039
FXUS65 KCYS 271119
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
519 AM MDT Thu Jun 27 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms are expected across much
  of southeast Wyoming and western Nebraska this afternoon and
  evening. A conditional threat for severe thunderstorms will
  exist across extreme southeast Wyoming and western Nebraska,
  with large hail and damaging winds the primary hazards along
  with the potential for an isolated tornado.

- A final round of strong to severe thunderstorms is expected
  Friday as a cold front sweeps across the CWA. At this time,
  the best chance for convection appears to be along and
  southeast of a line from Cheyenne to Alliance.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 215 AM MDT Thu Jun 27 2024

The primary focus over the next 12 to 24 hours continues to be a
threat for strong to severe thunderstorms across southeast Wyo &
the western Neb Panhandle during the mid/late afternoon into the
early evening hours. SPC has highlighted much of the western Neb
Panhandle in an ENH Risk for severe storms later today. However,
this is not without uncertainty as CAMs seem to be having a very
tough time handling the boundary layer moisture with significant
HREF spread noted in surface dew points, as much as 10 to 15 deg
F differences between 21z and 00z. This is likely the reason for
such discrepancies among the CAMs with convective coverage, with
the HRRR suggesting minimal activity today while the NAM Nest is
rather aggressive w/ CI and multiple UH Tracks. Tds from the 06z
NAM are around 5-7 deg F higher than the HRRR, contributing to a
more favorable thermal profile for robust/deep convection by mid
afternoon per forecast soundings. HREF probabilities are hinting
at around a 40% chance of dew points remaining above 55 deg F as
afternoon mixing commences this afternoon across the western Neb
Panhandle w/ higher probs just east of the CWA. If we can manage
to keep dew points in the mid to upper 50s, a conditional threat
for severe thunderstorms will exist with CAPEs up to 1500 J/kg &
35 knot effective bulk shear favorable for supercells capable of
very large hail, damaging winds, and perhaps a tornado. The best
chance of this (right now) appears to be in western Nebraska for
the time being, though if the dryline mixes less aggressively we
could see some southeast Wyoming zones get in on the action. The
big failure mode is a dryline which mixes rapidly eastward, with
moisture quality effectively being cut off before CI occurs.

Another round of strong/severe storms will be possible Friday as
the axis of the shallow northern plains upper-level trough moves
to the east. The primary focus here should be along & south east
of a line from Cheyenne to Alliance, based on expected positions
of the cold front.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 215 AM MDT Thu Jun 27 2024

Daily afternoon shower and thunderstorm chances continue through the
long term forecast period. Friday evening, upper-level troughing
will still be present across the region with a subtle 500mb
shortwave pushing through. Most of the convection that forms will
likely be earlier in the afternoon Friday, rather than Friday
evening. However, there is a Slight Risk of severe weather for
portions of the Panhandle. Upper-level troughing will slowly move
out of the area Friday night into early Saturday morning as an upper-
level ridge builds over the Intermountain West. With the cold front
moving across the region Friday, Saturday will be much cooler as
700mb temperatures drop into the 6-8C range east of the Laramie
Range, but west of the Laramie Range looks to be warmer as the upper-
level ridge slowly moves closer to the region. Highs on Saturday
will be in the mid-70s to low-80s east of the Laramie Range and mid-
70s to mid-80s west of the Laramie Range. Despite the strengthening
ridge overhead, some 500mb vorticity maxima will traverse
underneath, resulting in precipitation chances Saturday and most
days throughout the long term forecast.

The upper-level ridge will be at its maximum strength overhead on
Sunday, leading to rapidly warming 700mb temperatures into the 15-
17C range. Surface temperatures will increase into the upper-80s to
mid-90s east of the Laramie Range and upper-80s west, leading to a
very warm end to the weekend. A slightly cooler and more active
pattern returns for the work week as the upper-level ridge moves off
to the east and troughing pushes in. Disagreement begins to appear
in the long range models at this time. The GFS suggests zonal flow
moving in after the trough pushes through, while the ECMWF keeps
troughing overhead for Monday and Tuesday, with zonal flow arriving
Wednesday. Disagreement continues throughout the remaining long
term, but temperatures are likely to remain in the 80s and 90s, with
warmer temperatures arrive for the 4th of July.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 516 AM MDT Thu Jun 27 2024

VFR conditions expected for most of the 12Z TAF period. Primary
aviation concern will be strong to severe thunderstorms near all
terminals later this afternoon and evening. Exact timing is
difficult to pinpoint, as current model guidance is not in
agreement with significant differences between models. Wyoming
sites should start seeing thunderstorm activity around 22Z, with
Nebraska sites being a few hours later. All hazards are possible
today from gusty, erratic winds to large hail and an isolated
tornado or two.

&&

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

&&

$$

SHORT TERM...CLH
LONG TERM...AM
AVIATION...AM