607
FXUS65 KCYS 032328
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
528 PM MDT Wed Jul 3 2024

.KEY MESSAGES...

- Thunderstorms are expected this afternoon and tonight,
  especially across east central Wyoming and the western
  Nebraska Panhandle. Any developing storms will have the
  potential to become severe with all hazards possible.

- A cool and generally pleasant Independence Day is expected in
  the wake of a strong cold front set to sweep across the area
  later tonight. A few light showers are possible, but no major
  impacts are expected. Daytime highs on Thursday will only be
  in the 70s for most areas.

&&

.UPDATE...
Issued at 258 PM MDT Wed Jul 3 2024

Severe Thunderstorm Watch continues for western Nebraska. So will
keep our updated-short term discussion brief due to active
weather late this morning through this afternoon. Last night`s
short term discussion is still valid through Independence Day.

First round of severe thunderstorms persists across Cheyenne
County with several reports of large hail up to 2 inches in
diameter across Cheyenne and Morrill counties, gusty winds, and
even a few tornado reports. Expect this area to push off to the
south and east with relatively quiet conditions over the next
few hours. Strong cold front is quickly moving south across
northern Wyoming and southwest South Dakota. This will be the
start of round 2 across east central Wyoming and northwest
Nebraska this evening through tonight. Timing is a little
uncertain with high resolution models showing some variability
on coverage and the areal extent this afternoon. With the dry
airmass and elevated mixing layer already pushing across
Converse County and the I-25 corridor at this hour, expect the
2nd round of the severe threat to be confined to Niobrara county
and the northern Nebraska Panhandle between 7 PM through
midnight tonight with a few cells possibly developing across the
southern Panhandle after midnight due to high elevated CAPE.
Solid MUCAPE/elevated instability along with forcing along and
ahead of the front will be the primary ingredients. Would like
to note that the drier airmass has penetrated a little further
east than previously expected...which may result in a lower
thunderstorm coverage tonight. Not expecting any fog after the
front moves through the area due to the much drier airmass.
Otherwise, the previously advertised cooler temperatures late
this week are still on track.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 409 AM MDT Wed Jul 3 2024

The primary focus over the next 12 to 24 hours is the threat for
strong to severe thunderstorms across portions of the CWA. Storm
coverage is fairly uncertain today with a robust EML expected to
spread across the majority of the CWA by early afternoon w/ 700-
mb WAA contributing to 700-mb temperatures near +14 deg C by 18z
over most of south central & southeast Wyoming. As such, it does
appear likely that capping will be an issue for most of the area
w/ the best chance for CI over our far northern & eastern zones,
on the edge of the thermal ridge/EML. High-res guidance suggests
initial development may occur along/southeast of a line from IBM
to AIA by early afternoon as SSE boundary layer flow allows dew-
points to soar into the lower to middle 60s east of the dryline,
supporting rapid destabilization w/ MLCAPEs of 1500 to 2000 J/kg
by 18z. It is possible that additional development will occur in
the early afternoon across Converse/Niobrara counties, but would
expect the better chance to occur during the evening w/ the cold
frontal passage. Strengthening flow aloft & veering profiles may
support effective shear of 35 to 45 knots, suggesting the threat
for supercells capable of all facets of severe weather including
very large/damaging hail, strong winds, and even a few tornadoes
for any surface based storms in western Nebraska. Could see some
elevated storms persist well into the night behind the main cold
front, with a low-end risk for marginally severe hail overnight.

As 700-mb temperatures plunge to between +2 and +6C by Wednesday
night in the wake of the cold front, the stage will be set for a
very cool Independence Day w/most areas only seeing highs in the
70s. Isolated to widely scattered instability-driven showers are
expected, but overall do not expect to see much of an impact for
most areas with this activity.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 409 AM MDT Wed Jul 3 2024

Minimal changes made to the long term forecast. Still expecting a
cold front over the weekend which will lead to a brief cool down,
before strong ridging over western CONUS warms temperatures up again.

Biggest change in the long term appears to be the timing of the cold
front. Both the GFS and ECMWF show two progressive shortwaves
traverse the CWA within a 36 hour period. The ECMWF shows the first
shortwave as the stronger of the two, bringing colder 700 mb air
into the region by Saturday morning. This is followed by quick
warming of 700 mb temperatures as the first shortwave is quickly
ejected eastward. The second shortwave moves in during the day
Sunday, but lingers a bit longer, leading to cooler temperatures
during the day. The GFS differs in that it keeps the first shortwave
far enough to the northeast for the CWA to see limited effects. This
leads to a warm day on Saturday. Second shortwave moves through
during the day on Sunday, again, leading to cooler daytime highs.
Still have fairly high confidence that temperatures on Saturday will
be on the warm side, with a 5 degree increase in daytime highs
compared to Friday. Sunday will likely feature below average, but
pleasant high temperatures. With 700 mb temperatures as cold as +4C,
highs may not make it out of the 70s for most locations. Aside from
temperatures, could see some light precipitation develop with both
of these shortwaves due to increased mid-level moisture and forcing.

With a strengthening ridge building over western CONUS, expect
warming temperatures and dry conditions through the beginning of the
work week. Increasing 700 mb temperatures and rising 500 mb heights
will likely lead to the return of 90 degree temperatures by the
middle of next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 520 PM MDT Wed Jul 3 2024

Northwest flow aloft will continue, as a cold front moves across
the terminals tonight, ushering in cooler air.

Wyoming TAFS...Mostly clear skies will prevail, with scattered
clouds around 12000 feet at times. Winds will gust to 37 knots
at Rawlins and Laramie until 04Z, and to 25 knots after
15Z Thursday. Winds will gust to 35 knots at Cheyenne until
09Z, and to 25 knots after 15Z Thursday.

Nebraska TAFS...For Chadron and Alliance, scattered to broken
clouds from 5000 to 10000 feet will occur, with occasional
thunderstorms until 05Z producing wind gusts to 35 knots and
visibilities from 2 to 4 miles, along with ceilings near
3000 feet. Winds will gust to 25 knots until 05Z, and to
25 knots after 15Z Thursday.

For Scottsbluff and Sidney, scattered to broken clouds from
5000 to 10000 feet will occur, with thunderstorms in the
vicinity of Scottsbluff from 02Z to 04Z. Winds will gust to
30 knots until 06Z to 10Z, and to 30 knots after 15Z Thursday.

&&

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

&&

$$

UPDATE...TJT
SHORT TERM...CLH
LONG TERM...SF
AVIATION...RUBIN