Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 222332 AAA

Area Forecast Discussion
National Weather Service Great Falls MT
531 PM MDT Mon May 22 2017

update aviation...

Generally quiet conditions are expected this evening and into the
overnight hours, however, a few scattered light rain showers can
not be ruled out for areas south of a Great Falls to Lewistown
line. Low temperatures will be near to slightly above normal, with
readings generally into the upper 30s to near 50 degrees. Winds
are expected to remain light, and will become southerly during the
overnight as a warm front pushes northeast across Southwest and
North Central Montana.



Tonight through Tuesday...quiet conditions are expected during the
overnight hours, with low temperatures at or slightly above normal
values for this time of the year. Scattered showers will be possible
through the evening hours along the Continental Divide, but will
likely only produce light QPF amounts to around a hundredth of an
inch at most. At the surface, a warm front will begin to lift from
southwest to northeast across the region around or shortly after 03z
this evening. Upper level ridging will then try to build in during
the overnight hours, however, will be undergoing a break-down phase
through the day on Tuesday as the next upper level trough drops
quickly southeast from the Gulf of Alaska. Never-the-less, high
temperatures on Tuesday are expected to warm into the 60 to near 80
degrees, which is well above normal. As the upper level ridge breaks
down over the Northern Rockies on Tuesday, zonal/cross barrier flow
will begin to develop. This will help to promote dry and breezy
conditions (especially during the late afternoon and into the
evening hours when winds will quickly strengthen).

Tuesday night through Wednesday night....focus then turns to the
H500 trough, which will be just arriving onto the British Columbia
Coast by 00z Wednesday. This H500 trough and associated closed low,
will quickly dig south and east into Southern Alberta by 12z on
Wednesday. Strong cross barrier flow will develop across the
Northern Rocky Mountain Front by 06z Wednesday as H700 and H500
winds strengthen to around 50 to 75 and 80 to 100 knots
respectively. While these high winds will be occurring during the
overnight hours when stability from the nocturnal inversion
would/should ideally be strongest, the depth of this (Tuesday
nights) nocturnal inversion is very shallow per BUFKIT soundings
(in some instances only 500 feet deep AGL). With this in mind, it
will not take much to mix down higher wind speeds during the
overnight hours, especially up to H700 along the Rocky Mountain
Front. By 12z/15z on Wednesday, BUFKIT soundings within the
current High Wind Watch show winds of 50 to 55 kts within the
lowest momentum transfer region, with values at the top of the
momentum transfer region approaching 65 knots. I would not be
surprised if some favorable wind regions along the Rocky Mountain
Front realize 70 to 75 kt wind gust during this time frame. High
Wind criteria winds are then expected to persist through the day
on Wednesday, as area of low pressure rapidly strengthens to as
low as 975mb to 980mb across Alberta/Saskatchewan. Current model
solutions have this area of low pressure dragging a Pacific Front
first across the region during the mid morning/early afternoon
hours, with a secondary front then diving south from Canada during
the evening hours on Wednesday. This secondary cold front has me
worried about the potential for high winds extending into the
evening hours on Wednesday, as a strong barrier jet will be in
place behind it (the secondary cold front). For this reason, have
decided to extend the current High Wind Watch until 06z Thursday.
In addition to the strong winds on Wednesday, rain showers and
higher elevation snow showers will become ever increasingly
possible, especially from 18z Wednesday onwards. - Moldan

Thursday though Memorial Day...Low pressure centered across southern
Saskatchewan Thursday morning lifts out to the northeast. Breezy NW
winds will continue across much of N-Central MT on Thursday before
diminishing Thursday evening. Unsettled/cyclonic northwest flow in
the wake of the low will bring scattered showers Thursday. The main
upper low pulls out of the region Friday, however, broad troughing
still lingers across the region, bringing more scattered showers for
Friday, with the best chances coming across the south and west.
Broadscale trough and unsettled nw flow lingers to start the Holiday
Weekend on Saturday, with more scattered showers again possible. The
rest of the weekend we see ourselves in the wake of the large low
pressure system. A ridge tries to build although looks like it never
fully sets up. Weakly unstable nw flow could bring a few isolated to
widely scattered showers Sunday, although coverage doesn`t look as
widespread as Friday and Saturday. Similar setup will be found on
Memorial day, with a weak disturbance moving through the nw flow.
The result will again be chances for widely scattered showers.
Isolated thunderstorms will be possible during Memorial Day weekend
with the unsettled pattern, although at this point the lack of
strong lift will limit the strength of these storms. Thursday`s low
will keep temperatures below normal through much of the long term,
with a warmup to near normal expected for Sunday and Monday.


Updated 2331z.

An upper level disturbance is producing isolated showers over
southern Montana this afternoon. These will diminish this evening.
Another disturbance will be bringing increased wind speeds across
north central Montana Tuesday afternoon. Otherwise VRF conditions
are expected though Tuesday afternoon. db


GTF  46  79  55  65 /   0   0   0  10
CTB  47  78  48  62 /  10   0   0  20
HLN  48  80  52  70 /  10   0   0  10
BZN  40  75  45  71 /  10   0   0  10
WEY  32  63  37  64 /  10   0   0  10
DLN  43  75  48  70 /  10   0   0   0
HVR  42  79  50  69 /   0   0   0  10
LWT  39  72  48  67 /   0   0   0  10


High Wind Watch from late Tuesday night through late Wednesday
night Chouteau...Eastern Glacier...Eastern Pondera...Eastern
Teton...Hill...Liberty...Northern Rocky Mountain Front...
Southern Rocky Mountain Front...Toole.


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