Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 202047

Area Forecast Discussion
National Weather Service Missoula MT
247 PM MDT Sun Apr 20 2014

.DISCUSSION...Another clear and seasonably cool night is forecast
for the Northern Rockies as weak high pressure allows only
variable high clouds over the region. Monday will be quite nice
with temperatures once again reaching the upper 60s to low 70s
under a partly cloudy sky and afternoon breezes. A few more
cumulus clouds are likely late in the day with showers developing

Tuesday-Thursday...The storm system that we have been tracking for
several days now is still on track to impact the Northern Rockies
with moderate to heavy rain and snow at times for the region.  There
has been some changes in terms of timing and precip placement,
however the overall wording has not changed all that much from
previous forecast.

The ridge breakdown and progression of the trough will still
likely begin impacting central Idaho and southwestern Montana Tuesday
afternoon and evening and the atmosphere still looks favorable for
a few weak thunderstorms to develop in these areas putting down some
lightning. Heavy downpours will be the main threat with this initial
push, particularly for the Clearwater Mountains of Idaho County.
Rock and mudslides continue to be a concern for valley locations
along Highway 12. The heaviest precip is expected to be from
Tuesday evening into the overnight hours with some areas picking
up over 0.50 inches in a twelve hour period and possibly an inch
or more over a 30-36 hour period. Valleys will see all rain with
higher elevations getting 2-4 inches of snow during this time.

The main front and heaviest precip will enter western Montana by
Wednesday morning and continue through much of the day.  The
heaviest precip will be along the front where over a 0.25 inch of
rain isn`t out of the question during a short period of time.
Locations where there have been recent fire activity will need to
be closely watched for the potential of rock/mud or debris slides
due to the occasional heavy burst of rain. The biggest area of
concern will be in northwest Montana and along the Divide where
the heaviest valley rain and mountain snow will persist into
Thursday. Glacier Park (including Marias Pass) could see upwards
of 6 inches or more of snow by the time the system moves out of
the region late Thursday.

The overall picture looks like a very wet couple days for the
entire region mid week. 0.50 - 1.00 inches of rain for the
valleys, with the higher amounts in central Idaho, and 4 to 8
inches of snow for the mountains and Passes with the highest
amounts along the Continental Divide. More details to come!

Long range forecast models are in quite good agreement with their
depiction of a large, persistent trough extending from a deep Gulf
of Alaska low pressure system through the Pacific Northwest and
into the Northern Rockies from Friday into next week. This pattern
typically supports waves of energy breaking off the parent low,
spreading widespread rainfall across the entire northwestern part
of the United States (including much of Idaho and Montana).
At this time both the GFS and ECMWF support the best chance for
widespread rainfall occurring from Friday night through Saturday
evening. However this could change significantly over the next
several days considering the highly unpredictable nature of these
waves 5 to 6 days in advance. Thus have gone with better than
average chances for precipitation during this time frame.


.AVIATION...Breezy afternoon winds will gradually subside around
sunset and remain mainly calm through Monday morning. Afternoon
winds are expected to pick up again on Monday. But overall no
significant impacts to aviation are expected through the next 24




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