Flood in Montana, May 3 to 11, 2023

Area Forecast Discussion
National Weather Service Missoula MT
355 AM MDT Wed May 3 2023

.DISCUSSION... Showers and thunderstorms were more vigorous last
evening across the region. MRMS radar rainfall totals revealed a
few places in the northern Sapphire Mountains and Clearwater
Mountains received between 0.50 to 1.00 inches. If you want to see
the MRMS image, go to our Twitter feed as we tweeted it out
earlier on. Glancing at the hydrographs, it doesn't appear that
the rainfall last evening has translated into any substantial
river rises. Also the storms produced wind gusts between 35 and
45 mph, at least in the Bitterroot and Missoula Valleys. Around 5
pm last evening, a very large anvil cloud with embedded mammatus
moved over the Missoula Valley, blocked the sun and a noticeable
temperature drop at the airport from 88 to 84 was felt.
Incidentally, the high of 88 in Missoula broke the previous daily
record of 84 set in 1985. The high of 85 in Kalispell broke the
previous record of 80 set in 1985.

Satellite-derived atmospheric moisture content early this morning shows
around 200 percent of normal across northwest Montana. Combine
this moisture with the lifting mechanism of a shortwave will
trigger afternoon showers and thunderstorms. High resolution
ensemble guidance depict that activity could develop as early as 2
pm over the mountains northwest and north of Kalispell, then
drift over into the valleys thereafter. If you are planning to be
outside, especially in these areas, remember if you "Hear Thunder
roar, go indoors"...lightning can strike up to 10 miles away from
a thunderstorm. The peak wind gusts from the models show the
potential for up to 45 mph. The probability for the heavier
showers is the highest, greater than 60 percent, over the Salish
and Whitefish mountains.

Thursday is going to be an interesting weather day...
1) It will be the 6th and last day with record- breaking
heat across the region. Highs will rise into the 80s again.
2) East winds: Strong pressure falls are
expected to develop in response to both an incoming low pressure
system out of southern Idaho and the record heating. This will
create a strong easterly wind gradient across western Montana.
Initially the wind will increase in the higher terrain and on the
ridgetops, but as the day progresses, these winds will be able to
mix down to the surface. This is kind of similar to what occurred
on Monday afternoon and evening. High resolution models depict
east winds of 25 to 40 mph by Thursday evening. Areas that saw the
stronger winds on Monday evening included the foothills of the
Swan Range, and the Flathead Valley, the Missoula Valley and along
the Divide from Helmville to Interstate 90. Flathead Lake could
also be impacted by the winds.
3) A convectively- driven cold front with showers and thunderstorms
will track south to north from Lemhi County into Idaho County and
southwest Montana Thursday. The embedded thunderstorms will have
a southeast to northwest movement. With the anomalously high
moisture content, they will be able to produce heavy rainfall under
their cores. Gusty winds over 40 mph, small hail and lightning
will be other possible impacts with the storms.

On Thursday night, this slow moving cold front will be the focus
for moderate showers and thunderstorms moving from east to west
from west-central and southwest Montana towards north-central
Idaho. There is up to 55 percent probability for greater than
0.10 of an inch of rain to fall across Idaho, and also up to 45
percent from Butte to Philipsburg and the Sapphire mountains.

On Friday the upper level low will move ever so slowly from
southern Idaho to Lemhi County. This slow movement combined with
the trough being negatively tilted, and with above average
amounts of atmospheric moisture will set the stage for widespread
showers, some of which could be moderate in intensity at times.
The probability for greater than a tenth of an inch of rain is
80+ percent area-wide. It could actually feel kind of muggy
Friday afternoon across the region. Frequent moderate or even
heavy rain showers are possible across western Montana. Rivers
and creeks will continue to run high due to the recent warm
temperatures and this rainfall could push them higher.

Friday night The mid-level low will have moved ever so slowly
northwards towards north-central Idaho. With the influx of
moisture east of the divide, the moderate rain showers will
impact northwest Montana. Some of the deterministic rainfall
amounts range from 0.25 to 0.75 of an inch from 6 pm Friday
through 6 am Saturday. The probability for greater than 0.25
of an inch is up to 65 percent in the Glacier Region and also
the Swan and Mission mountains. The focus of widespread rain
continues across northwest Montana on Saturday. Elsewhere it
will be much cooler, especially felt in the mountains with
highs only in the 40s.

Beyond Saturday it stays unsettled with temperatures near
or below seasonable levels. Ensemble guidance are pointing
to another warm ridge developing later next week.

&&

.AVIATION...The focus for showers and thunderstorms shifts
over northwest Montana generally after 03/2000z this
Wednesday afternoon. The shower activity will first develop
over the mountains, especially surrounding the Flathead
Valley, then could shift over the valley with lightning and
gusty winds to 35 knots, potentially impacting GPI. Elsewhere
VFR conditions are expected other than a few isolated showers
or a thunderstorm.

&&

.HYDROLOGY...Warm, sunny weather this week followed by
widespread precipitation beginning on Thursday night will result
in further rises on streams and rivers across north-central Idaho
and western Montana.

Watch for localized stream/creek flooding from snowmelt in low-
lying areas through late this week, perhaps further enhanced by
locally heavy rain on Friday/Saturday. Mainstem rivers will
continue to gradually rise. A flood warning has been issued for
the Clark Fork above Missoula as minor flood stage is expected to
be reached as early as Wednesday. Other rivers that standout with
a 30-50% probability of reaching flood stage late this
week/weekend include the Bitterroot River and South Fork of the
Clearwater. The Yaak River may also approach flood stage in
several days.

&&

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