runoff in
flood prone areas across Glacier National Park, especially on
Going-to-the-Sun Road.

Precipitation will largely come to an end for Thursday into the
early part of the weekend, with temperatures largely rebounding to
seasonal normals in the upper 60s to mid 70s.

Looking ahead to next weekend, ensembles previously showed a
10-15 percent chance of a high impact, high elevation snow event
across northwest Montana. Recent model runs have backed off on
this, with probabilities for a foot of snow above 7,000 ft in
Glacier National Park dropping from 10-15% to 5% or less.
/Lukinbeal

&&

.AVIATION...Shower and thunderstorms will increase in coverage
through 17/0000Z, with storms developing across central Idaho and
slowly drifting northwestward into west-central Montana through
17/0600Z. With atmospheric moisture running well above normal,
storms will be capable of producing locally heavy rain, with
visibility reductions under 5SM under cores. A closed low will
bring widespread showers Tuesday into Wednesday. /Lukinbeal

&&

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