747
FXUS65 KCYS 240529
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1129 PM MDT Mon Jun 23 2025

.KEY MESSAGES...

- A few isolated showers or thunderstorms are possible this
  afternoon and evening.

- Scattered PM thunderstorms will have the potential to become
  strong to severe on Tuesday and Wednesday.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 315 PM MDT Mon Jun 23 2025

Cool and fairly tranquil conditions prevail across the area this
afternoon in the wake of yesterday`s strong summertime cold front. A
surface high meandering across the High Plains is keeping high
temperatures 10 to 15F below average for this time of year today.
Skies have mostly cleared, but scattered to broken low clouds remain
across parts of southeast Wyoming as steady southeast winds push
moisture against the higher terrain. The cool lower atmosphere has
limited instability today as the upper level ridge begins to rebound
aloft. The latest mesoanalysis shows the only pockets of instability
located in the vicinity of the Laramie range. As a result, today
should be a lull in the more active thunderstorm pattern, but we
still may see a few isolated thunderstorms develop in this narrow
unstable region. Low end PoPs continue into the evening here.
Slightly stronger forcing in the form of modest overrunning is more
prevalent to our south across northern Colorado. Thus, thunderstorm
coverage may be a little more widespread there, and we will need to
watch for storms moving northeast into our area as the evening
progresses. These storms are likely to be elevated by the time they
reach our area, but the southern Nebraska panhandle may still have
some thunderstorm potential into the evening.

The moisture will advance westward overnight with south to southeast
surface winds continuing. As dewpoints increase and temperatures
drop, we will have another round of low clouds and/or fog developing
over the High Plains Tuesday morning. The highest chances for
fog will be along the I-80 corridor from the summit towards
Sidney. This will put another capping inversion in place for
Tuesday morning. However, forecast models show clouds eroding
quickly with the higher moisture and warmer temperatures
allowing for significantly higher CAPE compared to today. A
strong southwest jet aloft will keep ample vertical wind shear
in place. All together, this looks to set the stage for a fairly
potent severe weather environment during the afternoon and
evening hours. The capping will likely reduce coverage, but
widely scattered thunderstorms that break the cap will have the
potential to quickly become strong to severe. While the primary
threat will be large to very large hail, all severe weather
hazards including isolated tornadoes, will be possible. Discrete
supercell thunderstorms appear likely. While the most activity
should be in the late afternoon hours, storms will probably
continue well into the evening, and possibly overnight along the
northern periphery of the forecast area.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Issued at 315 PM MDT Mon Jun 23 2025

A fairly active period expected Wednesday and for portions of the
weekend as unsettled weather moves in over head. Wednesday looks to
be the best day for severe chances in the long term forecast, with
relatively quiet weather Thursday and Friday. Wednesday will feature
a neutrally tilted upper-level trough across the Intermountain west
with a strong just downstream of the trough axis, just west of the
CWA. As the trough advects easterly, the upper-level jet will move
directly overhead, leading to favorable jet dynamics to support
afternoon convection. Lift from the upper-level jet will be coupled
with some additional lift from 500mb cyclonic vorticity advection as
multiple lobes of vorticity eject out ahead of the approaching
trough axis Wednesday afternoon. The low-level, 700b jet will not be
as favorable for convection as the jet remains mostly to the east of
the CWA throughout the day Wednesday. Modest warm air advection at
700mb will be present, though not along the same timeline as other
factors in this case.  A surface low will attempt to develop in
northeastern Colorado throughout the day, but will struggle to get
organized. As a result, southerly to southeasterly winds are
expected across the CWA through the afternoon hours, leading to
increase moisture advection into the region. Convection looks to be
initiated by a relatively weak cold front at 700mb that will knock
700mb temperatures from 13C down to around 9 to 10C. While this cold
front is relatively weak, the forcing from it will still be enough
to get storms to initiate along and ahead of the boundary. The NAM
suggests MLCAPE values around 1200 to 1500 Joules in the Panhandle,
with forecast NAM soundings showing fairly strong lapse rates in the
low- and mid-levels, with precipitable water values around 1 inch.
Increased DCAPE values will likely be present Wednesday afternoon as
the steep, low-level lapse rates favor gusty winds mixing down to
the surface, with steep mid-level lapse rates favoring hail
development as well. A Marginal Risk (Level 1 of 5) is in place
across much of the CWA on Wednesday, with hail and strong winds
currently looking to be the primary hazards. Daytime temperatures
will rise into the 70s and 80s across the region, with dewpoints
increasing into the 50s and 60s east of the Laramie Range, with the
highest values across the Panhandle. More details on the severe
weather potential on Wednesday will continue to come to light over
the next couple of days leading up to the event.

Thursday and Friday will serve more as transition days in the long
term pattern as southwesterly to westerly flow returns aloft behind
the departing trough. As a result, subsidence will be present aloft
across the CWA on Thursday as the upper-level trough leaves the area
and anticyclonic vorticity advection develops behind the trough. As a
result, dry conditions are expected Thursday and Friday as 700mb
temperatures warm back into the 13 to 16C range. Surface
temperatures will increase back into the 80s and 90s once more, with
Friday being the warmest day across the region. Expecting little to
no precipitation chances with fairly calm winds near the surface
ahead of the next approaching system for Saturday and Sunday.

Saturday will feature an upper-level shortwave pushing overhead,
which will lead to enough forcing to get afternoon showers and
storms once again. Multiple 500mb vorticity lobes will once again
eject out ahead of the approaching shortwave, leading to additional
synoptic lift from CVA across much of the eastern portions of the
CWA. These lobes will force convective initiation Saturday
afternoon. 700mb temperatures remain in the 15 to 17C range, leading
to surface temperatures in the 80s and 90s once again for Saturday.
Additionally another cold front with pass through the region,
initiating showers and storms along and ahead of the front, leading
to afternoon showers and storms. Messy flow develops once more for
Sunday with 700mb temperatures cooling slightly and afternoon shower
and thunderstorm chances remaining for much of the region.
Temperatures on Sunday will drop back into the 70s and 80s behind
the cold front passing through on Saturday.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1121 PM MDT Mon Jun 23 2025

Southwest flow aloft will continue. After a quiet night tonight,
Tuesday afternoon will see a significant increase in shower and
thunderstorm chances.

Wyoming TAFS...For Rawlins and Laramie, clear skies will prevail
through Tuesday morning, with scattered to broken clouds from
6000 to 12000 feet in the afternoon and evening, along with
thunderstorms in the vicinity of Laramie from 20Z to 02Z. Winds
will gust to 25 knots at Rawlins from 17Z to 01Z, and to
30 knots at Laramie after 15Z Tuesday.

For Cheyenne, ceilings will range from 2500 to 3500 feet until
09Z, then fog will prevail from 09Z to 15Z, reducing
visibilities to 3 miles and ceilings to 700 feet, then ceilings
will range from 4500 to 8000 feet after 15Z, with thunderstorms
in the vicinity from 20Z to 02Z. Winds will gust to 25 knots
from 15Z to 02Z.

Nebraska TAFS...For Chadron and Alliance, scattered to broken
clouds from 4000 to 8000 feet will prevail, with thunderstorms
in the vicinity from 20Z to 02Z Tuesday. Winds will gust to
22 knots until 08Z, and to 25 knots from 20Z to 02Z.

For Scottsbluff and Sidney, scattered to broken clouds from
2500 to 7000 feet will occur until 08Z, with ceilings from
700 to 1500 feet from 08Z to 16Z, then scattered to broken
clouds from 5000 to 8000 feet will occur after 16Z, with
thunderstorms in the vicinity from 20Z to 02Z. Winds will gust
to 22 knots at Scottsbluff until 09Z, and to 25 knots at both
terminals from 16Z to 02Z.

&&

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

&&

$$

SHORT TERM...MN
LONG TERM...AM
AVIATION...RUBIN