013
FXUS65 KCYS 190420
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1020 PM MDT Tue Jun 18 2024

.KEY MESSAGES...

- Cool temperatures will continue for tonight and Wednesday.

- Thursday will be much warmer with a chance of strong to
  possibly severe thunderstorms in the afternoon and evening. A
  chance of late day thunderstorms will continue for Friday.

- Hot and mostly dry weather will occur for Saturday through
  Monday.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 303 PM MDT Tue Jun 18 2024

Well below normal temperatures remain entrenched across the area
this afternoon. Light rain showers were present earlier, but
have propagated to the east. Surface observations show relative
humidity readings in the teens and 20% range for several areas,
despite the cooler temperatures. Satellite imagery shows a
growing grass fire in the central NE Panhandle as of 21Z. Breezy
northwest winds will continue to be an issue for that location
as the day progresses. Another interesting feature on the radar
this afternoon was a small meso-high just south of the WY border
close to I-25. This was largely influenced by the terrain, and
converging air from surface highs and the surface lo to our
south in CO.

Cloud cover will keep temperatures elevated overnight, but we
will remain under the influence of cold air advection. Several
areas will see overnight low temperatures bottom out in the
upper 30s to lower 40s. It will certainly cause folks to bring
an extra layer of clothing with them if planning on doing
outdoor activities shortly after sunrise. Weak upslope flow the
remainder of Wednesday for the I-80 corridor east of the Laramie
Range will keep temperatures chilly for the majority of the day.
A shortwave trough will move onshore from the California
coastline during Wednesday. This will influence lee cyclogenesis
across the Central Rockies. While we will not see a direct
influence from the synoptic surface low, wrap around moisture
and overrunning will cause an uptick in chances for rain showers
and thunderstorms for portions of the NE Panhandle and
South/Central Laramie Range.

Thursday will bring WAA to the area as we begin to be influenced
by the northern periphery of an amplified upper level ridge
further downstream. There will be a partial phasing of the
subtropical jet stream and the Polar jet stream on Thursday
across the Intermountain West. This will cause our chances for
convection to increase by the afternoon. PWATs, bulk effective
wind shear, dew points and modest instability should all assist
with diurnal heating and thunderstorms. SPC Day 3 doesn`t have
more than a general risk of Thunder at this time, but that could
change if consistency with models occurs as CAMs come into play
the next 24 hours. A couple strong to potentially severe
thunderstorms may become favored if model consistency holds
true. Overall, expect the cool temperatures today and tomorrow
to be short-lived as we have much warmer weather begin by
Thursday across the region.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 303 PM MDT Tue Jun 18 2024

Thursday evening could be an interesting setup especially with
regards to severe/heavy rainfall potential. The models are in
similar agreement on the position of the upper level jet moving
through northwest Wyoming, southeast Montana and the Dakotas,
placing us in the right entrance region of the upper level jet.
The other concern is the position of the lee side trof and the
associated frontal boundary. At this juncture, it appears like
the best forcing will be along and north of a Torrington to
Alliance line which is the approximate depiction where the low
level boundary will be in place. South of this boundary, there
appears to be quite a bit of capping potential. However, if some
storms can break the cap in the southern panhandle they will
most likely become explosive discrete supercells with any right
mover having the potential of producing large hail, torrential
rainfall due to the slow movement and isolated tornadoes.
Meanwhile, along and north of this boundary we could see the
potential for all modes of severe weather with multicell
clusters of showers and thunderstorms, damaging winds and
hail,due to higher effective shear values (40-60kts) and decent
instability with MUCAPES of 1500-2000j/kg. The bulk of these
storms should move out of this region by midnight or so with the
frontal boundary shifting south into the southern panhandle or
northern Colorado.

Friday-Saturday:
Another shortwave is expected to push through the northern
Rockies into the Plains on Friday. This shortwave will tend to
flatten the upper level flow on Friday which will mostly likely
weaken the cap. As a result, we may see more scattered showers
and thunderstorms on Friday. A few of these storms may also
become severe, but the instability will most likely be more
limited compared to Thursday. On Saturday, the upper level ridge
is expected to build over Rockies which may put is in more of
northwest flow regime with really warm temperatures. Cannot rule
some isolated showers and thunderstorms developing along the
northern periphery of our CWA due that area being in closer
proximity to the stronger flow aloft. We can also not rule an
isolated storm or two developing along the Laramie Range.

Sunday-Monday:
The models/ensembles/WPC clusters are displaying the upper level
ridge continuing to build over the area which will bring more
anomalous temperatures with only isolated convection chances. In
fact, this timeframe may need to be watched for potential heat
advisories. Stay Tuned!

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1005 PM MDT Tue Jun 18 2024

Southwest flow aloft will continue, while moist southeast low
level winds bring in low clouds, fog and possible rain Wednesday
evening east of I-25.

Wyoming TAFS...For Rawlins and Laramie, scattered to broken
clouds from 3500 to 15000 feet will prevail. Winds will gust to
38 knots at Laramie from 15Z to 00Z.

For Cheyenne, scattered clouds around 12000 feet will occur
until 14Z, then ceilings will be near 4000 feet until 00Z, then
ceilings will lower to 1500 feet. Winds will gust to 30 knots
from 14Z to 00Z.

Nebraska TAFS...For Chadron, Alliance and Scottsbluff, scattered
to broken clouds from 8000 to 10000 feet will prevail until 00Z,
then ceilings will lower to 1500 feet with visibilities of
3 miles in light rain and fog at Chadron and Alliance. Winds
will gust to 25 knots from 14Z to 00Z.

For Sidney, scattered clouds near 10000 feet will persist until
15Z, then ceilings will be broken from 3500 to 5000 feet until
00Z, then light rain and fog will develop reducing visibilities
to 3 miles with ceilings near 1500 feet.

&&

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

&&

$$

SHORT TERM...BW
LONG TERM...REC
AVIATION...RUBIN