642
FXUS65 KCYS 301141
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
541 AM MDT Thu May 30 2024

.KEY MESSAGES...

- Drier and cooler today with chances for showers and
  thunderstorms returning overnight across the Nebraska
  panhandle.

- Daily chances for precipitation continue into the weekend
  before turning drier early next week with above average
  temperatures.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 322 AM MDT Thu May 30 2024

Latest GOES WV imagery continues to show drying across much of
the CWA as a cold front continues to move across the area. Based
on latest observations, it appears that Cheyenne Co, NE is the
only portion of the CWA still on the warm side of the front,
though temperature gradients across the front are only on the
order of 5- 10F degrees. Lingering widely scattered showers will
continue early this morning before mostly clearing up.

Looking at slightly cooler weather to wrap up the week across
southeast WY and western NE, though temperatures will still be near-
normal for late May in the upper-60s and 70s. With this morning`s
frontal passage to the southeast, higher theta-e values will shift
into eastern CO where thunderstorm activity is expected later today.
Looking at dry, breezy conditions Thursday afternoon across much of
south-central WY where elevated fire weather conditions are likely
for much of Carbon and Albany Co.

Headed into the overnight hours, a secondary cold front associated
with the upper level low expected to be over the north-central CONUS
will increase low-level moisture convergence. Additionally, 700mb
WAA will help increase instability profiles supporting elevated
convection near the I-80 corridor between Cheyenne and Sidney. While
heavy rainfall and frequent lightning appear to be the main hazards,
cannot rule out some hail with the strongest cores as CAPE in the
-10C to -30C layer approaches 400 J/kg per the latest RAP. Latest
timing based on hi-res guidance suggests convective activity picking
up mostly after midnight and continuing through the early morning
hours on Friday.

Another round of showers and thunderstorms will be possible Friday
as additional shortwave energy crosses the central Rockies under
mostly zonal flow aloft. General lee troughing across the Front
Range will support southeasterly moisture advection across much of
the NE panhandle with MLCAPE values approaching 1000 J/kg. However,
lift could be a little more in question on Friday as better PVA
looks to remain farther south in CO where latest hi-res guidance
keeps the greater coverage of convective activity. Both the 00z and
06z runs of the HRRR even keep the entire CWA dry Friday afternoon
as a lingering mid-level warm nose may be too strong to
overcome.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 322 AM MDT Thu May 30 2024

Thunderstorm chances will continue into the weekend, but look
for a warm and dry weather pattern to take over by Monday and
last through much of next week.

The synoptic weather pattern will transition to a more zonal flow
pattern this weekend, with a few vorticity maximums and weak
shortwaves expected to traverse across the area. One disturbance
will draw the dryline eastward Saturday, spilling more westerly flow
and dry air into the area. However, there is uncertainty on just how
far east this will get. Areas east of the dryline will still have
southerly flow and sufficient moisture to have a supportive
environment for thunderstorms, with a chance to become strong to
severe. Areas to the west will have some marginal instability and
very steep lapse rates despite the lack of moisture, but this
indicates more of of a virga/wind threat. A stronger shortwave will
pass through the area Sunday, edging the dryline eastward once
again. While much of the area SW of a Douglas to Sidney line should
see very low dewpoints on Sunday, those NE of this will have chances
for another round of isolated to scattered showers and
thunderstorms. Steep lapse rates and added lift from the passing
shortwave may allow for some shower activity in the dry area, but
this will be pretty hard to get to the ground with forecast
soundings showing deep inverted-v profiles up to nearly 500-mb.
Expect a warming trend through the weekend also with 700-mb
temperatures Saturday around +10 to +12C, and Sunday up to +12 to
+14C. Sunday may bring a chance for the first 90F day of the season
in the typical hotter spots of far eastern WY and the NE panhandle.

Sunday`s shortwave will knock down 700-mb temperatures during the
evening and overnight, but recovery will be quick as stronger
ridging begins to build back into the area. Ensemble mean 700-mb
temperatures recover to +15C by 00z Tuesday over KCYS. This next
warmup will also feature a stronger upper level ridge than what
we`ve seen so far this season. As a result, temperatures aloft will
be on the increase, leading to a more stable atmosphere despite the
hot low-levels. Convective development will be hard to come by in
the expected environment. The next shortwave is expected around the
Tuesday/Wednesday period, and will bring what will probably be the
only chance for marginal precipitation of the week. The GFS and a
fair number of ensembles from both major ensemble systems keeps this
system too far north to deliver any precipitation, and only a brief
cool down. The ECMWF and about one third of members of the grand
ensemble have a much more potent shortwave dive further south, which
would result in a few scattered showers (but still nothing
impressive), and a much more pronounced cool down. ECMWF 700-mb
temperatures drop to near 0C for our northern counties by 00z
Wednesday, while the GFS keeps those areas around +10C. This results
in pretty substantial forecast uncertainty for temperatures on
Tuesday through Thursday. While essentially all ensembles are
showing a potent upper level ridge dominating the western US
beginning mid-next week, there is discrepancy on exactly where the
axis will set up, ranging from near the West Coast on the western
end to directly over our area on the eastern end. The latter would
lead to a period of potentially near-record heat, while the former
would keep the core of the heat to our west with a more northwest
flow pattern over our area. The latest ensemble suite has about 25%
of members in support of the ECMWF northwest flow regime, while the
remaining 75% are considerably warmer.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 533 AM MDT Thu May 30 2024

A cold front is pushing across the region this morning, with
some modestly gusty NW winds pushing in behind. An area of low
clouds is also visible on satellite. This is expected to bring
MVFR CIGs for a few hours to KCDR and KAIA. This is not favored
to reach KBFF and KSNY, but can`t rule out the cloud deck
extending that far. This won`t last long, with clearing skies
anticipated by late morning.

A fairly quiet afternoon is expected today, but things will
trend more active this evening. A reinforcing cold front will
push south during the evening, bringing a brief period of gusty
N to NE winds to the High Plains. Then, an area of thunderstorms
is expected to develop mainly over the southern NE panhandle
between 05 and 07z. Added VCTS in for southern NE panhandle
terminals. This storms will have the potential to produce gusty
winds, hail, and brief IFR conditions in heavy rainfall. Lastly,
a low cloud deck is expected to develop out of these storms and
push back against the Laramie range. MVFR to IFR CIGs are
possible Friday morning at KCYS and KSNY.

&&

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

&&

$$

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