Area Forecast Discussion
Issued by NWS Cheyenne, WY

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670
FXUS65 KCYS 200337
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
937 PM MDT Sun May 19 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms are likely across
  southeast Wyoming and the western Nebraska Panhandle today.
  the main time frame for potential severe thunderstorms is
  between now and 5 PM.

- Another round of showers and thunderstorms possible Monday.
  Some may be severe, particularly in the southern Nebraska
  panhandle.

- Much cooler temperatures with more widespread to numerous
  showers and thunderstorms through Tuesday evening. Snow
  likely above 8000 feet, but toward Wednesday morning, a few
  snowflakes may be seen as low as 6000 feet.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 324 PM MDT Sun May 19 2024

Widespread showers and thunderstorms today across southeast Wyoming
and western Nebraska with the primary threat west of the Laramie
Range being gusty outflow winds and east of the Laramie Range,
strong winds and large hail. This is continuing to look like an
earlier show with many of the storms developing along a dryline near
the WY/NE border and moving eastward. As of now, we have received
reports of 50-60+ mph wind gusts with the showers out west and golf
ball size hail with a storm in the Nebraska panhandle. Any storm
development behind this initial line of storms will likely not be
severe, but may still produce gusty winds and hail as the
thunderstorm ingredients are no longer as robust as they were
earlier in the day. Hires guidance has shower and thunderstorm
activity dying down between 5 PM and 8 PM this evening.

Monday, a shortwave embedded in the flow ahead of a positively-
tilted long wave trough ejects through southeast Wyoming and western
Nebraska which may kick off scattered to widespread showers and
thunderstorms Monday late morning through afternoon. Hires guidance
has not locked onto a solution for the location of these showers and
thunderstorms, but the better moisture, instability, and shear are
south of the North Platte River Valley. Overall, highest confidence
is in cooler temperatures, gusty winds, and increasing shower and
thunderstorms chances through the day. Monday night into Tuesday,
the main trough swings through, resulting in even cooler
temperatures and more widespread shower and isolated thunderstorm
chances. Looking at current guidance, precipitation will likely
remain rain below 7500-8000 feet and snow above. Precipitation
chances continue all the way through Tuesday.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 435 AM MDT Sun May 19 2024

No major changes were made with this forecast package. The main
concern remains chilly temperatures and rain/snow potential in the
early part of the forecast period, but model guidance has remained
fairly consistent. For more details, see the previous discussion.

Previous Discussion: (435 AM MDT Sun May 19 2024)

An active pattern expected for the long term with the potential for
snow Wednesday morning and daily shower and thunderstorm chances
across the region. Tuesday into Wednesday, a strong trough will dig
across the area and bring colder temperatures, rain showers, and
potentially some snowflakes. 700mb temperatures drop well below 0C
on Wednesday morning and keep surface low temperatures in the upper-
20s and low-30s west of I-25 and mid-30s to low-40s east. Forecast
soundings from the GFS further support a saturated environment with
cool temperatures in the low-levels. Snow mixed with rain will be
favored all the way down to around 6000ft, with mainly snow favored
for the higher terrain. However, the exact amount of snow intermixed
with the rain will depend partially on the precipitation rates. With
higher rates, snow will be more likely with larger flakes mixed in
with the rain. With lower rates, expect showers to be mostly rain
with a flurry or two mixed in. Given the cold start on Wednesday,
along with cloud cover, and the potential rain/snow mix early,
decreased temperatures for Wednesday a few degrees. With the cloud
cover, recovery into the low-60s may be challenging, despite a quick
warmup in the 700mb layer expected.

Thursday looks to be a quieter, transition day as the upper-level
trough moves out of the region ahead of another trough building off
to the west. Southwesterly flow will dominate the upper-levels
Thursday with similar winds expected at 700mb. 700mb temperatures
will warm into the 4-7C range for most of the area, leading to a
quick warmup into the low-60s to upper-70s area wide. With
southwesterly flow at 700mb and the surface, could get a few showers
and maybe a thunderstorm develop off the higher terrain, so kept
some low PoPs (15-25%) in at this time. Models begin to disagree
about the incoming upper-level trough on Friday morning, with the
ECMWF suggesting a strong trough, while the GFS is weaker and
broader. The GFS tries to develop brief ridging later in the day
Friday, while the ECMWF keeps the region under troughing throughout
the day. Decided to lean more towards the ECMWF solution and dropped
temperatures on Friday into the mid-50s and upper-60s in association
with the upper-level trough. With the long ranges models suggesting
some amount of precipitation Friday, felt confident keeping PoPs in
the 25-35% range for Friday, as some precipitation is likely.

Saturday through the remaining long term forecast looks messier at
this time with significant model disagreement. Long range models
attempt to bring in an upper-level trough after brief ridging
(ECMWF) or zonal flow (GFS) on Saturday. The GFS is more aggressive
at this time, with a strong upper-level low positioned over the
Pacific Northwest and moving eastward turning into a bowling ball
low. The ECMWF on the other hand does not show a bowling ball low
and instead keeps a broader trough just to the north of the CWA. The
GFS solution would likely lead to more precipitation across the
region, while the ECMWF solution would be much drier and windier.
Due to disagreement between long range models, kept PoPs in the 20-
30% range throughout the weekend and into early next week. The
weekend will be warmer than the work week, with highs on Saturday
currently forecast to be in the low- to mid-70s area wide. However,
it is important to note that these warmer temperatures rely more on
the ECMWF solution panning out, with the GFS solution likely being
wetter and cooler for the weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 928 PM MDT Sun May 19 2024

Northwest flow aloft will continue as a cold front moves across
the terminals tonight.

Wyoming TAFS...At Laramie, ceilings will be near 8000 feet
until 09Z, then fog will reduce visibilities to 3 miles with
ceilings near 1500 feet until 15Z, then ceilings will be near
5000 feet until evening, when light rain and fog will reduce
visibilities to 2 miles with ceilings near 1500 feet. Winds will
gust to 30 knots from 15Z to 00Z.

At Rawlins, ceilings will lower to 4000 feet by 10Z, then
showers will reduce visibilities to 4 miles with ceilings near
3500 feet from 15Z to 00Z, then light rain and fog will reduce
visibilities to 2 miles with ceilings near 1500 feet after 00Z.

At Cheyenne, ceilings will lower to 5000 feet after 14Z, then
light rain and fog will develop after 00Z, reducing visibilities
to 3 miles with ceilings near 1500 feet.

Nebraska TAFS...Ceilings will lower to 5000 feet overnight and
by mid morning, then light rain and fog will develop around 00Z,
reducing visibilities to 2 miles with ceilings from 1500 to
2500 feet.

&&

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

&&

$$

SHORT TERM...LEG
LONG TERM...AM
AVIATION...RUBIN