384
FXUS65 KTFX 281205
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
600 AM MDT Tue May 28 2024

UPDATED AVIATION SECTION

.SYNOPSIS...

Today there will be showers and thunderstorms primarily
across the eastern portion of North-central, Central, and
Southwestern Montana. Some of these thunderstorms could have severe
wind gusts. On Wednesday during the day the showers and
thunderstorms spread eastward to the eastern portion of Southwestern
and North-central Montana by Wednesday afternoon. On Thursday the
weather remains unsettled with precipitation chances across North-
central Montana and the mountains of Southwestern Montana. Friday
begins a warming trend through the middle of next week with
precipitation chances for the weekend.

&&

.DISCUSSION...

Today... Today there is an upper-level ridge over North-
central, Central, and Southwestern Montana and so temperatures will
be warmer today than yesterday. Today across North-central Montana
there is a 90 - 100% chance for temperatures 80 or greater across
the lower-elevations of Central and North-central Montana and in the
Helena and Gallatin Valleys. From Fort Benton to Great Falls there
is 50 - 85% chance for temperatures of 85 or higher. Late
afternoon/early evening through Wednesday morning a cold front
begins to move east across North-central, Central, and Southwestern
Montana. This combined with southwest flow aloft and the warm
temperatures will cause the atmosphere to become unstable. This will
result in thunderstorms some of which could produce severe wind
gusts. As result the SPC has issued a marginal risk (5%) for severe
wind gusts this afternoon through the evening from Inverness south
to Bozeman and west. These thunderstorms have the potential to occur
beginning between 2 and 4 pm and continuing through rest of the
evening. Due to anomalously high precipitable water values some of
these thunderstorms will produce heavy rain. As a result and due to
mountain snow pack being warm for the extreme Western portion of
Southwestern Montana and just east of the divide the WPC has issued
a Marginal Risk (5 - 15%) for flash flooding through 6 AM Wednesday.
Today there is a 15 - 40% chance for locations across the western
portion of North-central, Central, and Southwestern Montana
receiving a tenth of inch of rain or greater. See the HYRDOLOGY
section below for more information.

Wednesday... On Wednesday the upper-level trough associated with the
cold front begins to move over north-central, Central, and
Southwestern Montana. This will bring cooler temperatures and rain
to most of the area on Wednesday. As the cold front moves eastward
Wednesday morning there will be instability generated across the
eastern portion of Southwestern Montana and the extreme eastern
portion of South-central Montana. This will create the potential for
thunderstorms in those areas Wednesday afternoon and early evening.
These thunderstorms are unlikely to be severe with the main threat
being heavy rainfall. As a result and due to warm mountain snow pack
the WPC has issued a Marginal Risk (5 - 15%) for flash flooding from
6 AM Wednesday to 6 AM Thursday for the eastern portion of
Southwestern Montana and the extreme eastern portion of South-
central Montana. Across the eastern portion of Southwestern Montana
there is 50 - 80% chance for locations to receive a quarter inch of
rain or greater. Across the rest of Southwestern Montana there is 40
- 80% chance for receiving a tenth of an inch of rain or greater.
North of the Highway 200 corridor there is a 15 - 30% chance for
locations receiving a tenth of an inch of rain or greater. South of
the Highway 200 Corridor and east of Great Falls in Central Montana
there is a 40 - 60 % chance for receiving a quarter inch of rain or
greater on Wednesday. See the HYDROLOGY section below for more
details. On Wednesday due to strong flow aloft and a strong surface
pressure gradient there is a 60 - 90% chance for wind gusts
exceeding 55 mph along the Rocky Mountain Front. Along the plains
adjacent to the Rocky Mountian Front there is a 85% chance for wind
gusts exceeding 47 mph on Wednesday. Due to only isolated locations
of the plains adjacent to the Northern Rocky Mountain Front having a
50 - 60% chance for reaching 58 mph wind gusts or 40 mph sustained
winds (High Wind Warning Criteria) no High Wind Products were
issued. This will need to continue to be monitored. Late Wednesday
evening the rain across the Rocky Mountain Front and the mountains
of Southwestern Montana will transition to a rain/snow mix.

Thursday through Friday... On Thursday the upper-level trough
remains in place over North-central, Central, and Southwestern
Montana. This will keep temperatures cool. On Thursday there is a 20
- 40% chance for locations along the central and eastern portion of
the Hi-Line south to Fort Benton receiving a tenth of an inch of
rain (total liquid amount for mountain locations) or more. On
Thursday for the mountains of North-Central, Central and
Southwestern Montana there is 15 - 40% chance for receiving a tenth
of an inch of snow or more. On Friday the upper-level trough slowly
begins to move to the east away from North-central, Central, and
Southwestern Montana. This will allow temperatures to warm up to
near seasonal averages. On Friday it will be dry across the area.

Saturday through next Tuesday... On Saturday a weak upper-level
ridge moves over North-central, Central, and Southwestern Montana.
This will warm up temperatures and keep most of North-central and
Central Montana dry with some precipitation chances for Southwestern
Montana. On Sunday cluster analysis shows that an upper-level ridge
remains over North-central, Central, and Southwestern Montana which
will allow temperatures to warm up on Sunday. On Monday cluster
analysis keeps the upper-level ridge over the area. Two of the
clusters have a very strong ridge over North-central, Central, and
Southwestern Montana. This could cause hot temperatures for the
area. This will need to continue to be monitored. Next Tuesday three
clusters (81% of ensemble members) keep the upper-level ridge over
North-central, Central, and Southwestern Montana. The other cluster
(19% of ensemble members) shows an upper-level trough beginning to
move over Montana. This indicates that the weather pattern has the
potential to change for the middle of next week. -IG


&&

.AVIATION...

28/12Z TAF Period

VFR conditions are expected to continue through at least 29/12Z
across North Central, Central, and Southwest Montana, unless
otherwise mentioned.

The high pressure ridge over the area will gradually move east out
of the area through the period. Mostly clear skies will become
mostly cloudy from west to east after 18Z and southerly winds will
become gusty ahead of an approaching cold front. The flow aloft will
also shift more southwesterly and bring in increased moisture and
instability, along with some mountain wave turbulence. Scattered
showers and thunderstorms are then forecast to spread east of the
Continental Divide after 22Z and over much of the area through 06Z.
Winds behind the frontal passage will shift more westerly after 02Z
along and just east of the Continental Divide, while increasing
southerly winds will cause low level wind shear at KHVR.
Thunderstorm activity should decrease somewhat after 06Z, but
scattered showers will likely linger. -Coulston

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

&&

.HYDROLOGY...

A Pacific weather system will bring a threat of heavy downpours with
associated showers and thunderstorms, first to the Continental
Divide and adjacent areas this afternoon and evening, then to the
remainder of North Central, Central, and Southwest Montana
tonight through tomorrow. The concern is that this precipitation
will fall on warming high mountain snowpack, which could increase
the amount and speed of runoff out of the mountains. While most
rivers should be able to handle this runoff with a low risk of
minor flooding, smaller creeks and streams may run out of their
banks. This situation will continue to be monitored for possible
flood highlights. -Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  84  52  64  42 /  10  40  50  10
CTB  81  48  60  38 /  20  50  20  10
HLN  85  54  64  42 /  20  50  60  20
BZN  82  47  64  35 /  10  40  90  50
WYS  72  42  60  29 /   0  40  90  50
DLN  80  47  64  33 /  20  50  80  20
HVR  84  54  69  42 /   0  30  50  10
LWT  78  50  63  37 /   0  20  90  40

&&

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

$$

http://www.weather.gov/greatfalls