Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
525 PM MDT Sun Jul 9 2017

...UPDATED AVIATION SECTION...

.SYNOPSIS...

Several upper level disturbances will move through the region
tonight through Tuesday. As a result, expect scattered showers
and thunderstorms to develop during this time, mainly during the
daylight hours. Some storms could be severe tonight and then
again on Monday, with the main severe threat being damaging wind
gusts. High temperatures will cool a few degrees through Tuesday,
before another significant heat wave develops by late next week.

&&

.DISCUSSION...

Rest of Today through Tonight...an embedded wave was ascending the
upstream side of an amplified ridge this afternoon, who's axis
extended north from Western Wyoming, through Central Montana, and
into the Canadian Provinces of Alberta/Saskatchewan. The
aforementioned wave will crest the ridge through the evening hours
tonight, providing weak large scale ascent across portions of
Southwestern and Central Montana. In addition, strong diurnal
heating over the higher terrain of these regions was also
providing orographic lift this afternoon. The combination of weak
instability, 0-6km bulk shear on the order of 20-35kts, and the
aforementioned lifting mechanisms, has led to the development of
scattered thunderstorms as of early this afternoon across
Southwestern Montana. These storms are generally expected to lift
northeast into portions of Central Montana through the evening
hours tonight, before slowly dissipating near the midnight hour.
Current thinking is that the northward extent of these storms will
generally be along and south of an Augusta, to Great Falls, to
Fergus line. With deep inverted-V soundings present due to very
dry air residing in the lowest levels of the atmosphere, am
thinking the primary threat with any storms this afternoon/evening
will be strong to damaging wind gusts. While some hailstones to
the size of nickles and potentially quarters are possible, given
wet-bulb zero heights of 13,000ft+, am thinking this severe
weather mode will be quiet limited. - Moldan

Monday through Tuesday...Multiple shortwave troughs are expected to
traverse the CWA during this period. At the surface, a weak low
centered in vicinity of Southeast AB Monday morning should reach
Southeast SK by the end of the period, allowing a weak cold front to
advance southeastward over the CWA Monday afternoon and evening.
This pattern looks to be favorable for multiple rounds of scattered
showers and thunderstorms over the region, especially Monday
afternoon through daybreak Tuesday. Moderate to strong vertical wind
shear and moderate instability will likely promote severe
thunderstorms. During the afternoon and early evening, a dry and
deeply-mixed boundary layer will result in damaging wind gusts being
the primary severe weather threat. Despite nocturnal boundary layer
stabilization, the main threat may then transition to severe hail up
to half dollar-size late Monday evening into early Tuesday morning.
During this time period, significant elevated instability may
materialize within the hail growth zone, especially over the Hi-
Line. Additional showers and thunderstorms, some strong, are then
possible the rest of Tuesday. A slight cooling trend is expected
during the period due to the aforementioned cold frontal passage and
series of shortwave troughs moving overhead.
Jaszka

Tuesday night through Sunday...The pattern during this period can
best be characterized as becoming seasonably hot and dry. Upper-
level high pressure builds over the region from mid-week through
Saturday, then remains fairly steady into Sunday. Surface-to-mid-
level thickness values by next weekend could surpass yesterday's
(Saturday's) values, which would result in high temperatures near or
surpassing 100 degrees in many locations outside of the mountains
next weekend, and likely resulting in matching or surpassing many
daily record highs. Any showers or thunderstorms that may form
during this time will likely be terrain-based and on the dry side,
providing little to no relief from the worsening drought over our
eastern plains counties. Exercising quality heat safety and fire
weather safety measures will be the primary messaging themes as we
approach the later part of the week into next weekend. CC

&&

.AVIATION...
Updated 2325z.

VFR conditions will mostly persist through the day on Monday. Some
stronger storms may produce brief MVFR ceilings, but am not
confident enough that this will happen to include in the TAFs. A
disturbance moving over Central and Southwest Montana will continue
scattered strong thunderstorms south of a KCTB-KHVR line through
around 05Z. The main threat from these storms will be wind gusts
potentially up to 45 kt at times. Brief showers and pea sized hail
are possible but not likely. The airmass will stabilize overnight,
but an increasingly moist and unstable southwest flow aloft will
move into the area after 18Z. A frontal boundary will linger over a
KLWT-KDLN line during this time, and another boundary will move
south over the Hi-Line. This set-up will help cause scattered
thunderstorms to develop over much of the area. Again, the main
threat will be strong wind gusts, but brief heavy rain and small
hail will also be possible.
Coulston

&&

.FIRE WEATHER...

The main fire weather concern for the rest of today and Monday
will be strong thunderstorms that produce frequent lightning and
strong/gusty erratic wind gusts. The best chance for stronger
storms to occur will be during the afternoon/evening hours.

Looking ahead...afternoon temperatures will cool a bit into the
80s for Tue/Wed...only for another significant heat wave to
develop on Thursday and continue into next weekend (afternoon
highs 90s/lower 100s). No significant/widespread rainfall is
expected after Tuesday at this time. Brusda/Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  58  90  56  83 /  20  30  30  10
CTB  56  85  53  78 /  10  20  30  20
HLN  60  90  57  86 /  40  40  30  10
BZN  55  90  52  83 /  40  30  20  10
WEY  43  81  44  77 /  40  20  20  10
DLN  55  87  52  82 /  40  20  10  10
HVR  60  94  59  83 /  10  20  30  30
LWT  59  89  55  81 /  20  30  30  20

&&

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

weather.gov/greatfalls

Special Weather Statement
National Weather Service Great Falls MT
606 PM MDT SUN JUL 9 2017

MTZ050-051-054-100030-
Fergus MT-Judith Basin MT-Meagher MT-
606 PM MDT SUN JUL 9 2017

...SPECIAL WEATHER STATEMENT...

At 603 PM MDT, Doppler radar was tracking showers and thunderstorms
over southern portions of Fergus and Judith Basin counties.

Winds in excess of 35 to 50 mph will be possible with these showers
and storms.

Locations impacted include...
Lewistown, Hobson, Moore, Grass Range, Forestgrove, Moccasin, Ackley
Lake State Park, Sapphire Village, Glengarry, Utica, Garneill,
Giltedge, Heath and Buffalo.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Persons in campgrounds or recreating outside should consider seeking
sturdy shelter until these showers and storms pass.

&&

LAT...LON 4675 10864 4675 10938 4670 10939 4669 10945
      4670 10982 4675 10996 4674 11003 4675 11005
      4671 11015 4672 11027 4663 11030 4690 11032
      4713 10977 4716 10874 4710 10873 4708 10860
TIME...MOT...LOC 0003Z 263DEG 26KT 4686 10960

Moldan

Special Weather Statement
National Weather Service Great Falls MT
611 PM MDT SUN JUL 9 2017

MTZ015-052-055-100045-
Madison-Jefferson-Gallatin-
611 PM MDT SUN JUL 9 2017

...A STRONG THUNDERSTORM WILL AFFECT NORTHEASTERN MADISON...
SOUTHEASTERN JEFFERSON AND NORTHWESTERN GALLATIN COUNTIES...

At 610 PM MDT, a strong thunderstorm was located near Harrison, or 25
miles northeast of Virginia City, moving east at 30 mph.

Winds in excess of 40 mph are expected with this storm. Brief heavy
rain is also possible.

Locations impacted include...
Harrison, Pony, Norris, Amsterdam, Gallatin Gateway, Church Hill and
Madison Buffalo Jump State Park.

LAT...LON 4549 11194 4576 11196 4587 11120 4547 11119
TIME...MOT...LOC 0010Z 265DEG 27KT 4564 11173

Coulston

Special Weather Statement
National Weather Service Great Falls MT
615 PM MDT SUN JUL 9 2017

MTZ012-014-052-054-100100-
Cascade MT-Jefferson MT-Central and Southern Lewis and Clark MT-
Meagher MT-
615 PM MDT SUN JUL 9 2017

...SPECIAL WEATHER STATEMENT...

At 613 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 18 miles southwest of Wolf Creek to 7 miles
northeast of Garrison. Movement was east at 25 mph.

Winds in excess of 30 to 40 mph will be possible with these storms.

Locations impacted include...
Helena, East Helena, Marysville, Rimini, Spring Meadow Lake State
Park, Canyon Creek, Black Sandy State Park and Austin.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If on or near Hauser Lake or Lake Helena, get out of the water and
move indoors or inside a vehicle. Remember, lightning can strike out
to 10 miles from the parent thunderstorm. If you can hear thunder,
you are close enough to be struck by lightning. Move to safe shelter
now! Do not be caught on the water in a thunderstorm.

Persons in campgrounds should consider seeking sturdy shelter until
these storms pass.

&&

LAT...LON 4686 11234 4692 11157 4632 11216 4638 11237
      4641 11232 4662 11230 4666 11233 4666 11241
      4670 11242 4670 11252 4674 11253
TIME...MOT...LOC 0013Z 273DEG 22KT 4681 11232 4657 11268

Moldan

Special Weather Statement
National Weather Service Great Falls MT
631 PM MDT SUN JUL 9 2017

MTZ050-051-100100-
Fergus MT-Judith Basin MT-
631 PM MDT SUN JUL 9 2017

...SPECIAL WEATHER STATEMENT...

At 629 PM MDT, Doppler radar continued to track weakening showers
and thunderstorms over southern portions of Judith Basin and Fergus
counties that were producing gusty outflow winds.

Winds in excess of 30 to 50 mph will be possible with these showers
and storms.

Locations impacted include...
Lewistown, Hobson, Moore, Grass Range, Forestgrove, Ackley Lake State
Park, Glengarry, Heath, Buffalo and Garneill.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Persons in campgrounds or recreating outside should consider seeking
sturdy shelter until these showers and storms pass.

These storms may re-intensify, so be certain to monitor local radio
stations and available television stations for additional
information and possible warnings from the National Weather Service.

&&

LAT...LON 4675 10864 4675 10938 4670 10939 4669 10945
      4670 10982 4675 10996 4674 11010 4671 11015
      4673 11021 4687 11021 4712 10955 4713 10874
      4710 10873 4708 10860
TIME...MOT...LOC 0026Z 273DEG 26KT 4685 10932

Moldan

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