Severe Thunderstorm Warning 46, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
136 PM MDT Fri Aug 18 2023

.SYNOPSIS...

Strong and gusty west winds will continue across the Northern
Rockies through the evening hours tonight before a cold front
drops south and switches winds to the north. Ahead of this cold
front, showers and thunderstorms are possible across Southwest
and Central Montana this afternoon and evening, with a few storms
potentially becoming severe and producing damaging wind gusts. A
period of gusty winds is possible behind this cold front, in
addition to a quick drop in temperatures.

&&

.DISCUSSION...

Rest of today through tonight...H500 ridge, which has brought very
warm temperatures and dry conditions to the Northern Rockies for the
past several days, continued to be flattened this afternoon by a
potent shortwave through that was sliding east over the Pacific
Northwest and Southwestern Canada. This shortwave trough will
continue to slide east and over the Northern Rockies and High Plains
through the evening and early morning hours on Saturday, resulting
in continued strong and gusty west winds across all of Southwest
through North Central Montana through approximately 03-06z tonight.
These westerly winds have been especially strong so far this
afternoon along the Rocky Mountain Front and across western portions
of the Hi-Line out to Toole County, with sustained wind speeds of 25-
35 mph and gusts between 50-60 mph being common. In addition to the
strong westerly winds, showers and thunderstorms were developing
across Western and Southwest Montana early this afternoon, with
these showers and storms expected to continue to move east across
Southwest through Central Montana prior to 06z this evening. While
the best coverage of these showers and thunderstorms is expected to
be along and southeast of a Dillon to Bozeman Pass line, shower and
storm chances will extend as far north as a Rogers Pass, to Great
Falls, to Hays line. A few of the thunderstorms across Central and
Southwest Montana this afternoon and early evening could produce
damaging wind gusts given the very dry boundary and mid-level air.

By the mid-evening hours tonight a strong cold front, which is
currently stalled along a line from Calgary to Brooks, Alberta, will
begin to advance south and across North Central through Central
Montana. This cold front is expected to advance south through areas
north of the Montana Highway 200 corridor between 03-06z this
evening and north of the I-90 corridor prior to 12z Saturday. This
cold front will bring a quick wind shift to the north following its
passage, in addition to a 1-3 hour period of gusty winds of between
20-40 mph. Also, temperatures are expected to fall quite quickly
behind the passage of the front, with current temperatures across
Central and Northern Alberta in wake of the front generally being in
the 50s this afternoon. - Moldan

Saturday through next Friday...The persistent upper-level ridging
remains overhead with flow aloft switching to a more predominate
southwesterly flow and reintroducing monsoonal moisture to much of
the Treasure State. Additionally, with the west-northwestward
propagation of Hurricane Hilary, currently situated south of the
southern tip of Baja California, effects of the hurricane will
make way toward the Treasure State as early as Sunday. Widespread
stratiform precipitation is forecast throughout the weekend as a
result, indicative of WPC's marginal risk for excessive rainfall
forecast Saturday and Sunday, with a slight risk clipping portions
of Southwest Montana for Sunday. Note that excessive rainfall is
defined as the risk of rainfall exceeding flash flood guidance
with a 25 mile vicinity. Any flooding that may occur as a result
of excessive rainfall will generally occur in low- lying areas, in
areas of poor drainage, or along small streams and creeks.

Probabilistic guidance depicts right around a 60 to 70% chance of
locations along the Montana-Idaho border receiving 0.25" Sunday,
decreasing to 30 to 50% chance for 0.50" precipitation. Locations
generally along and north of the MT Hwy 200 Corridor will see
considerably less precipitation during this time. Furthermore,
increased cloud coverage (and aforementioned precipitation) as a
direct result of the storm will aid in keeping afternoon
temperatures 10 to 15 degrees cooler than seasonal averages. Many
locations will be in the upper 70s to lower 80s throughout the
weekend.

Unstable southwesterly winds aloft will persist through much of
early next week, leading to daily chances for showers and
thunderstorms. Latest blended model guidance continues showing
temperatures throughout next week to be near or below seasonal
averages for most locations, generally keeping 70s and 80s in the
forecast. Cluster Analysis portrays some inconsistencies with
regards to the location of the upper-level ridging, though, with
some members suggesting the ridge will eventually move eastward and
be replaced by a weak upper-level trough. Most cluster members
showcase the ridge rebuilding overhead by midweek, which will
promote warmer and drier conditions for the remainder of the outlook
period. - Pierce

&&

.AVIATION...
1108 AM MDT Fri Aug 18 2023 (18/18Z TAF Period)

VFR conditions are primarily expected through 06z Saturday; however,
showers and thunderstorms along and southeast of a KHLN to KLWT line
may bring temporary periods of MVFR/low-VFR conditions. The bigger
concern from any shower or thunderstorm will be the potential for
strong and erratic winds, with the potential for isolated wind gusts
to approach 50kts. Otherwise, westerly surface winds will continue
to strengthen through the early afternoon hours across all of
Southwest through North Central Montana, with the strongest winds
expected between 20z this afternoon through 02z Saturday. Mountain
wave turbulence is possible, especially prior to 06z, in lee of any
terrain feature. By 03z Saturday a cold front will begin to advance
south from Canada and across the Hi-Line, Central Montana between 06-
09z Saturday, and into Southwest Montana between 09-15z Saturday.
This cold front will bring a wind shift to the north, along with
lowering CIGS over the plains of Central and North Central Montana. -
Moldan

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

&&

.FIRE WEATHER...

Critical fire weather conditions will continue across all Fire
Weather Zones due to a combination of hot, dry, windy, and/or
thunderstorm activity through the mid-evening hours tonight. The
strong upper level ridge that has brought the hot and dry conditions
to the Northern Rockies over the past several days was being
flattened by a potent shortwave moving east from the Pacific
Northwest and Southwest Canada. As the shortwave slides east and
over the Northern Rockies this evening and through the morning hours
on Saturday an associated cold front will begin to advance south
from Canada. This strong cold front will bring a quick wind shift to
the north, in addition to a period of gusty winds of between 20-40
mph. While fire weather concerns will largely decrease in wake of
the cold front through Saturday morning due to increasing relative
humidity values and decreasing temperatures, a short period of
critical conditions may linger this evening given the sudden wind
shift across zones 112/113/114/115/117. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  46  72  54  73 /  10  10  30  40
CTB  41  69  48  70 /  10   0  20  30
HLN  53  80  57  75 /  20  20  40  60
BZN  54  82  53  70 /  30  20  50  60
WYS  45  74  47  65 /  20  40  50  80
DLN  49  82  53  67 /  10  30  40  70
HVR  48  75  53  72 /  20   0  40  30
LWT  47  72  50  68 /  30  10  60  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 9 PM MDT this evening for Eastern
Glacier/Toole/Central/Eastern Pondera/Liberty.

High Wind Warning until 9 PM MDT this evening for Eastern
Glacier, Western Toole, and Central Pondera-Northern High Plains.

Heat Advisory until 8 PM MDT this evening for Bears Paw
Mountains and Southern Blaine-Fergus County below 4500ft-
Northern Blaine County.

Red Flag Warning until 9 PM MDT this evening for Central and
Eastern Lewis and Clark National Forest Areas-Chouteau and
Fergus Counties-Hill and Blaine Counties-Lewis and Clark
National Forest Rocky Mountain District-Rocky Mountain Front.

Red Flag Warning until 9 PM MDT this evening for Helena and
Townsend Ranger Districts of the Helena National Forest-Lincoln
Ranger District of the Helena National Forest.

&&

http://www.weather.gov/greatfalls

Preliminary Local Storm Report
National Weather Service Great Falls MT
150 PM MDT Fri Aug 18 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1121 AM     Non-Tstm Wnd Gst 8 SE East Glacier Park  48.36N 113.11W
08/18/2023  M57 MPH          Glacier            MT   Mesonet

            Mesonet station DCRM8 Deep Creek RAWS.

&&

Moldan

Preliminary Local Storm Report
National Weather Service Great Falls MT
151 PM MDT Fri Aug 18 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1112 AM     Non-Tstm Wnd Gst 3 SW Cut Bank           48.60N 112.37W
08/18/2023  M52 MPH          Glacier            MT   ASOS

            ASOS station KCTB Cut Bank Airport.

&&

Moldan

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