Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 161627
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1027 AM MDT Sun Jul 16 2017

.SYNOPSIS...

A cold front will gradually settle south into Southwest Montana by
tonight. Along the front, it will be breezy across central Montana
this afternoon and early evening. Meanwhile, scattered thunderstorms
are expected further south across Southwest Montana. The front
eventually pushes through with a lower chance of thunderstorms by
next week. Temperatures stay warm into next week.

&&

.UPDATE...Previous forecast required quite a bit of changes this
morning, mainly to remove or adjust PoPs in a few areas and to
increase forecast high temperatures for today. Dry, warm and windy
conditions are expected across most of central and north central
Montana today. Only exception in this area will be a 20 to 30
percent chance for afternoon and evening thunderstorms across the
southern third of central Montana. Southwest Montana remains the
focal point for convection today and tonight. Have also made some
significant changes to raise PoPs in the far southwest adjacent
to Yellowstone Park where short term model consensus tools are
indicating will be the most favored location for precipitation.
Have also made adjustments to sky condition as based on satellite
imagery and PoP forecast. The Red Flag Warning continues to look
on-track so have continued it through this evening. mpj

&&

.AVIATION... Updated 1131Z.

The main aviation concerns today will be 1) gusty winds developing
across central MT and 2) scattered TSRA across parts of SW MT. The
wind will steadily increase this morning, remaining gusty through
this evening. Across SW MT, confidence is highest across far SW MT
in the vicinity of KWYS/KEKS. Along and south of a KHLN to KLWT
line, TSRA will also be possible, but confidence is too low to
include at KHLN/KLWT/KBZN for now. Trends will be monitored through
the afternoon hours. Any TSRA that develops today will be capable of
gusty/erratic winds, hail, and brief reductions in VIS. TSRA should
push east by Midnight, although an isolated SHRA or TSRA cannot be
ruled out across SW MT with a front lingering in the area. MARTIN

&&

.FIRE WEATHER...

Critical fire conditions continue to be expected today across much
of central Montana and a small part of SW MT. Hot temperatures
and increased lightning activity over the past couple of days will
be followed by increasing winds by this afternoon, especially
central Montana. The increasing winds should offset the lack of a
higher fire danger class and is the reason the ongoing Red Flag
Warning for fire zones 112, 113, 114, and 115 was expanded to
include fire zone 116 and part of 117. Of note, Meagher and Park
Counties are NOT included in the Red Flag Warning for zone 117 as
those two counties have a lower fire danger class and winds are
not expected to be quite as strong there. That said, elevated fire
concerns are certainly possible there, especially in any area that
saw more lightning activity over the past couple of days. Beyond
today, the wind will decrease through early next week, but breezy
conditions may return mid to late week along with warming
temperatures, potentially creating additional fire weather
concerns. MARTIN/Coulston

&&

.PREV DISCUSSION...
/ISSUED 531 AM MDT Sun Jul 16 2017/

Today through Monday Night...Latest SFC analysis shows a Pacific
cold front arcing from SW AB through western MT/western ID as of
4am. Ahead of it, thunderstorms will primarily be focused over SW MT
(along and south of a Helena to Lewistown line). Like yesterday, the
better shear will be somewhat removed from the better instability.
That said, shear across SW MT should be more than sufficient for
organized thunderstorms with gusty winds and hail possible. The
threat of a few severe storms would likely be higher if better
instability can be realized. At this time, though, a lack of better
shear/instability should keep the threat isolated. A few
thunderstorms may linger in this area on Monday, but instability
looks even weaker then.

Across central Montana, increasingly dry low/mid levels should allow
deep mixing to occur, bringing down the increasing winds aloft. All
this spells a breezy late-morning through early evening period with
gusts of 25-35 mph common. This should also lead to downsloping and
limit the thunderstorm potential. Winds aloft relax on Monday with a
subsequent decrease in SFC winds. See the fire section below for how
this will impact those with fire interests. MARTIN

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  49  83  51 /   0   0   0  10
CTB  81  45  77  45 /   0   0   0   0
HLN  89  54  85  54 /  20  10   0  10
BZN  89  49  84  50 /  30  30  10  20
WEY  81  41  79  41 /  30  30  20  10
DLN  87  49  82  50 /  20  10  20  10
HVR  93  49  84  51 /   0   0   0   0
LWT  89  51  81  51 /  20  30  10  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 9 PM MDT this evening Central and Eastern
Lewis and Clark National Forest Areas...Chouteau and Fergus
Counties...Hill and Blaine Counties...Lincoln Ranger District of
the Helena National Forest.

Red Flag Warning until 9 PM MDT this evening Eastern
Glacier/Toole/Central/Eastern Pondera/Liberty...Lewis and Clark
National Forest Rocky Mountain District-Rocky Mountain Front.

&&

$$

weather.gov/greatfalls



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