High Wind in Montana, September 24, 2019

Area Forecast Discussion
National Weather Service Great Falls MT
1155 PM MDT Mon Sep 23 2019

...UPDATED AVIATION SECTION...

.SYNOPSIS...

Breezy to windy conditions will continue tonight through the day
Tuesday. A few showers are possible across the Rocky Mountain
Front during this same time period, with a weak disturbance
possibly bringing some scattered activity on Tuesday to the
northern and central plains. Winds and precip will diminish
Tuesday night. After a near normal and less breezy day on
Wednesday, active and cool weather may return to the region later
in the week and possibly into the upcoming weekend.

&&

.Evening Update...

Evening update has been published, with some adjustments made to
winds and cloud cover to account for latest trends, observations,
and guidance. Expansive Chinook Arch has formed over North Central
and even Southwest Montana this evening as lift associated with a
strong mid- and upper level jet, which is transporting abundant
Pacific moisture aloft eastwards, overspreads the region. As the
jet contiues to push east over the region during the overnight
hours, expect little to no change in the mid- to upper level cloud
deck, which will result in an overall partly to mostly cloudy
night over the entire region. At the surface, a weak area of low
pressure was sliding east southeast across Southern Alberta and
into Saskatchewan, with high pressure attempting to build east
from the Pacific Northwest. This has led to relative sharp lee-
side surface trough and subsequent pressure gradient along the
Continental Divide, which combined with the mixing of the near
surface winds has led to a breezy evening across much of the
region, especially along the Front. As the night continues,
expected to pressure gradient along the Rocky Mountain Front to
decrease as the lee- side surface trough relaxes due to the
departing surface low over Southern Canada. However, mid- and
upper level cross barrier flow will be on the increase over the
Divide throughout the night, especially along the Southern and
North Rocky Mountain Fronts. This increasing cross barrier flow
and some ridgetop stability will continue to lead to strong
westerly winds, especially in wind prone areas and over ridgetops
of the Rocky Mountain Front. Would not be surprised if a few
isolated High Wind reports materialize tonight, especially where
mountain wave activity develops/persists. Will forgo any High Wind
highlights with the evening update given the expected isolated
nature of the strongest wind gusts near/over High Wind criteria,
but we will continue to monitor throughout the night. - Moldan

&&

.AVIATION...
Updated 1155 PM MDT Mon Sep 23 2019 (24/06Z TAFs)

VFR conditions will continue through at least the next 24 hours
under a strong west-northwest flow aloft. The main impact for this
period will be strong westerly winds causing mountain wave activity,
as well as the winds translating down to the surface. Gusts will
generally be in the 25 to 40 kt range, especially over the plains,
but gusts to 55 kt will be possible closer to the Rocky Mountain
Front. Some areas will either be protected from these low/mid level
winds (valley areas including KHLN, KBZN, KEKS, KWYS) or there is
periodic decoupling of winds (KLWT), which will cause low level wind
shear, mainly through 17Z. After then, the winds should mix down to
the terminals. Overall, winds should start to decrease and decouple
somewhat after 01Z. Otherwise, the mid and high cloudiness will
generally decrease from the north after 12Z, but isolated showers
may linger in the mountains of far Southwest Montana until 12Z, and
some may develop in the KHVR area during the 18Z to 00Z time period.
Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  52  66  42  65 /  10  10  10   0
CTB  49  61  38  62 /  10  10  10   0
HLN  52  66  43  66 /  10  10  10   0
BZN  45  68  40  67 /  10  10  10   0
WEY  34  61  30  62 /  10  10  10   0
DLN  46  66  39  65 /  10   0   0   0
HVR  51  65  39  67 /  20  20  30   0
LWT  49  62  40  60 /  10  10  20  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

http://www.weather.gov/greatfalls