Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 212057
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
257 PM MDT Fri Jul 21 2017

.SYNOPSIS...

Dry and breezy conditions prevail today, along with areas of smoke
and haze. Temperatures will be close to average. Mostly clear and
gradually warmer weather continues into the weekend. A better
chance of thunderstorms may return next week.

&&

.DISCUSSION...

Rest of today through Sunday...The main concern in the short term is
fire weather and related impacts from smoke. Gusty westerly winds
continue this afternoon, with 20-35 mph gusts common across much of
central MT. Winds have been gusty at times across SW MT, but not as
strong as further north. Winds should drop off fairly quickly this
evening as winds aloft decrease and as the nocturnal temp inversion
sets in. RH values have once again dropped into the 8-15 percent
range and combined with the gusty winds makes the Red Flag Warning
valid, so no change there (see the Fire section below for additional
fire weather details). As for smoke impacts, it varies across the
CWA, but has been dense enough to reduce the visibility at times
(note the recent observation at Dillon where the visibility was down
to 4 miles). Smoke will continue to push into SW MT and southern
sections of central Montana through this evening. Tonight, smoke may
also move into the area from fires in Canada. Given all this, I kept
a mention of patchy smoke in for the entire area overnight. The
inversion tonight may actually help to lock in the smoke that
advects in this afternoon.

For the rest of the weekend, ridging builds back in with steadily
warming temperatures. Other than continued smoke and heat impacts,
significant weather impacts are not expected. MARTIN

Sunday night through Friday...As we get into the beginning of next
week, models are in good agreement in a break in the long duration
weather pattern. Beginning with Monday and Tuesday, the upper level
pattern will feature a quick moving shortwave to our north, combined
with weak cut off closed upper low spinning of the California
Coastline. At the surface, a cold front will swing through the area
during early hours on Monday, followed by gusty winds during the day
with only an isolated chance at a shower or storm. However by
Tuesday, the aforementioned closed upper level low will begin to get
absorbed in the westerlies thanks to an approaching trough off the
coast of British Columbia. At the surface, it is likely several
clusters or waves of showers and thunderstorms will develop well to
our west, delivering periods of showers and storms to the forecast
area given persistent SW to NE flow aloft, and increasing moisture.
Details on exact timing and how much rain these showers and storms
will produce is still in question. Additionally, some of the storms
may be strong to severe at times Tuesday afternoon and evening given
modest surface based instability and weak, but notable shear aloft.

This active pattern will transition into Wednesday and Thursday as
what`s left of the shortwave moves in delivering more rounds of
showers and storms. I`ve kept pop chances on the higher end of most
model guidance as precipitation at this time looks to be widespread,
with better chances over central and southern areas. The exciting
news is this will be helpful given the drought situation areawide.
However, fire weather conditions will need to be monitored closely
with increased lightning activity. By Friday, we return to a zonal
pattern aloft with slightly drier conditions returning. KLG

&&

.AVIATION...
Updated 1742Z.

VFR conditions prevail under mostly clear skies. Breezy west winds
develop through the early evening hours. Visibility may become
reduced to near 6 miles at times as smoke from regional wildfires
mixes through the atmosphere. For now, we kept FU (smoke) out of the
TAFs, but introduced a FEW050 layer to highlight the possibility.
Amendments may be necessary if confidence increases regarding
reduced VIS, especially tonight as the inversion traps
incoming/ongoing smoke. PN/MARTIN

&&

.FIRE WEATHER...

Critical or near-critical fire weather conditions continue to be
experienced across a large portion of our fire weather zones at this
time. Even in areas where the wind is more marginal, the RH has once
again fallen to very low levels, in some cases below 10 percent.
Critical fire weather conditions should continue into early this
evening, but a temp inversion should allow a quick decrease in the
winds after 2000 MDT. RH recovery looks to be a bit better over the
weekend compared to last night, but still not great. Temperatures
steadily warm over the weekend, but the wind does not look to be
nearly as gusty as the past couple of days. Although, Sunday could
be at least an elevated day with a slight increase in SW winds
combined with lower elevations warming into the 90s. It is worth
noting that confidence continues to increase in a pattern change
next week with a greater potential of wet thunderstorms (although,
still scattered in coverage). Lightning activity will increase as
well, so perhaps a mixed blessing. MARTIN

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  53  88  57  96 /   0   0   0   0
CTB  50  85  54  90 /   0   0   0   0
HLN  54  90  59  96 /   0   0   0   0
BZN  48  87  52  94 /   0   0   0   0
WEY  43  80  40  84 /   0   0   0  10
DLN  49  86  51  91 /   0   0   0   0
HVR  54  90  58  97 /   0   0   0   0
LWT  52  85  56  94 /   0   0   0   0

&&

.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...Eastern Glacier/Toole/Central/Eastern
Pondera/Liberty...Helena and Townsend Ranger Districts of the
Helena National Forest...Hill and Blaine Counties...Lewis and
Clark National Forest Rocky Mountain District-Rocky Mountain
Front...Lincoln Ranger District of the Helena National Forest.

&&

$$

weather.gov/greatfalls


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