Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 211127

Area Forecast Discussion
National Weather Service Great Falls MT
527 AM MDT Sun May 21 2017

...Update to Aviation...


Unsettled northerly flow aloft is expected today into Monday night.
Fair weather and noticeably warmer temperatures are then expected to
grace the region Tuesday and Tuesday night, as an upper-level ridge
moves eastward over the area. A cooler and unsettled weather pattern
should return by Wednesday as an upper-level trough settles
southeastward over the area and the accompanying Pacific cold front
moves through.



Today through Tuesday...Montana will remain beneath a steady north
to northwest flow aloft through Monday night as a closed upper
low remains anchored over the Great Lakes region. To the west, a
weak upper level ridge will remain along the West Coast today and
tonight before becoming positively-tilted and nosing inland over
the Pacific Northwest Monday through Tuesday. By Tuesday, the
entire upper level pattern will undergo a significant transition
as a Pacific weather system moves southeastward from the Gulf of
Alaska and flattens the upper ridge.

For Sunday and Monday, temperatures will remain near seasonal
averages with isolated/scattered showers and a few thunderstorms
expected. This activity will be relatively weak as limited
instability and weak disturbances aloft interact in the northerly
flow. Surface winds will remain generally light to breezy and
northerly in direction. By Monday night, surface winds begin to
veer clockwise in response to the approaching Pacific weather
system and surface pressure falls in southern British Columbia and
Alberta. Tuesday will remain dry but temperatures will climb above
seasonal averages as westerly winds aloft and at the surface
fuel increasingly strong downslope winds across the region. mpj

Tuesday Night through Sunday...Dry weather expected Tuesday night as
an upper-level high pressure ridge advances eastward over the CWA.
However, a potent 700 mb jet streak accompanying a lead shortwave
trough impinging on our region may generate strong zonal flow aloft
across and in the lee of the Rocky Mountain Front Tuesday night
through early evening Wednesday. This pattern, combined with
subsidence in the surface to 700 mb layer, may permit warning-
criteria surface wind gusts, especially along the Rocky Mountain
Front. Even so, confidence in this high wind scenario is low at this
time. In addition, models are now in somewhat better agreement that
a Pacific cold front will move eastward across the CWA on Wednesday
and may be accompanied by scattered showers, while insufficient
instability may curtail the potential of thunderstorms. Wednesday
marks the beginning of what looks to be an unsettled weather pattern
heading into the weekend, as an upper-level trough settles
southeastward over our area and multiple disturbances pivot through
the trough. Scattered showers are forecast Wednesday night through
the weekend, with the potential for isolated afternoon/early evening
thunderstorms Thursday through Sunday. Temperatures on Wednesday may
be near or slightly above-normal, prior to the cold frontal passage.
Temperatures should then trend below-normal Thursday through the


Updated 1127z.

Northerly flow aloft will prevail over Montana through early Monday
morning. Scattered to broken mid and high level clouds can be
expected through the forecast period for widespread VFR conditions.
Only exception will be local LIFR/IFR conditions due to fog in
Southwest Montana valleys through about 15Z. Isolated showers and
possibly a thunderstorms are possible over eastern zones 18Z to 03Z
which could produce variable gusty winds up to 30 knots. Local fog
may develop again after 06Z Monday. mpj


GTF  69  45  69  46 /  10  10  10   0
CTB  70  43  70  45 /  10  10  10   0
HLN  71  47  72  47 /   0   0  10  10
BZN  64  40  67  40 /  10  10  20  10
WEY  55  29  58  30 /  10  10  20  10
DLN  64  40  68  41 /   0   0  10  10
HVR  72  44  71  43 /  10  10   0   0
LWT  64  43  64  40 /  20  20  20   0



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