Flood in Montana, June 3 to 7, 2021

Area Forecast Discussion
National Weather Service Missoula MT
347 AM MDT Thu Jun 3 2021

...Record Hot Temperatures Today...

.DISCUSSION...Near record high temperatures are expected today
with readings well into the 80s and 90s, lower 100s for the lower
Hells Canyon in Idaho.

Yesterday evening a few storms were able to develop over west-
central Oregon. One station received 0.60 of an inch of rainfall
in only two hours which tells us that while the surface was really
dry, the atmospheric moisture levels aloft were sufficiently high
to support those rates. High clouds from this activity is seen on
satellite imagery streaming over Idaho early this morning. As the
upper level ridge shifts eastward today, the flow will become
more southwest. Typically this is a sign that our area could
experience thunderstorm activity. Combined with the heat expected
in the region, the incoming elevated moisture could help to
weaken the capping inversion or “cap” enough to allow for
shower development over mountainous locations. Confidence is
increasing that areas over the higher terrain of north- central
Idaho and points north and east towards the Missoula and
Bitterroot Valleys could experience the best chance for
development late this afternoon and evening. Wind shear is not too
strong for the today’s scenario, so storms may be relatively
short-lived but be able to produce heavy downpours, small hail and
possibly strong gusty winds. Areas across northwest Montana have
a lower chance for shower activity as model forecast soundings
suggest the cap is much stronger. As a side note, any cloud cover
associated with the shower activity could also prevent some areas
from reaching their full potential in terms of high temperatures.

There is a chance for night-time showers and a few thunderstorms
tonight generally from north-central Idaho towards west- central
Montana. The probability for shower activity increases towards the
Mission Valley as compared to Thursday afternoon. There is a low
probability that areas as far north as the southern Flathead
Valley could experience a shower. Impacts from any “nocturnal”
thunderstorms would be cloud to ground lightning, small hail and
gusty winds.

On Friday, a relatively dry front will move through the region.
While it will be noticeably breezier with wind gusts between 20
and 30 mph, high temperatures will be a tad lower in the valleys,
and much cooler in the mountains. For example, Lolo Pass will be
in the lower 80s Thursday, but only reach the mid 70s on Friday.
The area with the better chance for showers and thunderstorms will
move further east and south...generally from the southern
Bitterroot mountains to southwest Montana. There is slightly
higher wind shear in the environment, but with drier air mass
working its way into the region, the probability for strong storms
is lower in these areas.

As a large upper level trough tracks from the Gulf of Alaska east
into western Canada on Saturday, the wind flow over us
strengthens. Besides cooler temperatures area-wide, it will be
much windier as compared to Friday. Wind gusts between 25 and 35
mph are anticipated by the afternoon and perhaps gusts to 45 mph
are possible along and west of the Divide. An upper wave will be
the impetus for a few showers across northwest Montana.

For the remainder of the forecast period heading into next week it
appears that seasonal temperatures will reign while the chance for
precipitation increases slightly by mid-week, thanks to potential
short-wave energy embedded in south to southwest flow aloft.

&&

.AVIATION...Expect hot temperatures in the 80s and 90s at all
terminals today. Winds will generally be less than 10 knots. There
could be a few showers and thunderstorms that develop over the
Clearwater mountains of Idaho and move into western Montana
between 03/2100Z and 04/1500Z. Lightning, small hail and gusty
winds near 40 knots are possible with this activity. A relatively
dry cold front will move through by Friday and gusty west winds
are expected across the region.

&&

.HYDROLOGY...Despite the warm temperatures over the last several
days, the Clark Fork River above Missoula has been slow to rise.
It is forecast to rise above minor flood stage later today and
crest near to above 8 feet on Friday.

&&

.CLIMATE...Butte saw a high of 86 degrees yesterday June 2 which
broke the previous record of 84 set back in 1986. Kalispell saw a
high of 87 degrees which tied the previous record set in 1986. For
today, old records that could be broken include:
Kalispell 89 set back in 1970,
Missoula 91 set back in 2007,
Butte 85 set back in 1988,
Dworshak Fish Hatchery 97 set back in 1986,
Salmon 91 set back in 1988.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Heat Advisory from 2 PM this afternoon to 7 PM MDT this evening
     Bitterroot/Sapphire Mountains...Butte/Blackfoot Region...
     Flathead/Mission Valleys...Kootenai/Cabinet Region...Lower
     Clark Fork Region...Missoula/Bitterroot Valleys...
     Potomac/Seeley Lake Region...West Glacier Region.

ID...Heat Advisory until 8 PM PDT this evening Lower Hells
     Canyon/Salmon River Region...Northern Clearwater
     Mountains...Orofino/Grangeville Region...Southern
     Clearwater Mountains.

&&