A mountain wave over the Big Horn Mountains broke resulting in very high winds at mountain top level to accelerate to the surface.
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The episode as NWS BYZ wrote it up.
A mountain wave over the Big Horn Mountains broke resulting in very high winds at mountain top level to accelerate to the surface.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Billings, MT wrote its last area forecast discussion before the first event at Dec 20, 5:58 PM MST, 3 hours 2 minutes earlier.
Area Forecast Discussion...Updated
National Weather Service Billings MT
558 PM MST Mon Dec 20 2021
.UPDATE...
Band of light snow northeast of Billings has developed a little
earlier than expected, so raised PoPs and snow amounts in these
locations for the evening hours. Winds have been hitting Advisory
criteria at Livingston for several hours, with blowing snow
reducing visibility below a mile at times. Winds have not picked
up quite yet from Big Timber to Harlowton but that expected as the
evening wears on. No changes to existing highlights. TWH
&&
.DISCUSSION...
Tonight through Wednesday night...
Retreating cold air has allowed southwesterly winds to develop
over the western foothills this afternoon. Breezy southwest to
west winds will develop along the rest of the western foothills
this evening, ahead of the next disturbance and associated cold
front dropping in from the north. There will be a period of
blowing and drifting snow that will bring reduced visibilities to
the western foothills this evening, before temperatures climb into
the middle to upper 30s, then drop with the front. The
aforementioned disturbance will bring a return to overrunning
precipitation tonight, with light snow accumulations expected,
greatest for areas east of Billings where up to a couple inches
are possible. Snow will diminish over the area by the morning. As
the cold front drops through the area, winds should back off a bit
in the Livingston to Nye areas, before increasing once again
Tuesday afternoon and evening as the pressure gradients tighten.
Expect the gusty winds to continue along the western foothills
into Wednesday, with warm and mainly dry conditions expected,
outside of some snow over the southwest mountains.
The Wind Advisory for Livingston and Nye remains in effect into
Wednesday. Have also gone ahead and issued a Winter Weather
Advisory from Big Timber north through Judith Gap for this evening
and overnight to account for blowing snow concerns there. The
overnight snowfall is going to be an issue, with blowing snow
possibly producing very low visibilities at times as well as
drifting snow over roadways making them slick. Lows in the single
digits to lower 30s can be expected tonight, coldest over the
northeast. Highs on Tuesday and Wednesday mainly in the 30s to 40s
are forecast, with lows in the teens to around 30 Tuesday and
Wednesday night. STP
Thursday through Monday...
The extended models are in good overall agreement with the large
scale pattern but differ somewhat in the smaller details. The
consensus is the forecast area can expect unsettled weather for
the next week. There does not look to be any major storm on the
horizon but there will be several systems that will affect the
area bringing periods of snow.
Thursday and Friday, another upper level disturbance moves across
the forecast area bringing a chance for snow Thursday night. At
the same time, an arctic front will sweep through early Friday
morning which will be the beginning of very cold air filtering
across our forecast area through Christmas weekend and into next
week. Conditions look to be dry Friday but snow chances look to be
increasing for Christmas Day through the weekend.
Another arctic surge plunges southward Sunday night into Monday
bringing even colder air to end the holiday weekend, and this
frigid air will continue through next week. Pacific moisture will
periodically override this arctic airmass resulting in the
likelihood for on and off snow Sunday through at least the middle
of next week. Hooley
&&
.AVIATION...
Snow will overspread the region from N to S tonight. Expect
MVFR/IFR conditions with locally lower conditions due to bursts of
heavy snow and blowing snow. KLVM will have surface wind gusts of
50-60 kt, decreasing somewhat overnight. These winds will create
periods of LIFR conditions in blowing snow. Mountain obscurations
will increase overnight. Areas of obscuration will linger Tue.
morning. A cold front will push S through the region tonight,
shifting surface winds to the W and NW. Expect LLWS at KBIL and
KSHR tonight. Areas of fog are likely Tuesday morning, otherwise
VFR will return to the area. LLWS is expect at KMLS Tue. morning.
SSW surface winds will increase again at KLVM Tue. evening. Arthur
&&
.PRELIMINARY POINT TEMP/POPS...
Tue Wed Thu Fri Sat Sun Mon
-----------------------------------------------------------
BIL 017/040 026/041 026/042 021/033 013/024 004/013 901/014
61/U 00/B 02/O 32/S 21/E 22/S 11/E
LVM 031/045 030/044 028/044 022/034 020/035 015/028 008/025
31/N 01/N 15/O 43/S 33/S 43/S 22/S
HDN 012/037 017/042 021/043 017/034 007/026 901/014 906/014
81/B 00/E 02/O 32/S 21/B 22/S 11/B
MLS 002/026 014/037 018/037 009/024 000/013 906/008 912/005
81/U 00/B 01/B 11/B 21/B 11/E 11/B
4BQ 014/036 021/043 023/047 021/036 012/031 006/020 901/019
71/U 00/B 01/B 11/B 10/B 11/B 10/B
BHK 006/029 016/040 022/043 013/029 007/025 000/016 908/011
71/U 00/B 00/B 11/B 10/B 11/E 11/B
SHR 019/042 025/049 027/051 021/037 017/039 011/029 003/026
31/B 00/B 01/B 32/S 11/B 11/B 11/B
&&
.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory in effect until 5 AM MST Tuesday FOR
ZONES 63-141-172-228.
Wind Advisory in effect until noon MST Wednesday FOR ZONES
65-66.
WY...None.
&&
weather.gov/billingsPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE BILLINGS MT
445 PM MST MON DEC 20 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0400 PM SNOW 6 WSW PARK CITY 45.60N 109.03W
12/20/2021 M1.6 INCH STILLWATER MT COCORAHS
0400 PM SNOW 18 SW FISHTAIL 45.24N 109.73W
12/20/2021 M3.0 INCH STILLWATER MT CO-OP OBSERVER
&&
TWHPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE BILLINGS MT
522 PM MST MON DEC 20 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0415 PM NON-TSTM WND GST 6 WSW RED LODGE 45.16N 109.36W
12/20/2021 M51.00 MPH CARBON MT MESONET
0432 PM NON-TSTM WND GST 6 ENE LIVINGSTON 45.70N 110.45W
12/20/2021 M62.00 MPH PARK MT ASOS
&&
TWHPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE BILLINGS MT
659 PM MST MON DEC 20 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0500 PM SNOW 1 ENE HARDIN 45.74N 107.60W
12/20/2021 M1.0 INCH BIG HORN MT CO-OP OBSERVER
0612 PM NON-TSTM WND GST 17 NE WILSALL 46.16N 110.38W
12/20/2021 M52.00 MPH PARK MT MESONET
0618 PM NON-TSTM WND GST 17 S FORT SMITH 45.07N 107.89W
12/20/2021 M55.00 MPH BIG HORN MT MESONET
&&
TWHPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE BILLINGS MT
700 PM MST MON DEC 20 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0600 PM HEAVY SNOW 5 W MELVILLE 46.10N 110.05W
12/20/2021 M6.4 INCH SWEET GRASS MT CO-OP OBSERVER
&&
TWHPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE BILLINGS MT
705 PM MST MON DEC 20 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0633 PM NON-TSTM WND GST 6 ENE LIVINGSTON 45.70N 110.45W
12/20/2021 M63.00 MPH PARK MT ASOS
&&
TWHPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE BILLINGS MT
808 PM MST MON DEC 20 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0753 PM NON-TSTM WND GST 6 ENE LIVINGSTON 45.70N 110.45W
12/20/2021 M67.00 MPH PARK MT ASOS
&&
TWHThe national radar mosaic for the moment the first event began.
1 high wind. Local standard time, as filed.
A mountain wave ahead of a cold front broke resulting in very high winds at mountain top level to accelerate to the surface.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2021Every state in 2021This day in weather historyWarnings in Wyoming, December 2021
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 992459.