-- Discussion containing changed information from previous version are highlighted. --
586
FXUS65 KCYS 191127
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
527 AM MDT Sun May 19 2024

.KEY MESSAGES...

- Scattered to numerous showers and thunderstorms are likely
  across southeast Wyoming and the western Nebraska Panhandle
  today. A narrow window for a few strong to marginally severe
  storms may exist from the early/mid afternoon into the early
  evening hours, mainly over eastern portions of the Nebraska
  Panhandle. Hail up to one inch in diameter and wind gusts up
  to 60 MPH would be the primary hazards.

- Another round of showers and thunderstorms will occur on
  Monday as the stagnant mid and upper-level jet remains
  overhead.

- Lee cyclogenesis over southeast Colorado will contribute to a
  sub-998-mb surface low lifting across central KS into eastern
  NE from Monday evening into Tuesday morning, with substantial
  wrap-around moisture working its way into the CWA. Minor snow
  accumulations will be possible in the high country. A few snow
  flakes could mix in as low as 6000 feet into Cheyenne late in
  the forecast period.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 435 AM MDT Sun May 19 2024

Showers and thunderstorms may become quite numerous across south
east Wyo and the western Neb Panhandle this afternoon w/ several
mid-level disturbances embedded within west-southwest flow aloft
and a coupled northern/southern stream H25 jet suggesting a very
broad area of upper-level divergence. High-resolution models are
continuing to show a broad range of solutions with regard to the
overall timing of convection, but agree on the general trends w/
two primary focus areas for this activity. Most areas should see
weaker storms associated w/ the upper jet. A better chance for a
few stronger storms will exist mainly across the far eastern row
of Nebraska Panhandle counties, just ahead of the surface trough
and dryline if initiation can occur before it translates east of
the CWA. The primary hazard period would be from early afternoon
through early evening, though chances for rain showers will most
likely persist through the night. In eastern areas, surface dew-
points in the lower to middle 50s coupled w/ steep mid-level LRs
should support MLCAPEs around 500-1000 J/kg early this afternoon
before the dryline clears the area. 0 to 6 km shear around 30-35
knots would support organized updrafts for a time, albeit rather
brief this afternoon. The primary hazards will be gusty winds to
60 MPH and marginally severe hail. Scattered to numerous showers
should continue through much of the night, mainly along & north-
west of a Laramie WY to Chadron NE line given the orientation of
the H25 jet. By Monday afternoon, the CWA once again becomes the
focus for convective development with intense 60+ knot 0 to 6 km
shear along with unidirectional wind profile possibly supporting
organized linear segments capable of producing strong winds. May
see some activity develop off the central Laramie Range as well,
as the Chugwater cyclone ramps up during the afternoon.

A more widespread precipitation episode still appears likely for
Monday night through Tuesday as robust cyclogenesis across south
east Colorado contributes to a sub-998-mb surface low lifting to
the northeast across central KS and eastern NE between 00z - 12z
Monday night and Tuesday morning. Although there is still a good
bit of model uncertainty w/ regard to the storm track and actual
footprint of the wrap-around moisture field, this should be good
for at least measurable precipitation for much of the CWA w/ the
best chances for more robust totals western Neb. H7 temperatures
falling to 0 to -2 deg C should support minor snow accumulations
in the high country. It is possible for rain to mix with snow in
Cheyenne for a time, but no impacts are expected.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
Issued at 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 /12Z TAFS THROUGH 12Z MONDAY/...
-- Changed Discussion --
Issued at 527 AM MDT Sun May 19 2024

A cloudy night and early morning for southeast Wyoming and western
Nebraska. Clouds will continue to increase throughout the day with
gusty winds expected at all terminals. Isolated to scattered
thunderstorms will be the primary aviation concern today. The best
time for thunderstorm activity will be between 20 and 00Z. Gusty,
erratic winds are possible around any showers or storms that develop
as downbursts look possible today. Showers and storms clear out by
02Z.
-- End Changed Discussion --

&&

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

&&

$$

SHORT TERM...CLH
LONG TERM...AM
AVIATION...AM