Flash Flood Watch 1, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
353 AM MDT MON JUN 1 2015

...STRONG TO SEVERE THUNDERSTORMS TODAY...
...MAIN THREATS: HEAVY RAINFALL, HAIL, LIGHTNING, & GUSTY WINDS...

.DISCUSSION...As a precursor to what is likely going to be an
active thunderstorm day for the Northern Rockies, parts of north
central Idaho and west central Montana experienced a few rounds of
nocturnal convection overnight. These storms produced very heavy
rainfall, plenty of lightning, and small hail. This is all just a
hint of things to come today, as conditions become increasingly
unstable and thunderstorms further develop and intensify.

Details concerning today's thunderstorms are as follow:

1) Storm coverage will be widespread through today and overnight
as well. Most of central Idaho and western Montana, if not all,
will see some form of impact from thunderstorms today.

2) The biggest concern from storms today will be the amount of
rainfall they produce in very short periods of time. This will
lead to the very real potential for flash flooding and urban
flooding. Those who live anywhere near wildfire burn scars will
be especially susceptible to flash flooding from thunderstorm
rains. For more details, please see the HYDROLOGY discussion
below.

3) Hail, gusty winds, and frequent lightning are additional
concerns today. Select storms over the weekend produced hail up
to one inch in diameter in some places, and that wasn't even from
the best storm development that is likely to occur today. This,
plus gusty erratic winds, will be a major concern to those with
plans to be outdoors, or who have property or belongings that
remain unsecured/unprotected outside.

4) Anyone recreating, be it on land or on water, should take heed
of the points mentioned above, as conditions could turn dangerous
rather quickly.

Storms will weaken in intensity but not in coverage tonight into
Tuesday, with another chance for heavy rain, lightning, hail and
winds possible through the day Tuesday.

Wednesday will continue to be unsettled with a round of showers
and isolated thunderstorms for many locations, especially during
the afternoon/evening hours. Locally heavy rain and dangerous
lightning will be the primary concerns.

Gradual drying and warming will take place Thursday into Friday
but enough residual moisture will remain for isolated to scattered
showers/thunderstorms, mainly from southwest Montana westward
into north-central Idaho. Over the coming weekend into early next
week, forecast models indicate daily showers will occur but differ
on the amount of moisture that will be brought up from the south.
The amount of available moisture will greatly influence both the
coverage and intensity of the rain activity that occurs.

&&

.AVIATION...Terminals should expect an active day of
thunderstorms, which may continue overnight into Tuesday. Along
with frequent lightning, bouts of heavy rainfall, gusty and
erratic winds, and hail are all possible, if not likely, with
storms that develop today. The timeframe of biggest concern for
all terminals will be between the hours of 01/2000Z and 02/0300Z
for the above- mentioned impacts. More convection and disruptions
from associated impacts is expected Tuesday.

&&

.HYDROLOGY...The biggest concern today with thunderstorms
affecting the Northern Rockies will be the very real potential for
flash flooding and urban flooding. We expect thunderstorms to
produce torrential downpours in very brief periods of time, leading
to the threat for flash flooding. Amounts of 0.30 to 0.50 inches
in as little as 30 minutes is not out of the question.

IMPACTS: Rock slides and/or washouts over roadways are possible.
Those living near wildfire burn scar areas should prepare for
impacts from heavy rainfall in the form of mud and debris flows.
For urban areas, poor drainage will only exacerbate the amount of
rain that thunderstorms produce, causing localized flood
concerns. Have a plan in place to help alleviate any flash
flood/urban flood issues that could arise throughout the day
today.

&&

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