Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 011110

Area Forecast Discussion
National Weather Service Missoula MT
410 AM MST TUE DEC 1 2015

.DISCUSSION...Low stratus and patchy fog has redeveloped in many
valley locations of western Montana and central Idaho this
morning. Much of the fog will lift into the stratus deck later
this morning, with the stratus diminishing later this afternoon.
The Mission/Flathead Valleys will likely see the stratus deck
persist all day. Temperatures today will be similar to the past
few days, with the cold pools and inversions remaining in place.

A weak disturbance remains on track to impact portions of the
Northern Rockies beginning Wednesday afternoon, and continuing through
the night. Precipitation is expected to be light with this system,
however there is the possibility of warmer air overrunning the
cold pools creating a light freezing rain/drizzle. The amount of
warm air and resultant liquid precipitation remains uncertain,
though right now, northwestern Montana and central Idaho has the
highest likelihood of seeing this outcome. The most probable
scenario will be widespread light snow, with isolated pockets of
freezing rain. Most valleys will remain in the cold pools through
the day on Wednesday.

Thursday could be the day that many valleys will break out of the
inversions. A tightening of the surface pressure gradient ahead of
the next system, will allow for a southwest wind to develop. The
Missoula Valley and Seeley/Swan regions will have the least
chances of breaking out of the inversions Thursday.

The next weather system approaches Thursday night into Friday,
with precipitation expected for north-central Idaho and most of
western Montana. This system will initially start off with
moderately high snow levels of 4000-5000 ft but will quickly drop
Friday morning as colder air ushers into the region. As a result,
some lower elevations could see rain or a rain/snow mix prior to a
transition to all snow above 2500-3500 ft. However, valleys that
fail to mix out their cold pools by the end of this week could
experience freezing rain/drizzle that would result in very slick
road conditions. Elsewhere...the main impact will be a few inches
of accumulating snow for the mountains and mountain passes,
particularly for Lookout and Lolo Passes, as well as the Glacier
NP/Bob Marshalls region. Mid-level wind flow will start off fairly
breezy with this system that could support blowing snow
conditions for those in the mountains.

Relatively warmer and drier weather returns for the weekend, with
the best chance for returning precipitation late Sunday/Monday.
Long-range models continue to suggest a series of disturbances to
follow, though uncertainty on timing and strength of these waves
remain low at this time.


Low stratus with ceilings below 10,000 ft AGL will continue for
the valleys of northwest Montana, impacting KGPI throughout the
next 24 hours. KMSO will have freezing fog in the vicinity of the
airport that will continue to thicken and drift around throughout
the morning hours. This will lead to periods of reduced visibility
through late morning before lifting to low stratus and then
diminish by late morning/afternoon. KBTM will be in the vicinity
of dense freezing fog which could occasionally drift over the
terminal at times. All aviation sites that see the low stratus
deck will also have periods of occasional light snow


Due to persistent frigid temperatures, ice has begun
to form along area rivers and creeks from Idaho and across western
Montana. Observations have been reported of river ice beginning
to back up in localized areas, though no flooding is being
reported. With continued cold temperatures expected through the
middle of this week, ice may continue to grow even further and may
contribute to very minor localized flooding.


Continued stable conditions with little atmospheric mixing has
caused persistent valley inversions across the region. This has
contributed to a degradation of air quality across portions of
west central Montana. Air quality may be slow to improve given
that stable conditions are expected through at least the middle of
this week.



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