033
FXUS65 KMSO 050800
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
200 AM MDT Fri Sep 5 2025

.DISCUSSION...

KEY MESSAGES:

- East winds continue until Saturday or Sunday.

- Thunderstorm chances increase this weekend through early next
  week.

- Potential for very wet conditions the middle of next week.

High pressure east of the Continental Divide caused some strong
east winds across the region yesterday and last night. Today that
high pressure starts to migrate east across the continent, but it
remains strong enough to cause east winds well into the weekend.
Each day the east winds will get lighter until westerly winds
return. The westerlies should return to central Idaho on Saturday
and across western Montana Sunday afternoon.

A trough digging off the west coast is pushing the ridge over the
region further east. By Saturday the ridge axis is east of the
Continental Divide and southwesterly flow returns. Saturday into
Sunday will see a sharp rise in precipitable water, or the amount
of water vapor in the atmosphere available to be rained out. This
brings with it a marked increase in instability, and rising
chances for thunderstorms this weekend. The instability is
unusually high for this time of year as depicted by the Extreme
Forecast Index (EFI) available for the European Ensemble System of
models. This means it will be more like a summer thunderstorm
pattern than an early fall pattern which usually comes with a cold
front. With the increasing moisture mentioned above and a lack of
strong wind shear, once again there will be a potential for heavy
rain with these storms.

Ensembles still show a wide range of uncertainty in the
precipitation amounts for the middle of next week. One notable
recent change is that the potential for widespread moderate to
heavy rain has been pushed back to the middle of the week when
previous runs had shown good potential early in the week too. Now
it looks like thunderstorms with localized heavy rain will persist
into at least Monday with the potential for widespread rain coming
late Tuesday or Wednesday. One of the cluster analysis tools is
showing a 20% chance for 2-day rain totals mid-next week to come
in at over 0.75" for most of the region. Most of the region has an
80 to 85% chance of picking up 0.10" of rain mid-next week.

&&

.AVIATION...East winds have pushed much of the smoke west of our
aviation sites overnight. Northwest flow aloft will keep terminals
KHRF and KSMN in some smoke for today as well, and enough will
remain across the region to cause some haze, but impacts to
visibility are expected to be minor. By Saturday, thunderstorm
potential returns for central Idaho and then by Sunday for the
rest of the region as well.

&&

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

$$