Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 170431
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1030 PM MDT Sun Jul 16 2017

...Aviation Section Updated...

.UPDATE...

Isolated thunderstorms along a stalled frontal boundary across SW
MT have largely diminished as the boundary slowly sags south
toward the ID/WY border and with the loss of daytime heating.
Surface high pressure continues to builds in from the NW and W
with winds gradually diminishing across northern and central
portions of the forecast area. With winds diminishing, critical
fire weather conditions are improving, however humidity has been
slower to improve as the incoming airmass from the W/NW is still
very dry. A wind shift boundary has been moving steadily south
through southern AB this evening and should cross the border
around midnight, bringing a shift to northerly winds to areas
along the hi-line. Satellite imagery and observations also
indicate a fair amount of low level smoke behind this boundary.
Meanwhile, smoke plume from the Park Creek fire north of Lincoln
will continue to impact southern portions of north-central MT with
additional smoke also moving in from upstream fire activity in
central ID. Hoenisch

&&

.SYNOPSIS...

A stalled-out surface front along a line from near Monida Pass on
the Idaho border to Grass Range in Fergus County will remain in
place tonight and Monday. Scattered showers and thunderstorms
continue to develop along this boundary this afternoon and evening
and again on Monday afternoon. The remainder of the forecast area
is expected to remain mostly clear, dry and warm.

&&

.AVIATION...
Updated 0430Z.

VFR conditions are expected through the 1706/1806 TAF period across
all terminals in North Central and most in Southwest Montana (with
the exception of the KEKS terminal where afternoon showers and
thunderstorms may lower ceiling to near MVFR). Confidence in MVFR
conditions being realized at KEKS is low this far out, so have kept
this TAF package at VFR conditions. Otherwise, breezy winds are
slowly diminishing across the region, with generally light winds
expected for the remainder of the night and into the day on Monday.
A wind shift line was drifting south out of Alberta Canada, and will
primarily affect the KCTB and KHVR terminals with a slight northerly
component to the wind direction. In addition, smoke in the lower
levels was evident behind this wind shift line. Was not confident
enough at this time to introduce smoke for KCTB and KHVR, but did
introduce some lower ceilings to around 6000ft agl at KCTB where
smoke may pool up against the Rocky Mountain Front due to the
nocturnal temperature inversion. Additional smoke is possible across
the region from distant Western CONUS fires, however decreased
visibilities and/or low ceilings are not anticipated. - Moldan

&&

.PREV DISCUSSION...
/ISSUED 520 PM MDT Sun Jul 16 2017/

Tonight through Monday night...A stalled-out surface front remains
along a line from near Monida Pass on the Idaho border to Grass
Range in Fergus County. Scattered showers and thunderstorms will
continue to develop along this line through about mid-evening
before activity tapers off overnight. Another round of
thunderstorms is expected to develop in the same area Monday
afternoon before the front moves off to the southeast and out of
the Great Falls CWA. The remainder of the forecast area remains
under the influence of a (somewhat) cooler airmass associated with
a progressive upper level trough in British Columbia. Cooler high
temperatures (but still near or above seasonal averages) will
prevail on Monday along with mostly clear to partly cloudy skies
and breezy west to northwest winds. mpj

Tuesday through Sunday...A disturbance moving through the westerly
flow aloft may bring a few thunderstorms to the mountain areas of
Southwest Montana Tuesday afternoon/evening. Otherwise, the upper
level high pressure ridge will rebuild north into the forecast area
into Thursday, keeping the area dry and warming temperatures back up
to between 5 and 10 degrees above normal. Medium range models are
showing some disagreement for the Thursday into Friday period
regarding the timing and placement of a shortwave trough moving
along or just north of the Canadian border. The GFS keeps it more so
along the border and moves it through Thursday into Thursday night,
which would bring a good chance of breezy westerly winds, especially
to the Rocky Mountain Front. The ECMWF and CMC models keep it a bit
farther north of the border and move it through on Friday. Recent
trends have leaned a bit closer to the GFS solutions, so have
focused the increased winds to the Thursday afternoon/evening time
frame. After the passage of the shortwave, the high pressure ridge
remains flattened over Montana through Sunday, resulting in a
relatively dry westerly flow aloft. Because of the differences in
timing of the shortwave, the forecast models differ somewhat on
temperature trends as well. The ECMWF keeps temperatures on the warm
side Thursday into Friday, then cools temperatures a bit for the
weekend. The GFS cools temperatures for Thursday and Friday, then
warms them up for the weekend. With this kind of discrepancy, will
stay with the model blend solution of a more broad-brushed period of
5-to-10-degree-above-normal temperatures (i.e. mostly upper 80s to
lower 90s for highs at lower elevations - still quite warm, but not
as hot as recent temperatures). Will look to fine tune this portion
of the forecast once models come into better agreement. Regardless,
this kind of an extended dry and warm period will only increase fire
weather concerns. Fortunately, at this point, there is very little
chance of additional thunderstorms.
Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  90  52  84  51 /   0   0   0  10
CTB  84  47  80  45 /   0   0   0   0
HLN  91  57  85  54 /   0   0   0  10
BZN  91  51  84  50 /  10  10  10  20
WEY  82  45  80  41 /  80  30  20  20
DLN  86  52  83  50 /  20   0  20  10
HVR  96  53  85  51 /   0   0   0   0
LWT  93  53  81  51 /  60  10  10  20

&&

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

$$

weather.gov/greatfalls



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