Winter Weather Advisory 29, NWS Cheyenne, WY

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
1150 PM MDT Sat Apr 17 2021

.UPDATE...
Issued at 1142 PM MDT Sat Apr 17 2021

Please see updated 06Z Aviation discussion below.

&&

.SHORT TERM...(late this afternoon through Monday night)
Issued at 235 PM MDT Sat Apr 17 2021

Early afternoon WV satellite imagery showed a upper level trough
from the Upper Midwest into CA. A weak shortwave was dropping south
across WY/NE, producing widely scattered showers. Precipitation
was light and confined over the far southeast WY plains and western
NE Panhandle. This convection will persist through late afternoon
before ending as the wave shifts south into CO/KS early this evening.
Latest HRRR/SREF indicates the potential for patchy fog, mainly along
the North Platte River Valley in the central NE Panhandle for late
tonight. A shortwave ridge will move across WY and NE Sunday, with
temperatures moderating into the 50s and low 60s during the afternoon.

Upper trough and strong cold front to the north will plunge south
into WY/NE Sunday night and early Monday. Current models and ensembles
remain in good agreement with widespread accumulating snow developing
behind the cold front Sunday night into Monday. Any rain that develops
initially will quickly change to snow Sunday night as 700mb temperatures
plummet below -10 Celsius. There is sufficient dynamic forcing aloft
and low level upslope flow to enhance snowfall, especially over the
higher terrain. Latest SREF plumes and ensembles are trending 2 to
5 inches lower elevations, with 5 to 10 inches for the mountains.
Winter Weather Advisory criteria is looking more likely given the
progressive nature of the upper trough. High temperatures Monday
will be 20-30 degrees colder as 700mb temperatures remain in the
negative low to mid teens Celsius. Snow will taper off Monday evening.
Lows Monday night will be in the teens, except single digits for the
higher terrain and valleys west of I-25.

.LONG TERM...(Sunday through Friday)
Issued at 309 AM MDT Sat Apr 17 2021

Impacts: Well-below normal temperatures likely starting off next
Tuesday after a cold frontal passage on Monday. Tuesday morning low
temperatures will be 15-20 degrees below normal with widespread
teens and 20s. Single digit lows could be possible in higher terrain
and valley floors. Wind chills will be single digits to near zero
for brief moments early Tuesday morning. A passing chance of snow
could occur late Wednesday into Thursday but impacts look minimal at
this time.

Discussion: A strong Alberta Clipper combined with solid cold Canadian
airmass will shift south across the region Monday and by Tuesday will
usher in -12C to -16C H7 temperatures. This will lead to well-below
normal temperatures Tuesday with a widespread significant freeze.
Record lows look safe as those are in +8F to -8F range and we'll
be 5-10F above those values at current forecast projections.
Lingering positive tilted troughing from the Monday H5 system
combining with a few shortwave troughs Wednesday will bring
slightly warmer H7 temperatures of -10C to -5C. QG lift will
increase late Wednesday into early Thursday along the WY/CO border
as a shortwave merges but deamplifies into zonal flow across the
Central Plains. Multi-model guidance suggests more precipitation
in CO but some mountain snows could be possible for Snowys/Sierra
Madres. Confidence in forecast decreases significantly late week
into the weekend with ECMWF showing general troughing across the
Rockies while the GFS indicates H5 ridging. Stay tuned as late
next week becomes more clear over time with future model runs.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1142 PM MDT Sat Apr 17 2021

VFR and tranquil conditions overnight and through much of Sunday
across the region. Limited, patchy fog possible across portions of
the NE Panhandle 10-13Z Sunday morning but should not impact TAF
sites. Surface winds will increase from the W/NW to 10-15 knots
with gusts in the low 20 knots during the afternoon under FL200
cirrus.

Impact focus shifts on a strong cold frontal passage impacting
north TAF sites (CDR/AIA) 02-06Z Monday (8pm-12am Sunday local
time). Lowering MVFR to IFR ceilings will occur quickly behind the
front into early Monday morning along with light to moderate
snow. Visibilities will drop to 1-3 miles and briefly lower just
after current 24 hour TAF period into Monday morning. Poor flying
conditions will continue through the day Monday due to clouds and
snow with IFR to LIFR conditions likely.

&&

.FIRE WEATHER...
Issued at 235 PM MDT Sat Apr 17 2021

Fire weather concerns will be minimal for the next several days.
A weak disturbance aloft will move south across the districts this
afternoon, accompanied by isolated light showers or sprinkles.
Milder and drier weather is forecast Sunday, followed by a strong
cold front Sunday night. Much colder temperatures and widespread
accumulating snow will develop along and behind the front into
Monday. Snow will end from north to south Monday evening. Cool and
unsettled weather will prevail for the remainder of the week.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

UPDATE...JSA
SHORT TERM...MAJ
LONG TERM...JSA
AVIATION...JSA
FIRE WEATHER...MAJ

Hazardous Weather Outlook
National Weather Service Cheyenne WY
516 AM MDT Sat Apr 17 2021

NEZ095-WYZ101>119-181130-
Northern Sioux County-Converse County Lower Elevations-
Niobrara County-North Laramie Range-
Ferris/Seminoe/Shirley Mountains-Shirley Basin-
Central Laramie Range and Southwest Platte County-
East Platte County-Goshen County-Central Carbon County-
North Snowy Range Foothills-Southwest Carbon County-
Sierra Madre Range-Upper North Platte River Basin-Snowy Range-
Laramie Valley-South Laramie Range-South Laramie Range Foothills-
Central Laramie County-East Laramie County-
516 AM MDT Sat Apr 17 2021

This hazardous weather outlook is for southeast Wyoming and
portions of the northwest Nebraska panhandle.

.DAY ONE...TODAY AND TONIGHT

The probability for widespread hazardous weather is low.

.DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY

A strong cold front for this time of the year will move across the
region Sunday night and Monday bringing widespread light to
moderate snow and much colder temperatures. Travel impacts are
possible.

.Spotter information statement...

Spotter activation will not be required.

NEZ002-003-019>021-054-055-096-181130-
Dawes County-Box Butte County-Scotts Bluff County-Banner County-
Morrill County-Kimball County-Cheyenne County-
Southern Sioux County-
516 AM MDT Sat Apr 17 2021

This hazardous weather outlook is for the western Nebraska
panhandle.

.DAY ONE...TODAY AND TONIGHT

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY

A strong cold front for this time of the year will move across the
region Sunday night and Monday bringing widespread light snow and
rain along with much colder temperatures to western Nebraska.

.Spotter information statement...

Spotter activation will not be required.