Severe Thunderstorm Warning 9, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1159 AM MDT Sat Jun 15 2024

.SYNOPSIS...

Showers and thunderstorms through this afternoon will give way to
increasing winds, especially along the Rocky Mountain Front from
this evening through Sunday morning. Temperatures cool to well
below normal for the first half of next week, with widespread
precipitation expected across much of the Northern Rockies.

&&

.UPDATE...

The primary focus of my update, this morning, is to adjust
Probability of Precipitation (POP), generally, reducing POPs
through midnight, tonight. Most adjustments were made across
portions of north central and central Montana. Aside from this, no
significant changes are made, at this time.
- Fogleman

&&

.AVIATION...
15/18Z TAF Period

A cold front approaching North Central and Southwestern Montana from
the west will continue to push into the area this afternoon,
bringing a few showers and thunderstorms, along with gusty winds as
it moves through, especially along and north of I-90. Watch for
turbulence and gusty surface winds as these showers and storms move
through, especially east of a line from KGTF to KLWT. Behind this
front, winds will be on the increase across the plains, particularly
at KCTB where mountain waves may result in 50 kt wind gusts this
evening/overnight. Winds will gradually weaken across the area
tomorrow. Ludwig

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

&&

.PREV DISCUSSION...
/ISSUED 545 AM MDT Sat Jun 15 2024/

Today through Sunday...primary forecast concerns throughout the
weekend are thunderstorms through early this evening, and the threat
for High Winds from this afternoon through Sunday morning along the
Rocky Mountain Front. Moist, southwest flow aloft combined with the
approach of an upper level jet will lead to increasing coverage of
showers and thunderstorms from the late morning through afternoon
hours, most notably along the Continental Divide north of the
Montana Hwy 200 corridor and across eastern portions of Central and
North Central Montana. Nocturnal convection has been on-going most
of the night across portions of the CWA, with this convection
expected to increase in areal extent by the late morning with the
approach of the aforementioned upper level jet. While most
thunderstorms are likely to only produce gusty/erratic winds and/or
small hail, a few of the storms across eastern portions of Fergus
County could become severe and produce damaging wind gusts and large
hail. Given this threat, the Storm Prediction Center as placed
eastern portions of Fergus County under a Marginal Risk.

The other primary concern throughout the period will be the threat
for High Winds from the mid-afternoon hours today through Sunday
morning along the Rocky Mountain Front. NBM probabilities for wind
gusts in excess of 48kts for much of the East Glacier Park Region,
Northern and Southern High Plains, and Eastern Glacier/Western
Toole/Central Pondera continue to range from 80-90%, with favorable
temperature profiles for mountain wave activity from this evening
through early Sunday morning. Given these higher probabilities and
the likelihood of mountain wave activity, have decided to upgrade
the High Wind Watch for the Northern High Plains and Eastern
Glacier/Western Toole/Central Pondera, along with adding the East
Glacier Park Region and Southern High Plains. Travelers, especially
those not used to the strong winds of the Rocky Mountain Front, will
need to exercise caution from this afternoon through Sunday.

Monday through Wednesday...Ensemble clusters continue to support
longwave troughing moving over the Northern Rockies throughout the
period, with the leading modes of uncertainty being the amplitude
of the trough as it moves over the Great Basin on Monday and then
the position and/or timing of the trough on Tuesday and Wednesday as
it lifts northeast to over the High Plains. With respect to Monday's
uncertainty, main impacts to North Central through Southwest
Montana will be just how "cold" and wet conditions will be, with
high temperature spreads across lower elevations between the 25th
and 75th percentiles being approximately 10-15 degrees from the
upper 40s/low 50s (25th percentile) to the upper 50s/low 60s.
Additionally, precipitation spreads for Monday and t Monday night
range from a couple hundreths over northern and southern portions
of North Central and Southwest Montana (respectively) to
0.10"-0.25" elsewhere for the 25th percentiles, with 0.25" to just
over 1" for the 75th percentiles. These large spread between
temperatures and precipitation also leads to uncertainty with
respect to the type of mountain precipitation, with NBM
probabilities for 6" or more of snow from Monday through Tuesday
generally ranging between 30-50%. Have continued the Winter Storm
Watch from Monday through Tuesday given this uncertainty, but
regardless if the Watch is upgraded to an Advisory or Warning or
cancelled...recreationists should be prepared for cold and raw
conditions. Additionally, should higher end snowfall amounts
materialize over this timeframe, damage to tree limbs and power
outages would be possible given the heavy, wet nature of the snow.
High temperatures will warm slightly for Tuesday and Wednesday,
but remain below normal. Light precipitation is expected linger
through the morning and early afternoon hours on Tuesday, with
overall drying conditions expected thereafter.

Thursday through next Saturday...Ensemble clusters largely support
and unsettled southwest flow regime over the Northern Rockies for
the second half of the work week and into next weekend, which would
translate to moderating temperatures and daily chances for showers
and thunderstorms. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  71  43  66  41 /  60  10  10  40
CTB  64  41  57  35 /  60  20  30  20
HLN  74  42  71  44 /  60  10  20  50
BZN  77  36  67  40 /  60  20  20  30
WYS  71  36  61  36 /  20  10   0  10
DLN  75  34  64  38 /  30   0  10  30
HVR  74  45  63  43 /  60  20  20  20
LWT  74  39  65  39 /  70  20  10  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from 3 PM this afternoon to 9 AM MDT Sunday
for East Glacier Park Region-Eastern Glacier, Western Toole, and
Central Pondera-Northern High Plains-Southern High Plains.

Winter Storm Watch from Monday morning through Tuesday evening
for East Glacier Park Region-Southern Rocky Mountain Front.

Winter Storm Watch from Monday evening through Tuesday evening
for Big Belt, Bridger and Castle Mountains-Elkhorn and Boulder
Mountains-Gallatin and Madison County Mountains and Centennial
Mountains-Little Belt and Highwood Mountains-Northwest
Beaverhead County.

&&

http://www.weather.gov/greatfalls

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