Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 201017

Area Forecast Discussion
National Weather Service Great Falls MT
417 AM MDT Fri Oct 20 2017


A frontal boundary crosses the region today, bringing a chance
for showers for the Rocky Mountains and Southwest Montana. Windy
conditions persist through the weekend with cooler temperatures.



Today through Sunday...Breezy conditions overnight maintained mild
temperatures and remarkably low humidity overnight for much of
the forecast area. A cold front associated with an Alberta Clipper
system will advance across the area this morning through early
afternoon. Hence, forecast high temperatures may be reached early
this morning, with cold advection driving temperatures gradually
lower through the day. Pressure rises behind the front will
produce a period of gusty west winds. Wind reports will be
monitored closely today, but the vertical wind profile does not
support any more than a few isolated gusts nearing 50 mph. Despite
another breezy day, fire concerns are slightly lower today due to
increasing humidity. Along with these rapid changes in temperature
and winds today, expect rain and mountain snow to develop up to,
and along the Continental Divide, and across southwest Montana
with the passing trough. Sprinkles are possible across the
N-central plains, but otherwise suppressed by the downslope winds.
All this activity tapers downward tonight into Saturday as the
Clipper system pulls further away to the east. A remarkably strong
zonal jet pushes across the PacNW and reaches across the Northern
Rockies Saturday night. Confidence continues to increase that a
widespread, potentially damaging high wind event could develop
late Saturday night through the day Sunday. A high wind watch is
appropriate for this potential event, but opted to hold off for
now to avoid confusion compared to today`s passing front. PN

Sunday night through Thursday...The main concerns during this period
center around wind and the potential for continued elevated fire
weather concerns.

An impressive upper level jet and associated moisture/precip aimed
at the Pacific NW will gradually shift south away from the area
Sunday night, with precip diminishing in the mountains. NW flow
takes over aloft as the upper level trough pushes into the central
US. This is followed by an anomalously strong ridge (for late-
October) TUE/WED which will likely yield a return to above normal
temps (60s/70s at lower elevations) both days. The models are then
in very good agreement in bringing a sharp trough SE out of Canada
later WED into THU. In this transition, another wind event appears
possible late Tuesday night into Wednesday, especially along the
Rocky Mtn Front where a 60-80kt mid-level jet and strong pressure
gradient are forecast to be co-located. Out over the plains, high
wind criteria may not be met, but as has been seen in previous
breezy/windy days, relative humidity values may fall low enough to
warrant elevated to near-critical fire weather concerns. The
shortwave forecast to dive SE out of Canada will likely be
accompanied by a strong northerly jet. Where the models differ is
where the upper jet and coldest air is directed (ie. right over our
area or further east across eastern MT and the Dakotas). Regardless,
the mild temps TUE/WED should quickly be replaced by much cooler
temps late-week. The potential then exists for yet another wind
event late in the week as well. MARTIN


Updated 1125Z.

Scattered to broken high cloud-cover will prevail overnight. A cold
front moves through the forecast area by mid-day Friday with winds
shifting to the west and increasing behind the front. Widespread
snow and mountain obscurations expected along the Continental Divide
after 12Z Friday, while isolated to scattered showers accompany
frontal passage through the plains. VFR conditions expected to
prevail at all forecast terminals through the period, except for
brief IFR/MVFR conditions possible in showers. Mountain wave
turbulence and low level wind shear expected to redevelop in favored
locations as west winds increase behind frontal passage. mpj/PN


GTF  63  37  51  45 /  20  10  10  20
CTB  50  37  48  38 /  10  10  10  20
HLN  61  35  51  41 /  30  10  20  20
BZN  64  29  50  38 /  50  30  10  20
WEY  45  24  35  27 /  90  60  40  60
DLN  57  28  45  36 /  30  20  20  20
HVR  66  39  54  36 /  30  20  10  20
LWT  67  34  49  38 /  20  10   0  20



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