Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 220256
AFDBYZ

Area Forecast Discussion...Updated
National Weather Service Billings MT
856 PM MDT Sat Oct 21 2017

.UPDATE...

Updated the forecast to adjust winds and temperatures for current
trends and to increase precipitation chances for the Cooke City
area. Winds have been gusting around 60 mph for the past couple of
hours over the gap wind areas from Livingston to Nye. As strong
jet stream approaches it will add some right front quad subsidence
and additional pressure falls to the mix to ramp wind gusts up to
around 75 mph by sunrise. Areas from Big Timber to Judith Gap will
feel the influence of this jet subsidence as well combined with a
favorable pressure gradient and moderate mountain wave activity to
ramp winds up along the US191 corridor after midnight. Going High
Wind Warnings for these western zones look to be in good shape and
no adjustments made. Temperatures have already fallen to forecast
lows for mountain and foothills ares so made some adjustments to
the forecast to lower those locations. Snowing in Cooke City at
this time and time heights from various models in good agreement
with a continued light snow pattern there through tomorrow
evening. Adjusted precipitation chances up to categorical to
account for this and bumped snow accumulations up just an inch or
two as well.

Models for tomorrow still showing very windy conditions for all
areas tomorrow into tomorrow night as a 150-160kt jet stream
pushes over the area. Not seeing enough ingredients to issue any
additional High Wind Warnings at this point, but gusts over 50 mph
will occur over the plains and can`t rule out a few 60mph
readings as a cold front moves through late in the day. Mid shift
will continue to monitor this windy pattern for any changes that
would necessitate additional highlites. Chambers

&&

.SHORT TERM...valid for Sun and Mon...

A 130-160 kt westerly jet in the upper levels will nose into the
northern Rockies tonight into Sunday. The combination of mid level
warm advection (i.e. stabilizing), lee side pressure falls and
increasing westerly mid level flow on the order of 65-70 kts will
result in a significant increase in SW gap winds at Livingston and
Nye tonight. Current indications are that we`ll see advisory
level gusts begin this evening. Peak wind speeds are expected
between 4 AM and 10 AM with a period of 70+ mph gusts possible.
Wind in the gap areas will gradually decrease somewhat through the
day afternoon as winds become mixed and directions veer westerly.
As this occurs wind directions will begin to favor the Big Timber
to Harlowton corridor along Highway 191. Unusually strong 700MB
wind speeds moving right over central Montana and will likely be
mixed to near the surface resulting in wind gusts 55 to 65 mph.
Further east, strong mixed west winds will spread across the
lower elevations on Sunday, with expectation of widespread 40-55
mph gusts. Do not see a synoptic mechanism to bring the stronger
mid level winds down, but if any light showers develop a
convective gust may produce a local gust to 60-65 mph.  In any
event, Sunday will be quite windy across the cwa and those with
outdoor/travel plans will need to take note. The situation in the
Livingston-Big Timber-Harlowton area looks especially dangerous
for travelers.

Passage of Pacific shortwave occurs late Sunday and Sunday night,
and we should see some light showers along mid level baroclinic
zone drop south across our CWA. Our western mountains will see a
few inches of snowfall the next couple days, beginning tonight
with a period of moist isentropic ascent and then late Sunday and
Sunday night with shortwave/instability. Any precipitation on the
plains looks to be light late tomorrow and tomorrow evening, but
should be rather light and of little to no impact.

Temps will be warmer Sunday with highs in the 60s to near 70
degrees. Cold advection and breezy NW winds can be expected Sunday
night. BT

.LONG TERM...valid for Tue...Wed...Thu...Fri...Sat...

Warm and pre-frontal day expected on Wednesday as a strong cold
front begins to shift southward from Canada. Ahead of the front
temperatures should warm into the low 70s. Adjusted the winds up
on Wednesday and Thursday especially in Livingston and the western
foothills, this is more in-line with the MOS forecasts. As the
front moves across the area, expecting rain to transition to snow
by Thursday morning across most of the area. Not expecting major
accumulations as both the Euro and GFS are now showing the front
progress through fairly quickly, however some minor accumulations
are possible in the lower elevations. Behind the front
temperatures will be 20 to 30 degrees cooler through Friday.
Models begin to diverge as we head into next weekend with the GFS
bringing a weak system into the area from the north, and the Euro
remaining dry. Walsh

&&

.AVIATION...

Winds begin to decrease this evening except for at LVM where
winds will increase with gusts to 65 kts after 06z. Strong winds
expected to persist through the day Sunday at LVM. LLWS concerns
at BIL, SHR, MLS in the pre-dawn hours with strong westerly winds
at 1500 to 2000 ft above weaker surface flow. Cloud decks and
visibility are expected to remain VFR at all sites through the
period. Walsh

&&

.PRELIMINARY POINT TEMP/POPS...

    Sun     Mon     Tue     Wed     Thu     Fri     Sat
-----------------------------------------------------------
BIL 041/066 045/060 037/068 045/070 036/043 026/043 030/054
    11/N    20/N    00/U    01/N    52/W    11/B    22/W
LVM 041/062 044/058 036/067 044/069 033/043 024/046 028/054
    23/W    30/N    00/N    01/N    73/W    11/U    11/B
HDN 036/069 043/062 033/069 040/072 034/045 023/043 025/055
    11/N    20/N    00/U    01/U    53/W    11/B    22/W
MLS 035/066 042/059 032/064 040/070 034/042 022/040 023/052
    12/W    20/N    00/U    01/U    43/W    11/B    22/W
4BQ 035/068 042/058 032/064 041/072 033/041 022/039 024/052
    11/N    20/N    00/U    00/U    42/W    11/B    22/W
BHK 033/066 040/056 031/060 038/066 032/039 021/036 022/048
    11/N    20/N    00/U    01/U    33/W    11/B    22/W
SHR 033/067 040/057 032/066 037/073 032/041 021/041 022/052
    01/B    20/N    00/U    01/U    63/W    11/B    22/W

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...High Wind Warning in effect from 3 AM to 9 PM MDT Sunday FOR
      ZONES 28-41-63.
     High Wind Warning in effect until 6 PM MDT Sunday FOR ZONES
      65-66.
WY...None.

&&

$$
weather.gov/billings



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