Flooding in Montana, May 23 to 31, 2018

Area Forecast Discussion
National Weather Service Missoula MT
209 PM MDT Wed May 23 2018

.DISCUSSION...Another unstable afternoon with plenty of high
level moisture has led to more thunderstorms popping up over the
terrain of west central and southwest Montana, as well as north
central Idaho. Storms will be slow-moving, as they have been the
past several days, so expect heavy downpours to occur, in addition
to lightning, small hail, and gusty outflow winds. Storms
yesterday afternoon across central Idaho produced very heavy rain
in brief periods of time, and this can be expected with storms
that form today and again on Thursday.

Friday and Saturday: another large low pressure system will once
again develop over northern California and slowly move eastward
through the Great Basin. The resultant southerly flow will usher
in some pretty moist air and unstable conditions, resulting in
continued afternoon and evening showers and thunderstorms. These
rounds of convection will be more widespread and potentially
stronger. Storms will contain heavy rain, which could pose
flooding issues for both small streams and potentially mainstem
rivers. Gusty winds and some hail are also possible with storms

Sunday and Memorial Day: the upper low moves slightly east, which
will move the focus for most showers and storms in Lemhi county
and southwest Montana, though there is the slight potential
across west central Montana. Northwest Montana may actually have
some pleasant weather during this time with just a few
showers/storms possible.

Next week looks to be more of the same, with unstable and moist
conditions over the region. Continued afternoon/evening showers
and storms are expected each day next week. Coverage and
intensity of storms are uncertain at this time.

&&

.AVIATION...Expect another round of afternoon and evening
showers/thunderstorms across the northern Rockies today. KMSO and
KSMN will have the best chance of being impacted by the slow-
moving, mainly terrain-locked storms. Storms will be capable of
producing heavy rainfall reducing visibility and ceilings. The
convection will diminish shortly after sunset by 03-04 UTC,
though showers are possible over northwest Montana through
24/1800Z. More shower and thunderstorm activity is expected after
24/1800Z across much of the region.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Flood Watch through Friday morning Flathead/Mission Valleys.

ID...None.
&&