639
FXUS65 KTFX 300610
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1210 AM MDT Tue Apr 30 2024

.Aviation Section Updated...

.SYNOPSIS...

A slow moving Pacific system will bring below average
temperatures, periods of rain and snow, and even a few
thunderstorms through mid- week. Breezy to windy conditions will
accompany the unsettled conditions. Temperatures moderate some
heading towards the weekend, but the typically active spring
weather pattern will persist for the foreseeable future.

&&

.Update...

Evening update has been published, with the primary adjustments
being to raise overnight temperatures across much of Central and
North Central Montana and (raise) PoPs along and east of a
Bozeman, to Great Falls, to Havre line. Showers, with embedded
thunderstorms, continue to lift east and northeast across portions
of Southwest through North Central Montana this evening,
predominately east of the I-15 corridor. Rain has switched over
to snow across some valleys of Southwest Montana this evening as
intense precipitation rates have cooled the lower levels
efficiently enough to allow for a changeover to all snow. Snow
accumulations across most lower elevations of Southwest Montana
will generally remain below 2 inches where precipitation changes
over to all snow, with 2-6" being possible in the
mountains. Further north over the plains of Central and North
Central Montana, precipitation will predominately remain as all
rain; however, a little wet snow may mix in near the end of the
precip shield exiting the area near Tuesday morning. With
widespread cloud cover and continued breezy and gusty southwest to
west surface winds expected through the overnight hours tonight,
I have raised low temperatures by approximately 2-4 degrees. While
temperatures aloft will be cooling throughout the night in wake
of the Pacific cold front, expect the aforementioned winds and
cloud cover to inhibit radiational cooling and keep temperatures a
little "warmer". - Moldan

&&

.AVIATION...
1210 AM MDT Tue Apr 30 2024 (30/06Z TAF Period)

North-central Montana (KCTB, KHVR): KCTB will stay dry through the
first 6 hours of the TAF period while rain showers linger around the
vicinity of KHVR as moisture continues to push to the east. After a
brief lull in activity, another disturbance moving through this
afternoon will bring more precipitation across the hi-line after
30/18Z. A rain/snow mix is possible for KCTB while further east at
KHVR temperatures will be warm enough for rain through the entire
TAF period. Gusty winds up to 40kts for KCTB and 30kts for KHVR will
be possible through most of the TAF period. Rain and snow combined
with gusty winds may work to bring temporary periods of MVFR/IFR
conditions to both north-central Montana terminals.

Central Montana (KGTF, KHLN, KLWT): Periods of MVFR/IFR will
continue at KLWT through 30/09Z before the rain moves out of the
area to the east. Otherwise, VFR are expected to prevail at all
three central Montana sites through the TAF period. There is a
chance for mixed precipitation at KHLN between 30/17Z and 30/21Z but
otherwise all three terminals are expected to remain dry through the
TAF period with gusty winds up to 32kts possible during the daytime
hours.

Southwest Montana (KBZN, KEKS): Snow showers will slowly taper off
by 30/12Z across southwestern Montana with a roughly 6 hour lull
before another round of precipitation returns to the area and
remains through the rest of the TAF period. During the day,
predominately rain is expected with winds gusting up to 26kts. The
transition to snow is expected to start after 30/22Z, becoming
predominately snow after 01/03Z. Periods of MVFR and mountain
obscuration are possible across the region after 30/18Z.

-thor

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

&&

.PREV DISCUSSION...
/ISSUED 645 PM MDT Mon Apr 29 2024/

Shower and isolated thunderstorm activity associated with an
approaching Pacific trough continues to increase from the west
/southwest. Although current mesoanalysis data only show around
100 to 250 J/kg of CAPE, cooler air aloft is keeping mid-level
lapse rate around the -8C/km mark. Some of the stronger cells may
contain gusty winds over 40 mph, brief downpours, and a few
lightning strikes. This is mostly likely to occur for
central/southwest areas along and south of a Helena to Lewistown
line.

A period of wet snow is also expected this evening, mostly over
and near the central ranges and the Madison/Gallatin ranges.
Although some of these areas may pick up a quick 1 to 3 inches of
snow on grassy surfaces, no winter weather highlights are being
considered at this time given the short period of occurrence and
the warmer antecedent ground conditions; however, the situation
will continue to be monitored and the appropriate actions will be
taken should things take an unexpected turn. Precipitation
diminishes and ends in a west to east fashion later this evening
into tonight.

The aforementioned broad Pacific trough will slowly traverse the
region Tuesday through early Thursday, bringing periods of
rain/snow, gusty winds, and well below average temperatures.
Guidance has been fairly consistent with the positioning of the
system, with one closed low moving along the Canadian border and a
secondary circulation splitting off farther south along the MT/ID
border. However, timing and borderline temperatures have added
forecast challenges in regards to overall snow impacts.

H700 temperatures in the -10 to -15C degree range are definitely
supportive to dendritic snow growth, but the latest trends are
delaying the optimal northerly flow aloft to the
Wednesday/Wednesday night timeframe for some areas. Given that the
we are at the end of April, the higher sun angle increases loss
of snow from insolation and lowers roadway impacts, even in the
mountains. This leaves a brief opportunity for higher impacts
Wednesday morning and then again Wednesday night. The inherited
Winter Storm Watches do highlight the locations where the highest
impacts are expected and snow exceedance probabilities are running
above 50% for 6 inches of snow or more Tuesday through Thursday.
The East Glacier zone was upgraded to a Winter Storm Warning and
Winter Weather advisories were posted for the adjacent Northern
Plains and Southern Rocky Mountain Front zones given higher
confidence of impacts late Tuesday night into Wednesday morning.
The Little Belt Mountain Winter Storm Watch was left as is for
now, since the timing of impacts looks to have shifted later on
Wednesday/Wednesday night. Winter Weather Advisories will be
considered for other locations over the next 24 hours, with the
most likely candidates being the Madison/Gallatin mountains and
perhaps the Big Belts and the northerly upslope areas along the
highway 200 corridor between Great Falls and Lewistown.

Regardless of snowfall amounts, anyone with outdoor plans will
encounter cold and wet conditions, especially Tuesday night
through Thursday. In addition to that, low temperatures will be
near or below freezing each morning for the remainder of the work
week, with the highest probabilities (70% +) for 32F or lower
readings along the Rocky Mountain Front and central/southwestern
areas along and south of the highway 200 corridor.

Ensembles maintain general troughing and below average
temperatures over the Northern Rockies for the remainder of the
work week, with another passing shortwave to bring another round
of lower elevation rain showers and mountain snow Friday through
early Saturday. This system doesn`t appear to be quite as wet and
cold as the previous one and this is reflected in the NBM
probabilities of a quarter inch of rainfall/liquid equivalent
precipitation for the 24 hour period ending 6 pm Friday currently
running less than 40% for all areas. Transient ridging dries
things out Saturday through early Sunday before another Pacific
system moves in for the first part of next week. Despite the
overall unsettled conditions, Temperatures slowly creep toward
seasonal averages this weekend into early next week. - RCG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  37  52  32  43 /  80  20  30  90
CTB  34  50  32  38 /  10  40  50  90
HLN  36  52  30  47 /  60  30  10  70
BZN  27  49  24  46 /  80  40  40  50
WYS  21  40  19  38 /  50  50  50  60
DLN  25  47  23  45 /  50  30  20  30
HVR  39  56  33  50 /  50  40  50  80
LWT  32  49  27  45 / 100  20  10  80

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from midnight Tuesday night to noon MDT
Thursday for East Glacier Park Region.

Winter Weather Advisory from midnight Tuesday night to noon MDT
Thursday for Northern High Plains-Southern Rocky Mountain Front.

Winter Storm Watch from Wednesday morning through Thursday
morning for Little Belt and Highwood Mountains.

&&

$$

http://www.weather.gov/greatfalls