-- Discussion containing changed information from previous version are highlighted. --
265
FXUS65 KCYS 201119
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
519 AM MDT Mon May 20 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms are likely across
  southeast Wyoming and the western Nebraska Panhandle today.
  The potential for strong to severe thunderstorms exists from
  3PM and 11PM as thunderstorms move from west to east. Large
  hail up to two inches in diameter, strong winds up to 60mph,
  and an isolated tornado can not be ruled out. The Storm
  Prediction Center has a Day 1 risk of Marginal to Enhanced
  across the southern portions of our cwa.

- 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.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 400 AM MDT Mon May 20 2024

The surface cold front swept through our cwa by 6Z earlier this
morning, and behind it we have observed cool northerly winds as
temperatures have adiabatically cooled down to the upper 30s to
middle 40s for most areas. GOES-16 satellite imagery with the
nighttime microphysics spectrum has additional cloud cover
propagating to the northeast, originating from the Great Basin
area courtesy of the upper level jet stream. The polar jet and
subtropical jet will begin to phase later today, as a potent
upper level longwave trough continues its propagation downstream
while being positively-tilted from the Pacific northwest. A few
light rain showers are evident on the radar as of 9Z this
morning, due to diffluent flow aloft combined with the lingering
semi-stationary frontal boundary draped along and near our
CO/WY border. This frontal boundary is expected to stall as it
progresses southward along the Front Range of Colorado.

As today progresses, a tricky and difficult forecast will
quickly evolve. Overnight cloud cover will be difficult to pin
down as to how much it will impact the potential atmospheric
instability for convective initiation later this afternoon and
evening. At this time of inspection, the upper and mid-levels of
the atmosphere will adiabatically cool at the 300mb/500mb
heights. The anomalously potent upper level trough will cause
tropospheric folding to ensue as the day continues, especially
over the Central Rockies when analyzing the 1.5 and 2.0 PVU
layers from model guidance. Steep low-level and mid-level lapse
rates are anticipated to be enhanced as the atmosphere
adiabatically cools aloft. The subtropical jet stream and polar
jet stream are anticipated to phase between 12z this morning,
and 0Z this evening. The nose of a 100+ knot jet streak is
signaled by model guidance around 0Z this evening Lee-side
cyclogenesis is expected to occur across central and northeast
Colorado today. As the rapid cyclogenesis occurs today, we will
see the aforementioned semi- stationary frontal boundary lift
northward as the surface low deepens across CO. This will most
likely bring a surge in warmer temperatures to areas where the
frontal boundary lifts northward, particularly along and near
the I-80 corridor from southeast WY toward the NE Panhandle.
Much cooler temperatures are expected further to the north,
especially in Converse and Niobrara County where temperatures
will struggle to get out of the 40s due to it being behind the
cold front. In addition to the steep lapse rates exceeding 6.5
degrees C after 21Z, we will see the moisture levels from PWATs
greater than 0.5 inches and dew points creeping upwards of 45
degrees F as we approach the 0Z timeframe. 0-6km bulk shear
values greater than 40-55 knots are favored east of the I-25 and
I-80 corridor. Helicity values at 0-1km also creep up in excess
of 100+ m2/s2 between 21Z and 3Z this evening. CAMs of the HRRR
and NamNest have UH tracks across northern CO and southeast WY
extending into the NE Panhandle after 0Z this evening.
Inverted-V soundings are favored for southeast WY, showing DCAPE
values of 500-750 J/kg. Gusty winds are favored for several
areas due to scattered to numerous rain showers.

All of this being mentioned, the potential for strong to severe
thunderstorms is anticipated along and east of the Laramie
Range. A couple of organized thunderstorms could also initiate
along the I-80 corridor west of the Laramie Range, and push east
across the Laramie Range. Discrete supercells are possible, with
Bunkers right-motion favored as the thunderstorms become
potentially severe. Model guidance highlights the strongest
possible convection occurring near and along the WY/NE state
line, and then propagating to the east. All modes of severe
weather hazards are possible, with the highest confidence of
large, damaging hail up to two inches or more in diameter being
possible. Strong, gusty winds in excess of 60mph are also
favored, with an isolated tornado being possible due to the high
propensity of wind/speed shear being present later today.
Localized heavy rain may also be favored behind the initial
convective line, especially in the western Nebraska Panhandle.
Lingering rain showers will be present after 6Z Tuesday, but
the surface cold FROPA will have moved south by this time. The
cold air aloft will take its time to mix down to lower
elevations, but light snow showers will begin to accumulate for
elevations above 8000 feet.

Another wave of energy associated with the upper level trough
will propagate across the area on Tuesday. The trough axis will
be over our cwa by late Tuesday afternoon. Scattered to numerous
rain showers and thunderstorms are favored once again for areas
east of the Laramie Range, but colder temperatures are favored
west of the Laramie Range. The light snowfall will struggle to
accumulate during the daytime in the higher terrain due to the
high sun angle and warm ground temperatures. However, Tuesday
night will bring another window of light snowfall accumulations
the Snowy/Sierra Madre mountain ranges. The potential for
winter headlines exists for those areas, but we will need to
take another look once the window for severe weather dissipates.
The rain/snow mix line will slowly decrease in elevation Tuesday
night, but it isn`t favored to push east of the Laramie Range at
this time of inspection. Model guidance is struggling to get a
good handle on this due to the potential for cloud cover to
linger through 12Z Wednesday morning. Additionally, with
surface high pressure and decreasing cloud cover anticipated, we
may see frost/freeze occur for the high plains. Frost advisory
headlines may be favored for early Wednesday. Stay tuned as we
have an active weather pattern continuing this week for the
short term forecast and beyond.

&&

.LONG TERM /TUESDAY NIGHT 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 /12Z TAFS THROUGH 12Z TUESDAY/...
-- Changed Discussion --
Issued at 515 AM MDT Mon May 20 2024

A tricky forecast for the 12Z TAF period. Primary aviation
concern will be thunderstorms throughout the afternoon and
evening hours, likely staying around into the overnight. Gusty,
erratic winds will be possible near any storms that form and
these storms may contain large hail. A tornado or two cannot be
ruled out. The strongest storms are expected between 20 and 05Z
today. Gusty winds will be present ahead of the storms and
continue afterwards behind the passing cold front. Included
VCTS for all terminals, with TSRA in for the terminals with the
highest confidence. Once storms move out and the cold front
passes through, expect IFR to LIFR ceilings to develop late.
-- End Changed Discussion --

&&

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

&&

$$

SHORT TERM...BW
LONG TERM...AM
AVIATION...AM