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000
FXUS65 KCYS 232321
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
521 PM MDT Tue Apr 23 2024

.KEY MESSAGES...

- Expect warmer temperatures and an increase in shower and
  thunderstorm coverage for Wednesday.

- A progressive disturbance aloft and at the surface will help produce
  scattered to numerous showers and scattered thunderstorms
  Thursday afternoon, with some thunderstorms possibly becoming
  severe east of a Cheyenne to Torrington to Alliance line.

- Cooler, wet and unsettled weather is likely for Friday and
  the weekend.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 255 PM MDT Tue Apr 23 2024

Tonight...Satellite imagery and WSR-88D reflectivities indicating
showers developing across southern portions of Carbon and Albany
Counties as a weak disturbance moves across in the low amplitude
ridge aloft. Will paint isolated to widely scattered showers and a
few thunderstorms this evening south of a Rawlins to Wheatland to
Scottsbluff line. A few showers may linger past midnight along and
just north of the Colorado state line. Not as cold tonight as the
airmass slowly moderates.

Wednesday...Expecting an increase in afternoon and evening shower
and thunderstorm coverage with an attendant increase in low and mid
level moisture availability. Low level convergence on southeast
winds will aid in lift for precipitation development. Warmer
temperatures with 700 mb temperatures near 7 Celsius.

Wednesday night...As the next energetic shortwave trough aloft moves
into southern Nevada, and the flow aloft backs to southwest, the
models indicate the low level southeast winds will help low level
moisture increase after midnight across our eastern counties, and
especially for far southeast Wyoming and the southern Nebraska
Panhandle, where boundary layer progs suggest a decent chance for
fog and low cloud development, so will add fog to our gridded
forecast through the late night hours.

Thursday...The negatively tilted and potent shortwave trough aloft
moves into central Colorado in the afternoon, with low level
convergence maximized along a surface trough from northeast Wyoming
to northeast Colorado. Although bulk shear values are not too
impressive, steep mid level lapse rates and adequate low and mid
level moisture along with mesoscale and synoptic lift suggests
scattered to numerous showers, and scattered thunderstorm
development in the afternoon, especially along and east of
Interstate 25. The Storm Prediction Center forecasts a marginal risk
of severe thunderstorms east of a Cheyenne to Torrington to Alliance
line, and thus we have added the mention of a few severe storms to
the gridded forecast. Maximum temperatures will be somewhat limited
by the widespread cloud cover and precipitation coverage.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 255 PM MDT Tue Apr 23 2024

Unsettled weather is expected to continue through the weekend, but
another warming trend will begin early next week.

Two potent shortwaves will eject out of a longwave trough positioned
over the Western US between Friday and Sunday, keeping daily chances
for showers, a few thunderstorms, and high elevation snow in the
forecast each day. Beginning on Thursday night, the first upper
level shortwave trough axis will pass across the area. Initially
this disturbance will cross the Rockies as an open wave, but most
guidance has the upper level low rapidly deepening as it moves into
the High Plains overnight. This will allow the system to establish
some wrap-around precipitation as the night progresses, even though
this will not be present initially. As a result, the best chance for
more appreciable stratiform precipitation in the wrap-around area
late Thursday night into Friday will be further north and east in
the forecast area. This will be a fairly warm system, with limited
access to cold air. Thus, expecting rain for most areas, but areas
above 9000 ft are still expected to see snow, and snow levels could
creep down towards 8000 feet Friday morning. As with the last
several events like this one, expect a period of gusty northwest
winds in the wake of this system across the area on Friday. Most
ensemble members have 700-mb winds around 40 to 50 knots along the I-
80 corridor from the summit east towards Sidney, supporting
confidence is a windy day. At this time, it looks like winds should
remain below High Wind criteria for the plains, but the southern
Laramie range and foothills could be closer. In-house guidance based
on the GFS has been fairly consistent in showing about a 40-50%
chance of high winds for this area beginning early Friday morning
and continuing through midday. Expect a cooler day Friday as well,
but highs will still be fairly close to average or just a few
degrees below.

The first low departs Friday night, leaving our area sandwiched in
between the two lobes of upper level vorticity on Saturday. During
this period, a surface high pressure is expected to slide down the
east side of the Rockies and turn the flow to northeasterly over the
High Plains. The cold air is again fairly lackluster here, with this
front only knocking 700-mb temperatures down to -2 to 0C or so.
Plenty of cloud cover will lead to cooler highs on Saturday,
probably struggling to get out of the 40s to mid 50s. As the second
shortwave begins to traverse across the Rockies, look for another
round of cyclogenesis across Colorado Saturday afternoon into the
overnight hours. This is expected to develop further south than the
first round, and also close off and develop a solid wrap-around flow
a little earlier in its evolution. There are some timing differences
between models, but after a brief lull, precipitation is expected to
fill back in sometime between the late morning hours and sunset,
with the highest confidence in widespread precipitation occurring
overnight. Subtle differences also still exist in the exact track of
the upper level low, which lead to disparate rainfall amounts across
the I-80 corridor. It`s a little early to get into rainfall amounts,
but high totals are favored further south and east in the forecast
area for the second event. Precipitation type should be mainly rain
once again, but this system will have a little more cold air than
the first. With 700-mb temperatures dropping closer to -4C by
sunrise Sunday, snow is expected to at least mix in above 6500 ft,
and we could see some snow down to about 5000 ft. For now, just
introduced a mention of rain/snow mix for the 5000 to 6500 ft
elevation range, though confidence in any snow impacts here is low.
Winter weather headlines could be needed again for the mountains and
I-80 summit area.

Look for a gradual end to precipitation and clearing Sunday and
Sunday night with another cool day expected. Once the second upper
level low departs, we abruptly return to a zonal flow pattern by
Monday, allowing temperatures to trend back above normal.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 517 PM MDT Tue Apr 23 2024

VFR conditions expected through the 00Z TAF period. Primary
aviation concern will be showers developing along I-80 this
evening and potentially impacting KCYS, KLAR, and KRWL through
the evening hours. A second band of showers is expected to
develop in western Nebraska. Included VCSH for most western
Nebraska sites and all Wyoming sites. Winds expected to increase
tomorrow morning through the remaining TAF period.

&&

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

&&

$$

SHORT TERM...RUBIN
LONG TERM...MN
AVIATION...AM