Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 201011
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
311 AM MST Mon Nov 20 2017

.SHORT TERM...valid for Tdy and Tue...

Surface pressure gradient from Idaho Falls to Lewistown was 18mb
this hour. Gradient is expected to tighten just a bit more very
early this morning, before decreasing late this morning and this
afternoon. Winds have been hitting Advisory criteria all night in
Livingston and Nye. Guidance and local tools suggest winds will
increase through dawn with gusts approaching 75mph. For Big
Timber, 700mb winds increase to 65kts by dawn, with an increase in
surface pressure falls. This should cause an increase in wind
speeds there too. Clouds do look to increase early this morning
and this may take an edge off the strongest winds. Will keep
warnings in place. Strongest surface pressure falls surge well to
the east and south early this afternoon, so winds should be on the
decrease this afternoon, which is consistent with the going
forecast.

Attention then turns to a cold front advancing from the north and
the potential for rain and snow showers with this front. The GFS
was more aggressive with the front, in timing, strength, and
amount of QPF generated with it, compared to the ECMWF. The GFS
has also pushed the band of precipitation developing along the
frontal boundary farther west and centers Billings right in the
heaviest QPF with around .25 inches of moisture falling tonight.
The ECMWF was not quite that aggressive, but does put down .15
inches. Timing has slowed just a bit with the onset of ascent, but
confidence still growing of a band of rain showers developing
this evening over western and central zones, so raised PoPs. Cold
air is progged to slide in by mid evening, so rain should change
to snow then. Coldest air holds off until around midnight and
metro model keeps roads mainly wet until then, but could turn
slushy overnight. Air temperatures fall into the 20s and roads may
become slick toward Tuesday morning. Snowfall does taper off
pretty quickly after midnight, so there is a disconnect between
precipitation and the coldest air settling in, so roads may get a
chance to dry off a bit. Will have to watch this event. Snow
accumulations will be very tricky with rain changing to snow, but
initial thought was around an inch, with local higher amounts
possible, mainly over grassy surfaces.

Need to talk about the mountains here. Surge of moisture into
this system was adequate for decent snow rates today, with the
deeper moisture sliding south tonight. System was not terribly
cold and this should hold accumulation down some. That being said,
expect to see amounts of 6-8 inches, possibly up to 10 in spots.
Will issue a HWO with occasional heavy snow in the mountains, and
the possibility of a couple inches over the plains, with maybe
some slick roads tonight and for the Tuesday morning commute.

Snow will taper off quickly Tuesday as flow quickly becomes
downslope with an upper ridge from the west building back in.
Tuesday will be warmer with 850mb temperatures approaching 9c.
Weak energy rotates around the building ridge to the west and
brings moisture and lift back into western and northwest zones
Tuesday night. TWH

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

The extended forecast continues to be on track, so only minor
changes.

UPper level ridging and strong downslope flow will bring temps
well above normal for Wednesday and Thanksgiving Day. Highs in the
60s will be common, with near 70F possible on turkey day in some
locations. Highs on the 23rd may challenge current daily records.

SW winds at Livingston and Nye could reach advisory criteria both
Wednesday and Thanksgiving Day, with gusts approaching 60 mph
possible. Mid level winds to near 50 kts, a stable atmosphere, and
a favorable sfc gradient are all strong indicators of advisory
level gusts.

Models continue in good agreement on timing a Pacific shortwave
across the region late Thursday night and Friday. Wave appears to
be pretty quick moving at this time, but should bring a shot of
lower elevation rain showers and some mountain snowfall. Continued
with inherited pops at this time, with minor tweaks for
coordination purposes.

The shortwave will also bring some cooler air into the region, but
certainly not cold air. Upper level ridging returns by Saturday
morning, bringing dry weather and a general warming trend through
the end of the period. Expect temps to remain above late November
normals, with highs in the 50s. AAG

&&

.AVIATION...

The main aviation threat in our region is the very strong surface
winds from KLVM to K6S0 continuing through much of today. Winds
gusting from 50 to over 60 kts is expected, strongest wind
expected through 18Z. In addition, some LLWS is possible around
KMLS and KBHK tonight. VFR conditions will prevail through this
morning, but expect increasing obstructions to the
Absaroka/Beartooth Mountains this afternoon. A cold front will
cross the region this evening and overnight. Precip will start as
rain in lower elevation and change to snow late this evening.
Expect reduced VIS and CIGS in precip, with localized IFR to LIFR
conditions possible. AAG

&&

.PRELIMINARY POINT TEMP/POPS...

    Tdy Tue     Wed     Thu     Fri     Sat     Sun
-------------------------------------------------------
BIL 052 026/041 038/063 043/064 042/054 031/048 032/052
    3/W 72/J    21/N    00/B    23/W    00/B    11/B
LVM 049 030/050 042/060 043/063 041/050 030/046 032/051
    7/W 62/W    41/N    11/N    45/W    11/B    11/B
HDN 054 024/039 033/064 038/066 039/056 029/049 029/053
    3/W 62/J    11/N    00/B    23/W    00/U    11/B
MLS 054 018/033 026/060 039/066 040/054 028/046 027/048
    2/W 30/B    12/W    00/B    12/W    00/U    11/B
4BQ 057 021/038 031/063 040/067 042/055 029/048 028/052
    2/W 40/B    11/N    00/B    14/W    10/U    01/B
BHK 055 013/031 021/060 034/064 039/053 025/043 022/046
    2/W 20/U    02/W    00/B    12/W    00/U    11/B
SHR 056 026/042 032/062 036/066 038/054 026/049 027/054
    2/W 62/J    11/B    00/B    13/W    10/B    01/B

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...High Wind Warning in effect until 6 PM MST this evening FOR
      ZONES 41-65-66.
WY...None.

&&

$$
weather.gov/billings



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