Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 240440
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1040 PM MDT Wed Aug 23 2017

...Update to Aviation...

.SYNOPSIS...
With an upper level ridge moving east tonight and a weak upper
level trough approaching the region, a few isolated thunderstorms
will be possible over Southwest MT. The storms will become a bit
more widespread and also affect areas further northward on
Thursday. Afternoon highs for Thursday and Friday will be slightly
cooler than they have been, with highs in the upper 70s to lower
80s. However, warmer temperatures and dry conditions return over
the weekend and into the first half of next week.

&&

.UPDATE...

Latest satellite imagery shows a small circulation over eastern
ID, possibly thunderstorm-induced. This feature should continue to
lift slowly north overnight with isolated showers and perhaps a
thunderstorm remaining possible over SW MT through the night.
Overall, though, the loss of daytime heating looks to limit the
coverage of showers as there doesn`t appear to be significant lift
associated with this feature. The previous forecast appears to
have a good handle on the situation and no major changes were
needed for the update.

Taking a quick look at Thursday, the best chance of seeing a
stronger thunderstorm looks to be along/south of a Great Falls to
Lewistown line where the best shear/instability combo is likely.
Gusty winds should be the primary threat. Of note, though,
precipitable water values look to exceed the 90th percentile for
this time of year and any sustained storm could produce locally
higher rainfall amounts (maybe up to 0.25"+). MARTIN

&&

.AVIATION...
Updated 0440Z.
Westerly flow aloft will continue to bring scattered to broken mid
and high level clouds to the forecast area with widespread VFR
conditions prevailing. However, some areas of reduced visibility due
to wildfire smoke could cause periods of MVFR conditions, mainly at
KHLN and KGTF as well as in the valleys of southwest Montana.
Scattered showers and thunderstorms are expected across mainly
western and southern portions of the forecast area after 18Z
Thursday and continuing through about 06Z Friday. mpj

&&

.FIRE WEATHER...
The wind has continued to decrease across most fire zones this
evening and the Red Flag Warning that was in effect for fire weather
zones 112, 114, and 117 has been cancelled early. Of note, though,
breezy conditions will continue through the night across the western
half of fire weather zones 112 and 114. Relative humidity will
recover some in that area, but remain generally poor.

On Thursday, thunderstorms are possible but no highlights are
planned for at this time. After some slightly cooler temperatures
on Friday, it then turns mostly dry and warm again over the
weekend and through the first half of next week. Brusda/MARTIN

&&

.PREV DISCUSSION...
/ISSUED 551 PM MDT Wed Aug 23 2017/
Tonight through Friday...The upper level ridge is moving eastward,
as a an upper level disturbance moves into Southwest MT. Most of
the storms this evening will affect areas mainly south of
I-90...but an isolated storm could develop as far north as the
Helena area. On Thursday, an upper level trof will move through,
cooling temperatures a bit and allowing for a slightly better
chance for storms to develop over portions of Central and
Southwest MT. A few of the storms will linger on Friday over the
eastern portions of the region...but much of the area will be
dry...along with seasonable temperatures. Brusda

Friday night through Wednesday...Overall a warming and dry
pattern will be found through the long term, with elevated fire
weather danger to continue. Friday night through Saturday we will
find ourselves in the wake of Thursday/Friday`s disturbance. This
will bring weakly unstable zonal flow with a weak jet max
overhead. The result will bring some lingering chances for
showers and thunderstorms across southern and eastern portions
during this time period. A large area of high pressure will then
build across much of the western US starting Sunday and
continuing through at least the rest of the long term period. The
result will be warming temperatures and mainly dry conditions.
Afternoon RH values will continue to be low. This combined with
normal afternoon breezes may continue at least elevated fire
weather danger through the long term. A slight shift in location
of this high pressure on Tuesday and Wednesday may bring just
enough moisture and instability for isolated thunderstorms across
western and southern portions. With pwats expected to be low,
this looks to be mainly a dry thunderstorm threat, perhaps
further enhancing fire weather danger. Temperatures will start
near normal on Saturday, then start a warming trend with the
mentioned high pressure. Highs in the 90s are possible by next
week...especially Monday through Wednesday.
Anglin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  61  85  51  81 /   0  20  20   0
CTB  58  82  46  77 /   0  20  10   0
HLN  60  83  54  83 /   0  30  30  10
BZN  55  81  49  81 /  20  50  40  20
WEY  48  70  35  73 /  10  40  40  20
DLN  54  80  45  80 /  20  20  30  10
HVR  60  91  53  81 /   0  20  30   0
LWT  60  87  53  79 /  10  40  40  10

&&

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

$$

weather.gov/greatfalls


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