034
FXUS65 KTFX 181753
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1155 AM MDT Wed Sep 18 2024

.SYNOPSIS...

Widespread rain continues through Thursday morning across Central
and North Central MT. Drier conditions settle in for the weekend
and beginning of next week, with a few disturbances moving through
causing low chances for precipitation throughout the forecast
period.

&&

.UPDATE...

The flood advisory for Teton County has been cancelled due to
heavy rain having ended and there is no longer a risk for
flooding. For other Hydrology impacts see the HYDROLOGY section
below. Rain will continue along and east of the I-15 corridor in
North-central and Central Montana through the day today. The
heaviest rainfall is currently along the Cascade and and Judith
Basin County lines. This will shift eastward this morning. Today
it will also be windy across the plains of North-central and
Central Montana.

For the update winds and wind gust were increased. Hourly
temperature and sky grids were adjusted. Pops this afternoon and
evening in Blaine and Fergus Counties was adjusted. QPF this
morning west of the I-15 corridor was decreased. The rest of the
forecast is on track. -IG

&&

.AVIATION...
18/18Z TAF Period

Widespread rain will continue through the remainder of the day on
Wednesday as a notable weather system continues to affect North
Central and Southwestern Montana. Currently, a band of moderate to
heavy rain was along a line extending north of KGTF and then east to
KLWT, and this band is not expected to move much through 0z or so.
Expect IFR to LIFR conditions in and near this band, particularly at
KLWT. Outside of this band, expect gusty winds but generally better
cigs and vis, with occasional -RA brining cigs below 025 and vis
under 6sm. Rain will taper off overnight, though some pockets of fog
may develop at various airports, though confidence is not high
enough to mention in the TAFs quite yet. Ludwig

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

&&

.HYDROLOGY...

A Flash Flood Watch remains in effect for the Horse Gulch fire
burn scar area through the afternoon. Saturated soils from rain
yesterday with the concerns for periods of steady moderate
precipitation today will maintain a risk for flash flooding and
debris flow in the burn scar area.

Local minor flooding concerns remain across eastern portions of
North-Central MT and Central MT with steady moderate rainfall.
This is especially true in the Central MT mountain ranges where
the highest rainfall totals are forecasted. Current flash flood
guidance is 1-2.5"(higher amounts in the mountains) over 3 hours,
with some areas as low as 1" over 1 hour in the plains. The main
concern will be moderate steady rain can cause concern for
excessive runoff in low lying areas and poor drainage. Some
standing water built up is a concern as a result, so the Flood
Watch remains in effect through the evening. -Wilson

&&

.PREV DISCUSSION...
/ISSUED 610 AM MDT Wed Sep 18 2024/

Current radar shows areas of widespread rain with a few embedded
thunderstorms being wrapped around the low station in Eastern MT.
This low hangs around the region through out the day, keeping the
TROWEL region centered along and east of the Great Falls to
Chester line. This will keep the steady rain with pockets of
moderate to heavy rainfall through Wednesday night. Snow levels
look to continue to stay above 8,500ft this morning, keeping snow
accumulations to the high mountain peaks. Convective showers that
occurred yesterday evening along with additional lighter rainfall
through the night has posed a concern for minor flooding/road
ponding in flood prone areas in portions of eastern/central Teton
County. Therefore a flash flood advisory was issued through 9 a.m
this morning.

Models have continued to uptrend in additional rainfall amounts
through Thursday morning. The HREF model guidance along with
other deterministic models suggests there`s a 50-80% for an
additional 2" of rain across the plains east of a Great Falls to
Hingham line. The main concern today will be areas of minor
flooding in low lying areas/poor drainage and road ponding with
moderate to heavy rain bands. Some moderate rain bands through the
night have produced local areas of 1-2" of rain over the span of 3
hours across the North-Central MT plains. Additional flood
advisories may need to be monitored throughout the day as a result
of these moderate rain bands continuing throughout the day. See
the HYDROLOGY section for more details.

Another area of concern is the Central/North Central mountain
areas and lower elevations across Judith Basin and Fergus
Counties in receiving additional moderate to heavy rainfall
today. The HREF and deterministic model guidance suggests that
some areas may receive and additional 3-4" of rain. Isolated
higher amounts are possible, with model guidance suggesting
higher end amounts of up to 6" (20-40% possible) across the Bear`s
Paw, Juidth`s/Snowy`s, and the Little Belts/Highwoods. The Flood
Watch for those regions remain in effect through Wednesday
evening. See the HYDROLOGY section for more details.

Besides the concern for flooding, a strong mid/low level jet sets
up along the backside of the low pressure. This will cause gusty
northwest/west winds across the North-Central/Central MT plains
today. The High Wind Warning in effect for Judith Basin and Fergus
counties remain in effect through Wednesday night, where there`s a
70% and greater of 55 mph wind gusts. Other areas across North-
Central MT have less than a 40% of 55mph wind gusts, but still
cannot rule out the possibility of an isolated 55mph gust.

The low pressure system pushes off to the east Wednesday night,
and rain begins to wind down, before fully exiting Thursday
morning. Northwest flow aloft moves in behind. A weak upper level
trough moves into the region over the weekend, producing low end
chances for precipitation. Ensemble clusters suggests some upper
level ridging moves in beginning of next week, keeping the
forecast drier. However, some guidance suggests a few disturbances
bring low end precipitation chances to the area. -Wilson

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  64  47  73  47 /  90  50   0   0
CTB  67  45  71  45 /  40  10   0   0
HLN  65  47  74  47 /  80  20   0   0
BZN  61  42  71  42 / 100  40   0   0
WYS  51  32  61  31 /  90  40  10   0
DLN  60  39  68  40 /  60  10   0   0
HVR  65  49  73  48 / 100  80   0   0
LWT  56  45  68  45 / 100  80   0   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late tonight for Bears Paw Mountains and
Southern Blaine-Fergus County below 4500ft-Judith Basin County
and Judith Gap-Little Belt and Highwood Mountains-Snowy and
Judith Mountains.

High Wind Warning until midnight MDT tonight for Fergus County
below 4500ft-Judith Basin County and Judith Gap-Snowy and Judith
Mountains.

Flash Flood Watch until 6 PM MDT this evening for Big Belt,
Bridger and Castle Mountains.

&&

$$

http://www.weather.gov/greatfalls