Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 271820 AAA
AFDCYS

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
1120 AM MST Fri Jan 27 2023

.UPDATE...
Issued at 1042 AM MST Fri Jan 27 2023

Please see updated 18Z aviation discussion below. Ongoing winter,
visibility, and ceilings are occurring and will persist through
the next 26-48 hours at area terminals.

Outside of the aviation aspect, moderate to heavy snow is
occurring across Carbon County and portions of Albany County,
especially in the mountains above 8500ft. Snow and blowing snow
will be the main impact in these areas today with lower impact
east across the High Plains outside of a few scattered flurries
and pockets of blowing snow. Worsening conditions occur Saturday
with arctic airmass surging south along with snow shifting east
across the High Plains. Residual travel impacts are likely. Wind
chills will drop through the day and stay below zero through
Wednesday across the entire area. Please be prepared for this cold
spell. More details later on the main afternoon update.

&&

.SHORT TERM...(Today and tomorrow)
Issued at 429 AM MST Fri Jan 27 2023

Hazards Discussion: Numerous highlights ranging from High Wind
Warnings, Blizzard Warnings, Winter Storm Warnings, and Winter
Weather Advisories are currently in effect for much of SE Wyoming
and parts of the northern Nebraska Panhandle. The following
sections will discuss updates and changes made to the active
highlights in this forecast package.

Observation platforms within the High Wind Warnings have shown a
downward trend over the past 3 hours which was expected to occur
this evening with a shift in the wind vector and slight weakening
in the gradients. In-house modeling guidance captured this lull
as well, as probabilities for a 2-3hr period beginning around 2 AM
this morning dropped some 30 percentage points. Analyzing both
in-house modeling analogs and HI-RES models of the HRRR, FV3 and
the ARW, consensus is for a ramp up in the winds, first across the
Arlington wind prone area by sunrise then spreading across the
Bordeaux wind prone and the I-80 summit by the late morning.
Gradients begin to tighten and increase across these areas through
the morning and periods of high winds can be expected. The
strongest winds will be across the Arlington wind prone as gusts
could approach 70 mph. In addition to these winds, snow will
increase today and leftover snowpack will create blowing and
drifting snow in this zone from Arlington to Elk Mountain.
Blizzard conditions have been observed in these areas and
confidence remains high for blizzard conditions to continue as the
winds ramp back up and the snow increases throughout the day.
Breezy conditions are likely for areas east of the Laramie Range,
along the I-25 corridor, but winds look to be below High Wind
criteria at this time.

A prolonged period of moderate to heavy snow will first impact the
mountains and Carbon county today before gradually move east.
Taking a look at the active Winter Storm Warnings currently in
effect, no significant changes were made to these zones. Main
adjustments were to analyze the snow ratio`s across the mountain
zones and the Carbon county valleys. Temperatures were decreased
slightly across Carbon county which shifted snow ratios up into
the 15-16:1 range for the activity expected today. This adjusted
snow amounts slightly but not enough to warrant an upgrade to a
Winter Storm Warning at this time and the Winter Weather Advisory
romaines on track.

The highest confidence for Winter Storm conditions, will be
across the mountains. A long duration heavy snow event seems
likely for the mountains of SE Wyoming beginning today and
continuing through the weekend. A moist airmass will remain
entrenched and be met with a series of clipper systems in the form
of progressive shortwave troughs, that will slam a shallow arctic
airmass into this moisture plume. Snow ratios start out around
17:1 but as that cold air moves in, snow ratios should climb
toward 19-20:1 by Saturday. Enhanced orographic/upslope mechanisms
should aid in maximizing snowfall across the mountains,
especially across the Sierra Madre and Snowy ranges. Storm total
snow accumulations could approach 4 feet for both the highest
elevations and the prime upslope north facing slopes in the Sierra
Madres, with 1 to 2 feet possible for the northern Laramie Range
and the Snowys. Winter Storm Warnings remain on track for these
areas.

The biggest shift in the forecast were for both Converse and
Niobrara counties. Things are beginning to come into better
alignment for a better moisture surge to occur in conjunction
with the passage of the strongest shortwave in the series. CAA
will easily filter into the region jointly with some robust
moisture. Overall snow totals across these two counties shifted
the most and increased the storm total a good 2 inches from the
previous forecast. Considered putting these two counties in a
Winter Storm Watch, but total snow expected on Saturday falls just
short. If trends continue an upgrade may be needed in later
forecast updates.

Weather Details: Overall, we can expect an active weather period
over the next 36 hours across SE Wyoming. Prolonged periods of
moderate to heavy snow is expected, that will add to the ongoing
snowpack across the region. Toward the end of the snowfall, we
will focus on the cold, arctic airmass that will move into the
region and drop temperatures well below average.

.LONG TERM...(Sunday through next Thursday)
Issued at 429 AM MST Fri Jan 27 2023

Minimal changes made to the long term forecast. The main story
remains snowfall continuing through Monday with very cold
temperatures, then a brief period of strong winds Tuesday.

Snow amounts have remained relatively consistent. Greater
accumulations are expected Sunday, with an additional 2 to 4 inches
across lower elevations of Carbon, Converse, and Niobrara counties;
1 to 3 inches across the High Plains; 8 to 12 inches in the Snowy
and Sierra Madre Mountain Ranges. Monday, the chances of snow
continue, but current forecast amounts are anywhere from a trace to
an inch outside of the mountains, and not much more in the
mountains. During this time, southeast Wyoming and the Nebraska
panhandle are sitting in the arctic air mass, keeping temperatures
quite cold through Monday, only starting to warm up Tuesday.
Confidence is high in the current forecast temperatures due to the
model guidance staying locked in on this solution. GFS and ECMWF
Ensemble Members have remained tightly clustered in the
temperatures. Wind Chill Advisories (wind chills -20 to -29) will
likely be needed for Carbon, Albany, Converse, Niobrara, Goshen,
Sioux, and Dawes counties for Sunday night and Monday night. The
arctic air mass is finally pushed out as the trough axis swings east
across southeast Wyoming and the Nebraksa panhandle Monday night,
allowing warmer air to move back in. As one last shortwave moves
across southeast Wyoming and the Nebraska panhandle Tuesday, there
is a period of strong winds possible that could result in widespread
westerly wind gusts 45 to 55+ mph with the strongest winds in the
wind-prone areas of southeast Wyoming. The GFS shows a tight
pressure gradient against the Laramie Range co-located with a band
of subsidence on the eastern slopes. Internal guidance is showing
850 mb CAG to CPR pressure gradients jumping up to 65 to 80 meters
between Monday night and Tuesday afternoon, which is a good indicator
of high wind potential. Wednesday into Thursday, a ridge develops
along the western CONUS, then shifts eastward, putting southeast
Wyoming just on the eastern edge of the ridge. This will result in a
nice warm up in temperatures, back to near-normal for the start of
February.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1042 AM MST Fri Jan 27 2023

Elevated to significant aviation impacts ongoing or set to occur
within 12-24 hours through Sunday with snow, IFR or worse
ceilings, and reduced visibility down to 1/4 SM at times. These
conditions wont be experienced at all terminals but elevated
impacts likely at least and MVFR to IFR at all terminals with SN
and reduced visibilities.

Ongoing significant winter and blowing snow impacts reducing
visibility down to 1/2 mile at KRWL and this will persist through
mid-day Saturday. KCDR will be the next site to experience
impacts tonight with RASN then SN as an arctic cold front plunges
south through early Saturday morning. IFR or worse will occur KCDR
and then spread south and east through Saturday. KCYS and KSNY to
KLAR will be last hold outs of impacts until Saturday afternoon
with SN coverage increasing. Winds today and overnight will be
elevated for KRWL at 25-35 knots with blowing snow and slightly
less at KLAR with 20-30kt and KCYS gusting upwards of 25 knots.
Lower wind speeds of 20kt in NE sites from the west then switching
north behind the cold front Saturday morning and remaining
elevated.


&&

.FIRE WEATHER...
Issued at 202 AM MST Fri Jan 27 2023

Prolonged period of widespread snowfall is expected today and
continue through the weekend. With additional new snow on top of
an already existing snowpack, fire weather concerns will be low
over the next few days. Winds will be gusty across SE Wyoming
today with strong winds across the wind prone areas and higher
elevations. By next week, temperatures will drop to well below
average.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory from 5 PM this afternoon to 11 PM MST
     Saturday for WYZ101-102.

     Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
     Saturday for WYZ106>108.

     Winter Storm Warning until 5 PM MST Saturday for WYZ103.

     Winter Weather Advisory until 5 PM MST Saturday for WYZ104-105-
     109-111.

     High Wind Warning until 3 PM MST this afternoon for WYZ106.

     Blizzard Warning until 8 PM MST this evening for WYZ110.

     High Wind Warning until 8 PM MST this evening for WYZ110-116-117.

     Winter Storm Warning until 11 PM MST Saturday for WYZ112-114.

NE...Winter Weather Advisory from 5 PM this afternoon to 11 PM MST
     Saturday for NEZ002-095.

     Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
     Saturday for NEZ096.

&&

$$

UPDATE...JSA
SHORT TERM...AW
LONG TERM...LK
AVIATION...JSA
FIRE WEATHER...AW


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