290
FXUS65 KCYS 122117
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
317 PM MDT Wed Jun 12 2024

.KEY MESSAGES...

- Hot today. Scattered to numerous high-based showers and a few
  rumbles of thunder in the afternoon and evening, possibly
  associated with 45+ MPH wind gusts.

- Isolated thunderstorms Thursday along and east of the Laramie
  Range.

- Widespread showers and thunderstorms are expected across the
  high plains on Friday. Some storms may be severe, with all
  hazard types possible.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 311 PM MDT Wed Jun 12 2024

Hot, dry weather so far today across southeast WY and western
NE with an upper level ridge building over the central Rockies.
Most areas have reached the upper 80s to 90s with KBFF hovering
around 100F degrees at this hour. Latest GOES visible imagery
shows weak convection over south-central WY resulting in showers
with occasional lightning near Baggs and the Sierra Madres.
Based on latest RAP soundings west of the Laramie Range, LCLs
extending well above the freezing level with dry low-levels will
support the potential for gusty, erratic winds around 45 mph in
the vicinity of showers through late this afternoon. Hi-res
guidance continues to push convection farther east along the
I-80 corridor through early this evening with minimal precip
accumulation.

Better chances for showers and thunderstorms arrive later this week,
starting on Thursday. Post-frontal upslope flow with an upper level
system passing over the central CONUS will increase moisture
convergence across southeast WY and northeast CO. Despite
southeasterly low-level flow, wind profiles are fairly
unidirectional aloft supporting of straight hodographs and splitting
storms. However, slightly better turning of the wind profile is
noted closer to the Laramie Range near Cheyenne with the latest
NAMNest showing higher 0-3 km SRH values near 200 m^2/S^2. MLCAPE
with latest HREF guidance is generally below 1000 J/kg, but
additional moisture pooling along the Front Range with 40+ kt shear
could support strong to severe thunderstorms Thursday afternoon,
especially near the CO/WY/NE border.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Issued at 311 PM MDT Wed Jun 12 2024

Friday still looks to be the most active day in the long term
forecast. Throughout the day Friday, a potent, upper-level,
negatively tilted, shortwave trough will dig northeasterly out
of the Four Corners region across the Intermountain West. Ample
synoptic lift will be present ahead of the shortwave as it digs
towards the region. Strong 500mb cyclonic vorticity advection
will move overhead, further enhancing lift across the region.
Further down, at 700mb, a compact low-level jet attempts to
develop across far northeastern Colorado and southwestern
Nebraska. There is slight disagreement on the exact placement of
this jet, with the GFS suggesting a further west jet than the
ECMWF and NAM. However the NAM suggests a secondary, weaker,
low-level jet positioned across the Wyoming/Nebraska border. If
either the GFS or NAM solution comes to fruition, the CWA will
be under additional synoptic left from the jet dynamics surround
the low-level jet. However, if the ECMWF solution pans out, the
jet will probably be too far east to provide any additional
lift to the region. The big story is found closer to the
surface, where a moisture tongue will move overhead into the
region. Dewpoints across the Panhandle will increase into the
low- to mid-60s, with dewpoints into the low- to mid-50s across
southeast Wyoming. Southeasterly surface winds will continue to
advect moisture into the region throughout the morning and
afternoon hours, keeping ample moisture in the region. A dryline
is expected to develop across the Laramie Range and progress
easterly through the day before interacting with these high
dewpoints across the Panhandle. This will likely act as the
storm initiation mechanism, with storms moving into an excellent
environment as the fire and move off the dryline. MLCAPE and
MUCAPE values are progged to skyrocket towards the 3000 J/kg
across the Panhandle, according to the NAM. The GFS is much less
excited with the instability, but suggests CAPE values to be in
the 2500 J/kg range.

The one failure mode for severe storms on Friday will be the
relative lack of shear depending on exactly where storms fire across
the Panhandle. Northern portions of the Panhandle suggest favorable
shear with SRH values in the 150-200 range. However, further south
where the moisture will likely be better, limited shear is shown in
forecast soundings with minimal SRH values. Effective shear to the
south is around 30kts, which is still enough to get storms, but
would like to see more shear for a more significant threat of hail
and/or tornadoes. Further north could see a higher potential for
hail and, maybe, tornadoes. Therefore, we could see two separate
modes of convection depending on where storms form. The northern
threat may be hail producing storms, while the southern threat will
be heavy rain and flooding. According to the NAEFS, PW values will
be in the 95-99th percentile on Friday afternoon for much of the
Panhandle, with forecast soundings suggesting PW values well over
the 1 inch mark. Therefore, slow moving storms, with ample moisture
will lead to heavy downpours and the threat for flash flooding
across the Panhandle. Large hail could still be a threat with any
storms that form due to the strong instability throughout the
atmosphere and decent lapse rates, but flash flooding will likely be
the largest threat. SPC has most of southeast Wyoming and western
Nebraska in a Marginal Risk for severe weather, with a sliver of a
Slight Risk clipping Cheyenne County. WPC currently highlights a
Marginal Risk for excessive rainfall across a small portion of the
CWA. It is possible that these threats will progress further west
with future updates, but this setup will be something to watch over
the coming days.

The remaining long term forecast looks to be quieter than Friday as
southwesterly flow aloft moves overhead. An upper-level trough is
progged to move into the western CONUS with southwesterly flow ahead
of it over the CWA. There is some differences between long range
models as far as the progression of this trough. Both the ECMWF and
GFS suggest this low deepening across the western CONUS by Monday.
However, the GFS flattens this trough by Tuesday evening and ejects
if further to the northeast, keeping the CWA under weaker
southwesterly flow. The ECMWF on the other hand continues to deepen
the trough through Wednesday, with a large trough across much of the
western CONUS with strong jets on either side. With the GFS
solution, 700mb tank into the 1-8C range, while the strong upper-
level flow with the ECMWF heats 700mb temperatures into the 15-18C
range, before dropping them back into the 10C range as the trough
moves out of the region Wednesday. This leads to significant
uncertainty in the Tuesday temperature forecast. The GFS solution
suggests highs in the mid-60s while the ECMWF keeps them closer to
the upper-70s and low-80s across the region. Decided to keep the
Tuesday forecast closer to the GFS solution, but this will need to
be monitored. Daily precipitation chances return Monday through the
remaining long term due to the upper-level trough to the west.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1107 AM MDT Wed Jun 12 2024

VFR conditions expected across all terminals today. Winds will
be gusty this afternoon with many terminals gusting between 25
and 30 kts. The main aviation concern today will be the
development of showers that may produce gusty and erratic winds
near most of the terminals. Winds should decrease overall
around sunset tonight.

&&

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

&&

$$

SHORT TERM...MB
LONG TERM...AM
AVIATION...AM