Helena Valley Northwest, MT 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

139
FXUS65 KTFX 211746 CCA
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1047 AM MST Thu Nov 21 2024

Aviation Section Updated.

.KEY MESSAGES...

 - Periods of minor mountain snow with rain/snow across lower
   elevations today and Friday.
 - Minor ice accumulations possible Friday across North-Central
   MT.
 - A cold front Saturday will transition rain in the lower
   elevations to all snow through Sunday.
 - Cold temperatures settle in for the weekend and beginning of
   next week.

&&

.UPDATE...

The main reason for an update this morning was to freshen
temperature and precipitation chance trends through the day into
tonight. There is a low (Generally less than 15% chance) for some
freezing drizzle as early as this afternoon across portions of the
plains. Given this chance is so low, I did not opt to extend the
Winter Weather Advisory at this time. Trends will need to be
monitored this afternoon and early evening for any short term
changes in Winter Weather Advisories.

The chance for freezing drizzle rises heading into tomorrow, which
is when most of the Winter Weather Advisories go into effect.

The only other change was to change the timing of the Winter
Weather Advisories along the Rocky Mountain Front to better
reflect latest thinking on timing of a break between snow events
that looks to occur late tonight. -AM

&&

.DISCUSSION...
/Issued 434 AM MST Thu Nov 21 2024/

 - Meteorological Overview:

This broad upper-level trough stationed across the northwest U.S
will bring periodic minor mountain snow and light rain/snow across
the region today. Heading into Friday, warm air advection
overrides the stationary boundary draped across North-Central MT.
This in combination with freezing surface temperatures Friday
morning, will allow for snow and/or light freezing rain to
develop across North-Central MT. Snow/ice accumulations look to
remain light. Precipitation continues across the region Friday
night and into Saturday morning. On Saturday, a cold front pushes
southward through the state, which will transition lower
elevation ran/wintry mix to all snow along and behind the front.
Behind this front, temperatures drop quickly as the North Central
plains will drop into the teens to low 20s by the evening.
Accumulating snow winds down Sunday. Northwest flow aloft will
keep the cold temperatures for the first half of next week.
Ensemble models hint at most places staying dry Monday and
Tuesday, with some upper-air disturbances keeping snow in the
mountains.


 - Forecast Confidence & Scenarios:

Models have been in good agreement of this upper-level trough
bringing in periods of rain/snow today and Friday. In terms of
accumulations, another 1-2" of snow are likely along Marias Pass
today. In terms of precipitation Friday, Hi-resolution models
have been in good agreement for minor accumulations along the
Northern Rocky Mountain Front and light snow accumulations along
the North Central plains. There`s a 40-50% chance for 6" along
Marias Pass. There`s a 30-70% chance for 1" of snow along and
north of the MT Highway 200 corridor. There is medium confidence
for light freezing rain to develop Friday across North-Central MT.
NBM guidance gives a 20-50% chance for 0.01" of ice for areas
north of the Helena to Lewistown line. Ice accumulations are
expected to remain light, with a light glaze with freezing rain.
There`s some uncertainty in the southern extent of the freezing
rain corridor. Models have been a little inconsistent with how
much warm air advection occurs, with higher-resolution model
vertical profile soundings having a more defined mid-level warm
nose across North Central MT. Some medium to longer range models
have less of a warm nose, and keep the extent of freezing rain
more minimal. With a light wintry mix and/or freezing rain
impacting traveling, a Winter Weather Advisory was issued for
North- Central MT and northern Continental Divide Friday.

In terms of snow amounts Saturday, guidance has slightly increased
in moisture amounts and thus snowfall accumulations. Along the
Northern Continental Divide, ensemble models have a 50% chance
for Marias Pass to get at least 9" of snow. If models continue to
trend higher a Winter Storm Watch may be needed down the line.
Across lower elevations, there`s a 40-80% chance for 3" of snow
across North-Central MT, and up to a 40% chance for 1" of snow.
Winter highlights will likely be needed down the line, but as of
this moment, snow amounts look to remain below winter storm
criteria at lower elevations.

There`s high confidence in the cold temperatures for the weekend,
with Sunday and Monday in particular. There`s a 70-90% chance in
areas along the Hi-Line region have morning lows Monday dropping
below freezing. There`s a 70-90% chance for the rest of North-
Central MT having single digits low temperatures Monday morning.
-Wilson

&&

.AVIATION...
21/18Z TAF Period

The initial concern will be for a period of snow along the
Northern Rocky Mountain Front out to the KCTB area late this
afternoon and early evening. Then concerns turn more toward a
period of light freezing drizzle and additional periods of snow
across portions of Central and North-central Montana late tonight
into the day Friday. Mountains will become increasingly obscured
this evening into Friday. A breezy south wind will continue at
KEKS through the TAF period. -AM

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  37  20  27  22 /  20  30  90  60
CTB  29  13  21  15 /  30  70 100  80
HLN  46  28  41  29 /  40  20  70  50
BZN  45  29  47  30 /  30  20  30  40
WYS  36  25  40  22 /  70  60  30  40
DLN  43  27  45  30 /  30  20  20  40
HVR  34  17  29  20 /  20  40  90  70
LWT  46  21  36  24 /  20  10  70  40

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 10 PM MST this evening for East
Glacier Park Region.

Winter Weather Advisory from 5 AM to 11 PM MST Friday for Bears
Paw Mountains and Southern Blaine-Cascade County below 5000ft-
East Glacier Park Region-Eastern Glacier, Western Toole, and
Central Pondera-Eastern Pondera and Eastern Teton-Eastern Toole
and Liberty-Fergus County below 4500ft-Gates of the Mountains-
Hill County-Judith Basin County and Judith Gap-Little Belt and
Highwood Mountains-Northern Blaine County-Northern High Plains-
Snowy and Judith Mountains-Southern High Plains-Southern Rocky
Mountain Front-Western and Central Chouteau County.

Winter Weather Advisory from 5 AM to 11 AM MST Friday for Helena
Valley-Meagher County Valleys-Upper Blackfoot and MacDonald Pass.

&&

$$
http://www.weather.gov/greatfalls