430
FXUS65 KCYS 161725
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1125 AM MDT Wed Apr 16 2025

.KEY MESSAGES...

- Elevated to near-critical fire weather conditions for
  Wednesday under warm, dry, and windy conditions.

- A pattern change is expected at the end of the week, bringing
  cooler temperatures and precipitation chances, including
  thunderstorms and the return of widespread snow.

- Remaining unsettled during the weekend through early next week
  with chances of showers each day.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 304 AM MDT Wed Apr 16 2025

Fairly quiet tonight across southeast Wyoming and western Nebraska.
A few high-level clouds are drifting across the region, but mostly
clear conditions prevail. Temperatures have remained quite mild
overnight, with temperatures currently in the mid-30s to mid-40s,
with the warmest temperatures in areas experiencing some downslope
component to the winds. Upper-level water vapor satellite imagery
shows the base of the incoming trough just entering into the
northern most reaches of the GOES-19 CONUS domain. This trough will
bring widespread precipitation to much of the region as it slowly
slides across Thursday through the weekend.

One more warm and dry day is in store for southeast Wyoming and
western Nebraska today as upper-level riding remains overhead as an
upper-level trough slowly slides into the northern CONUS throughout
the day. A more westerly component to the winds develops this
afternoon, leading to gusty conditions across the region and
elevated to near-critical fire weather conditions, especially across
western Nebraska. Afternoon relative humidity values will drop into
the 10 to 15% range with wind gusts approaching and potentially
exceeding 25mph. However, these conditions are not expected to be
continuous for multiple hours at this time, so decided against fire
weather headlines with this update. If confidence grows in critical
conditions lasting multiple hours, a quick Red Flag Warning or Fire
Weather Watch may be needed for these locations. During the
afternoon today, temperatures will increase well into the 60s and
70s once more, with some locations in the Panhandle potentially
seeing the low-80s. Some rain showers are possible this afternoon
across western portions of the CWA before spreading eastward through
the late afternoon and early evening hours. This is due to multiple
lobes of 500mb vorticity ejecting out ahead of a nearly-cut off,
upper-level low just off the coast of California. Very weak
instability may develop this afternoon during peak heating, further
favoring the development of isolated to scattered afternoon showers.
Cannot completely rule out some thunder with any shower than can
stay together this afternoon.

The more active weather begins Thursday as the upper-level trough
over southern Canada begins to push towards the region Thursday
morning. As this trough moves into the Intermountain West Thursday
morning, the CWA will be under the left exit region of the
subtropical jet. This jet will continue to funnel in Pacific
moisture into the region, with dynamic synoptic lift, allowing the
showers and storms to persist overnight Wednesday into the morning
hours Thursday. In addition to this, lobes of 500mb vorticity will
advect into the area, further supporting gradual, synoptic lift
across the region and increased precipitation chances. The best,
most impactful, chances for precipitation will arrive later in the
afternoon and evening as a strong cold front associated with the
upper-level trough pushes through the region, dropping 700mb
temperatures well below average into the 10th percentile of
climatology per the NAEFS climatology. Ahead of this cold front, due
to some destabilization throughout the day from daytime heating,
modest MLCAPE approaching 500 J/kg will develop ahead of the cold
front, leading to the potential for showers and maybe a strong
thunderstorm ahead of the advancing cold front west of the Laramie
Range eastward towards Cheyenne. Severe weather is not expected with
this front, but a strong thunderstorm or two cannot be ruled out
ahead of the cold front. With the strengthen of the approaching cold
front, a quick transition from rain to snow is expected Thursday
evening. However, accumulation is not expected to start until
overnight Thursday into early Friday morning due to the warm
conditions we have enjoyed over the past several days. Therefore,
storm total snowfall amounts are more representative of how much
snow is expected to fall, rather than how much snow will accumulate.
The best snow accumulation is expected in Friday morning onwards, so
see the long term discussion for more details.

With the most recent forecast snow total accumulation with this
event, decided to issue Winter Storm Watches for portions of
southeast Wyoming, including northern Carbon County, the Sierra
Madre, Snowy, and North Laramie ranges, as well as the I-80 Summit.
Other headlines will likely be needed for the lower elevations, but
will let day crew take another look as confidence is not as high for
the lower elevations due to the warm ground and road temperatures.
Confidence is not high at this time that the lower elevations will
see impactful snowfall with this system and much of the initial
rounds of snow will melt as it hits the ground. Stay tuned!

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 304 AM MDT Wed Apr 16 2025

A more unsettled weather pattern looks likely in the medium to long
range. Models are in good agreement with the overall pattern across
the Intermountain West and the Front Range through the forecast
period as several chances for precipitation and cooler to near
average temperatures for this time of the year. It definitely looks
like a more typical spring pattern as we head into the last full
week of April.

For Friday and Friday night, we will still be dealing with the last
half of the northern Pacific storm system, which is forecast to dive
southeast into the area for Thursday and Friday. Upper level trough
axis will be over eastern and southeast Wyoming in the afternoon
hours with some very cold air between the surface and 500mb. 700mb
temperatures are forecast to remain below -10c through the day with
mostly cloudy skies and periods of snowfall continuing through the
day. Thus, high temperatures on Friday will struggle to reach
freezing across most of southeast Wyoming, with highs around 40 for
western Nebraska and the lower North Platte River Valley. Ensemble
members are hinting at a possible resurgence in snowfall rates
during the afternoon and evening hours as the southern upper level
disturbance stalls near the four corners region and lifts northeast
as it pivots around the main trough axis. Kept POP between 30 to 50
percent through Friday night...mainly along the I-80 corridor due to
persistent upslope flow and some low level frontogenesis.

As we head into the weekend, all models show a slow warming trend
even though temperatures will likely remain below average for mid
April. Northerly flow aloft will remain over the area through
Saturday night with the upper level flow backing into the west on
Sunday. Remaining chilly on Saturday with skies becoming partly
cloudy. However, low level instability and some daytime convection
will result in cloudy skies in the afternoon with scattered rain and
snow showers expected. Sunday looks somewhat drier with moderate
downslope westerly wind expected. It will be milder on Sunday with
temperatures close to average for April 20th (50s to low 60s).

Next week looks mostly unsettled with several upper level
disturbances, embedded in the mean westerly flow aloft, impacting
the forecast area nearly every day. The most potent disturbance and
associated jet max aloft looks to be later Monday and Tuesday, with
some respectable QPF amounts over portions of the forecast area.
Thankfully, precipitation type will be mostly rain outside of the
mountains as 700mb temperatures increase above -5c to 0c. The
mountains typically do well with spring snowfall in these patterns
which some much needed moisture for most of southeast Wyoming and
western Nebraska. Also added thunderstorms to the forecast Monday
and Wednesday with ensemble members and deterministic guidance
showing decent low level instability and increasing moisture/surface
dewpoints.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1125 AM MDT Wed Apr 16 2025

A storm system crossing the Rocky Mountains will bring some
changes to eastern Wyoming and western Nebraska terminals over
the next 24 hours or so. Fair conditions with gusty west winds
today will transition to thick cloud cover and gusty north winds
tonight. Some terminals (KRWL, KCDR) will likely drop to MVFR
or IFR by tomorrow morning as low ceilings set in. A few
sporadic rain showers are possible late this evening in
southeast Wyo, but more widespread moisture appears probable
beginning late morning tomorrow.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Watch from late tonight through Friday morning
     for WYZ103-112-114.
     Winter Storm Watch from late tonight through late Thursday
     night for WYZ104.
     Winter Storm Watch from Thursday morning through Friday
     morning for WYZ116.
NE...None.

&&

$$

SHORT TERM...AM
LONG TERM...TJT
AVIATION...DS