A strong upper level disturbance within an unstable northwest flow aloft resulted in heavy snow over the Big Horn Mountains and adjacent foothills.
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The episode as NWS BYZ wrote it up.
A strong upper level disturbance within an unstable northwest flow aloft resulted in heavy snow over the Big Horn Mountains and adjacent foothills.
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 Apr 18, 7:12 PM MST, 18 minutes earlier.
Area Forecast Discussion
National Weather Service Billings MT
812 PM MDT Sun Apr 18 2021
.UPDATE...
Precipitation has filled in across southern Montana and northern
Wyoming as upper jet divergence has now moved over the area, and
between it and the upslope is providing ample ascent for moderate
rain and snow. Temperatures have plunged over the past hour and
is quickly converting rain to snow for most locations, hence snow
will be the dominate precipitation type for the rest of the
night.
Snow amounts remain similar to what was previously defined. The
case still boils down to what type of surfaces snow falls on...
cooler surfaces like grass will be able to quickly accumulate
snow, while road surfaces will have a lot of melting occur, but
some snow cover is still possible. There is still a small chance
for a window of icy road conditions to occur tomorrow morning,
especially along I90 both from Columbus through Livingston, and
from Crow Agency south through Sheridan. However, all roads will
be slushy and slick tomorrow morning, and snowfall may still be
occurring for the morning commute.
Vertz
&&
.DISCUSSION...
Today through Monday night...
A warm start to the day, and rapid warming ahead of the
approaching cold front have allowed high temperatures to
over-achieve today. Highs thus far have climbed into the lower 60s
in Billings, Livingston, and Sheridan. The cold front passed
through Billings at about 1pm, so temps have started to fall.
The strong cold front moving south across the region today should
reach Sheridan by 4PM. This cold front will be accompanied by
showers and may produce an isolated thunderstorms, and will shift
winds to the north. Heavier and more extensive precip will lag
behind the frontal passage. Precip is expected to start as rain
over lower elevations, but should quickly transition to snow, with
temperatures expected to wet bulb down quickly. Heaviest precip
amounts are expected to be over west and southern zones, with
eastern areas (Northern Rosebud, Custer and Fallon Counties),
shorted once again. That said, these drier, eastern zones should
still see between 0.15" and 0.20" of precip, and will likely not
see as much snow. Otherwise, remaining lower elevations and
foothills should see 0.25" to 1.0", with the highest amounts in
the foothills. The mountains could see upwards of 1.5" of QPF.
As for snow amounts, the foothills will see 5 to 10 inches, with
8 to 14 inches possible in the mountains with the highest amounts
on the north facing slopes. Therefore, Winter Storm Warnings have
been issued for these areas. Lower elevations, extending from
Harlowton, to Billings, to Alzada will see 2 to 5 inches of snow,
mainly accumulating in the overnight hours. Therefore, Winter
Weather Advisories have been issued for these areas. Areas further
northeast, including most of Northern Rosebud, Custer, and Fallon
Counties will see around an inch of snow, which is expected to
melt fairly quickly. Warm temperatures ahead of the front on
today, will melt any snow into this evening, but as temperatures
continue to cool overnight, snow is expected to begin to stick on
roadways. Therefore, expect roads to become slushy overnight and
possibly slick by morning, and for the morning commute,
especially on bridges. Heaviest snow is expected to taper off
Monday afternoon/evening, but additional shortwaves will shift
across the region, bringing periods of additions snow showers
across the region through the Monday night.
Tuesday will see a continuation of well below normal temps, under
persistent northerly cyclonic flow. The cold front appears to
stall a bit across southern zones, for Tuesday. Additionally, a
few weak pieces of energy slide through the flow, to bring a
little additional forcing, keeping shower potential over the area
through the day Tuesday.
Low temperatures tonight will be in the 20s in most locations.
High temperatures on Monday will in the 30s to around 40 degrees,
with mainly 40s expected for Tuesday.
Gilstad
Wednesday through Sunday...
Weak ridging will build into the area on Wednesday with warmer
and drier conditions into Thursday. A shortwave and associated
cold front look to move through the area Thursday night into
Friday, bringing additional chances for rain and snow, along with
cooler temperatures. The weather pattern looks similar to the
system that will affect the region tonight into Monday. Unsettled
weather looks to continue through the weekend with the potential
for additional showers.
High temperatures are forecast to range from the lower 50s
Wednesday to the low 60s by Thursday. High temperatures will cool
back into the 40s for Friday with 50s to lower 60s over the
weekend. STP
&&
.AVIATION...
A mix of rain and snow will become all snow by 06Z, and continue
overnight. Conditions will continue to deteriorate to IFR this
evening and continue overnight into Monday. Mountains obscurations
will continue. Expect conditions to begin improving generally
after 18Z. STP/Reimer
&&
.PRELIMINARY POINT TEMP/POPS...
Mon Tue Wed Thu Fri Sat Sun
-----------------------------------------------------------
BIL 026/038 025/042 022/053 030/061 034/047 033/060 039/064
+5/O 31/B 00/U 01/U 34/O 21/B 22/R
LVM 025/035 021/042 021/050 026/058 033/050 033/059 038/061
+6/S 21/B 00/U 01/U 44/O 22/R 23/R
HDN 024/039 023/042 021/053 026/061 032/048 031/060 037/064
+5/O 32/S 00/U 00/U 34/O 21/B 12/R
MLS 023/040 023/042 022/053 028/061 031/048 030/057 037/063
63/O 22/S 00/U 00/U 12/S 10/B 12/R
4BQ 024/036 023/038 021/049 027/058 031/048 031/057 037/064
94/S 22/S 00/U 00/U 12/O 10/B 11/B
BHK 022/038 020/039 018/051 026/059 029/048 028/055 034/061
42/O 22/S 00/U 00/U 12/O 10/B 11/B
SHR 024/033 018/037 015/046 021/054 028/046 028/057 034/062
+8/S 32/S 00/U 00/U 13/O 21/B 12/R
&&
.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory in effect until noon MDT Monday FOR
ZONES 29>31-36-37-173.
Winter Storm Warning in effect until 3 PM MDT Monday FOR
ZONES 34-56-66>68-138-141-169-171-172-228.
Winter Weather Advisory in effect until 3 PM MDT Monday FOR
ZONES 40-42-57-58-63>65-139-170-235.
WY...Winter Storm Warning in effect until 3 PM MDT Monday FOR
ZONES 198-199.
&&
weather.gov/billingsThe national radar mosaic for the moment the first event began.
2 winter storm. Local standard time, as filed.
Snowfall of 12-13 inches fell over the Big Horn Mountains. No unnecessary travel was recommended.
Snowfall of 6 to 11 inches fell across the area. This resulted in high impacts on Interstate 90.
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, April 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: 947518, 947519.