Severe Thunderstorm Warning 6, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
250 PM MDT Mon Aug 17 2020

.DISCUSSION...A plume of subtropical moisture is evident on
satellite imagery, with origins off the northern California coast.
This moisture will continue to be directed into western Montana
and central Idaho tonight through Tuesday, setting the stage for
thunderstorms across the region. A few thunderstorms will be
possible this afternoon and early evening across southwest Montana
and Lemhi County in Idaho due to afternoon heating. However, the
better wave of energy doesn't arrive into central Idaho until
late tonight and into western Montana until early Tuesday
morning. This wave will be responsible for showers and isolated
thunderstorms, with little beneficial rainfall expected. Recent
high resolution forecast models have been trending further north
with this wave, now showing the potential for some early morning
storms even into northwest Montana by Tuesday morning.

Tuesday will continue to be well above normal for temperatures,
with many locations running roughly 10 degrees above seasonal
normals. Forecast models continue to show a more significant wave
of energy to move into the region Tuesday afternoon and evening.
This wave will interact with the subtropical moisture already in
place to produce scattered thunderstorms. The main focus is
generally expected from Missoula southward, although there have
been recent trends showing convection further north into northwest
Montana as well. A few storms could become strong, with the main
threat being gusty outflow winds and lightning. Storms will likely
be a mix of wet and dry, with wetter storms more likely across
northwest and west central Montana.

The high pressure ridge weakens a bit on Wednesday as the ridge
axis moves slightly east of the region. This will allow
temperatures to cool several degrees, though still running above
normal for this time of year. Slightly increased winds are
anticipated, along with another round of thunderstorms. Storms
looks mostly likely across central Idaho and west-
central/southwest Montana, where gusty outflow winds and lightning
will again be a concern. Drier air does return to northwest
Montana as the ridge shifts eastward, meaning thunderstorms will
not be as likely for that area.

High pressure does seem to re-establish over the area to end the
week, though current indications are that the ridge will not
rebound quite as strongly as seen early this week. Therefore,
slightly above normal temperatures will continue, with little to
know precipitation expected into the weekend.

&&

.AVIATION...Hot conditions will continue into Tuesday due to
strong high pressure aloft, resulting in high density altitudes.
Unstable conditions and incoming moisture from the south-
southwest will create a chance for isolated thunderstorms through
central Idaho and southwest Montana after 18/00Z including at
KSMN and KBTM. The potential for showers and isolated
thunderstorms will enhance through the night into early Tuesday
morning, possibly spreading as far north as KMSO and KGPI by
18/1500z. Gusty outflow winds along with lightning will be the
main threats with storms through the next 24 hours.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening Bitterroot...Deerlodge/West Beaverhead...East
     Beaverhead.

     Heat Advisory until 9 PM MDT Tuesday Flathead/Mission Valleys...
     Kootenai/Cabinet Region...Lower Clark Fork Region...
     Missoula/Bitterroot Valleys.

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

&&