665
FXUS65 KTFX 291149
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
549 AM MDT Wed May 29 2024

...Updated Aviation Section...


.SYNOPSIS...

A cold front with scattered showers and thunderstorms will move
east over the area throughout the morning, leaving gusty westerly
winds in its wake. Its associated disturbance will continue
showers and thunderstorms south of a line from Helena to Havre
through early this evening. The main threats from the storms are
strong erratic wind gusts and some heavy downpours. The gusty
westerly winds with a chance of light showers will persist through
the day on Thursday, as temperatures remain slightly below normal.
After a relatively quiet day on Friday, mild and unsettled
are forecast for the weekend into at least early next week.

&&

.DISCUSSION...

Today through Thursday... The main concern today into this evening
will be the passage of a cold front across North Central, Central,
and Southwest Montana and its associated disturbance. Along the
cold front itself, scattered showers are moving across the area
early this morning. Westerly winds will then increase and become
gusty behind the front, and the resulting downsloping will help
limit further activity in the afternoon on the plains adjacent to
the Rocky Mountain Front. Have therefore increased National Blend
of Model (NBM) mean forecast winds to between them and their 90th
percentile to better represent widespread 20 to 30 mph winds with
gusts to 40 mph at times over the plains. Overall, temperatures
will struggle to warm back up to seasonal normals.

However, increased moisture and lift on the leading edge of the
main disturbance will combine with daytime heating to focus
increased shower and thunderstorm activity along and east of a
line from Dillon to White Sulphur Springs to Lewistown. The
enhanced winds aloft also on the leading edge of the disturbance
will increase the chance of strong winds translating down to the
surface, so the Storm Prediction Center (SPC) has put a Marginal
risk (5-15%) that a few severe thunderstorms may occur, mainly due
the strong wind gust potential. Also, ensemble guidance indicates
that the precipitable water over that area will be 1 to 2
standard deviations above normal, so the Weather Prediction Center
(WPC) also has that area under a Marginal risk (5-15%) that
localized minor flash flooding could occur (please see the
HYDROLOGY section for more details).

As the disturbance starts exiting the area overnight,
precipitation will decrease from west to east, but a strong
surface pressure gradient at the surface and continued moderate
westerly winds aloft, breezy westerly surface winds should persist
over the plains. Overall, though, ensemble guidance only gives a
50 percent chance that areas on the Rocky Mountain Front will
exceed 55 mph wind gusts. Also, snow levels will mostly lower to
between 4500 feet and 5500 feet overnight, but the tapering off of
precipitation will limit snow accumulation to 2 to 4 inches above
7500 feet.

On Thursday, the northwest flow aloft on the back side of the
disturbance will bring a chance of additional rain and mountain
snow showers, but any additional amounts will be minor. However, a
persistently strong surface pressure gradient and winds aloft will
cause breezy northwest winds to become gusty again by afternoon,
as temperatures remain below normal.

Friday through next Wednesday... A weak high pressure ridge is
forecast to move into the area on Friday, bringing at least one
day of drier conditions, lighter winds, and a return to near
normal temperatures. Model clusters indicate that a more
progressive westerly flow pattern will develop for the weekend
into the middle of next week, but there is some disagreement on
when the disturbances will move through the area. As a result, the
NBM keeps the area slightly warmer than normal with a chance for
showers and thunderstorms each day, especially over the mountain
areas of Central and Southwest Montana. -Coulston

&&

.AVIATION...

29/12Z TAF Period

VFR conditions will prevail for the KHLN, KGTF, KCTB, and KHVR
terminals for this TAF period. At the KBZN, KEKS, KWYS, and KLWT
terminals thunderstorms are likely (60%) to occur and reduce
visibility to MVFR levels Wednesday afternoon and evening. During
thunderstorms there will be gusty and erratic winds and small hail.
Wednesday afternoon at the KBZN, KEKS, and KLWT terminals ceilings
will be reduced to MVFR levels. At the KBZN, KEKS, KWYS, and KLWT
terminals there will be rain showers Wednesday afternoon and evening
due to a front moving through the area. At the KLWT and KWYS
terminals winds will gust up to 24 kts Wednesday afternoon and
evening. At the KLWT terminal there is a 20% chance for showers and
thunderstorms during the first 6 hours of this TAF period. At the
KHVR, KHLN, and KGTF there will be rain showers Wednesday morning.
At the KGTF and KHVR terminals winds will gust up to 28 kts from the
middle of this TAF period through the end. At the KCTB terminal
during this TAF period through the end winds will gust up to 40 kts.
At the KHLN terminal from the middle of the TAF period through the
end winds will gust up to 29 kts. There will be mountain obscuration
during this TAF period. During this TAF period there will be
mountain wave turbulence. Across North-central Montana at the end of
this TAF period there is the potential for isolated occurrences of
low-level wind shear. -IG

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

&&

.HYDROLOGY...

A disturbance will move through the area today into this evening,
with the focus for potentially heavy downpours from thunderstorms
being over the area along and east of a line from Dillon to White
Sulphur Springs to Lewistown. The main concern is that the rain
will fall on warm mountain snowpack, causing rapid runoff. Most
rivers should handle the extra water, as river levels are not
exceptionally high, but some creeks and streams will run high and
possibly out of their banks in some spots especially out of the
Madison, Gallatin, Little Belt, and Big and Little Snowy mountain
ranges. We are not anticipating widespread issues, but will
monitor the situation closely for possible localized flooding.
-Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  64  40  61  38 /  80  10  20   0
CTB  61  38  59  37 /  70  10  20   0
HLN  64  40  64  40 /  70  10  10   0
BZN  61  34  60  34 / 100  60  10   0
WYS  59  30  53  28 /  90  70  10   0
DLN  61  33  59  33 /  90  20   0   0
HVR  69  41  62  41 /  80  10  50  10
LWT  61  36  56  35 / 100  40  20   0

&&

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

$$

http://www.weather.gov/greatfalls