Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 211525

Area Forecast Discussion...Updated
National Weather Service Billings MT
925 AM MDT Sat Oct 21 2017

Main change to the forecast was to upgrade the High Wind Watch
from Judith Gap to Big Timber. This area will see an impressive
upper and mid level jet work over it late tonight through Sunday.
Strong downward motion and mixing of the boundary layer supports
the idea of strong to dangerous wind gusts for high profile and
light vehicles. We started the warning at 3 AM tonight for this
area just to give it some room as winds will increase and peak
probably around midday Sunday. End time is 9 PM.

Rest of forecast in good shape. We did back off PoP`s a bit for
the mountains the rest of today. Snow showers will develop over
mountains tonight. BT


.SHORT TERM...valid for Tdy and Sun...

Water vapor imagery shows a weakening trof over eastern MT, and a
fast westerly flow upstream moving into the Pac NW. Light showers
over our cwa have mostly ended as surface front has pushed into
the central Dakotas, leaving gusty westerly downslope winds across
our cwa. Cannot rule out a few snow showers over the mountains
this morning, and maybe an early morning sprinkle in our north,
otherwise it will be dry today under subsidence aloft and with
persistent gusty W-NW surface winds. Boundary layer will remain
well-mixed and gusts of 35 to 50 mph will be widespread. Temps
today will be cooler with highs in the 50s to lower 60s.

A 130-150 kt H3 westerly jet 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 before midnight, and peak winds overall should occur
between 09-15z Sunday. Confidence continues to increase in a
period of 70+ mph gusts, so have issued a High Wind Warning for
the Livingston area and Beartooth Foothills from mid evening
through Sunday. Winds will gradually decrease through Sunday
afternoon as winds become mixed and directions veer westerly.

Northern Sweet Grass and Wheatland Counties will also see
increased SW-W winds late tonight and Sunday, with peak gusts
beginning once pressure fall axis moves east of north central MT,
i.e. starting 09-12z. Cannot rule out some mountain wave
activity, but most widespread and consistent gusts will occur
during the day on Sunday with mixing into core of strong mid level
flow. Will keep the High Wind Watch in effect for these areas and
let day shift make a final decision on a potential warning.

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 will need to be watched. In any event, Sunday will be quite
windy across the cwa and those with outdoor/travel plans will need
to take note.

Passage of Pacific shortwave occurs late Sunday and Sunday night,
and we should see some light showers along mid level baroclinic
zone. 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

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


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

Some uncertainty has crept into the extended forecast as the
GEFS/ECMWF ensembles showed lower confidence with the midweek
system and the 00Z ECMWF was different that it`s previous run from
Thursday onward. In addition, Super Typhoon Lan, seen on water
vapor south of Japan early this morning, was forecast to move NE
as it becomes extratropical. This system adds uncertainty to the
downstream pattern that will affect our region.

On Monday, models were consistent with showing the Pacific jet
becoming oriented N-S as it shifts E of the area. Some moisture
lingers over the southern mountains early in the day for a slight
chance of snow showers, otherwise it will be dry. High
temperatures will be around normal. Windy conditions will continue
with the GFS soundings showing winds in the 30s through the
column. Upper ridging, dry weather and a warming trend then move
through the area through Wednesday. Expect periods of gusty winds
W of KBIL. Highs will top out around 70 degrees on Wednesday.

The ECMWF and GFS both bring a trough SE into the area Wed. night
with an accompanying cold front. GFS also showed an atmospheric
river moving through the area. There was good model agreement in
dropping 850 mb temperatures below zero degrees C as well. The
above supported a good chance of rain showers mixing with and
changing to snow showers overnight. The models then fall out of
agreement on Thursday with the ECMWF cutting off a low over NE
MT/W ND, while the GFS maintains a progressive system and ends the
precipitation from NW to SE. Given the previous run of the ECMWF
was very similar to the current run of the GFS, have followed the
GFS closely. Will be watching future model runs for a consensus
for Thursday. Model blends gave highs in the 40s for Thursday.
Kept Thu. night/Friday dry per the GFS. The GFS brings a shortwave
down in NW flow for Saturday, while the ECMWF pivots the midweek
system out of the region. Given the great deal of uncertainty,
used model blends and climo for the end of the period. Arthur



Gusty WNW winds 30 to 40 kts through the afternoon today at all
terminals. Expect decreasing winds other than at LVM where winds
will increase with gusts to 65 kts after 06z. Strong winds
expected to persist through the day Sunday at LVM. Gusty winds
will be the only hazard to aviation, as cloud decks and visibility
are expected to remain VFR at all sites through the period. Walsh



    Tdy Sun     Mon     Tue     Wed     Thu     Fri
BIL 058 042/069 045/060 037/067 045/070 036/045 029/050
    0/N 21/N    21/N    00/U    01/N    42/W    11/B
LVM 052 045/063 043/058 036/067 044/068 033/045 027/049
    1/N 33/W    51/N    00/N    01/N    72/J    11/B
HDN 059 036/070 042/062 033/067 039/071 033/046 025/050
    0/N 21/N    21/N    00/U    01/U    52/W    10/B
MLS 058 035/069 043/059 032/064 040/069 033/044 024/046
    2/W 11/N    21/N    00/U    01/U    32/J    11/B
4BQ 057 033/070 043/058 032/063 040/071 033/044 025/046
    1/N 01/N    21/N    00/U    00/U    42/W    11/B
BHK 056 033/067 040/056 031/060 038/066 031/042 023/043
    1/N 01/N    21/N    00/U    01/U    33/W    11/B
SHR 056 031/069 039/058 032/065 036/072 031/043 023/046
    0/N 11/B    21/N    00/U    00/U    63/W    11/B


MT...High Wind Warning in effect from 3 AM to 9 PM MDT Sunday FOR
      ZONES 28-41-63.
     High Wind Warning in effect from 9 PM this evening to 6 PM
      MDT Sunday FOR ZONES 65-66.


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