184
FXUS65 KCYS 281147
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
545 AM MDT Sun Apr 28 2024

.KEY MESSAGES...

-Lingering light snow showers will bring light snowfall
 accumulations through Sunday afternoon to the Sierra Madre and
 Snowy Range mountains above 8500 feet. Winter headlines remain
 in effect mainly due to those planning outdoor activities in
 the higher terrain during the weekend.

-Strong winds are anticipated early Tuesday morning through
 Tuesday afternoon in the wind prone and gap areas of the higher
 terrain of southeast Wyoming.

-Active weather pattern continues for the extended forecast as
 the next window for rain and accumulating snowfall exists for
 Wednesday night and Thursday.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 330 AM MDT Sun Apr 28 2024

A very slow moving upper level system responsible for multiple
cloudy days in a row, and rain and snow showers continues to
slowly trek to the east early this morning. Several locations
are reporting light rain showers, especially the western
Nebraska Panhandle, with chilly temperatures to boot. As of 9Z,
observations depict southeast Wyoming and the Panhandle areas in
the upper 20s to near 40 degrees, with the warmest temperatures
residing along the North Platte River Valley. Foggy conditions
and very low cloud ceilings from an upslope flow regime have
made for low visibility readings. Will continue the Dense Fog
headline through the morning hours of Sunday, and have the next
shift assess whether to discontinue it or not. SNOTEL
observations also showing light snow accumulations in the higher
terrain, so a continuance to the winter headlines appeared
prudent as we expect additional light snow in that area.

Cold air intrusion from the mid-levels will keep temperatures
chilly for the NE Panhandle today while combined with light rain
showers. Expect partial clearing to occur for areas across the
cwa, and with the higher sun angle in place despite the pesky
cloud cover, daytime highs should recover the near seasonal
normals for the southeast WY forecast zones, and the North
Platte River Valley of the NE Panhandle. A few areas will reach
the upper 50s to lower 60s where sunshine is present the longest
today, but do expect the remaining tier of the cwa to see upper
40s to middle 50s. This evening and overnight, there will we a
weak shortwave disturbance that is sheared off from a Pacific
Northwest system, bringing another shot at light snow flurries
and rain showers overnight to southeast WY. Not looking at
anything impactful for weather conditions, except the mountain
zones that may receive another inch or two of snow during the
overnight hours.This will be a quick-moving shortwave on the
heels of the departing cut-off upper level system making its way
towards the Upper Mississippi Valley.

Mostly sunny skies will become the main story on Monday. Daytime
highs will make for a nice recovery after the chilly
temperatures of late. Highs in the 60s and 70s for the lower
terrain will occur, with cooler temperatures in the mountain
zones. A new shortwave disturbance will propagate from the
Canadian Rockies toward our cwa by Monday night. A surface low
pressure over northern WY will move towards the Dakotas by
Tuesday morning. This will bring another opportunity for light
rain showers in the lower terrain and light snow in the higher
terrain to be present. Additionally, model guidance continues to
depict a potential mountain wave event across the Central
Rockies, including our cwa for Tuesday. This could create some
blowing snow issues for the higher terrain, but the higher sun
angle may limit this potential during Tuesday. The gusty winds
are favored to be strongest in the gap and wind prone areas. A
high wind headline may be necessary to reflect this in the
coming 24-36 hours. Downslope effects off the lee side of the
Laramie Range will adiabatically warm areas, and dry out fuels
for Tuesday afternoon. Have kept the forecast consistent with
low afternoon humidity for Tuesday as we see slightly cooler
daytime highs across the region, mainly upper 50s to lower 70s.
With the recent moisture over the past couple of days, and
spring green-up taking place, fire weather concerns should
remain low to elevated.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
Issued at 330 AM MDT Sun Apr 28 2024

Primary forecast concern for the medium to long range will be
the track and evolution of a Pacific storm system and associated
cold front forecast to push across the central and northern
Front Range late Wednesday and Thursday. Southeast Wyoming
should begin to see the first round of activity with this system
late Tuesday night as the initial frontal boundary stalls near
the Nebraska/Wyoming border and a 110kt jet max aloft suggest
nighttime showers and isolated thunder across the area through
early Wednesday morning. Kept elevated POP across most of the
area before the trough axis begins to accelerate eastward
Wednesday. Increased min temperatures Tuesday night due to cloud
cover and warm air advection just off the surface...which should
aid in developing weak nocturnal convection. High temperatures
on Wednesday should be seasonable and in the 50s to mid 60s
before the cold front pushes into the region later in the day.

For late Wednesday through Thursday, will be closely monitoring
the position and evolution of the storm system forecast to track
across the area. Models have continued to trend colder for this
system, so nudged max and min temperatures downward a bit since
all ensembles and deterministic models are showing much colder
temperatures with 700mb temps between -5c to -10c Thursday
afternoon. The 00z GFS solution is the most aggressive solution,
showing broad WAA aloft and some frontogenesis as the upper
level trough attempts to form a closed low across Colorado. The
ECMWF and about half of the GEFs and Euro ensemble members show
a much more progressive solution as the trough remains an open
wave as it moves eastward into the Dakotas, mainly because the
trailing system across the PAC NW is faster compared to the GFS.
The 00z ECMWF solution is also a bit further north, but still
shows widespread precipitation across the area although for a
shorter duration. Increased POP into the likely category (55% to
70%), but will hold off on mentioning snow level, potential
snow accumulations, etc until we get closer to the event due to
the amount of uncertainty. Would like to note that all models
show a cold enough airmass for widespread snow down to
elevations of 5000 feet Thursday. Did not go that aggressive in
the official forecast due to low confidence and the 00z GFS an
outlier when compared to the middle "most probable" 25th to 75th
percentile of all ensemble guidance. Lowered max and min
temperatures below average for the end of the week. At this
time, snow accumulations can`t be ruled out for elevations down
to 5000 feet, with mainly rainfall for elevations below 5000
feet. Still dealing with a relatively large spread in forecast
high temperatures for Thursday, but not as bad as yesterday.
Highs will likely be well below normal with the GFS showing the
coldest temperatures...generally in the 30s to near 40 early
Thursday morning, dropping into the upper 20s through the day.

For later this week and into next weekend, thankfully the cold
airmass will be short-lived...which is typical for May as 700mb
temperatures increase above 0c. All models show the front
lifting northeast as a warm front, with a pleasant weekend
possible as highs increase back to near average and possibly
above average by next Sunday. However, can`t rule out a daily
chance of showers and thunderstorms both Friday and Saturday
afternoon.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 545 AM MDT Sun Apr 28 2024

Pacific storm system will lift northeast into Nebraska today and move
out of the area tonight. Improving conditions are expected with rain
and snow showers becoming more isolated by this afternoon.

HAZARDS/WEATHER TRENDS: Steady snow/rain will come to an end later this
morning across western Nebraska, with IFR or near IFR conditions
improving to MVFR by this afternoon due to low CIGS. CIGS are expected
to lift above 5000 feet AGL between 21z today and 00z Monday for all
western Nebraska terminals except for KCDR...which may struggle to
improve due to a lingering upslope northeast winds. Further west across
southeast Wyoming, conditions will improve to VFR late this morning
with another round of rain showers expected through this evening.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until 6 PM MDT this evening for WYZ112.
     Winter Storm Warning until 6 PM MDT this evening for WYZ114.
     Dense Fog Advisory until 8 AM MDT this morning for WYZ116>118.
NE...None.

&&

$$

SHORT TERM...BW
LONG TERM...TJT
AVIATION...TJT