818
FXUS65 KTFX 180026
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
625 PM MDT Tue Sep 17 2024

UPDATED AVIATION SECTION

.SYNOPSIS...

A weather system will bring widespread rainfall to North Central
and Southwestern Montana over the next 48 hours, with some areas
seeing in excess of 2 inches, mainly in an area bounded by Great
Falls, Lewistown, and Havre. Rain will let up late Wednesday
afternoon and evening, with mostly clear skies expected on
Thursday.

&&

.DISCUSSION...

Key Points:

-Scattered severe thunderstorms this afternoon/evening in North
 Central Montana, particularly east of a line from Havre to Great
 Falls to Lewistown

-Significant (once in 10-20yr) rainfall amounts for September
 likely across portions of N-central MT tonight through Wednesday

-In addition, gusty winds are expected Wednesday, especially in
 Judith Basin and Fergus Counties

Short term (through Thursday)... A notable weather system will
continue to push into the area this afternoon and evening, which
will bring a fairly widespread soaking rain to the area over the
next 48 hours. For the rest of the day today, rain showers will
continue to gradually work their way further north from their
current location in Southwestern Montana, Wyoming, and Idaho, and
they will strengthen as they do so. Some of these showers may
eventually grow into strong to severe thunderstorms, particularly
east of a line from Havre to Great Falls to Lewistown, where a few
instances of gusty winds and large hail will be possible later
this evening and into the overnight hours. Elsewhere, expect rain
showers and a few thunderstorms through the remainder of the
afternoon, evening, and overnight hours, with some localized heavy
rainfall possible.

Wednesday will see rain continue and become mostly a widespread
stratiform/steady rain, with a few areas of embedded moderate to
heavy rainfall. Within this area of widespread rainfall, models
have been fairly insistent of a band of moderate to heavy rainfall
setting up across the plains of North Central Montana, and
ensembles show some areas seeing at least a 50% chance of seeing
2 of rain, mainly in the area from Havre to Great Falls, then
north of the Little Belt and Snowy Mountains to Lewistown in
association with this persistent band of more intense rainfall.
While soils are fairly dry given that we are not anticipating
widespread river flooding, but a Flood Watch has been issued for
the potential for flooding of low lying areas. In addition to the
rainfall, well have to watch for a brief period of gusty winds as
drier air works in behind this system Wednesday afternoon and
evening, particularly in Judith Basin and Fergus Counties, where
ensembles show at least a 60% chance of seeing wind gusts over 60
mph, which is backed up by a strong low level jet on the back side
of this system that will likely mix to the surface as the rain
comes to an end tomorrow evening. Behind this system, a surface
high pressure system will push in from the Southwest, allowing for
a dry day on Thursday. Ludwig

Extended (Friday through next Tuesday)... A more
progressive west to northwest flow aloft pattern is generally
supported by most medium range model ensembles, however
considerable differences exist in the location and timing of
troughing within this pattern, leading to lower than average
confidence in forecast details for this weekend into early next
week. Hoenisch


&&

.AVIATION...
18/00Z TAF Period

Widespread showers with mainly VFR conditions over Southwest (KWYS,
KEKS, KBZN) and Central (KHLN, KGTF, KHLN) Montana will continue to
spread north into North Central (KHVR, KCTB) Montana through 06Z
ahead of a low pressure system. However, scattered embedded
thunderstorms moving into North Central Montana may produce periods
of mountain-obscuring MVFR conditions in heavier rain, along with
erratic wind gusts. After 06Z, precipitation will remain widespread,
but it will become more stratiform as conditions lower to mountain-
obscuring MVFR/IFR levels with some high mountain snow. The low
pressure system will move east of the area, and winds will become
quite gusty out of the west to northwest across the area, especially
over the plains east of a KCTB-KGTF line. This overall increase in
wind will cause mountain wave turbulence and low level wind shear.
Conditions will very gradually improve after 18Z west of that line
and into Southwest Montana, but conditions will remain quite low east
of it. -Coulston

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 Wednesday with initial concerns for brief
higher intensity precipitation with thunderstorm activity later
this afternoon and evening. While the potential for intense
precipitation rates decreases tonight, steadier moderate
precipitation will maintain some threat for flash flooding and
debris flows in the burn scar area.

Local minor flooding concerns may also arise on Wednesday with
persistent moderate rainfall across portions of north-central MT.
This is particularly true where heaviest amounts are expected near
the central MT mountain ranges, such as the area around the
Little Belt, Snowy, and Bears Paw Mountains, where some low lying
areas and locations with poor drainage may begin to see standing
water buildup. With this in mind, I have issued a Flood Watch for
these areas. Hoenisch/Ludwig

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  52  62  48  73 / 100 100  40  10
CTB  51  65  45  71 /  60  30  10   0
HLN  51  64  48  74 / 100 100  40  10
BZN  46  61  43  71 / 100 100  50  10
WYS  34  52  33  62 / 100  80  60  20
DLN  44  59  40  67 /  50  50  20  10
HVR  56  66  49  73 / 100 100 100  10
LWT  49  56  46  68 / 100 100 100  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 6 PM MDT this evening through late Wednesday
night 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 from 9 AM Wednesday to midnight MDT Wednesday
night for Fergus County below 4500ft-Judith Basin County and
Judith Gap-Snowy and Judith Mountains.

Flash Flood Watch through Wednesday afternoon for Big Belt,
Bridger and Castle Mountains.

&&

$$

http://www.weather.gov/greatfalls