Flood Watch 1, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
314 AM MDT Tue May 19 2020

.DISCUSSION...Warm and moist southerly flow has lead to a mild
night across the Northern Rockies, along with some lingering rain
showers. The trough of low pressure that will continue to bring
rainfall to the region over the next few days will dig further
south, keeping western Montana and north central Idaho in general
southerly flow with an increasingly unstable atmosphere. As a
result, the Storm Prediction Center has placed a portion of west
central Montana in a slight chance category for strong to
potentially severe thunderstorm development later this afternoon
through this evening. Locations from the Missoula valley south
through Salmon, ID and points eastward are areas that could see
the best chance for thunderstorm development today. Impacts could
include hail, gusty outflow winds, lightning, and brief, heavy
downpours.

As this area of instability ends late this evening, more
persistent rainfall will usher back into the region. A focused
area of heavy rainfall will impact north central Idaho (Clearwater
and Idaho counties), and along the spine of the Continental
Divide. Confidence is moderate to high in the positioning of this
heavy rainfall, with 48-hour amounts ranging from 1.50 to 2.00
inches. Elsewhere, lesser amounts will range between 0.50 inches
and 1.00 inches during the same time frame.

Flooding of small streams and creeks will be likely. Mainstem
rivers will also see substantial rises, with several reaching or
surpassing their specific flood stage(s). Please refer to
water.weather.gov for more information on mainstem rivers and
their flood potential in the coming days. We will also provide
up-to-date flood information via weather.gov/missoula and our
social media pages @NWSMissoula.

The forecast remains wet through the weekend, with varying degrees
of rainfall intensity across all of north central Idaho and
western Montana. As such, the threat for flooding will continue,
particularly for smaller streams and creeks.

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.AVIATION...Precipitation will remain a steady focus over the next
24 hours for all aviation terminals across north central Idaho and
western Montana. Thunderstorm development after 19/2000Z will
likely affect KMSO, KBTM, and KSMN today, with the potential for
erratic gusty winds, lightning, and hail from storms that develop.
Convective activity will last through roughly 20/0400Z.
Thereafter, steady rainfall should be expected, along with low
ceilings/visibility from moderate to heavy rainfall.

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