Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 260456

Area Forecast Discussion
National Weather Service Great Falls MT
1055 PM MDT Mon Sep 25 2017



Based on radar trends and observations from our office before
sunset, decided to expand slight chance POPs for scattered showers
across western portions of North-Central MT, ahead of a subtle
disturbance embedded in NNW flow aloft. The best chance for these
showers will be before midnight and rainfall amounts should be no
more than a few hundredths of an inch. Otherwise, the rest of the
forecast appears valid.



Isolated showers will move southeast across parts of the area this
afternoon through Tuesday afternoon, mainly in the mountains and
just downwind of northwest facing slopes. After Tuesday, drier and
milder conditions are expected.



Rest of today through Wednesday...No high-impact weather is expected
over the next few days.

A slow-moving upper level trough stretches from Baja California NE
through the Northern High Plains. This trough has been slow to move
much in recent days, but will continue to slowly push east as
ridging builds in from off the Pacific. In between the two features,
NW flow aloft will prevail. Low to mid level moisture within this
flow will allow a few showers to develop this afternoon through
Tuesday afternoon, mainly focused in the mountains as well as
plains/valleys downwind of NW facing slopes. Any precip amounts will
be very light. Of note, there may be enough instability for a few
lightning strikes, but confidence is too low to include any mention
of thunder in the forecast for now. With the ridge building in,
temperatures will continue to moderate, peaking Wednesday and
beyond. If clouds clear out Tuesday night, patchy fog in SW MT
valleys is certainly possible. MARTIN

Wednesday Night through Monday...Medium range models agree on moving
a weak Rex-blocking pattern over the forecast area for Wednesday
night through Thursday night. The high pressure area remains over
Montana and the low over the Four-Corners area, keeping the forecast
area dry. It is forecast to weaken and move east out of the area
Friday into Friday night, but a lingering dry and stable airmass
will continue the dry conditions. The models also agree on moving a
longwave trough through the area Saturday into Sunday, which should
bring increasing southwest winds, and a chance of mainly mountain
showers, with possible thunderstorms in southwest Montana. There are
some differences in the solutions moving into the Monday timeframe.
The local model blend indicates the potential for continuing
wrap-around showers as the upper level low closes off to our
northeast. Temperatures will trend warmer for the mid to late week
period, going from near normal Wednesday night/Thursday to between 5
and 10 degrees above normal for Friday/Friday night. They will then
cool back below normal for the weekend with the passage of the
shortwave. Coulston/PN


Updated 0455Z.

VFR conditions are generally expected across the forecast area
during the next 24 to 36 hours as areas of mid and high cloudiness
move over the area in a continued north-northwest flow aloft. A
disturbance will keep mainly mountain top obscuring showers over the
area through 12Z with the potential of brief MVFR ceilings mainly in
the valleys of southwest Montana. Another disturbance moving along a
weak frontal boundary may bring a few showers again between 21Z and
03Z over the eastern terminals (KHVR KLWT). Winds will remain
relatively light.


GTF  42  65  42  68 /  20  10  10   0
CTB  41  66  40  68 /  20  10  10  10
HLN  42  64  41  68 /  20  10  10   0
BZN  37  61  37  66 /  20  10  10   0
WEY  27  53  27  58 /  10  10  10   0
DLN  36  57  34  63 /  20  10  10   0
HVR  42  69  42  71 /  10  10  10   0
LWT  40  63  40  66 /  10  10  10   0



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