Flooding in Montana, June 18 to 26, 2018

Area Forecast Discussion
National Weather Service Missoula MT
245 PM MDT Mon Jun 18 2018

.DISCUSSION...

The closed low pressure circulation over the Great Basin that has
brought rain showers and thunderstorms to the Northern Rockies
will finally move east out of the region after Tuesday evening.
Precipitation coverage and intensity will lighten up after sunset
this evening, however there will remain a threat of hydroplaning
for area motorists. Small streams across southwest Montana and
Lemhi County Idaho have increased flow in response to earlier
rainfall, however those streams should level off later tonight and
stay within their banks. Finally, breezy conditions will remain
over Flathead Lake through sunset this evening, with some fast
shifting winds and sudden wave activity that may be hazardous at
times for small watercraft.

Rainfall expectations for the Northern Rockies are anticipated to
shift to along the Continental Divide and decrease on Tuesday,
with the closed low pressure circulation close to Yellowstone.
Glacier Park and Continental Divide passes will continue to
experience periods of moderate rain showers. Thunderstorm
potential will be slightly higher for much of the Northern Rockies
(except north of Highway 2, Kalispell) on Tuesday compared to
Monday, but the continuous rainfall should mostly cease at low
elevations. Daytime temperatures will begin to warm a bit from
today to Tuesday.

Average temperatures will return across western Montana and
north-central Idaho Wednesday through the weekend. Valley
locations will see highs in the low to upper-70s. Low-clouds and a
few areas of patchy fog will be possible Wednesday morning. A few
isolated showers and thunderstorms will develop Wednesday
afternoon, but will likely remain centered across mountain areas,
with valley locations remaining mostly dry.

Our next weather disturbance enters the region Thursday. An area
of low pressure will move through the Pacific Northwest into
Montana Thursday evening. Scattered to numerous showers and
thunderstorms will develop and move through the region Thursday
afternoon and evening. Isolated showers and storms will be
possible across the area Friday and Saturday afternoon and
evening. Lingering moisture and a warm airmass will generate
enough instability to drive thunderstorm and shower activity.

High pressure will build into the area Sunday through early next
week. Warm and dry conditions are expected.

&&

.AVIATION...Stratiform precipitation across the Northern Rockies
air space will transition to mostly convective during the next 24
hours. Low ceilings and terrain obscurations will continue to be
common (particularly along the Continental Divide, across
southwest Montana and Lemhi County Idaho, including near KBTM and
KSMN), however the convective nature of the clouds will lead to
ceilings being more broken up. Visibility will also vary near KBTM
and KSMN, particularly with moderate rain showers and any
thunderstorm that develops. Rain will also continue near KGPI and
KMSO through the next 24 hours, albeit with ceilings remaining
higher compared to air space to the south and east. Ridge/pass
elevation winds are generally in the 10 to 20 knot range, but will
diminish after 19/0300Z. Thunderstorm threat for southwest Montana
and Lemhi County will cease after 19/0300Z as well, but will re-
emerge once more after 19/1800Z.

&&

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