Lewistown, Montana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

430
FXUS65 KTFX 160653
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1253 AM MDT Sat May 16 2026

.KEY MESSAGES...

 - Locally breezy conditions develop Saturday ahead of a colder
   Spring system moving in late Saturday into Sunday.

 - Mountain snow, with a mix of lower elevation rain and snow
   Saturday night into Sunday.

&&

.DISCUSSION...

 - Meteorological Overview:

Longwave troughing over much of the western CONUS will bring well
below normal temperatures and widespread precipitation to the
Northern Rockies through the weekend, with below normal temperatures
lingering into the first half of the upcoming work week along with
scattered showers. High temperatures in the 50s today will fall into
the upper 30s to mid-40s by Sunday thanks to a strong cold front
pushing south from Canada tonight through Sunday morning. This cold
front combined with increasing synoptic lift will help to support a
shield of precipitation (mountain snow and lower elevation
rain/snow) developing from late tonight through the day on Sunday.
Prior to this shield of precipitation a line of showers and isolated
thunderstorms is expected to develop by the early morning hours
today, with these showers and storms slowly sliding east through the
afternoon and early evening hours, especially for areas along and
south of the Montana Hwy 200 corridor. By the late afternoon/early
evening hours tonight snow levels will begin to fall across
Southwest and Central Montana, which will allow precipitation to
change over to snow for elevations above 5000ft. Snow levels will
continue to fall into the day on Sunday thanks to the aforementioned
cold front pushing south; however, the effects of a high May sun
angle is likely to limit snowfall accumulation across lower
elevations, especially between the hours of 10 AM and 8 PM Sunday.
Precipitation will gradually decrease in areal coverage and
intensity from west to east Sunday night, but daily chances for
showers will linger through Tuesday. Several nights, mainly Sunday
night and Monday night, are likely to see low temperatures fall into
the upper teens to 20s across all elevations. - Moldan

 - Forecast Confidence & Scenarios:

Precipitation (rain or snow) through Sunday night...

Latest ECMWF EFIs support the potential for an unusual precipitation
event (some snow mixing in at times across lower elevation) with
respect to climatology along the northern slopes of the
Gallatin/Madison Mountains and Gallatin Valley, portions of the
Montana Hwy 200/US Hwy 87 corridor from Great Falls to Lewistown,
northern slopes of the Bears Paw Mountains, and eastern slopes of
the Rocky Mountain Front. Below is the NBM5.0 probabilities for
rainfall accumulations from 6am Saturday through 6am Monday for
select cities from North Central through Southwest Montana.

                         48 Hour Rainfall Probabilities
LOCATION               0.10"  |  0.25"  |  0.50 "  |  1.00"
Browning                85%   |   65%   |   15%    |   <5%
Cut Bank                75%   |   40%   |   10%    |   <5%
Havre                   90%   |   85%   |   60%    |   25%
Great Falls             90%   |   80%   |   55%    |   20%
Lewistown              100%   |   95%   |   95%    |   55%
Helena                  90%   |   60%   |   25%    |   10%
Bozeman                 95%   |   90%   |   40%    |   10%
Dillon                  55%   |   35%   |   15%    |   <5%
Ennis                   95%   |   95%   |   75%    |   20%
West Yellowstone        50%   |   15%   |    5%    |    0%


Accumulating Snow from Tonight through Sunday evening...

Latest ECMWF EFIs support the potential for an unusual snowfall
event with respect to climatology for the northern slopes of the
Little Belt Mountains and eastern slopes of the Rocky Mountain Front
and immediate eastern plains out to the US Hwy 89 corridor from
Choteau to the Piegan Port of Entry. Here EFIs range from 0.5 to
0.75, with Shift of Tails (SOTs) of 1. Further east over the plains
near the northwestern slopes of the Bears Paw Mountains including
the Havre Area the SOTs climb to around 2.5, which indicates that at
least several ensemble members are suggesting the potential for an
"extreme" event with respect to climatology (May climatology vs
anytime during the winter). A closer look at the Havre Area reveals
that 5 ECMWF ensemble members do support snowfall accumulations of 2
inches or more here, with the most extreme member suggesting upwards
of 6 inches through the timeframe. While accumulating snow can`t be
ruled out for the Havre Area, especially the mid-slopes of the Bears
Paw Mountains, the overall timing of the snowfall (during most of
the day on Sunday) does not seem favorable for snowfall
accumulations on road surfaces to impact travel. For this reason a
Winter Weather Advisory was not issued. Further west and south
across the previously mentioned Rocky Mountain Front and Little Belt
Mountain Areas a Winter Weather Advisory has been issued for the
mountains as confidence is high enough that snowfall accumulations
will amount to enough to impact travel and outdoor recreation,
especially through Sunday morning. Over the adjacent plains out to
the US Hwy 89 corridor confidence was not high enough to issue an
Advisory at this time, but future shifts may consider to do so
should models trend higher with snowfall amounts and colder with
temperatures.

Below is the NBM5.0 probabilities for snowfall accumulations from
12am Saturday through 12am Sunday for select mountain passes across
North Central through Southwest Montana.

                        24 Hour Snowfall Probabilities
LOCATION                  2"   |   4"  |   6"   |   8"
Marias Pass              60%   |  15%  |   5%   |  <5%
Rogers Pass              80%   |  55%  |  25%   |  10%
Kings Hill Pass          75%   |  35%  |  15%   |  10%
Lewistown Divide         30%   |  15%  |   5%   |  <5%
MacDonald Pass           40%   |  15%  |  10%   |   5%
Deep Creek Pass          10%   |  <5%  |  <5%   |  <5%
Boulder Hill             25%   |   5%  |  <5%   |  <5%
Elk Park Pass            10%   |  <5%  |  <5%   |  <5%
Homestake Pass           10%   |  <5%  |  <5%   |  <5%
Bozeman Pass             15%   |   5%  |  <5%   |  <5%
Chief Joseph Pass        30%   |  10%  |   5%   |  <5%
Big Hole Pass            10%   |   5%  |  <5%   |  <5%
Monida Pass              <5%   |   0%  |   0%   |   0%
Raynolds Pass            <5%   |  <5%  |   0%   |   0%
Targhee Pass             <5%   |   0%  |   0%   |   0%

- Moldan

&&

.AVIATION...
16/06Z TAF Period

VFR conditions will prevail at most terminals during this TAF
period. The exceptions are KBZN and KLWT, where MVFR conditions
are possible with the onset of precipitation. Light rain showers
are expected to begin around 19Z 5/16 for KBZN and KEKS, with a
change to a rain/snow mix around 02Z 5/17. For KHLN, these light
rain showers are expected to begin around 10Z 5/16, while for
KGTF and KLWT they are expected to begin closer to 14Z 5/16. KWYS
will begin to see a rain/snow mix around 01Z 5/17. At this time
it is not anticipated that KCTB or KHVR will receive precipitation
during this TAF period. -Dzomba

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  55  33  40  28 /  40  40  90  70
CTB  55  30  38  25 /  10  60  80  10
HLN  50  33  42  31 /  80  40  80  50
BZN  58  29  45  28 /  80  90  30  30
WYS  52  25  35  20 /  30  60  30  20
DLN  57  26  44  22 /  30  40  50  30
HVR  59  35  43  29 /  30  50  80  70
LWT  53  31  39  27 /  80  90  80  90

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from midnight tonight to midnight MDT
Sunday night for Big Belt, Bridger and Castle Mountains-East
Glacier Park Region-Elkhorn and Boulder Mountains-Little Belt
and Highwood Mountains-Southern Rocky Mountain Front-Upper
Blackfoot and MacDonald Pass.

Winter Weather Advisory from 5 PM this afternoon to 6 AM MDT
Sunday for Gallatin and Madison County Mountains and Centennial
Mountains.

&&

$$
http://www.weather.gov/greatfalls