601
FXUS65 KTFX 271718
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1117 AM MDT Mon May 27 2024

Updated Aviation Discussion

.SYNOPSIS...

A high pressure ridge will help warm temperatures to between 10
and 15 degrees above normal by Tuesday across North Central,
Central, and Southwest Montana, while keeping the area mostly
dry. However, a Pacific weather system with an associated cold
front will bring scattered showers and thunderstorms to the area
beginning Tuesday afternoon through Wednesday. Some storms could
become strong with erratic wind gusts and heavy downpours.
Southerly winds ahead of the front will also shift more westerly
on Wednesday, as temperatures cool back to near normal. Cool and
unsettled conditions will then continue through the work week.

&&

.UPDATE...

A nice day ahead with no major changes to the going forecast.
Thunderstorm probability for tomorrow still looks decent but will
be looked at more in depth with the afternoon forecast package
later today.

&&

.AVIATION...

27/18Z TAF Period

Upper level ridge of high pressure shifts east across the Northern
Rockies through Tuesday morning, maintaining a dry west to northwest
flow aloft and just some scattered clouds with VFR conditions
prevailing at all terminals. Surface winds remain light across
southwest MT with west winds prevailing through this afternoon
across north-central MT before winds become light this evening and
turn more southerly by Tuesday morning. Hoenisch

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 Tuesday afternoon and
evening, then to the remainder of North Central, Central, and
Southwest Montana Tuesday night through Wednesday. 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.

&&

.PREV DISCUSSION...
/ISSUED 523 AM MDT Mon May 27 2024/

Today through Thursday... A high pressure ridge will continue to
strengthen over the Northern Rockies through Tuesday, which will
keep North Central, Central, and Southwest Montana mostly dry and
warm to end the Memorial Day holiday weekend. This warming will
be aided by a shift in the breezy winds to a more southerly
direction ahead of an approaching shortwave trough from the
Pacific Ocean and its associated cold front. Many lower elevation
locations have an 80 percent or greater probability of reaching 80
degrees for the first time this year on Tuesday.

However, the increasingly moist and unstable southwest flow aloft
ahead of the system will get some added lift as it becomes more
difluent, which should generate some added lift and cause
scattered showers and thunderstorms to form west of the
Continental Divide by late afternoon. These storms are forecast to
move east of the Divide into the evening hours. The warm and dry
air at the surface and increased wind shear due to the southerly
surface winds will make strong erratic wind gusts the initial
threat from these storms, so the Storm Prediction Center (SPC) has
this area under a Marginal Risk (5 to 15% chance) for severe
thunderstorms, primarily due to wind. However, the slightly higher
than normal precipitable water with them will transition the
threat more so to heavy downpours, possibly with some small hail,
during the late evening and overnight.

As the cold front moves east across the area Tuesday night into
Wednesday, so will the focus for additional showers and
thunderstorms, as gusty winds shift more westerly behind the
front. The increased moisture and cooler temperatures will be
firmly in place by Wednesday, continuing the main threat for heavy
downpours with the storms. This could cause increased runoff out
of the mountains of Southwest and Central Montana, so the Weather
Prediction Center (WPC) has that area under a Marginal Risk (5 to
15% chance) for flash flooding (for more details on this
potential, please see the HYDROLOGY section).

As the cold front exits the area Wednesday night into Thursday,
the main portion of the shortwave trough will move into the area.
This will further cool temperatures below normal, but keep a
chance of showers and isolated thunderstorms over the area. In
fact, snow levels are forecast to lower down as low as 5000 feet
in some areas, which may result in snow accumulations of between
1 and 3 inches mainly to areas above 7000 feet.

Friday through next Monday... Model ensemble clusters seem to
agree with keeping a broad upper level trough over the area on
Friday, likely continuing the near to slightly below normal
temperatures with a chance of showers and thunderstorms. Slightly
more than half of the clusters (56%) then start to bring weak high
pressure ridging in for Saturday, while the other 44% want to keep
the weak trough in place. There is then better agreement in
building in the weak ridge for Sunday into Monday, but 25% of the
clusters forecast the ridge to start shifting east with the
approach of another trough. The National Blend of Models (NBM)
seems to resolve this uncertainty well by warming temperatures
through the weekend into early next week, but bring in a slight
chance of showers and thunderstorms Sunday into Monday. -Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  71  45  82  53 /   0   0  10  40
CTB  69  42  79  49 /   0   0  10  50
HLN  75  48  82  54 /   0   0  20  50
BZN  71  41  80  48 /   0   0  10  30
WYS  63  33  71  41 /   0   0   0  30
DLN  71  42  78  47 /   0   0  10  40
HVR  73  44  82  54 /   0   0   0  30
LWT  66  42  77  52 /   0   0   0  20

&&

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

$$

http://www.weather.gov/greatfalls