Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1120 AM MDT Mon Jul 10 2017

UPDATED AVIATION

.UPDATE...

Updated forecast for today is out. Freshened temperatures and POPs
per latest observed trends and model guidance. Still expect
scattered showers and thunderstorms today, especially this
afternoon and evening. Some thunderstorms will likely be severe
with damaging wind gusts being the primary threat. This is
especially the case over Central and Southwest Montana per latest
model guidance and SPC convective outlook.
Jaszka

&&

.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 become severe today, especially
along and south of a Craig, to Belt, to Hays line. Damaging wind
gusts and large hail will be the primary threat today, especially
across the aforementioned region. High temperatures will cool a few
degrees through Tuesday, before another significant heat wave
develops by late next week.

&&

.AVIATION...
Updated 1720Z

Mainly VFR expected next 24-hours as multiple weather disturbances
cross the area from west to east. Scattered showers and
thunderstorms are expected, especially through 12Z/Tue. Severe
storms capable of hail up to quarter-size and surface gusts up to 60
knots are possible this afternoon and evening, especially over SW
and Central MT. Also, brief MVFR CIGS may accompany any shower or
storm.
Jaszka

&&

.PREV DISCUSSION...
/ISSUED 506 AM MDT Mon Jul 10 2017/

Today through Tuesday night...

Main concern today is with the potential for severe thunderstorms.
Several disturbances will move through the area, with
thunderstorms developing as early as this morning and then
becoming more numerous through the afternoon and early evening
hours, especially over central Montana. The primary severe threat
will be for damaging winds, but marginally severe hail can not be
ruled out with the more organized storms. The other concern for
today will be with the possibility of new fire starts due to
lightning, much as we saw yesterday.

Showers and thunderstorms will continue to diminish in intensity
and coverage through tonight and into Tuesday. With the upper
level trough passage during the day, temperatures will be the
coolest they have been in quite some time, and for at least the
next week. Clearing commences across the region Tuesday night. CC

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 now expected across the
northeastern areas as it appears clouds and thunderstorms will
delay their arrival in those locations. This will result in one last
day of hot and dry conditions with gusty winds. This will be
complicated further by the potential for thunderstorms by 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 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. Cassell

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  89  56  83  53 /  30  30  20   0
CTB  85  53  78  51 /  30  30  40  20
HLN  88  59  86  56 /  50  40  20   0
BZN  87  53  84  51 /  50  30  20  10
WEY  81  44  77  42 /  40  40  40  30
DLN  85  53  83  51 /  40  30  20  20
HVR  95  58  84  54 /  10  20  20  20
LWT  88  55  81  52 /  60  70  20  20

&&

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

&&

weather.gov/greatfalls

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