311
FXUS65 KTFX 191757
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1157 AM MDT Sun May 19 2024

...Updated Aviation Section...


.SYNOPSIS...
Seasonably cool temperatures will be around the next week or so,
with opportunities for precipitation most days. There is still
some uncertainty in amounts, but Wednesday into Thursday looks to
be the most impactful from a precipitation perspective.

&&

.UPDATE...
Updated forecast is out. Main adjustment was to increase pops to
100 this morning in areas where it is raining/snowing over Central
MT. Rainfall amounts around 0.10 to 0.20 are likely, with 1 to 3
inches of snowfall in the mountains. Accumulations on roadways
will generally be less than one half inch, thus just going to
continue to highlight the impact with an SPS. Otherwise, it will
be a cooler day today for most. The trend for much of this week
will be below normal temperatures and periods of rain and
mountain snow. Brusda

&&

.AVIATION...
19/18Z TAF Period

For all but the KBZN and KLWT terminals VFR conditions will prevail
during this TAF period. At the KBZN and KLWT terminals rain is
reducing ceilings to MVFR levels and will continue to do so for the
first few hours of this TAF period. At the KCTB, KGTF, and KHLN
terminals this afternoon and early evening there will be isolated
thunderstorms. At the KBZN, KEKS, KLWT, KHVR, and KWYS terminals
this afternoon and evening there is a slight chance (%20) for a
thunderstorm or two but there wasn`t enough probabilistic support to
include it in those TAFs. For all terminals light rain showers will
continue through this afternoon into the early evening with minimal
impacts to aviation operations. For the KBZN, KEKS, and KHLN there
is a 40% chance of rain/snow Monday morning which could reduce
visibility and ceilings to MVFR levels and so a PROB30 group was
included for those terminals. Tonight there is a 50% chance of
rain/snow reducing visibility and ceilings to MVFR levels at the
KWYS terminal and so a PROB30 group was included for it. For the
KLWT and KGTF terminals there is a 20% chance of rain on Monday
morning but there wasn`t enough probabilistic support to include a
PROB30 group for it. This afternoon and Monday morning there will be
mountain obscuration across Southwestern Montana. This afternoon and
early evening there will be instances of mountain wave turbulence. -
IG

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

&&

.PREV DISCUSSION...
/ISSUED 520 AM MDT Sun May 19 2024/

Through Tuesday... To start the period, a series of upper level
disturbances are in places from the Pacific NW northeastward across
much of southern Canada, forming a congealed mess of a broad upper
level trough. The next disturbance in the parade will shift eastward
from the Pacific NW toward the Northern Rockies this morning,
shifting eastward away from the region late today.

A brief period of frontogenesis in the 700-600 mb layer will
result in a band of precipitation early this morning, generally in
the Helena area east-northeastward toward Lewistown. This band
will slowly shift southeastward through the morning, dissipating
as mid-level support diminishes and shifts eastward late morning.
High resolution ensembles give high confidence in snow as the
precipitation type in terrain. The Kings Hill Pass area has the
best chance for a couple inches of snow with this band. A mix of
rain and snow looks to be a good bet at lower elevations of
Central Montana south toward the I-90 corridor through late
morning. Given warm roadways I am not expecting too many impacts
outside of reduced visibility at lower elevations where snow does
mix in.

Given the cool temperatures aloft working in with this system,
numerous showers and a few thunderstorms look to develop late
morning into the afternoon, especially in areas that see less cloud
cover early in the day. Most showers will diminish by sunset.

The next in the parade of upper level disturbances will largely miss
the region to the south cool Monday into Tuesday. Although this is
the case, cool temperatures aloft stick around into Monday and
Tuesday, allowing additional showers to develop late morning through
early evening across much of the region each day. Overall, not
expecting much impact from these showers. Cool temperatures at the
surface trend a touch warmer Monday, and get close to average for
Tuesday.

Wednesday into next weekend... Ensembles are converging on a
solution where a closed upper low dives southward from northern BC
late Tuesday into the Pacific NW Wednesday, eastward across
Southwest Montana Wednesday night into Thursday, finally exiting
eastward late Thursday night or early Friday. This solution would
bring a round of accumulating precipitation to the region, largely
rain at lower elevations, with high mountain snow. The chance for a
half an inch of total precipitation Wednesday through Thursday night
is largely greater than 50% across much of the plains and across
terrain.

Ensembles get a bit murky on the Friday through Sunday period,
though temperatures near to slightly below normal with chances for
lingering precipitation look reasonable at this point. -AM

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  55  35  59  37 /  80  30  60  20
CTB  55  33  57  35 /  70  30  50  10
HLN  57  37  60  39 / 100  30  60  20
BZN  55  32  56  34 /  60  20  70  40
WYS  52  27  51  28 /  40  30  70  50
DLN  53  31  55  33 /  30  20  60  30
HVR  59  37  60  38 /  60  20  40  10
LWT  52  32  54  33 /  70  30  70  20

&&

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

&&

$$

http://www.weather.gov/greatfalls