Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 180317

Area Forecast Discussion...Updated
National Weather Service Billings MT
817 PM MST Sat Feb 17 2018

Snow has filled in west and north of Billings. Web cams showing
snow coming down at a pretty good pace from Reed Point to Big
Timber to Judith Gap to Roundup. Have adjusted PoPs and snow
amounts slightly to account for radar and model trends. Added
northern Stillwater county to the Advisory based on above. Other
updates made earlier this evening look on track. Winds have picked
up at Livingston and were now just below Advisory criteria.
Expect an continued uptick in winds through the early overnight
hours, then dropping off toward morning. Advisory there in good
shape. TWH

Earlier update: Made a quick update to raise PoPs (the potential
for snow) over much of the western and central zones (including
Billings). Radar was filling in aggressively and short range
models were picking up on that. Also raised snow amounts to the
north as snow was filling in quickly there too. TWH


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

Water vapor imagery this afternoon shows a potent wave traveling
through northern Washington with flow aloft backing from
northwesterly to westerly over southern Montana and northern
Wyoming. This is promoting an onset of downsloping and gap flow
winds along the mountains and generating some moderating
temperatures. Lower spots along the rivers are stuck in the
colder air but VWP in Billings shows southerly winds just above
the first gate and a boundary is shifting northeast along a
Lewistown to Hysham light producing brief snow showers.

Wave over Washington sends a piece of energy that travels across
northern Montana while more energy lingers back over the PacNW and
helps develop a slower moving trough. This pattern sets up a
period of active weather tonight...followed by a relatively lull
late tonight into early Sunday...with another round of weather
Sunday afternoon into Monday. Tonight a strong surface low
expected to track across southern Montana but model solutions vary
by about 80 miles in its track which is a huge difference between
a couple of inches of snow and no snow. GFS is furthest south with
the surface low which appears to be biases by trend to blend the
surface inflection induced by the shortwave into a strong lee
mountain wave signature on the east side of the Beartooth/Absaroka
mountains. RAP NAM and SREF Models all track a low further north
and maintain more downsloping for portions of south central
Montana. Models do agree heaviest area of snowfall will be over
and north of the Bull Mountains extending over towards the Crazy
Mountains. Have lowered precipitation chances and amounts for the
Billings area and locations like Hardin and Hysham but think that
winter weather advisory in affect for areas north and west of
Billings looks good.

As the surface low moves into the region this will support a
period of strong gap flow winds transitioning to a westerly
mountain wave wind situation for the Livingston area and Beartooth
Foothills. Have been watching to see if this will translate to a
wave setting up along the Big Horn mountains but suspect that low
level airmass will cool off enough early this evening to keep it
from mixing down and impacting Sheridan. Do have strong winds at
Burgess Junctions. Winds will let up as the surface low passes by.

Cold front progged to cross the region between 9 pm and 2 am which
will bring the best chance of snow for areas not in weather
advisories but any snow with this front will be short lived.
Exception will be over northern portions of southeast Montana
where surface front may not pass as quickly supporting a bit
longer snow. Advisory for this area still looks good but watching
to see if the snow may not last that long depending on low track.

Sunday morning sees a break in the snowfall but by afternoon
backing flow aloft becomes More southwesterly and pushes moisture
over the area while the arctic airmass deepens. This allows a
light snowfall to develop mainly for Sunday night into Monday
morning which is the greatest period of snowfall for areas like
Billings. The upper flow is showing a bit stronger trough passing
across Montana and moving a bit slower and this sets up better
precipitation amounts for southeast Montana. Will have to watch
that area for a possible advisory Monday.

Other big story is the cold which will be nearly 40 degrees below
normal for Monday. The high temperature is going to struggle to
break zero. borsum

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

The GEFS showed uncertainty with the pattern beginning midweek,
while the ECMWF ensembles showed high confidence with the pattern.
That being said, the deterministic models were not in phase
beginning on Wednesday, so used blended guidance for the PoPs.

Generally, the extended period will be quiet with a warming
trend. High amplitude trough from the short-term period will
be over the region on Tuesday. 850 mb temperatures will be a few
degrees higher than they will be on Monday, but highs will still
be in the single digits. Lingering moisture will be over the SE
zones and mountains for a slight chance of snow in these areas.
Models still have troughiness over the area on Wednesday, but are
out of phase with the energy in the trough. GFS showed a slight
chance of snow over the SW mountains, otherwise the forecast will
be dry. Temperatures will climb into the 20s in the W with teens
E. Lee troughing Wed. night into Thursday will support possible
Advisory winds in the gap flow areas. GFS showed a reinforcing
trough for Thursday, while the ECMWF had brief upper ridging.
Blends gave low PoPs N and W of KBIL, and highs in the 20s to low
30s. General troughiness continued on both models Friday through
Saturday with chances for snow showers, mainly in the mountains,
and highs in the 20s and 30s. Arthur



Expect strong southwest winds with gusts to 50kts around KLVM will
continue through around 09z. Around KSHR could see strong gusts
to 45 kts briefly as a front moves through after 04z. Expect snow
to continue overnight at all terminals with MVFR to IFR conditions
expected. Heavier showers could produce localized LIFR
conditions. Expect improving conditions at all terminal except
KSHR after 12z. Reimer


    Sun     Mon     Tue     Wed     Thu     Fri     Sat
BIL 012/013 906/002 918/009 906/021 008/031 011/031 014/033
    77/S    74/S    21/B    11/U    11/B    11/B    11/B
LVM 017/020 910/003 917/010 901/023 013/032 015/033 018/034
    77/S    84/S    22/J    11/N    22/J    22/J    22/J
HDN 013/014 902/005 921/010 909/021 004/028 007/029 010/031
    66/S    74/S    22/J    11/U    11/B    11/B    11/B
MLS 009/012 905/004 915/005 911/015 001/021 002/022 005/026
    82/S    64/S    22/J    11/U    11/B    12/J    11/B
4BQ 016/018 903/005 916/007 911/018 002/027 006/028 009/030
    64/S    76/S    32/J    11/U    11/B    11/B    11/B
BHK 010/012 905/003 914/004 912/015 000/022 002/023 004/027
    92/S    65/S    22/J    11/U    11/B    12/J    11/B
SHR 020/025 906/005 919/008 914/020 000/028 007/030 010/033
    56/S    76/S    32/J    11/U    11/B    11/B    11/B


MT...Winter Weather Advisory in effect until 9 AM MST Sunday FOR
      ZONES 28>34-41-42-63.
     Wind Advisory in effect until 2 AM MST Sunday FOR ZONES
     Winter Storm Warning in effect until 9 AM MST Sunday FOR
      ZONES 67-68.


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