Severe Thunderstorm Warning 40, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
155 PM MDT Mon Jul 4 2022

.SYNOPSIS...

Persistent southwesterly flow aloft will continue to provide daily
afternoon and evening opportunities for showers and thunderstorms
through the week, some of which may be severe. Temperatures
slowly trend above average heading into the end of the week.

&&

.DISCUSSION...

Water vapor satellite, radar, and models indicate a well-defined
disturbance moving from eastern Washington and Oregon into the Idaho
panhandle early this afternoon. This feature is progged to move
northeastward through the northwestern half of Montana on its way
into southern Canada tonight. A moist low-level environment remains
over north-central and most of southwest Montana, with dewpoints
ranging from the 50s to lower 60s. As the early July sun burns off
the low cloud cover this afternoon and the mid-levels cool with the
arrival of the shortwave, expect fairly rapid destabilization of the
atmosphere from south to north from southwest into north-central
Montana. CAMS suggest a fairly robust broken line of convection
developing west-northwest to east-southeast along/near the
Continental Divide of western Montana and moving northeast through
the late afternoon and evening across central and north-central
Montana. A Slight Risk of severe thunderstorms exists from the Great
Falls and Helena areas extending east across the Lewistown area,
with a Marginal Risk to the south and north. Best timing for
convective initiation is between 2 and 5 pm this afternoon, with the
large cluster of thunderstorms moving northeast through the Great
Falls area between 5 and 8 pm, but possibly not reaching the
Havre area until closer to mid-evening. Damaging hail and winds
are the primary severe weather threats, but a tornado can not be
completely ruled out especially along the north side of the
mountains from near Great Falls east to Lewistown. Torrential
rainfall is also likely, with flash flooding especially a concern
with any supercell that tracks across an urban area, burn scar, or
rugged terrain.

For more information on today's severe thunderstorm threat, please
see SPC's Mesoscale Discussion #1358 covering parts of southwest
and central Montana valid until 4:30 pm this afternoon.

Rest of this Afternoon through Wednesday...

To start the period, an unusually deep upper trough for early July
remains along the PacNW coast. Over the central CONUS, a broad upper
ridge is in place. Fast southwesterly flow aloft is the result over
the Interior Northwest, including Montana. Overall, this pattern
will hold, with just slight perturbations each day, through the much
of the week. A series of waves traversing through the trough will
eject towards Montana through Wednesday, resulting in daily
opportunities for showers and thunderstorms that generally start in
the terrain mid-to-late afternoon and spread into adjacent areas
late in the afternoon and early evening.

Given favorable dynamics and thermodynamics, a severe threat will
exist each day through at least Friday. Vertical wind shear and
instability look to be most favorable over much of Central Montana
through Tuesday, expanding into North-Central Montana Wednesday.
Gusty winds to 60 mph, quarter sized hail, and brief heavy downpours
are the greatest concerns with thunderstorms that do form.

Thursday and into the weekend...

The upper ridge over the central CONUS slowly nudges westward,
supporting temperatures warming to slightly above average by Friday.
Southwesterly flow aloft persists however, maintaining chances for
afternoon and evening showers and thunderstorms across much of the
region through the end of the week.

Ensembles and cluster guidance are in good agreement that the upper
ridge will continue to nudge westward into Saturday. Models at this
range have a difficult time resolving subtle features that may lead
to afternoon showers or thunderstorms, however. Given a weak trough
forecast to be near/just off the West Coast over this time period, I
have maintained a slight chance for showers and thunderstorms this
weekend in the afternoon and evening hours. -AM

&&

.AVIATION...
1112 AM MDT Mon Jul 4 2022 (04/18Z TAF period)

MVFR cigs will gradually burn off through the early to mid-
afternoon. Instability increases through the afternoon as low-level
moisture burns off and skies at least partially clear, with
thunderstorms becoming likely by mid-late afternoon over the
mountains around KHLN and possibly KBZN. These storms will move
northeast through KGTF, KCTB, KLWT, and KHVR through mid-evening. A
few storms may become severe with damaging wind gusts and hail along
with torrential rain the primary severe threats. Mountains will be
obscured across much of the region for much of the TAF period.

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  50  77  52  84 /  80  30  10  40
CTB  46  74  48  78 /  90  10  10  40
HLN  53  79  57  86 /  50  40  10  50
BZN  52  81  51  87 /  40  40  10  40
WYS  42  75  42  80 /   0  20   0  20
DLN  49  77  49  84 /  10  30   0  40
HVR  55  77  55  85 /  70  30  20  30
LWT  52  74  52  81 /  80  60  30  50

&&

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

http://www.weather.gov/greatfalls

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