228
FXUS65 KCYS 031751
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1151 AM MDT Thu Apr 3 2025

.KEY MESSAGES...

- Below average temperatures will continue late this week and
  into the weekend.

- Periods of accumulating snow expected today through early
  Saturday morning. Travel impacts are possible, especially over
  the higher elevations.

- Warmer and drier conditions begin on Sunday and likely linger
  through much of next week.

&&

.UPDATE...
Issued at 1123 AM MDT Thu Apr 3 2025

We are sending out a quick update to delay precipitation chances
a bit. Latest radar loop was showing the mid level column of air
is starting to saturate across northern Colorado, but the
snowfall has not occurred yet. Would expect things to continue
saturating during the 2 to 3 hours and start pushing into the
I-80 corridor by 21z and 00z south and east of an Alliance to
Scottsbluff to Chugwater line. At this point, it appears like
the higher snowfall rates will start developing in southern
panhandle late this afternoon into this evening where the
highest omega fields will be in place. We may need to consider
an advisory in this area with the next update.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 340 AM MDT Thu Apr 3 2025

Primary forecast concern will be the potent upper level
disturbance, currently near the Four Corners region, lifting
northeast across the area later today and tonight...with a
secondary cold front sliding south out of western Canada
resulting in another round of snowfall later Friday through
Friday night. Models are trending a little more aggressive with
these two events as short range ensemble guidance and high res
guidance have trended wetter and colder compared to 24 to 48
hours ago.

For early this morning, mostly clear skies continue over much of
the area based on current IR Satellite loop. Cloud shield
associated with the upper level disturbance/trough is still
well to the south across Colorado. Fog has not formed yet across
the western Nebraska Panhandle. Will give this a few more hours
before we take it out of the forecast as surface winds have not
backed into the southeast yet. Dewpoint depressions are still
between 1 to 4 degrees C, so will keep fog in the forecast for
now. Otherwise, expect increasing clouds from south to north
this morning as a stationary front remains near the Laramie
Range. Low temperatures look on track...with readings in the mid
teens to mid 20s. As winds eventually shift into the east and
southeast, a period of fog is possible for the I-80 Summit in
this pattern, so added some fog west of Cheyenne until the snow
begins later this afternoon.

The upper level shortwave disturbance to our south, which is
part of the overall long wave trough over most of the western
third of the United States, will lift northeast today across
northern Colorado and southeast Wyoming/western Nebraska this
afternoon and this evening. Although the feature doesn`t look
too impressive, a closer look at the 700mb level and jet
dynamics show some moderate low to midlevel forcing later this
afternoon through midnight tonight. These features can surprise
us with moderate to locally heavy precipitation this time of the
year with any low level instability. In the upper levels, a 90
knot jet max will quickly move northeast ahead of the primary
vorticity max which will be located somewhere near the
Colorado/Wyoming border. Closer to the surface, a 700mb low is
forecast to develop and lift northeast...eventually stalling
near the Wyoming/Nebraska border before weakening late tonight.
In addition, the stationary front will remain near the Laramie
Range and drift east tonight, resulting in additional low level
convergence. Increased POP across most of the area with higher
QPF totals than previously expected. Precipitation will start
off as mainly rain below 5000 feet, but snowlevels should slowly
drop 3500 feet by early Friday, so most places will see all
snow by sunrise Friday. Can see a few places, especially those
above 5500 feet, getting some moderate snow bands with a half an
inch per hour rates at times. Believe these will be short-lived
as the first system looks pretty compact with two separate
peaks in precipitation and a lull in the middle (first one later
today with the second one Friday associated with the polar cold
front digging south). The question then becomes will snowfall
rates overwhelm surface/pavement temperatures for travel
impacts/hazards? This is where the most uncertainty is with the
forecast over the next 48 hours. Given the time of the day most
of the snowfall is expected to fall...which is mainly during
the daytime or evening hours along the I-80 corridor, decided
against a Winter Weather Advisory due to the pretty high early-April
sun angle. Even in the mountains, two separate periods of snow
are expected which will likely result in 4 to 9 inches of total
snowfall for the Sierra Madre Range and Snowy Range. This is
just below Advisory thresholds, so will hold off on any
headlines. Will have to monitor snowfall rates closely and areas
north of Interstate 80 once the secondary cold front arrives.
High res guidance shows a more persistent band of snowfall
developing from Lusk to Chadron and Alliance late tonight and
into Friday morning.

Pretty cold temperatures expected, especially on Friday, as the
surface cold front dives southward out of Canada. Forecast highs
are expected to be in the low to mid 30s for most of southeast
Wyoming above 5000 feet...with highs in the upper 30s to near 40
across western Nebraska and the lower elevations of far eastern
Wyoming. Another round of snow is expected with the arrival of
this front, but most models and ensembles show light snowfall
rates and maybe an other inch or two of snow. However, most of
this will fall during the day and will likely struggle to
accumulate. Very cold temperatures Friday night with clearing
skies with lows in the single digits and teens, but still not
approaching record lows for the day.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 305 AM MDT Thu Apr 3 2025

Starting Sunday morning the long term forecast is dominated by
upper level ridging and downslope flow to keep temperatures in
the 60`s to 70`s during the day and 30`s to 40`s into the overnight
hours. However, mainly on Tuesday the Intermountain West experiences
an upper level shortwave that temporarily disrupts our warmer
temperatures. The main upper level low traverses the Canadian
Providences creating a shortwave the helps to flatten our stout
ridge. This shortwave creates some small precipitation chances
for our area but currently the only area forecasted to experience
any precipitation would be mainly the mountains. This mainly
has to do with the stout dry air near the surface caused from
the downsloping off the mountains while in previously persistent
westerly flow. There is a brief instance that we enter
northwesterly flow to give a slight chance for precipitation out
of the mountains. But the most likely scenario is that the sky
becomes cloudy and most of the forecast area sees Virga in the
afternoon. The cloudy skies and virga may cool us off slightly ,
but the ensemble means still keep temperatures in the 60`s. To
add confidence in our overall dry forecast, the moisture content
at 700mb looks to be sufficient to see some light accumulations
(1-2 inches) in the mountains (mainly due to upslope), but
insufficient to produce any measurable precipitation elsewhere.
By Wednesday morning the ridge once again starts to rebuild and
temperatures return back into the 60`s.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1139 AM MDT Thu Apr 3 2025

Look for precipitation alongside a quick uptick to winds to move
into the region early this afternoon, continuing through the
evening into tomorrow. Some breaks in precipitation could be
possible overnight into early tomorrow, but sites KBFF and KCDR
may see sustained precipitation through the entire TAF period
once it begins. Cloud decks will fall drastically as the system
moves in, with widespread ceilings between 500-2500 feet. During
falling snow, expect visibilities below 6SM, with heavier snow
dropping visibilities below 3SM. All sites are likely to see IFR
to LIFR conditions outside of KLAR and KRWL, but even these
sites could see brief reductions to IFR or below if a band of
heavier snow does move across.

&&

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

&&

$$

UPDATE...REC
SHORT TERM...TJT
LONG TERM...MM
AVIATION...CG