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Area forecast discussion
National Weather Service Great Falls Montana
523 PM MDT Sat Jul 22 2017

Updated aviation

Synopsis...
mostly dry and warm to hot conditions continue through Sunday, but
a couple of cold fronts will lower temperatures some going into
early next week. Thunderstorms will be isolated Sunday and Monday,
but the chance of thunderstorms may increase some by mid-week.

&&

Discussion...

Rest of today through Monday night...a progressive ridge overhead
this afternoon will keep conditions dry through Sunday morning.
Smoke hasn't been quite as widespread today as yesterday, but is
more dense in localized areas, especially Meagher County east of the
Chinn fire. On Sunday, a warm front will push NE into southern
Canada, allowing temperatures to be the warmest of the weekend. A
Lee-side surface trough will then shift east during the late
afternoon/early evening hours and may provide just enough lift for a
few thunderstorms with gusty winds to develop, mainly central Montana
east of I-15. Sunday evening/night, a sharp cold front/wind shift
will drop south out of Alberta, possibly enhanced by convective
outflow from storms in central Canada. Low/mid level moisture
briefly increases behind this front and, combined with low level
upslope flow may allow a few thunderstorms to develop from gnp to
Cut Bank. Breezy conditions will accompany the front as well.

On Monday, the front will likely stall around the central mountains,
then return north as a warm front, or wash out more or less. Ahead
of the front, there may be enough lift/instability for isolated
thunderstorms along/south of a Helena to Lewistown line. Yet another
front drops south out of Canada Monday night, ushering in seasonably
cooler temperatures. Martin

Tuesday through Saturday...the forecast continues to trend
drier and more stable Tuesday in the wake of low pressure over
Canada speeding away from the area. The one likely exception would
likely be south of Interstate 90, where there will likely be better
moisture and more focused instability toward the Idaho border.
Meanwhile, a closed upper low will continue spinning off the coast
of northern California. This low will help to continue to pump
increasing moisture our way, especially toward Wednesday into early
Thursday, resulting in increasing shower and thunderstorm chances
during that time. Increasing downslope flow off of the Rocky
Mountains will likely result in any convective activity confined to
far eastern and southern areas Thursday afternoon. Continued active
westerly flow aloft through Friday and Saturday is likely to result
in at least some potential for showers/thunderstorms during
afternoon/early evening peak heating. Cassell

&&

Aviation...
updated 2323z.

Mainly VFR expected next 24-hours as high pressure ridging aloft is
gradually broken-down by a weather disturbance approaching from the
pac northwest. Isolated showers and thunderstorms may form over SW Montana after
18z/sun, but the terminals will likely be unaffected. Areas of
wildfire smoke will also persist. However, confidence in visibility
reduced to MVFR or worse remains low. Nevertheless, mountain
obscuration in smoke remains possible.
Jaszka

&&

Fire weather...

Decent mixing is expected once again this afternoon, but winds
aloft are weak/modest and even with deep mixing, winds at the surface
only look to reach 10-15 mph for most areas. Additionally, relative humidity may
only fall to 15 to 25 percent. All this paints more of a marginal
threat, fire weather wise, for today, so we'll hold off on any
red flag warnings.

On Sunday...expect increasing winds over north central Montana along
with hot temperatures and low to very low rh, possibly down into
the single digits for a few locations. Wind gusts may be a bit
more marginal on Sunday, but given the very low relative humidity values
expected, we opted to upgrade the Fire Weather Watch to a red flag
warning and have extended it to include much of central Montana.
Elevated to near-critical fire conditions may return on Monday as
gusty northwest winds develop in the wake of tonight's front. Of note,
that front will bring a sharp wind shift to central Montana Sunday
evening through Sunday night along with a period of gusty northwest/north
winds, especially for zones 112 and 113. Isolated dry
thunderstorms cannot be ruled out ahead of this front in zones 113
and 115 Sunday evening. Next week, the chance of wet thunderstorms
may increase by mid-week. Martin

&&

Preliminary point temps/pops...
gtf 59 95 54 85 / 0 0 10 0
ctb 56 90 50 79 / 0 0 20 0
hln 59 96 58 90 / 0 10 10 20
bzn 53 94 55 90 / 0 10 10 20
wey 44 85 49 82 / 0 10 10 30
dln 52 91 53 88 / 0 10 10 20
hvr 59 98 55 83 / 0 0 20 0
lwt 57 94 57 83 / 0 10 10 10

&&

Tfx watches/warnings/advisories...
red flag warning from noon to 9 PM MDT Sunday central and
eastern Lewis And Clark National Forest areas...Chouteau and
Fergus counties...eastern Glacier/Toole/central/eastern
Pondera/Liberty...Hill and Blaine counties...Lewis And Clark
National Forest Rocky Mountain district-Rocky Mountain front.

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