Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 240356
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
955 PM MDT Sun Jul 23 2017

UPDATED AVIATION

.SYNOPSIS...
A Canadian cold front will move across the plains tonight and result
in cooler and breezy conditions for Monday. Thunderstorm chances
return to start the week especially over central and southwest
Montana. Temperatures will remain slightly above-normal before more
hot weather returns by late week.

&&

.UPDATE...Latest models, satellite imagery and surface
observations continue to support a weak and dry surface cold
front pushing south from Canada by morning. So far, quiet
conditions have prevailed this evening and early evening models
have dropped any suggestion of thunderstorms overnight. Have
therefore updated forecast to remove PoPs and mention of dry
thunderstorms, although Monday afternoon and evening could still
see an isolated storm or two, mainly south and east of the Little
Belt Mountains. I also made some minor adjustments to overnight
lows and forecast high temperatures for Monday as well as sky
condition tonight through late Monday afternoon based on short
term model consensus tools. Finally, allowed the Red Flag Warning
(RFW) to expire at 9 PM MDT as planned. RFW and a Fire Weather
Watch remain in place for Monday afternoon and evening for the
Great Falls fire weather forecast area. I am not convinced that
widespread critical fire weather conditions will be met, but will
leave them in place to allow the midnight shift to make the final
decision. mpj

&&

.AVIATION...
Updated 0355Z

VFR expected next 24-hours. Southwesterly to westerly flow aloft
will persist as a disturbance advances E`ward over western Canada
and another approaches from the Pac NW toward the end of the TAF
period. A surface cold front accompanying the former of the two
disturbances should move SE`ward across the Hi-Line between 06Z and
10Z/Mon before reaching the Central MT mountains by 15Z/Mon, where
the front should dissipate soon thereafter. Surface winds will veer
from southwesterly to northwesterly with the front`s passage. While
a stray shower or thunderstorm accompanying the cold front cannot be
ruled-out, the probability is very low. A better chance for isolated
showers and thunderstorms exists over/near the higher terrain of
Southwest and Central MT after 18Z/Mon.
Jaszka

&&

.FIRE WEATHER...
A cold front will move southward through the region during the
overnight hours tonight. Expect a wind shift to the north over the
plains east of Interstate 15. The winds will be strong/gusty at
times overnight...especially over North Central MT. On Monday
expect breezy west winds to develop by the afternoon hours along
with low humidity, with critical fire weather conditions expected
along the Rocky Mountain Front and along and west of the Big Belt
Mountains, including Helena National Forest and western Lewis and
Clark National Forest. Additionally there is a small chance for a
passing dry thunderstorm, mainly south and east of the Little
Belts. Some storms could produce strong gusty erratic wind gusts.
Brusda/Cassell

&&

.PREV DISCUSSION...
/ISSUED 446 PM MDT Sun Jul 23 2017/

Rest of today through Monday night...Isolated thunderstorms are
developing at this time over the mountains surrounding Ennis and
Bozeman in association with a compact upper low moving through the
area. This evening, a few thunderstorms may develop further north
across parts of central Montana. The threat is conditional on there
being just enough sufficient moisture, as well as the timing of any
lifting mechanism in the form of a shortwave disturbance or possibly
a weak lee-side SFC trough. Over southern Alberta, a SFC low
continues to develop and will push east tonight, sending a cold
front quickly SE across the area. Breezy winds will likely accompany
the front late tonight/early Monday morning across central Montana
and will usher in lower temperatures (ie. about 10 degrees or so
lower than today). While SW MT may see a slight lowering of temps on
Monday, it doesn`t look nearly as breezy as farther north.

On Monday, a s/w moving quickly through the area may kick off a few
thunderstorms, mainly along/south of a Helena to Lewistown line. The
primary threat with any thunderstorm through Monday should be gusty
winds. At this time, severe weather is not anticipated. CC/MARTIN

Tuesday through Sunday...Latest model guidance continues to
illustrate a notable drying trend through the long range. Beginning
with Tuesday and early Wednesday, a closed upper low rotating off
the northern Coast of California will begin to get pulled slowly
northwards in response to weak ridging breaking down in SW Canada.
This shortwave will help feed increasing tropospheric moisture into
the area overnight Tuesday with small, perhaps terrain enhanced
nocturnal clusters of showers and storms making its way as far north
as SW parts of the CWA through daybreak Wednesday. It is looking
like by the daytime hours on Wednesday that most areas will remain
dry as the aforementioned weak ridge holds tight over the majority
of the area. However, by later in the afternoon and evening,
increasing spotty convection area wide will become a good bet, with
a few storms being strong given modest instability across central
and southern areas. Another important note will be lightning
activity as recent drought conditions may enhance the possibility of
new fire starts. Also, breezy conditions are looking possible
Thursday afternoon and evening with the passage of the shortwave
trough that may exacerbate fire weather conditions.

We begin to see a pattern shift beginning on Friday and lasting into
next weekend with the return of the dominant four-corners ridge in
the desert southwest. With good agreement on most long range model
guidance, I`ve started to bump temperatures up enough to reflect
this with highs easily reaching the lower to mid 90s area wide, with
perhaps upper 90`s for some through next weekend. Of course this
will continue to be fine-tuned as we get closer in the week. KLG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  58  85  51  84 /  10  10  10   0
CTB  52  80  46  81 /  10  10   0   0
HLN  62  91  57  88 /  10  10  10  10
BZN  56  90  52  86 /   0  10  20  10
WEY  46  81  45  77 /  10  20  20  30
DLN  55  87  53  84 /   0  10  20  20
HVR  59  83  51  85 /  10  10  10   0
LWT  60  84  53  82 /  10  10  20   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 1 PM to 9 PM MDT Monday Eastern
Glacier/Toole/Central/Eastern Pondera/Liberty...Helena and
Townsend Ranger Districts of the Helena National Forest...Lewis
and Clark National Forest Rocky Mountain District-Rocky Mountain
Front...Lincoln Ranger District of the Helena National Forest.

Fire Weather Watch from Monday afternoon through Monday evening
Central and Eastern Lewis and Clark National Forest Areas...
Chouteau and Fergus Counties...Hill and Blaine Counties.

&&

$$

weather.gov/greatfalls


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