Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
220 PM MDT Fri Sep 13 2019

.SYNOPSIS...

A fast moving weather disturbance tracking east along the
international border will drag a cold front east across the region
late this afternoon and evening. Gusty west to southwest winds
will continue into this evening before diminishing and a few
showers or isolated thunderstorms are also possible across north-
central Montana this evening. Dry and breezy conditions are likely
again Saturday before temperatures warm well above average Sunday
and Monday.

&&

.DISCUSSION...

Tonight through Saturday...

A fast moving shortwave trough tracking east along the International
border will continue to bring breezy to windy conditions through early
this evening before winds diminish after sunset. A cold front
associated with the shortwave moving east off the Rockies this
afternoon will continue across northern and central portions of MT
through early this evening while surface low pressure tracks east
across southern AB/SK. Widely scattered showers and isolated weak
thunderstorms are beginning to develop across western portions of
north-central MT this afternoon as cooling aloft moves east with
the shortwave and instability increases. Despite only weak/shallow
instability, the presence of an upper level jet along the AB/MT
border and moderate wind shear could lead to some strong winds
associated with these showers and thunderstorms, while fast motion
and only limited storm height should limit rain/hail potential.
These showers and storms track east across primarily north-central
MT through early this evening and and hi-res models continue to
target Choteau, Hill and Blaine counties for the bulk of this
activity along with greatest potential for associated strong wind
gusts. Surface winds diminish tonight behind the departing
shortwave but a fairly strong westerly flow aloft remains in place
through Saturday with breezy to windy conditions likely again
Saturday afternoon, especially across north-central MT. Hoenisch

Sunday through Monday night...primary forecast concerns are high
temperatures on both Sunday and Monday, and increasing precipitation
chances Monday afternoon through Monday night.

Sunday morning begins with a large H500 closed low off of the
Pacific Northwest/Southwestern British Columbia Coasts and broad
ridging over the Northern High Plains. As the day progresses on
Sunday, 100-150m H500 height falls will spread south southeast along
the Pacific Coast as the trough digs towards Southern California. As
the H500 trough digs, the upper level ridge will amplify and broaden
further over the center of the CONUS, placing Southwest and North
Central Montana on the upstream side of the ridge. 1000-500mb
thicknesses will climb to around 5700-5800m over the region, which
will result in high temperatures on Sunday in the 80s to near 90
degrees, which is approximately 10 to 20 degrees above normal for
the middle of September. As the trough axis slides east northeast
into the Great Basin during the day on Monday, southwest flow aloft
will begin to advect Pacific moisture in across Southwest Montana.
This will lead to increasing chances for showers and storms over
Southwest Montana and along the Divide from Monday afternoon and
through the overnight hours. In addition to the chances for
precipitation, high temperatures on Monday will once again soar some
10 to 20 degrees above normal across most of Southwest and North
Central Montana.

Tuesday through Friday...period begins with H500 trough lifting
northeast across the Northern Rockies and into the Northern Rockies
by Wednesday morning, however, models begin to diverge with the
strengthen and placement of the trough axis/closed low over this
timeframe. None-the-less, expect temperatures to fall back closer to
climatological norms over this timeframe, with at least some slight
chance/chance PoPs across the CWA. Models diverge even further
beyond Wednesday, which has led to an overall low confidence
forecast for the second half of the next work. - Moldan

&&

.AVIATION...
Updated 1157 AM MDT Fri Sep 13 2019 (13/18Z TAFs)

Mostly VFR conditions are expected throughout this TAF period but
brief periods of MVFR conditions may be introduced with any
shower/thunder activity this afternoon and evening. A passing
disturbance will keep gusty southwest to west winds going through 3Z
this evening. As mentioned above, there is still a chance for
isolated showers and perhaps few thunderstorms this afternoon
through the evening hours across Central and North Central Montana.
Left the mention of thunder out for all tafs except KHVR, where
confidence is highest for thunderstorm activity. Mountain wave
turbulence is possible through this evening hours along the Rocky
Mountain Front, with some mountain obscuration possible due to
increasing mid- to upper level cloudiness. Gusty southwest to west
winds will return on Saturday, especially for the north-central
terminals.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  50  76  52  88 /   0   0   0   0
CTB  48  71  50  82 /   0   0   0   0
HLN  49  78  50  87 /   0   0   0   0
BZN  43  79  45  85 /  10   0   0   0
WEY  33  71  34  77 /   0   0   0   0
DLN  44  77  46  82 /   0   0   0   0
HVR  48  78  51  89 /  20   0   0   0
LWT  46  75  50  85 /  30   0   0   0

&&

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

http://www.weather.gov/greatfalls

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