High Wind Watch 12, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1125 AM MDT Wed Sep 23 2020

.SYNOPSIS...

Dry, warm, and breezy conditions prevail today as weak high
pressure aloft moves through the region. An approaching weather
system will bring rain to the western mountains late tonight
through Thursday with strong and gusty winds expected across much
of north central Montana. Cooler and unsettled conditions can then
be expected into the weekend.

&&

.Morning Update...

Forecast has been published, with only change being to increase
cloud cover to account for mid- to upper level clouds moving
across the region. Otherwise the remainder of the forecast for
today is still on track, with increasing winds across the region,
especially along the Rocky Mountain Front. Primary focus and
concern with the afternoon forecast will be the threat for high
winds and critical fire weather conditions on Thursday as a
Pacific cold front moves eastward across the Northern Rockies. -
Moldan

&&

.AVIATION...
1125 AM MDT Wed Sep 23 2020 (23/18Z TAF Period)

VFR conditions are expected throughout the 2318/2418 TAF period
beneath mid- to upper level cloudiness; however, mountains along the
Continental Divide will become increasingly obscured late this
evening and into Thursday morning. Precipitation will begin to
increase in coverage along the Continental Divide late this evening
and persist into the day on Thursday ahead of an approaching Pacific
cold front. Winds will also be on the increase throughout the entire
TAF period, especially after 06z Thursday over Central and North
Central Montana where winds gusts of 35-55kts are possible (most
notably from the mid-morning to mid-afternoon hours on Thursday).
Low level wind shear is possible across the entire region from late
this evening through Thursday morning, especially along the
Continental Divide and in the valleys of Southwest Montana. In
addition to low level wind shear, mountain wave turbulence is also
possible, especially immediately east of the Rocky Mountain Front
over Central and North Central Montana. - Moldan

Refer to weather.gov/zlc for more detailed regional aviation weather
and hazard information.

NOTE: Amendments not scheduled for Ennis (KEKS) due to ongoing
runway closure.

&&

.FIRE WEATHER...

Weather disturbance aloft and an associated surface cold front
sweep through the forecast area on Thursday. Strong westerly winds
behind the front will combine with minimum RH values of 20-30
percent to create significant fire weather concerns across the
grasslands Thursday afternoon and evening. A Fire Weather Watch
remains in effect for this period.

&&

.PREV DISCUSSION...
/ISSUED 540 AM MDT Wed Sep 23 2020/

Weak shortwave ridging aloft will build over the forecast area
today. This will bring warm, dry, and breezy conditions along with
variable mid and high level clouds at times. By late tonight, an
approaching weather system will bring increasingly strong mid to
high level winds through northern Montana. Downslope pressure
gradients will also strengthen as a strong surface low pressure
system develops in central Alberta overnight. Finally, a strong
surface cold front will sweep through north central Montana
Thursday morning with winds becoming westerly and increasing in
its wake. Consequently, I have issued a High Wind Watch for
Thursday morning through evening in northwestern portions of
central Montana.

Precipitation develops over the mountains of Glacier National
Park early Thursday morning then spreads southward along the crest
of the Continental Divide. However, the strong downslope winds
along the east slopes of the Rockies will limit precipitation to
the RMF and immediate adjacent plains. Additionally, the
combination of strong winds with minimum afternoon humidity
ranging from 20 to 30 percent has prompted issuance of a Fire
Weather Watch for all of Great Falls Fire Weather zones. Please
see the Fire Weather section (below) for more detailed
information. Broad cyclonic flow aloft will remain over western
Canada and northwestern USA for Friday and Saturday. This will
keep temperatures near or below seasonal averages with breezy and
unsettled conditions. mpj

Unsettled northwest flow aloft develops for Sunday and Monday as
the aforementioned weather system moves east of Montana.
Temperatures rebound slightly but isolated to scattered showers
remain possible during this time frame, especially over the
mountains. Medium range diagnostic models and ensembles are in
very good agreement for Tuesday and Wednesday of next week. A
highly amplified upper level pattern is expected to build with a
ridge along the West Coast and a deep upper level trough over the
eastern US. This will place Montana beneath northwest to northerly
flow aloft as the ridge slowly edges inland. This should bring
mostly clear to partly cloud skies with temperatures warming above
seasonal averages. No precipitation is anticipated during this
period and winds should be generally light to breezy.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  77  54  68  46 /   0  10   0   0
CTB  72  50  63  42 /  10  10  20  10
HLN  77  51  68  42 /   0  10  30  10
BZN  78  45  73  41 /   0   0  20  30
WYS  70  33  66  32 /   0   0  20  30
DLN  75  48  69  41 /   0   0  10  10
HVR  76  54  72  44 /   0  10   0   0
LWT  76  52  72  43 /   0   0  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Fire Weather Watch from Thursday afternoon through Thursday
evening Central and Eastern Lewis and Clark National Forest
Areas...Chouteau and Fergus Counties...Eastern
Glacier/Toole/Central/Eastern Pondera/Liberty...Helena and
Townsend Ranger Districts of the Helena National Forest...Hill
and Blaine Counties...Lewis and Clark National Forest Rocky
Mountain District-Rocky Mountain Front...Lincoln Ranger District
of the Helena National Forest.

High Wind Watch from Thursday morning through Thursday evening
Eastern Glacier...Eastern Pondera...Eastern Teton...Liberty...
Northern Rocky Mountain Front...Southern Rocky Mountain Front...
Toole.

Fire Weather Watch from Thursday afternoon through Thursday
evening Gallatin National Forest.

&&

http://www.weather.gov/greatfalls