Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
545 PM MDT Mon Jul 10 2017

...UPDATED AVIATION SECTION...

.SYNOPSIS...

Several upper-level disturbances will move through the region
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 today, especially
over Southwest and Central Montana. Damaging wind gusts and
quarter-size or slightly bigger hail will be the main threats
from any severe storm. High temperatures will cool a few degrees
through Tuesday, before another significant heat wave develops by
late next week.

&&

.DISCUSSION...

Tonight through Tuesday Night...Multiple weather disturbances will
traverse our region from west to east tonight through Tuesday.
Thereafter, upper-level high pressure ridging will rebound over
the CWA Tuesday night. Accordingly, scattered showers and
thunderstorms today through tonight will become more intermittent
on Tuesday. Fair weather should then return Tuesday night. Severe
thunderstorms capable of damaging wind gusts and quarter-size or
slightly bigger hail are possible this afternoon and evening. In
addition, a few additional thunderstorms accompanied by quarter-
size or slightly larger hail cannot be ruled-out over/near the Hi-
Line overnight tonight. Near-average temperatures are expected
tonight through Tuesday night.
Jaszka

Wednesday through Sunday...After a brief reprieve from the warmest
temps of late, the western US ridge looks to rebuild in earnest
later this week. Models are in good agreement through about
Saturday, but then begin to diverge. The main point of contention is
how quickly an upper trough over the NRN Pacific will shift inland,
if at all. The EC/CMC have started to trend quicker, but as WPC
points out in their extended forecast discussion, the quicker
solutions have not worked out particularly well. So, for now, I kept
highs well into the 90s to near 100 late in the week and over the
weekend. If, however, the EC/CMC are correct, temps would likely not
be quite as hot by Sunday due to a flattening of the ridge. Of note,
ensemble guidance suggests the strength of the ridge forecast this
week has a return interval of about 5-10 years, although 850mb/700mb
temps do not show quite as high of a return interval. All this
points to unseasonably hot temperatures likely returning areawide
with near-record highs possible in a few locations. We encourage
everyone to plan for another extended period of well above normal
temperatures.

Each day, especially Wednesday through Saturday, isolated
thunderstorms may develop just about anywhere (as was seen this past
week) as weak shortwaves move beneath the ridge. Trying to pin down
any of these waves is next to impossible this far out, especially
since some may be driven by convection from previous days. For this
reason, I trended pops towards climo, with a slight favoring of the
SW MT mountains. No one day sticks out any more than another
regarding the threat of stronger storms, but all storms will likely
be capable of gusty winds with drier low levels/inverted V
environments.
MARTIN

&&

.FIRE WEATHER...

Critical fire weather conditions are expected across the
northeastern areas, where hot temperatures, low relative humidity,
and gusty winds will persist into this evening. This situation
will be complicated further by the potential for thunderstorms by
this late afternoon or early evening. For much of central
Montana, the primary fire weather threat 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 this afternoon and 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.
Cassell

&&

.AVIATION...
Updated 2345Z.

VFR conditions are generally expected to continue at least through
the day on Tuesday, unless otherwise mentioned. Scattered showers
and potentially strong thunderstorms will continue to develop along
and south of a KHLN-KLWT line through 03Z due to disturbances moving
through the moist and unstable southwest flow aloft over a nearly
stationary frontal boundary. These storms may produce occasional
gusts to 45 kt, small hail, and brief heavy rain with periods of
MVFR conditions. Mid clouds will then linger there overnight and
into Tuesday. A frontal boundary will also bring showers and a few
similarly strong evening thunderstorms to the Hi-Line. Occasional
MVFR ceilings are possible with these showers. Showers will likely
decrease during the day on Tuesday.
Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  56  83  53  90 /  20   0   0   0
CTB  53  78  50  86 /  40  20  10   0
HLN  57  86  56  91 /  30   0   0   0
BZN  53  84  51  88 /  20   0  10   0
WEY  48  76  41  79 /   0  10  20  10
DLN  53  82  51  85 /  10   0   0   0
HVR  60  84  54  91 /  20  20  10  10
LWT  55  81  52  85 /  60   0  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 11 PM MDT this evening Chouteau and
Fergus Counties...Hill and Blaine Counties.

&&

weather.gov/greatfalls

Special Weather Statement
National Weather Service Great Falls MT
613 PM MDT MON JUL 10 2017

MTZ015-055-110045-
Madison MT-Gallatin MT-
613 PM MDT MON JUL 10 2017

...SIGNIFICANT WEATHER ADVISORY FOR EAST CENTRAL MADISON AND EAST
CENTRAL GALLATIN COUNTIES UNTIL 645 PM MDT...

At 613 PM MDT, Doppler radar was tracking a strong thunderstorm 18
miles east of Ennis, or 26 miles southwest of Bozeman, moving east at
35 mph.

Dime size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Big Sky.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

&&

LAT...LON 4541 11148 4559 11093 4553 11090 4552 11092
      4536 11092 4535 11104 4529 11104 4524 11140
TIME...MOT...LOC 0013Z 251DEG 30KT 4536 11134

Coulston

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