-- Discussion containing changed information from previous version are highlighted. --
819
FXUS65 KCYS 191127
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
527 AM MDT Wed Jun 19 2024

.KEY MESSAGES...

- Cool again today with gusty south to southeast winds expected,
  especially in the Laramie Valley.

- 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.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 400 AM MDT Wed Jun 19 2024


Much of the area is off to a very chilly start this morning for mid
June in the wake of Monday night`s strong cold front. The
shortwave responsible for that front is now well off to the
east, moving into Canada via Minnesota, but a broad longwave
trough remains positioned over the Pacific Northwest. Mid to
high level cloud cover is streaming over northern and western
portions of the forecast area associated with a jet streak
crossing from Utah into Wyoming, while additional cloud cover is
located to our southeast lingering from widespread convection
that went up along the cold front today. In between, we have
clear skies over portions of the High Plains, which is allowing
for pretty cold temperatures in the Cheyenne to Chadron
corridor, where widespread low 40s are showing up. Elsewhere,
cloud cover is holding temperatures up slightly.

A surface high pressure is now located in western South Dakota, and
is expected to strengthen and expand southeastward as the day
progresses. This will turn the surface flow more southeasterly
across the area this morning and increase westward moisture
advection. By midday, models show a fairly strong reverse pressure
gradient showing up across the area, even into the High Plains.
Thus, expect widespread south to southeast winds gusting 30 to 40
MPH for much of the day. This flow will align with the gap at the
southeast side of the Laramie Valley, leading to locally higher
winds in this area. Wouldn`t be surprised to see a few gusts of 50
MPH or greater around Laramie today. Meanwhile, the potent ridge
aloft to our east is poised to begin to restrengthen today. As its
western fringe amplifies to the north, we`ll have fairly widespread
isentropic lift and positive theta-e advection moving up from the
south. This, combined with the low-level upslope flow as the winds
shift to southeast, should kick off the first shower activity within
a few hours after sunrise along the southern Laramie range.
Overrunning lift will continue through much of the day today, but
recovering the moisture lost after the very dry airmass pushed
through yesterday will take some time. Initially, this will result
in more cloud cover along and east of the Laramie range and much
cooler temperatures, held to the 60s especially in the I-80
corridor. By the afternoon hours, we may have enough moisture to get
some shower activity going. While the profile will be inverted,
there will be some elevated instability to tap into, which should
produce some thunderstorms as well. The location and timing of
precipitation activity this afternoon is pretty uncertain as HiRes
models are handling this quite differently. As a result, the current
forecast has fairly low end PoPs starting around the Laramie range,
then moving east, then spreading north overnight. Continued
southeast flow with much higher moisture in place may lead to some
fog along the I-80 corridor through Thursday morning.

After a full day or so of warm, moist air advection, we`ll have a
very different setup for Thursday. The surface warm front will lift
northward to roughly the US-20 corridor, allowing for temperatures
to climb back to near normal for this time of year south of that.
Additionally, we`ll have much deeper moisture in place after the
warm/moist air advection aloft all day today and tonight. Lingering
low clouds in the morning Thursday will put a fairly potent capping
inversion in place, but almost all models show this eroding south of
the North Platte River by midday. Steep mid-level lapse rates in
excess of 8 C/km, veering wind profiles, and strong shear in the 40-
50 knot range in the 0-6 km layer will set the stage for an active
day of convection across the area. The spark will come from multiple
pieces, including continuing isentropic lift on the western
periphery of the strong ridge aloft, synoptic lift associated with
the right entrance region of a jet streak to our north, and a modest
vort-max ejecting out of the western CONUS trough. With all of these
lifting mechanisms in play, we shouldn`t have any trouble getting
convection going despite the presence of the capping inversion.
Storms will have the possibility to become strong to severe,
especially in their initial phases moving off the Laramie range and
into western Nebraska. All severe hazards will be possible during
this phase. However, with so much synoptic lift in play, storm
coverage is expected to be fairly high, and rapid expansion in
coverage/upscale growth may limit the time for discrete supercells.
This may result in a quick transition to more of a wind threat as
storms reach the eastern portion of the forecast area. Plentiful
moisture will also result in a locally heavy rain threat. While
storm motions should be fairly progressive, the right-mover
vectors are fairly slow so this threat will also be more
elevated during the early phase when discrete supercells are
more likely.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 400 AM MDT Wed Jun 19 2024

Friday could pose as another active severe weather day, however the
upper level trough is expected to breakdown as it approaches the
central Rockies leading to weaker flow aloft. As shear profiles and
mid-level lapse rates look to decrease compared to Thursday, overall
parameter space still suggests strong storms are possible with the
shortwave passage. Vertical profiles remain fairly moist with PW
values greater than 2sigma over the climatological normal for late
June around 1-1.25". Latest guidance suggests best moisture
convergence will be along the warm front along the Pine Ridge area
in the northern NE panhandle extending westward into east-central WY
where MLCAPE values approach 1000 J/kg with 30-35 kt of bulk shear.

Headed into the weekend and into early next week, temperatures are
expected to climb above normal with 500mb height rises across the
Intermountain-West as a ridge builds in underneath the polar jet
positioned near the Canadian border. Ensembles are showing good
agreement of warming temperatures with 700mb temps reaching 16-18C
(nearing climatological 98th percentile value in spots)off of the
latest NAEFS. Afternoon temperatures will climb into the mid-90s
east of the Laramie Range (upper-80s west), especially Sunday and
Monday. Additionally, some areas could reach the triple digits
including along the North Platte River valley where latest NBM
probabilities of exceeding 100F degrees ranges from 30-50% between
Torrington and Scottsbluff. Ensembles begin to diverge slightly for
the middle of next week with the timing of a few shortwave passages
to the north that could impact temperatures, however still looking
at above normal temps next week. The overall upper air pattern could
stay slightly active next week with a few weak disturbances passing
overtop of the ridge, however widespread precipitation is not
likely. We could see isolated to scattered storms that would develop
along the higher terrain during the afternoon.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
-- Changed Discussion --
Issued at 520 AM MDT Wed Jun 19 2024

VFR conditions currently across area terminals with BKN upper
level cloud cover west of the Laramie Range and SCT-BKN mid-
level clouds over portions of the NE panhandle per latest
satellite imagery. Gusty southeast winds expected throughout the
day with KLAR possibly gusting near 50 kt with a rare downslope
off the South Laramie Range westward into the valley. Headed
into late this afternoon and tonight, moist southeasterly flow
will bring low clouds and potential fog leading to IFR/LIFR
conditions continuing into Thursday morning for KCYS and NE
panhandle terminals.
-- End Changed Discussion --

&&

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

&&

$$

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