Snow pack up to 200 percent of normal combined with above normal temperatures contributed to flooding across western Montana.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS MSO wrote it up.
Snow pack up to 200 percent of normal combined with above normal temperatures contributed to flooding across western Montana.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Missoula, MT wrote its last area forecast discussion before the first event at May 11, 1:40 PM MST, 7 hours 5 minutes earlier.
Area Forecast Discussion
National Weather Service Missoula MT
240 PM MDT Fri May 11 2018
.DISCUSSION...Scattered showers will continue the remainder of the
afternoon as a large closed low drops south of the region. A few
thunderstorms containing brief downpours and small hail are also
possible mainly across north-central Idaho. Widespread river
flooding will continue across much of west-central Montana with
numerous warnings in effect, see the hydrology section for more
details. Smaller streams and creeks which rose from today's rain
are expected to gradually recede into Saturday.
Precipitation will become focused over Lemhi County tonight into
Saturday morning, with snow levels around 6000 ft. Occasional
roadway snow accumulations are possible around Gilmore Summit and
Bannock Pass but the main impact will be to backcountry users.
Prepare for raw conditions due to gusty winds, falling snow, and
below freezing temperatures.
The low pressure system is expected to meander around southern
Idaho this weekend into early next week. Weak disturbances will
combine with moisture and instability to produce a scattering of
showers and a few thunderstorms each day. Precipitation will be
focused in southwest Montana and across Lemhi County. Temperatures
are expected to gradually warm to above seasonable levels.
Forecast models show a general pattern of daily showers and above
normal temperatures for the middle and latter part of next week
but disagree considerably on the details.
&&
.AVIATION...Scattered showers will continue into this evening with
the most likely terminal sites to be impacted including KMSO,
KBTM, and KSMN. Isolated thunderstorms containing brief downpours
and small hail will also be possible across north-central Idaho.
It will be quite breezy the rest of today into Saturday as the
pressure gradient increases especially across southern sections of
the region. Occasional gust of 20-25 kt are anticipated. Scattered
showers will continue on Saturday, most focused around KBTM and
KSMN.
&&
.HYDROLOGY...Area rivers and streams continue to flood as
forecast. Lowering snow levels today and Saturday will help slow
down the snow melt and cause area rivers to briefly recede.
However, this is competing against the run off from the recent low
elevation rainfall and the practical result is that flooded areas
will see little or no relief from current flood impacts.
The recent outage of a river gauge on the Blackfoot river, which
has since been fixed, is causing some difficulty with the river
forecast on the Clark Fork above Missoula. You may notice more
forecast to forecast variation than usual in the long term.
However despite the variation in the numbers, confidence remains
very high that the Clark Fork above Missoula will remain above
major flood stage for the foreseeable future.
The high water on the Clark Fork is also causing a backwater
affect on the Bitterroot River near Missoula. Our hydrologic
forecast models cannot handle the backwater, so the river forecast
there will not match the observations until the Clark Fork recedes
enough to not affect it. A flood warning remains in effect for
the Bitterroot more as a result of water from the Clark Fork
affecting it than due to flow down the Bitterroot River.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...Winter Weather Advisory from midnight tonight to 9 AM MDT
Saturday ABOVE 6500 FEET for the Eastern Lemhi County.
&&The national radar mosaic for the moment the first event began.
1 flood. Local standard time, as filed.
Low lying areas were flooded by the Clark Fork River between Thompson Falls and Plains.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2018Every state in 2018This day in weather historyWarnings in Montana, May 2018
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 753949.