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501
FXUS65 KCYS 192041
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
241 PM MDT Wed Jun 19 2024

.KEY MESSAGES...

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- Southeasterly upslope winds expected this evening that could
  lead to widespread low clouds and fog in the Nebraska
  Panhandle to the east slopes of the Laramie Range to include
  Cheyenne.

- Warmer temperatures and more widespread shower and
  thunderstorm activity with the potential for severe storms
  will return on Thursday. This may continue on Friday.

- Hot and mostly dry weather will occur this weekend into early
  next week with low chances (10-20%) for showers and storms and
  afternoon highs likely (>85%) above 90F degrees east of the
  Laramie Range.
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&&

.SHORT TERM /THROUGH FRIDAY/...
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Issued at 241 PM MDT Wed Jun 19 2024

Inverted surface trough lays across northwestern
Colorado...north into eastern Carbon County this afternoon. This
has created a fairly tight...reverse Craig to Casper 850/700mb
height gradient. 18Z Craig to Casper 850/700mb height gradients
at -18/-3 mtrs that increase to -31/-5 mtrs at 00Z this
afternoon. Pretty rare reverse gradient winds up to 55 mph have
been reported in the Laramie Valley and over the Summit. Even
Bordeaux reporting southeast wind gusts to 45 mph. Definitely
something we don`t see very often here. Southeast winds also
pulling higher dewpoints north with low 50s in northeast
COlorado and upper 50s/low 60s in east central Colorado and
western Kansas. We have had a persistent cloud deck over the
southeast WYoming plains through the day today...which has
limited temperature rises. Currently 63 here at Cheyenne...with
low 70s out across Carbon County.

With lack of surface heating today...not expecting much in the
way of thunderstorm development across the CWA. HRRR/RAP
simulated radar do show some light QPF over the Panhandle this
evening. So did continue some low chance PoPs (30-40%) chances
for the Panhandle this evening. Perhaps of more significance is
the increased low level moisture as GFS 850 humidity increases
to 90 percent across the southern Panhandle by 00Z and upper 90
percent into the southeast WYoming plains at 03Z. Looks like a
pretty widespread stratus event for tonight and possibly
widespread fog overnight. Did add areas of fog into the forecast
for this evening and overnight. Forecast soundings support
widespread stratus and fog tonight at Cheyenne and the
Panhandle...so evening shift may need to go more widespread than
what is currently in the weather grids.

Thursday is looking good for thunderstorms with the increased
low level moisture across the area. Upper shortwave moves
through Thursday afternoon. Ample 0-6km shear with southeasterly
low level winds and upper westerly winds of 50-60kts. NAM MUCAPE
of 2500 to 3000 J/KG with similar instability on GFS soundings. HRRR
simulated radar going very aggressive on thunderstorm
development Thursday afternoon. For now...SPC has a Marginal
Risk identified for southeast Wyoming and Nebraska Panhandle.
But would not be surprised if they upgrade that of near future
updates.

Friday looks similar to Thursday as low level moisture will
still be in place. Dewpoints in the Panhandle still up in the
low to mid 60s Friday afternoon with next upper shortwave
tracking across the CWA.
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&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
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Issued at 241 PM MDT Wed Jun 19 2024

A very warm weekend and start of the week is expected for southeast
Wyoming and western Nebraska. Saturday looks to be the coolest day
at this time as an upper-level trough slowly traverses east out of
the region and ridging begins to build in behind it. As the ridge
builds, it will slowly move to the east, with the ridge axis over
the region by Sunday afternoon. The GFS and ECMWF have some slight
differences in the strength of the 250mb ridge, with the ECMWF
suggesting a stronger ridge than the GFS at this time. However, at
500mb, the GFS and ECMWF are nearly identical for the exact strength
of the ridge at this level. The ridge will remain firmly in place
across the region from Sunday, with another ridge building across
the southwestern CONUS as the first ridge begins to track easterly.
Therefore, a very warm week is expected for much of the region.
700mb temperatures soar into the  15 to 20C range by Monday evening,
with corresponding surface temperatures well into the 90s east of
the Laramie Range, with many 100s possible across western Nebraska.
Mid- to upper-80s are likely west of the Laramie Range during the
weekend and into the start of the work week.

Despite the stout ridge overhead, several 500mb shortwaves will
traverse through the ridge and lead to daily shower and thunderstorm
chances across the region. With these passing shortwaves, the
temperature forecast is somewhat less confident as the shortwave
could knock a few degrees off the current forecast temperatures.
Additionally, any cloud cover produced by these shortwaves will
further act to reduce the surface temperatures. Overnight lows will
be rather warm across the region under the influence of the upper-
level ridge, with overnight lows consistently in the 50s and 60s
across the region. Luckily, dewpoint values will remain near average
for this time of year, so the hot temperatures will not any more
unpleasant from a surplus of moisture. Overall, a warm and mostly
dry long term forecast is expected. Any showers and thunderstorms
that develop throughout the week will likely be isolated to
scattered in nature, leading to some locations receiving
significantly more precipitation than others.
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&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1115 AM MDT Wed Jun 19 2024

Concern for the 18Z TAFs will be widespread low clouds and fog
as we continue under southeast upslope flow through Thursday.
Continued trend towards lowering flight conditions for airports
east of the Laramie Range. Latest HRRR and LAMP guidance hitting
these airports pretty hard with LIFR to VLIFR conditions. WOrst
airport may be KCYS where we could see dense fog developing
tonight. Will continue to monitor and revise timing as we get
closer to the event. But right now...looking for lowering
conditions around the 03Z timeframe.

&&

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

&&

$$

SHORT TERM...GCC
LONG TERM...AM
AVIATION...GCC