Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 280536

Area Forecast Discussion
National Weather Service Great Falls MT
1130 PM MDT SAT AUG 27 2016

Aviation Section Updated.

Have made minor adjustments to the PoP grids. Westerly flow aloft
will continue to bring scattered to broken mid and high clouds
across the area overnight. Satellite imagery showing a chinook
arch that has developed in the lee of the Rockies. Radar returns
showing echoes moving across the Hi-Line but any precipitation is
not likely reaching the ground due to the extremely dry lower
atmosphere and the downslope effects of the westerly flow aloft.
Gusty southwest to west winds will continue to decrease.


Westerly flow aloft will continue to bring scattered to broken mid
and high clouds across the area. A weak frontal boundary will
shift winds to northwesterly on Sunday. VFR conditions prevail.



Tonight through Monday...Weak energy and moisture in a westerly
flow aloft will keep areas of clouds over the forecast area
through Sunday with very little chance of showers or
thunderstorms. The breezy westerly winds currently across the area
will decrease gradually tonight, but overnight low temperatures
will only fall to between 5 and 10 degrees above normal. Areas of
smoke have also been blown east of the Continental Divide and
continue to move into parts of Southwest Montana (with an Air
Quality Alert in effect for the West Yellowstone area and reduced
visibility in Dillon). Have therefore added mention of areas of
smoke to these areas through at least this evening. However, the
winds will shift more northerly across Central Montana on Sunday
with the passage of a weak Canadian cold front. This wind shift
will decrease the smoke and cool temperatures to around normal
with just a slight chance of an afternoon/evening shower or
thunderstorm over the mountains of Southwest and Central Montana.
Continued westerly winds in the southwest will keep smoke there at
least into the day on Sunday. An upper level ridge of high
pressure will then build into the state Sunday night into Monday
for an overall decrease in cloudiness, allowing night time
temperatures to cool back closer to normal. However, as the ridge
axis moves east of the area on Monday and an inverted surface
trough develops over the northern Rockies, temperatures will warm
back into the 80s for lower elevations. The resulting
southwesterly flow aloft will combine with weak instability during
the afternoon to bring a very slight chance of a thunderstorm to
the Rocky Mountain Front. Afternoon relative humidity will fall
into the teens again on Monday, but relatively light winds will
limit fire weather concerns.  Coulston

Monday Night through Saturday...Period begins with dry and very
warm conditions as high pressure ridge that was over us on Mon
drifts into the Dakotas on Tues. This puts our forecast area in
the warm southwest flow aloft on the west side of the ridge,
pushing high temps back into the upper 80s and low 90s on Tues
aftn. Remnants of the ridge will linger over central MT into Wed,
but from midday Wed into the weekend, our main weather-maker will
be the broad Pacific trof that will extend from the AK/BC coast
all the way to southern California. Minor shortwaves moving
through the southwest flow ahead of the trof axis will combine
with a mix of Pacific and monsoonal moisture to generate isolated
to scattered thunderstorms each aftn from Wed-Fri. Models still
struggling to find agreement on when the main portion of the
Pacific system will come ashore and how much moisture will
accompany it. In yesterday`s model runs, the GFS was slower in
advancing the trof inland, but is now fairly similar to the ECMWF
solution for timing, but is considerably wetter than the ECMWF on
precip coverage and amount. Latest forecast raises the
probability of precip on Saturday, with 25-35% values covering
the entire county warning area through Sat night. Temperatures
will trend steadily downward through the latter half of the week
as each succeeding shortwave passage brings increased cloud cover
and slightly cooler air down from Canada. Highs will be back in
the mid 80s by Thurs and the low 70s by Sat.


GTF  60  79  52  88 /   0   0  10   0
CTB  52  72  47  84 /   0   0  10  10
HLN  59  83  54  89 /   0  10  10   0
BZN  51  85  48  88 /   0  10  10   0
WEY  42  74  41  77 /   0  10  10   0
DLN  51  83  47  85 /   0   0  10  10
HVR  57  77  52  86 /   0   0   0   0
LWT  57  80  52  84 /   0  10  10   0



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