360
FXUS65 KCYS 211755
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1155 AM MDT Fri Jun 21 2024

.KEY MESSAGES...

- Widespread showers and thunderstorms expected once again this
  afternoon into the early evening with a few severe
  thunderstorms capable of producing large hail and strong
  winds. Localized flooding is also possible with heavy
  rainfall.

- Warmer and drier trend starting on Saturday, but cannot rule
  out gusty winds with high-based storms in the afternoon near
  the CO/WY border.

- Hot temperatures in the upper 80s to 90s are expected for
  Sunday through much of next week. Elevations below 4500 feet
  will have approximately a 30 to 60% chance of exceeding 100F
  on Monday.

&&

.SHORT TERM /THROUGH SATURDAY NIGHT/...
Issued at 345 AM MDT Fri Jun 21 2024

Latest GOES imagery shows widespread cloud cover across eastern
WY and western NE with large scale lift out ahead of an upper
level trough positioned near the Great Basin. Latest radar shows
rain showers developing over the southern NE panhandle with
ongoing WAA associated with a leading shortwave with this
system. The upper level trough over the Great Basin will
continue to progress eastward today toward the central Rockies
that will support another round of widespread showers and
thunderstorms across the CWA as the trough axis passes through.

A stationary boundary positioned along the inverted surface trough
extending out of northeast CO through central WY may not be the only
focal point for CI early this afternoon with favorable PVA ahead of
the upper level trough, but will likely see an increase in storm
intensity as they cross the moisture boundary with upper 50F degree
dew points off to the northeast. Storm coverage will increase as
early as late morning along and west of the Laramie Range before
moving east of the I-25 corridor mid-to-late this afternoon.
Moisture profiles again are rather rich with PWs between 1-1.25" (2-
3 sigma above climatological normal) supportive of heavy rainfall
producing storms. Latest RAP forecasts 1500+ J/kg of MLCAPE and 40
kt of effective shear northeast of the boundary with wind profiles
fairly unidirectional above the surface layer. This will lead to
numerous cell splits that could lead to messy widespread coverage of
storms and cold pool congealing quickly. While large hail may be a
threat early on as the storms cross over into the more favorable
instability, the hazards threat looks to quickly transition over the
strong winds and heavy rainfall late this afternoon into the
evening. Main area to watch for strongest storms today will be along
and immediately northeast of a line from Wheatland to Douglas, but
SPC has highlighted most of the CWA along and east of the Laramie
Range into the NE panhandle with a Slight Risk (Level 2/5) for
Severe Thunderstorms. Portions of the northern NE panhandle may
remain capped with this morning`s low-clouds, but cold pool
organization could still support storm potential later in the
evening.

500mb flow will turn more zonal over the CWA Saturday as an upper
level ridge begins to amplify over the central and southern Rockies.
Temperatures will continue the warming trend as we dry out with
afternoon highs mostly in the 80s. A weak shortwave passing overtop
of the ridge could bring high-based showers and thunderstorms (15-
20% chance) mainly to the higher terrain near the CO/WY border
including around Cheyenne and Laramie. Little to no precipitation is
expected, however high-based storms could bring gusty, erratic winds
in their vicinity.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
Issued at 345 AM MDT Fri Jun 21 2024

A potent ridge will build in over the Four Corners states this
weekend and usher in a much warmer and drier weather pattern that
will carry through a good portion of next week. Ensemble median 700-
mb temperatures climb to about +16 to +18C by Sunday and hold there
through Monday. This will support high temperatures reaching the
upper 80s to 90s for the majority of the lower elevations both days.
Monday still has a decent chance (30 to 60%) for 100F or higher
temperatures in certain hotter locations. The highest probabilities
are in the Scottsbluff area, but probabilities of 30 to 40% exist
for the rest of the North Platte River valley and the Chadron area.
With the dryline pushed to the eastern border of the forecast area,
precipitation opportunities will be limited Sunday and Monday.
However, precipitable water values remain at least near average for
this time of year due to decent mid/upper level moisture. Therefore,
expect to see a few radar echoes showing up each day, but
precipitation reaching the ground will be difficult. Of course, we
may need to watch for dry microbursts with this weather pattern.

A shortwave will push over the top of the ridge late Monday into
early Tuesday, compressing the ridge slightly. A weak cold front may
come along with this shortwave. Impacts will be confined to areas
east of the Laramie range, and mainly along and north of the North
Platte River valley. The US-20 corridor could be as much as 10F
cooler on Tuesday compared to Monday, but elsewhere the difference
will be more subtle. Despite the frontal forcing, precipitation
chances will remain limited with a similar story expected to Sunday
and Monday. The ridge re-consolidates over the west by Wednesday
allowing temperatures to warm once again. But, most ensemble members
allow the dryline to slide back westward on Wednesday which will
boost low-level moisture, and possibly allow for slightly higher
thunderstorm chances for Wednesday and Thursday.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1154 AM MDT Fri Jun 21 2024

Active weather expected across the area this afternoon and into the
overnight hours. First, low stratus continues across much of the
Nebraska panhandle early this afternoon. Gradual clearing of the low
stratus will likely take place over the next few hours. Storms will
begin to develop this afternoon across southeast Wyoming and
eventually push eastward into Nebraska late this afternoon and
evening. This first round of storms could contain large hail,
damaging wind and heavy rain which could reduce visibility. Another
round of storms is expected later tonight, with lower probabilities
for severe weather, especially across southeast Wyoming terminals.
Low stratus may return to the Nebraska panhandle again overnight.

&&

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

&&

$$

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