435
FXUS65 KTFX 251127
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
528 AM MDT Wed Sep 25 2024

...AVIATION SECTION UPDATED...

.SYNOPSIS...

It will be a dry and very warm day today across North Central and
Southwest Montana, as southerly winds increase ahead of an
approaching Pacific cold front. The frontal passage tonight will
bring some precipitation to the mountains of the Rocky Mountain
Front, but the main impact will be very windy conditions over the
plains overnight into Thursday afternoon. Temperatures will cool
up to 15 degrees on Thursday, but they will warm again for Friday,
with another chance for windy conditions.

&&

.DISCUSSION...

Key Points:

- Very warm and dry conditions today with increasing southerly
  winds

- Strong Pacific cold front on track to bring strong westerly
  winds tonight into Thursday, especially on the Rocky Mountain
  Front and adjacent plains

- Gusty winds possible again on Friday

- Warm temperatures through the weekend with brief cooling on
  Monday


Today through Thursday... High pressure aloft over the Rocky
Mountain region will start to shift east out onto the Great Plains
today due to an approaching low pressure trough. The resulting
dry southerly flow aloft over Southwest and North Central Montana
will warm temperatures up to 20 degrees above normal today with
increasing southerly winds ahead of an approaching Pacific cold
front. The front is forecast to move east across the area
overnight, bringing a round of light precipitation to the
mountains of the Rocky Mountain Front.

However, the larger impact from the frontal passage will be
winds, as southerly winds shift more westerly and increase ahead
of the weakening low pressure trough and behind the frontal
passage. Cold air advection behind the front and likely mountain
wave activity could bring strong wind gusts and periods of blowing
dust to the plains at times; the two favored periods for strong
winds appear to be between 2 am and 4 am tonight after the initial
frontal passage, and then again between 7 am and 11 am Thursday
morning with another surge of strong mid- level winds. The area
with the greatest potential for exceeding High Wind criteria
(sustained winds of 40+ mph and/or gusts of 58+ mph) will be along
the Rocky Mountain Front and its adjacent plains, then east along
the Hi- Line to around Liberty County. A favorable set-up will
develop there, as a strong ridge-top inversion develops with winds
up to 50 kt being forced down to the surface underneath the
inversion. Although model blend guidance only has a 40 to 50
percent probability of gusts exceeding 55 mph, am thinking the
frontal passage, cold air advection, and expected mountain wave
activity increase the potential for brief periods of strong winds.
Have therefore upgraded the High Wind Watch for that area to a
Warning. The exit of the weakened shortwave trough Thursday
afternoon should help decrease winds. Temperatures on Thursday
should also cool 10 to 15 degrees over today.

Overall, these dry and windy conditions will elevate fire weather
concerns for the plains, but the Hot-Dry-Windy Index mostly stays
below the 95th percentile, which is a threshold for critical fire
weather conditions. Will therefore hold off on any fire weather
highlights for now.

Friday... A moderate westerly flow aloft will persist over the
forecast area through Friday, and the breezy downslope winds will
help warm temperatures back up another 5 to 10 degrees over
Thursday. A ridge-top inversion with strong mid-level winds is
forecast to develop once again on the Rocky Mountain, similar to
tonight into Thursday, which may bring another round of strong
winds there. However, there is a lack of a cold frontal passage
with cold air advection, so confidence in another High Wind
situation is low at this time, but the situation will monitored.

Saturday through next Wednesday... The ensemble model clusters are
in good agreement with rebuilding a weak high pressure ridge back
into the forecast area on Saturday, which should help keep the
area dry and warmer than normal, but with lighter winds. Some
disagreement develops for Sunday into Monday, as 20 percent of the
clusters indicate that a weak shortwave trough will move through
the area, while the remainder of the clusters bring through little
ripple that may only bring a period of westerly winds. As a
result, the NBM is only bringing a 20 percent chance of
precipitation to the plains of North Central Montana Sunday night
into Monday with a brief period of below normal temperatures. The
majority of the clusters then rebuild the high pressure ridge
Tuesday into Wednesday, likely keeping the area dry with above
normal temperatures to begin the month of October. -Coulston

&&

.AVIATION...
25/12Z TAF Period

VFR conditions prevail with south to southwesterly surface winds
increasing at most terminals by this afternoon. Increased flow over
the Rockies may also lead to some mountain wave turbulence before a
Pacific cold frontal passage brings periods of strong, gusty winds
and a few isolated convective showers tonight into Thursday.
Mountain wave turbulence and instances of low level wind shear
increase after 26/00Z. - RCG

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  89  53  73  49 /   0   0   0   0
CTB  85  51  66  48 /   0  20   0   0
HLN  90  53  77  49 /   0   0   0   0
BZN  88  49  80  44 /   0   0   0   0
WYS  75  38  71  33 /   0   0   0   0
DLN  83  48  76  43 /   0   0   0   0
HVR  90  54  73  47 /   0  10  10   0
LWT  86  51  72  46 /   0   0   0   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from 9 PM this evening to 3 PM MDT Thursday
for East Glacier Park Region-Eastern Glacier, Western Toole, and
Central Pondera-Eastern Toole and Liberty-Northern High Plains-
Southern High Plains-Southern Rocky Mountain Front.

&&

$$

http://www.weather.gov/greatfalls