Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 241841

Area Forecast Discussion
National Weather Service Missoula MT
1241 PM MDT Tue May 24 2022

.DISCUSSION...A weak, flat ridge is slowly building over the
region this afternoon through tomorrow morning. The building ridge
will compete with a mildly unstable, cool air mass and some of the
strongest sunshine we get throughout the year. The result will be
widespread afternoon fair weather cumulus with isolated showers.
The building ridge will push back on the instability and limit the
strength of the updrafts so we won`t see any thunderstorms.
Wednesday the ridge will be stronger and we aren`t expecting any
showers, though there will be another round of afternoon cloud

Thursday afternoon and evening the flow aloft turns southwest as a
trough approaches the Pacific Northwest coast. The trough`s
progression is slow, so the region will be under unstable
southwest flow both Thursday evening and Friday. This is setting
us up for some thunderstorms, with about a 20 percent chance for
some of the storms lasting through Thursday night in central
Idaho. Current forecasts keep the wind shear and instability low
enough that severe thunderstorms are not expected.

Saturday through about Tuesday a closed low develops and moves
across the Pacific Northwest and the northern Rockies. Ensembles
are starting to agree well that the precipitation will come in two
waves. The first will be Friday night through late Saturday
morning, and then the next wave will be Monday and linger into
Tuesday morning. Some showers will continue between the waves, but
they will be more isolated and weaker showers.

Overall, the probability of picking up a quarter of an inch of
rain or more with the first wave is around 60% in Grangeville, ID
and far northwest Montana around Noxon and Troy. The valleys of
western Montana have a 20-40% chance of picking up a quarter of an
inch of rain, with the exception of the area from Garrison
Junction to Butte which is around 10%. In the mountains, a median
value for the forecast ranges from 0.50 to 0.65 inches for most of
central Idaho and western Montana. Lemhi County, Idaho will be
much drier with only a few hundredths of an inch expected in the
first wave, but their chances improve for at least a tenth of an
inch or more with the second wave early in the week.

While this is some of the wettest weather we`ve had in the area in
quite a while, May is climatologically one of the wettest months
of the year for the region and this storm isn`t exceptional in any
way. So while it will be cool and wet, we don`t expect significant
negative impacts from it for things like flooding or landslides.


.AVIATION...A weak ridge building over the region today is helping
to put a cap on convection in an otherwise cool, unstable air
mass. Several cumulus clouds have formed in the area with light
showers coming from some of the stronger updrafts, mainly over the
mountains. A lack of a steering flow under the developing ridge
means the showers will just pop up and drop out again after about
half an hour. This will continue into the evening hours, or until
about 02Z.



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