861
FXUS65 KTFX 180559
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1158 PM MDT Tue Sep 17 2024

Updated Aviation Discussion

.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.

&&

.UPDATE...

Main update to the overnight forecast period was to better time
the exit of that first band of showers and storms to the north
and the entrance of more widespread precipitation with a lingering
chance of thunderstorms from the southeast. Much of Central and
Southwest Montana are in a lull between the areas of precipitation
as of mid-evening, but the northeast to north flow around the low
pressure system will cause the precipitation to become more
widespread from the southeast and east overnight. Overall, the
current Flash Flood / Flood Watches seem to handle the situation
well, as does the High Wind Warning, so will leave them as is for
now. -Coulston

&&

.AVIATION...
18/06Z TAF Period

Areas of rain and a few emebedded thunderstorms will increase in
coverage overnight as an fairly potent storm system lifts north into
eastern MT. MVFR/IFR conditions will become widespread as
precipitation coverage increases after 08z with periods of LIFR
possible in areas of moderate to heavy precipitation intensity. Low
ceilings and periods of lower visibility will continue through much
of the day Wednesday across N-central MT with more widespread
improvement slowly developing from SW to NE across the area late
Wednesday afternoon and evening. Gusty west to northwest surface
winds also develop later tonight and increase through Wednesday
morning with gusts in excess of 35 kts possible across primarily
north-central MT terminals. In addition to the overall poor flight
conditions from precipitation and low ceilings, areas of mountain
wave turbulence and aircraft icing concerns will develop later
tonight through Wednesday before the system exits. Hoenisch

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

&&

.PREV DISCUSSION...
/ISSUED 625 PM MDT Tue Sep 17 2024/

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  49  62  48  73 / 100  90  60   0
CTB  51  65  45  71 /  80  50  30   0
HLN  50  64  48  74 / 100  90  40   0
BZN  46  61  43  71 / 100 100  50  10
WYS  34  52  33  62 /  90  80  50  20
DLN  44  59  40  67 /  70  70  20  10
HVR  54  66  49  73 / 100 100  90  10
LWT  49  56  46  68 / 100 100 100  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch 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