318
FXUS65 KCYS 132123
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
323 PM MDT Thu Jun 13 2024

.KEY MESSAGES...

- Scattered to numerous showers and thunderstorms with small
  hail and gusty winds continue this afternoon into the early
  evening hours, especially along the I-80 corridor from Laramie
  to Kimball.

- Strong to severe thunderstorms are possible across the high
  plains of southeast Wyoming and the western Nebraska Panhandle
  on Friday. Torrential rainfall and the potential for flash
  flooding will be the primary concern. However, large hail and
  damaging winds will be possible with any storms developing early
  in the afternoon prior to storm mergers.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 315 PM MDT Thu Jun 13 2024

Latest satellite imagery shows scattered showers and thunderstorms
over southeast WY extending into the southern NE panhandle. Latest
SPC mesoanalysis shows around 500 J/kg of MLCAPE along and along the
foothills of the South Laramie Range with a frontal boundary
extending southeast into portions of CO. Storms will continue into
this evening as a shortwave continues to pass along north of the
upper level ridge in the southern Rockies. However, coverage should
remain fairly isolated with storms weakening as they approach the NE
border where greater convective inhibition is present. Additional
shower development will be possible early Friday morning.

Looking at increasing storm coverage headed into Friday afternoon
associated with the previously closed low off the southern CA coast
beginning to move across the central Rockies. Strong southerly
moisture advection out ahead of the trough will support upper 50 to
low 60 degree dew points for portions of the NE panhandle with moist
vertical profiles and PWs near climatological 99th percentile
values. This will lead to SBCAPE values exceeding 2000 J/kg east of
the WY/NE border as indicated by the latest HREF guidance with
40 kt of 0-6 km shear. However, previous shifts noted 700mb WAA
resulting in the quite a bit of capping that needs to be
overcome for storms. While this may limit discrete convection
early in the afternoon, larger scale lift with the approaching
upper level trough will be supportive of widespread
precipitation developing. Any discrete storms that could form
early in the afternoon will be capable of producing large hail
before upscale growth begins. It is important to note that
latest model trends have been suggesting the best support may
remain over eastern CO, as the upper level trough tries to dig
farther south, potentially leaving southeastern portions of the
CWA within the stratiform region of the convection in CO. While
strong winds and hail will be possible with any stronger cells,
heavy rainfall continues to be the main hazard with WPC
highlighting the I-80 corridor east of Cheyenne with a marginal
risk for excessive rainfall through early Friday evening.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Issued at 303 AM MDT Thu Jun 13 2024

The long term forecast will start off with relatively tranquil
weather through the weekend with increasing chances of showers and
thunderstorms early next week. Throughout the period, general
southwesterly flow will be in place aloft, with stronger flow to the
north and much weaker flow to the south. The outcome of
Friday/Friday night`s convection will likely have a large impact on
the forecast for Saturday, as a larger, more robust MCS/thunderstorm
complex will likely clear a large majority of the surface moisture
in place well to the east of southeast Wyoming/NE Panhandle. Global
models likely do not yet have a handle on these impacts, however
generally speaking surface moisture is currently progged to be much
lower on Saturday afternoon. Still, with a bit of faster southwest
flow aloft impinging on the CWA from the north, a few isolated high-
based showers will be possible in the Laramie Range and through east-
central Wyoming. A surface frontal boundary evident in both the GFS
and ECMWF will dive southward on Sunday morning, squeezing surface
moisture into a narrow band somewhere over the Nebraska Panhandle
and perhaps as far west as southeast Wyoming. Typically, these post-
frontal moisture-squeeze setups can produce severe weather in the
high plains. However, we will once again be struggling to find much
in the way of favorable wind shear as the bulk of faster flow aloft
will remain to the north and west. Isolated multicell and outflow-
dominant clusters will likely be the favored storm mode wherever the
moisture corridor sets up given strong surface heating, inverted V
soundings, and warm air in the mid-layers of soundings.

By Monday and Tuesday, precipitation and thunderstorm chances look a
bit more interesting across the CWA as a more robust trough attempts
to swing through the northern Rockies embedded in the broad
southwesterly flow regime. This system has seen quite a bit of
spatiotemporal variability in the past few days of guidance cycles.
As of early this morning, the main trough axis appears to be just a
bit too far west to result in a more widespread thunderstorm threat
on Monday afternoon. Still, moisture return in south southeasterly
flow across the high plains is noted. Additionally on Monday
afternoon into Tuesday, southwest surface flow west of the Laramie
Range will also increase dramatically. Will have to watch the Monday
evening and especially Tuesday timeframe for possible high winds
over south-central Wyoming as this system approaches. As far as the
thunderstorm threat goes, Tuesday looks a bit more favorable given
the closer proximity of faster flow aloft associated with this
trough. However, model guidance has shifted northward with the
position of the center of the low, keeping cooler temperatures aloft
and better wind shear/thermodynamic profiles farther north into
northeast Wyoming and the northern Plains states. Again, with a
plentiful reserve of surface moisture just off to the east and a
potent trough in the vicinity, we will have to keep a close eye on
this system over the next few days. Any shift farther south with the
main low pressure system would certainly result in a more
significant severe weather threat for eastern Wyoming and western
Nebraska. Overall, temperatures will run above normal this weekend,
and gradually cool off early next week. Forecast confidence is high
in the temperature forecast, but only moderate with the
precipitation forecast given the variable nature of model guidance
from Monday onward.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1140 AM MDT Thu Jun 13 2024

The main aviation concern today is again the afternoon
convective activity. Showers and storms are expected to develop
in the vicinity of KCYS around 19z, then develop westward
towards KLAR and KRWL shortly after. This activity will move
east through the afternoon and early evening, though may not
quite reach KCDR and KAIA before dissipating. Gusty and erratic
winds are possible if a shower or storm moves over a terminal.

There remains a slight chance for low CIGs Friday morning in the
NE panhandle, but the probability is too low to add to the TAF
at this time.

&&

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

&&

$$

SHORT TERM...MB
LONG TERM...MAC
AVIATION...MN