Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 221059
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
455 AM MDT Tue Aug 22 2017

...UPDATED AVIATION SECTION...

.SYNOPSIS...

A high pressure ridge will keep the region dry and warmer than
normal with light winds into Wednesday. However, an increasingly
moist and unstable southwest flow aloft will bring a chance of
thunderstorms to southwest Montana on Wednesday, then to most of
the region for Thursday. Westerly winds will also become breezy
along the Rocky Mountain Front on Wednesday, then they will spread
over most of the area on Thursday behind a Pacific cold front.
Highs in the 80s to lower 90s are likely today and Wednesday, but
they will cool into the mid 70s and 80s on Thursday. Lows will be
in the mid 40s and 50s.

&&

.DISCUSSION...

Today through Thursday...High pressure aloft will keep the area
dry with light winds and temperatures around 10 degrees above
normal into the day on Wednesday, but more unsettled conditions
are expected Wednesday afternoon into Thursday. A disturbance will
eject from a weak low pressure area along the California coast
Wednesday morning and move through the ridge Wednesday afternoon
and evening, bringing a chance of thunderstorms to southwest
Montana. A low pressure trough will also move east from the
British Columbia coast on Wednesday onshore into BC and Alberta on
Thursday. As a result, the flow aloft will increase and become
more southwesterly over the forecast area, initially bringing
breezy westerly winds to the Rocky Mountain Front on Wednesday.
The flow aloft will then become increasingly moist and unstable
Wednesday night into Thursday. A Pacific cold front moving
through the area on Thursday will also cool temperatures closer
to normal and bring a chance of showers and thunderstorms. The
best chance will be along and south of a Great Falls to Lewistown
line. Moderate instability and wind shear may cause some storms to
produce strong gusty winds and some hail. Precipitable water
values are predicted to be higher than normal for this time of
year, so heavy rainfall could accompany the storms as well.
Another concern on Thursday will be post-frontal winds. Forecast
models indicate that mid level winds of around 40 kt are possible,
which could translate down to the surface along the Rocky
Mountain Front as wind gusts Thursday afternoon. Although high
winds are not expected, the dry and breezy conditions Wednesday
into Thursday could warrant fire weather highlights (please see
the Fire Weather section for more details). Coulston

Thursday Night through Tuesday...The aforementioned upper trough
still looks to continue digging over our region Thursday night
through Friday morning, allowing isolated to scattered showers
and thunderstorms to persist. Thereafter, the trough should begin
lifting northeastward Friday afternoon as high pressure aloft
builds- in from the south and west. Accordingly, the potential
and coverage of showers and thunderstorms should dwindle as
Friday elapses. Drier weather is then expected during the
upcoming weekend through next week Tuesday as the aforementioned
high pressure ridge aloft continues building over the area.
Temperature-wise, near-normal lows are expected throughout the
period. Behind the aforementioned cold front, Friday will feature
highs mainly in the 70`s to near 80 degrees, which will be
several degrees below-normal for many places. The building high
pressure ridge should then yield near-normal to slightly above-
normal highs this weekend through Tuesday of next week, with many
locations experiencing readings in the 85 to 90 degree range.
Jaszka

&&

.AVIATION...
Updated 1055Z.

VFR conditions are expected to continue at least through 12Z
Wednesday. High pressure aloft will keep skies clear but hazy with
light winds. However, wildfire smoke will reduce visibility somewhat
across southwest Montana, possibly as low as 5SM (high MVFR) at
times. FYI: the KEKS site is observing a mid level cloud layer (040
to 070 AGL) between the hours of 02Z and 13Z, but it is likely not
an actual cloud layer but a smoke layer aloft that is detected
during the overnight inversion.
Coulston

&&

.FIRE WEATHER...

Fire weather concerns heighten this week due to hot and dry
conditions through Wednesday and the passage of a Pacific cold
front on Thursday. Above normal temperatures will cause afternoon
relative humidity values to fall well into the teens today and
Wednesday, and maybe approaching single digits for some areas on
Wednesday. Overnight humidity recovery at mid-slopes to ridgetops
will also become increasingly poor through Wednesday night. A low
pressure trough moving into British Columbia Wednesday into
Thursday will bring an increasing southwest flow aloft. These
winds may start to translate to the surface along the Rocky
Mountain Front on Wednesday, then spread out over the rest of the
plains on Thursday and possibly into Friday behind a Pacific cold
front. Fire weather concerns on Wednesday lean more heavily on the
very dry conditions than on the breezy winds, while winds on
Thursday will be stronger than those on Wednesday, but with less
critical dryness. The southwest flow will also become moist and
unstable, bringing a good chance of thunderstorms to the area
Thursday into Thursday night. Some of these storms may cause new
lightning ignitions, but the precipitable water is forecast to be
above normal during this time, which may cause heavy rainfall.
This situation will be monitored closely over the next few days
and we will coordinate with fire officials to determine if fire
weather highlights are warranted.  Hoenisch/Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  90  54  93  57 /   0   0   0   0
CTB  88  50  91  56 /   0   0   0  10
HLN  90  55  92  59 /   0   0   0  10
BZN  89  50  89  53 /   0   0   0  10
WEY  80  35  76  37 /   0   0  20  20
DLN  87  49  85  51 /   0   0  10  20
HVR  90  52  94  57 /   0   0   0   0
LWT  87  53  90  56 /   0   0   0  10

&&

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

$$

weather.gov/greatfalls



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