Severe Thunderstorm Warning 67, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
535 PM MDT Sat Jul 6 2019

Updated Aviation Section

.SYNOPSIS...

An unstable southwest flow aloft will keep mostly afternoon and
evening shower and thunderstorm activity going through early next
week. Thunderstorms may become strong to severe today and Sunday,
with hail, gusty winds, and heavy rain possible. Drier and warmer
conditions are expected by mid-week.

&&

.DISCUSSION...

Remainder of this afternoon through Sunday Night...Our region
continues to be situated in a moist and unstable flow ahead of a
closed low over British Columbia. A disturbance rotating around this
feature is initiating shower and thunderstorm development over the
Rocky Mountain Front and Southwest Montana and this activity will
eventually expand deeper into central and north-central locations
by late this afternoon into the evening. Hi-res models continue
to suggest that gusty winds up to 60 mph will be the most
widespread impact from these storms, although small hail and brief
heavy rain may accompany these storms. Areas along the Rocky
Mountain Front and eastward toward the golden triangle will likely
observe the greatest impacts as storms diminish in strength as
they move east from there. Most activity should move out of the
area by 6Z but some isolated shower/thunder activity can't be
ruled out, especially along the Rocky Mountain Front. By Sunday,
the aforementioned low pressure trough will move east over the
Pacific Northwest, which will increase the southwest flow aloft
over the region as a weak Pacific cold front traverses the region.
Showers and thunderstorms may initiate much earlier than today,
perhaps as early as the late morning. The NAM and HRRR, suggest
that a wave of more moderate showers/thunder may develop ahead of
the anticipated stronger activity as early as 13-15Z. Good
moisture, steep mid-level lapse rate, wind shear should offer a
good chance for severe storm development with large hail and
damaging winds over the plains. Portions of the CWA will be under
a marginal risk for excessive rainfall, so localized flooding due
to heavy thunderstorm rains will be possible. Storms will
encounter an increasingly more favorable thunderstorm environment
as they move toward the east and northeast. One thing to keep in
mind is that if shower activity begins earlier in the morning then
some of the severe threat may be mitigated by convective
contamination, especially for the central and western portion of
the plains. As the Cooler air moves into the area behind the cold
front late Sunday evening and overnight, the severe storm threat
should diminish. Note, that scattered showers may linger into the
overnight hours.

Monday through Saturday... The Pacific Northwest trough will start
to weaken as it moves more so into the Northern Rockies on Monday.
Although the threat for severe weather will shift east of the area,
more widespread rain showers with weak thunderstorms will persist
through Monday into Monday night, as temperatures remain cooler than
normal. The showers will decrease from the west Tuesday into Tuesday
night as the low pressure trough moves east out onto the Great
Plains, but temperatures will remain below normal one more day. A
high pressure ridge will then build into the area on Wednesday,
which should bring drier conditions with warmer than normal
temperatures. This ridge will then be flattened somewhat by weak
disturbances moving over Montana from the Pacific Ocean, leaving the
area under a less stable westerly flow aloft with only slightly
cooler temperatures. The disturbances will combine with
afternoon/early evening instability to cause a few showers and
thunderstorms to develop over the higher terrain and move out over
the plains. Although this does not appear to be a pattern that would
cause severe weather at this time, the situation will continue to be
monitored.  Coulston

&&

.AVIATION...
Updated 0535 PM MDT Sat Jul 6 2019 (00Z/Jul 7 TAF issuance)

Showers and Thunderstorms will result in brief MVFR conditions
though the evening before diminishing, especialy around GTF and HLN.
Some of the storm could produce strong winds and large hail. Mainly
VFR conditions are expected after midnight across the area.
Additional showers and isolated thunderstorms will develop tomorrow
morning, with a few strong to severe thunderstorms possible in the
afternoon.
Langlieb

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  53  79  52  73 /  60  50  50  40
CTB  49  73  47  69 /  50  40  40  30
HLN  52  80  52  75 /  50  50  50  70
BZN  50  81  52  74 /  30  50  50  80
WEY  41  78  44  68 /  20  20  30  80
DLN  48  76  48  71 /  40  50  40  80
HVR  55  82  54  75 /  30  50  40  30
LWT  51  80  51  68 /  30  60  70  60

&&

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

http://www.weather.gov/greatfalls

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Special Weather Statement
National Weather Service Great Falls MT
614 PM MDT Sat Jul 6 2019

MTZ014-052-053-070030-
Broadwater-Jefferson-Central and Southern Lewis and Clark-
614 PM MDT Sat Jul 6 2019

...A STRONG THUNDERSTORM WILL AFFECT SOUTHEASTERN LEWIS AND CLARK...
NORTHWESTERN BROADWATER AND NORTHEASTERN JEFFERSON COUNTIES...

At 613 PM MDT, a strong thunderstorm was located over Helena, moving
northeast at 30 mph.

Winds in excess of 30 mph and pea size hail are possible with this
storm.

Locations impacted include...
Helena, East Helena, Montana City, Clancy and Spring Meadow Lake
State Park.

This includes the following highways...
 Interstate 15 between mile markers 182 and 205.
 Highway 12 between mile markers 34 and 61.

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm, and may cause
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

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

To report severe weather, contact your nearest law enforcement
agency. They can relay your report to the National Weather Service
office in Great Falls.

LAT...LON 4642 11183 4652 11222 4683 11222 4664 11151
TIME...MOT...LOC 0013Z 206DEG 28KT 4658 11197

TP