659
FXUS65 KCYS 202018
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
218 PM MDT Mon May 20 2024

.KEY MESSAGES...

- The potential for strong to severe thunderstorms exists from
  4PM and 11PM in far southeast WY and southwest NE. Large hail
  up to two inches in diameter, strong winds up to 60mph, and an
  isolated tornado can not be ruled out. The strongest storms
  will likely develop after 6pm.

- Much cooler temperatures with more widespread to numerous
  showers and thunderstorms on Tuesday. Accumulating snow likely
  above 8000 feet, with a rain and snow mix possible down to
  6000 feet late Tuesday night into early Wednesday morning.

- Daily shower and thunderstorm chances continue through the
  long term. Could see some elevated winds in the wind prone
  regions throughout the week and weekend.

&&

.UPDATE...
Issued at 1142 AM MDT Mon May 20 2024

Noontime visible satellite imagery reveals a thick band of cloud
cover over much of southeast Wyoming as well as the far
northwest Nebraska panhandle. Pockets of lower-level clouds were
observed streaming northward and eastward, providing some visual
clues to an increase in lower level moisture in upslope flow.
Surface observations across the southwest Nebraska panhandle
and southeast Wyoming have shown a steady change in wind
direction over the past 2-3 hours as northerly post-frontal flow
shifts to easterly. However, of note is the rather paltry
moisture in that surface easterly upslope flow. Td values in
the Pine Bluff area remain in the low to mid-30s as of the
noontime hour with obs farther east in the SNY vicinity showing
Td values in the low 40s. A few pockets of elevated instability
over the mountains of southeast WY and far northwest NE have
resulted in some isolated weak thunderstorm activity, primarily
tied to the differential heating boundary between the cloud
shield to the west and clearer skies to the east.

Over the next several hours, continued easterly upslope flow
will slowly transport higher surface moisture values in from the
east. This pool of deeper moisture is evident as a band of
thicker near-surface stratus over northwest Kansas and the MCK
area of Nebraska. Given the rather high T/Td spreads, initial
thunderstorm development in southeast Wyoming will be high-based
with a marginal hail and wind threat through around 22z. After
around 23z or so, CAM guidance does remain consistent supporting
the development of some deeper convection over far southeast
Wyoming and moving into the NE panhandle. While deeper surface
moisture and better low-level directional shear will likely
remain south of the Colorado state line, RAP guidance does
indicate long, straight hodographs over the NE Panhandle which
would support an early supercell storm mode. With a deeply mixed
boundary layer and plenty of cold air aloft, storms will likely
tend to be outflow dominant and may quickly transition to
bowing line segments with primarily a high wind threat. This is
particularly evident in the past few runs of the HRRR over
southwest Nebraska in the Scotts Bluff, Banner, and Morrill
county corridor from around 6-9pm this evening. Areas along I-80
may ultimately remain shielded from stronger convection as any
dominant supercells in northeast Colorado may block deeper
moisture return. Regardless, it is looking like the 6-9pm
timeframe will be the time window to watch today for storms,
especially from the Wyoming state line eastward in the SW NE
panhandle.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 206 PM MDT Mon May 20 2024

Lingering rain showers are possible early Tuesday morning
across much of the NE panhandle after Monday night`s convective
activity with the initial leading shortwave continuing off
towards the upper midwest. Additionally, ongoing precipitation
will continue over east-central WY associated with the slow
moving vort max over western WY. This feature will continue to
drop 700/500mb temps across the area on Tuesday supporting of
high elevation snowfall, mainly above 8000 feet elevation. This
will lead to near advisory snow accumulations in the higher
terrain of the Snowy and Sierra Madres throughout the day
Tuesday.

Cooler temperatures will continue into Tuesday night as skies begin
to clear, especially after midnight, leading to much of south-
central WY dropping near the freezing mark. East of I-25, lows will
drop into the mid-30s with a few localized areas dropping near the
freezing mark. Latest NBM probability of dropping to/below 32F
degrees is 30-50%, especially extending eastwards off the Cheyenne
and Pine Ridges. However, with the average date of the last spring
freeze being in mid-to-late May across most areas east of I-25,
no frost/freeze headlines are expected at this time.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 400 AM MDT Mon May 20 2024

Models continue to trend towards an active long term forecast for
southeast Wyoming and western Nebraska. Several upper-level
disturbances are progged to traverse across the region. Wednesday
morning, an upper-level trough will move off to the east as another
builds the coast of Washington and Oregon. Forecast soundings from
the GFS for Wednesday morning no longer suggest a potential for snow
across the region. Dry low-levels will likely not allow much
precipitation to fall to the surface Wednesday morning. Therefore,
removed the mention of snow from the Wednesday forecast. With the
lack of snow expected now, temperatures should increase as 700mb
increase to the 4-6C range. Surface temperatures should warm back
into the low-60s to low-70s across the region. Could see some
isolated showers throughout the day as residual energy remains
overhead behind the departing trough, but widespread precipitation
is not anticipated at this time.

The second of many upper-level troughs digs through the region
Thursday, with long range models in good agreement on the evolution.
This trough is progged to quickly swing across the area with decent
precipitation chances in the afternoon hours. At 700mb, a low is
expected to develop across western Montana and move overhead
throughout the day. A weak cold front will move through with the
passage of this low, though the coldest air is expected to remain
off to the north in eastern Montana. However, the cold front will
act as a catalyst for more showers and potentially thunderstorms to
develop in the afternoon hours before dropping temperatures for
Friday. Thursday`s highs look to be in the low-60s to upper-70s
again, with highs on Friday in the upper-50s to low-70s across the
area. The trough will move out the region Friday with brief riding
developing for Saturday before a third trough pushes towards the
area. Behind the cold front, winds will pick up significantly and be
further amplified by the 700mb low passing to the north. Height
gradients will tighten throughout the day, favoring strong winds
across the wind prone regions and likely spilling out into the
adjacent plains.

On Saturday, brief ridging is progged to develop aloft, allowing
700mb temperatures to increase back into the 4-6C range once more.
Residual energy from the departing trough, as well as some energy
ejecting out ahead of the incoming trough will be enough to spark
some showers and thunderstorms during the afternoon hours.
Temperatures will warm into the mid-60s to mid-70s, with isolated to
scattered precipitation possible. With the passage of a 700mb low to
the north of the CWA, height gradients will tighten across the
region once more. Could see some elevated winds in the wind prone
regions of southeast Wyoming and likely see some winds spill out
into the adjacent plains. Daily shower and thunderstorm chances
continue through the weekend, with the incoming upper-level trough.
Some disagreement appears between long range models for Sunday
evening, as the GFS has a closed upper-level low and the ECMWf has
an open wave across the northern CONUS. However, both models still
suggest precipitation chances so kept PoPs in the 20-30% range for
the remaining long term forecast.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1142 AM MDT Mon May 20 2024

Main aviation concern remains the thunderstorms currently near KRWL
and soon KLAR, then KCYS, KBFF, KCDR, KAIA, and KSNY later this
evening and overnight. Gusty and erratic strong winds, large hail,
and lightning will be the primary hazards in these thunderstorms.
CIGS and VIS will drop to low MVFR to IFR in these thunderstorms.
The thunderstorms will transition to more of a stratiform rain early
tomorrow morning with low IFR CIGS persisting and gusty north to
northwesterly winds.

&&

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

&&

$$

UPDATE...MAC
SHORT TERM...MB
LONG TERM...AM
AVIATION...LEG