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

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
707 PM MDT SUN JUL 09 2017

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

0646 PM     TSTM WND GST     6 E MARYSVILLE          46.73N 112.16W
07/09/2017  M53 MPH          LEWIS AND CLARK    MT   TRAINED SPOTTER

            TRAINED SPOTTER REPORTED A MEASURED WIND GUST OF 53
            MPH.

&&

KLG

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

MTZ014-052>054-100200-
Broadwater MT-Jefferson MT-Central and Southern Lewis and Clark MT-
Meagher MT-
715 PM MDT SUN JUL 9 2017

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHEASTERN LEWIS AND CLARK...
NORTHERN BROADWATER...CENTRAL MEAGHER AND NORTHERN JEFFERSON COUNTIES
UNTIL 800 PM MDT...

At 711 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 20 miles east of East Helena to near Deer
Lodge. Movement was east at 30 mph.

Wind gusts up to 50 mph, occasional lightning, brief heavy rain, and
hail up to pea size will be possible with these storms.

Locations impacted include...
Helena, Townsend, Boulder, White Sulphur Springs, East Helena,
Montana City, Basin, Fort Logan On The Smith River, Wickes, York,
Rimini, Alhambra, Corbin, Spring Meadow Lake State Park, Austin, Fort
Logan, Elkhorn State Park, Newlan Creek Reservoir, Winston and Silos.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If on or near Lake Helena, Hauser Lake, or Canyon Ferry Lake, 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.

&&

LAT...LON 4635 11249 4633 11246 4639 11240 4641 11232
      4662 11230 4666 11233 4666 11242 4667 11241
      4689 11160 4688 11108 4669 11058 4628 11128
      4623 11218 4630 11257
TIME...MOT...LOC 0111Z 274DEG 27KT 4669 11151 4641 11267

Coulston

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