421
FXUS65 KCYS 201126
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
526 AM MDT Thu Jun 20 2024

.KEY MESSAGES...

- Severe thunderstorms possible this afternoon with hail, strong
  winds, and heavy rainfall. Cannot rule out a tornado with any
  longer track supercells that develop. Severe thunderstorm
  potential continues Friday.

- Hot temperatures are expected for Sunday through the middle of
  next week. Highs exceeding 100F will be possible (40-60%
  chance) particularly on Monday.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 403 AM MDT Thu Jun 20 2024

Latest GOES imagery shows a weak shortwave passing over the
central Rockies with showers and thunderstorms from central CO
extending northeast towards Cheyenne Co, NE where latest GLM
picked up a couple flashes. Strong southeasterly moisture
advection has brought near 60F degree dew points to the NE
panhandle along with low clouds and patchy fog. A Dense Fog
Advisory remains in effect this morning for the South Laramie
Range including I-80 between Cheyenne and Laramie where reported
visibilities have been under one-quarter mile all night.

This afternoon is shaping up to be an active severe weather day
across southeast WY and western NE. With the aforementioned
southeasterly moisture advection into the area along with WAA, a
notable capping inversion will need to be broken for CI across the
NE panhandle today. However, CI over the higher terrain further west
with a passing leading shortwave looks to kick things off today with
storms developing as early as late this morning across the Laramie
Range. MLCAPE values exceeding 1500 J/kg appear likely with the
capping inversion breaking in areas of clouds clearing later this
morning through the early afternoon. With southerly flow across the
Cheyenne Ridge, enhanced convergence will be present across southern
Platte/Goshen Co with a Chugwater cyclone setup reminiscent of the
setup late last June. Hail, strong winds, and heavy rainfall
are likely today with the stronger cells, and cannot rule out a
tornado with any longer track supercells that develop. Vertical
profiles lack any significant dry layers limiting hail
potential, but overall moist soundings with 1-1.25" PW values
would be supportive of efficient rain producers and localized
flooding with any training storms. Southeasterly surface winds
veering aloft will support modest curvature of the low- level
hodograph, through SRH values are slightly limited by a shallow
southerly flow layer before quickly transitioning to
southwesterly aloft.

While initially discrete, hi-res guidance grows storms upscale
across the NE panhandle late in the afternoon with storms pushing
east into central NE early this evening. HREF 40 dBZ paintballs show
a notable difference in timing with a member (NAMNest) being
slightly delayed with CI and eastward propagation. It appears
overnight convection from the model in eastern CO may try to enhance
the capping inversion in our area and therefore delaying CI.
However, model depiction compared to latest radar imagery would
suggest overdone coverage of storms, but will need to continue to
monitor through the morning.

Friday will bring another chance for showers and thunderstorms
across the CWA with more widespread coverage as the trough axis
passes through. A stationary boundary positioned along the inverted
trough extending out of northeast CO through central WY will enhance
moisture convergence with upper 50F degree dew points off to the
northeast. Storm coverage will increase by late morning with
favorable lift associated with 100 kt 250mb jet extending into the
north-central CONUS. Storms will likely increase in intensity but
remain elevated upon crossing the moisture boundary by mid-afternoon
where the latest RAP forecasts 1500+ J/kg of MLCAPE and 40 kt of
effective shear. Storms will likely grow upscale quickly with larger
scale lift present, but could see a brief window for large hail
production before shifting the focus to a wind and heavy rainfall
threat. SPC has highlighted portions of the CWA with a Day 2 Slight
Risk for Severe Thunderstorms.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 403 AM MDT Thu Jun 20 2024

A hot and largely dry weather pattern will dominate much of the
week ahead. Friday`s shortwave trough will be off to our east
by early Saturday morning, leaving dry air advection over much
of our area in its wake. The shortwave moving east will break
down the potent ridge that has been dominant over the central
and eastern US this week. However, the ridge will then
retrograde westward and expand over the Four Corners states
through the weekend. Models are in good agreement showing the
ridge remaining fairly stationary but amplifying through the
middle of next week. Ensemble mean 500-mb heights over KCYS
climb from around 587-dm at 12z Saturday to a maximum of 592-dm
by Tuesday evening. Temperatures should return above normal on
Saturday as 700-mb temps climb to around +15C over the Wyoming
portion of the forecast area. Widespread 80s to lower 90s will
still probably be the coolest day of the long term forecast
period. By Sunday, 700-mb temperatures edge up to +16 to +18C
and remain there or higher through at least Tuesday, and
possibly beyond. Look for widespread 90s on Sunday, and possibly
a few degrees higher on Monday. Some areas will have a good
chance at surpassing 100F. Current probabilities of 100+F
temperatures on Monday are sitting around 40% for Chadron,
Torrington, and Bridgeport, but over 60% for Scottsbluff.
Forecast confidence decreases somewhat beyond Monday, as the
amplifying ridge may open the door to a few weak shortwaves
sliding over the top. Some of these could have the potential to
bring surface cold fronts through the area. While there is
nearly every ensemble member has extremely hot temperatures on
Sunday and Monday, about 20% of members bring a front through
our northern counties by Tuesday, and this proportion rises for
Wednesday and Thursday. Overall, the forecast still leans quite
hot, but by mid-week there is a little bit more uncertainty.

Other than the heat, the week ahead looks fairly dry too.
Precipitable water values don`t look too bad, hovering around
average for Saturday into early next week. However, the combination
of dry low-levels (due to the dry-line getting pulled eastward) and
warm air aloft will reduce precipitation chances. We should still
see some activity each day, but it will be much less widespread than
Thursday and Friday. With inverted-v soundings showing up in the
forecast in place of decent mid to upper level moisture lingering,
expect each day to put up some scattered radar echos, but the chance
for measurable precipitation on the ground looks fairly low.
Showers may lead to dry microbursts through this hot period. The
majority of ensemble members allow the dryline to pull back to the
west by Tuesday or Wednesday, increasing low level moisture and thus
the chance for more noteworthy precipitation and thunderstorm
activity for the middle of the week.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 521 AM MDT Thu Jun 20 2024

Low clouds are producing MVFR to IFR conditions for High Plains
terminals this morning. This is expected to gradually
lift/break-up between about 14z and 16z. After that, attention
quickly shifts to an active day of afternoon thunderstorms.
Showers and storms are expected to begin as early as 17z in
Wyoming, spreading into Nebraska after 20z. Storms today will
have the potential to produce large hail and strong, gusty
winds, as well as VIS reductions to MVFR or IFR in heavy
rainfall. The exact timing of direct impacts to terminals
remains uncertain, but multiple rounds of showers and
thunderstorms will be possible through about 03z. Another round
of low clouds will impact KCDR and KAIA Friday morning.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Dense Fog Advisory until 6 AM MDT early this morning for
     WYZ116-117.
NE...None.

&&

$$

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