Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 131954

Area Forecast Discussion
National Weather Service Missoula MT
154 PM MDT Thu May 13 2021

.DISCUSSION...The Northern Rockies will be under northwesterly
flow aloft for the remainder of today. A jet streak aloft will
contribute to some degree of instability over northwest Montana
this afternoon and evening. Forcing is weak, though, and moisture
is limited, so the chance for thunderstorms will be slight. A dry
lower atmosphere will be conducive to gusty outflow winds where
thunderstorms are able to form. Flow turns westerly by Friday.
Continued instability courtesy of the upper level jet streak and a
series of fast-moving shortwave disturbances will keep a slight
chance for showers and thunderstorms in the forecast. The focus of
precipitation will slide further south on Friday compared to
today, so look for showers and thunderstorms to pop up in north-
central Idaho, as well as west-central and southwest Montana.
Terrain-based showers along the Continental Divide will not be out
of the question. Like today, instability and moisture will be
limited on Friday, so expect storms to be on the weak side. The
lower atmosphere remains dry, so expect gusty winds to accompany
any storms that are able to develop.

On Saturday, an elongated trough of low pressure forms along a
line from about San Francisco, CA to Great Falls with a closed
low over San Francisco. This feature will shift upper level winds
to the north, even the northeast in southwest Montana and Lemhi
County, ID. As a result, Lemhi County and southwest Montana could
see scattered showers in the afternoon under an easterly moisture
feed. Sunday and Monday are looking to be the warmest, driest
days of the week as valleys in Montana reach the lower 80s. Some
of the lower valleys in central Idaho such as Riggins and Orofino
will flirt with 90.

Some questions remain as to the timing of our next weather maker.
Recent ensemble solutions have slowed the arrival of a deep trough
of low pressure that was looking to arrive by Tuesday. Latest runs
are beginning to agree on low pressure, but they are delaying the
onset. We will keep the forecast for Tuesday on the dry side for


.AVIATION...Terminal sites should expect continued VFR conditions
today and Friday. The only impact to aviation is a chance for
thunderstorms today at KGPI after about 13/2100Z. There is not
enough confidence of thunderstorms to include thunder in TAFs.
Any storm that develops will be capable of gusty outflow winds.
Friday afternoon will bring a similar situation, with a slight
chance of thunderstorms affecting KBTM and KSMN in the afternoon.



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