-- Discussion containing changed information from previous version are highlighted. --
203
FXUS65 KCYS 241739
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1139 AM MDT Mon Jun 24 2024

.KEY MESSAGES...

- Near record high temperatures are expected again today. Heat
  Advisories remain in effect for the central Nebraska Panhandle
  and Goshen County.

- Scattered thunderstorms developing around noon today will have
  the potential to produce strong, gusty winds and lightning
  with limited rainfall.

- Moisture will return to the High Plains on Wednesday, leading
  to the potential for some strong thunderstorms in the
  afternoon and evening.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 345 AM MDT Mon Jun 24 2024

The axis of the upper level ridge has shifted to just east of the
eastern edge of the forecast area this morning. While mostly clear
skies are covering the vast majority of the area again, we have a
little bit more going on today. First, a subtle shortwave moving
over top of the ridge can be seen moving into North Dakota as this
hour as an area of high clouds. A surface low associated with the
wave is located near the MT/WY/SD intersection,with a subtle trough
trailing to the south towards the CO front range. As a result, we
have a modest cross-barrier pressure gradient keeping southwest flow
active through the night. This is leading to a very warm night in
some of the breezier areas affected by southwest flow. This includes
much of the I-25 corridor through Platte and Converse counties, the
I-80 corridor in Carbon county, and the US-20 corridor from Lusk to
Chadron. A 3AM temperature of 81 degrees was reported at Bordeaux,
with much of the rest of this area stuck in the 70s. As the surface
trough progresses eastward, expect southwest flow to ease over the
next few hours. This will set the stage for another hot day today.
700-mb temperatures were analyzed at +16C or higher across the
entire forecast area this morning, slightly warmer than yesterday.
We should warm up very rapidly this morning, with several areas
breaching the 90F mark by 9 or 10AM. However, another feature
apparent on water vapor satellite imagery this morning is the
pseudo- monsoonal moisture plume over the Four Corners states
encroaching into southern Wyoming this morning. Mid to upper
level moisture is expected to be higher than yesterday, despite
the very dry low- levels remaining. A little more cloud cover
will be the limiting factor for the heat today. Thinking that
the better forcing for heat and increased cloud cover will
roughly cancel each other out, resulting in fairly similar highs
today compared to yesterday. Those west of the Laramie range
may be a touch cooler, while those to the east may be a touch
hotter. Due to the uncertainties with the cloud cover, decided
not to expand the Heat Advisory. Forecast highs today are
generally within a few degrees of daily record highs, and also
several degrees above guidance (which verified well yesterday).

Another result of the increased mid level moisture today will be a
more active afternoon convective day. A little extra synoptic lift
from a subtle vort-max moving through the area in the presence of
slightly higher moisture should result in greater storm coverage
than yesterday. Forecast soundings by the afternoon hours show
a deep inverted-v up to about 500-mb, but with fairly good
moisture in the 300 to 500-mb layer. This will allow the high-
based storms to grow a little taller than yesterday. However,
rainfall will still have a very deep and very dry boundary layer
to pass through, so rainfall expectations are pretty low today.
While probably not totally dry, the footprint of winds and
lightning will be larger than the rainfall footprint today. The
wettest storms may drop a few hundredths to an isolated tenth of
an inch or so. Expect convective activity to get a pretty early
start today, kicking off around noon. Storms will have the
potential to produce dry microbursts with a few scattered wind
gusts of 60+MPH possible. The SPC has much of the High Plains in
a Marginal risk, primarily for the dry microburst threat. The
hail threat will be limited to mainly the northern NE panhandle
where storms will encounter a little better low level moisture.
If thunderstorm activity manages to continue after nightfall
(which some HiRes models indicate), wouldn`t be surprised to see
a heat burst between sunset and midnight.

As another surface low gets carried off to the east across Nebraska
on Tuesday, expect a light northwest breeze to develop across much
of the area, with a subtle frontal boundary stalled just northeast
of the North Platte River valley. Areas north/east of that will see
temperatures close to 10F cooler than today, but the rest of the
area will probably climb to within a few degrees of today`s highs on
Tuesday. Forecast soundings show more limited upper level moisture
Tuesday, and also less lift as the ridge re-amplifies just to our
west. The resulting WNW flow aloft will suppress the monsoon-like
moisture to the south and west, leading to lower PM thunderstorm
coverage. Isolated storms may produce gusty winds and lightning
again, but this should be lower coverage than today. By Tuesday
night, the stalled frontal boundary will push back towards the
Laramie range as the surface high pressure over the northern plains
progresses south and east. This will allow low-level moisture back
into the picture by Wednesday morning.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 345 AM MDT Mon Jun 24 2024

The long term forecast period will start out on an active note with
a good chance for showers and thunderstorms on Wednesday and
Thursday. On Wednesday, southeasterly surface flow to the east of
the Laramie Range will transport rich surface moisture westward with
dewpoints near 60 degrees likely from US 85 eastward. This
southeasterly flow will be accompanied by an increase in westerly
flow aloft, with forecast soundings indicating around 40 knots of
bulk shear or greater by mid-afternoon. While eastern Wyoming and
western Nebraska will be on the northern periphery of a broad ridge
of high pressure aloft, a compact shortwave/vort-max is
currently progged to pass through the region during the late
afternoon hours. This is particularly evident in recent runs of
the NAM, which show a well-timed shortwave ejection over the
high plains concurrent with greatest surface
heating/instability. If the timing of this shortwave remains on-
track, we could be looking at a few strong to severe
thunderstorms with all hazards possible on Wednesday afternoon
and early evening, generally from the Laramie range eastward
into the Nebraska panhandle. While the GFS does not indicate
quite as obvious a ridge-riding shortwave, the ECMWF does
(similar to the NAM) lending some confidence to this forecast.

By Thursday the broad ridge over the southwestern CONUS will have
broken down with as a longer-wave trough moves throughout the
central and northern Rockies. This trough will spread faster flow
aloft eastward by later in the day. Lingering surface moisture in
southeastern Wyoming and western Nebraska will again result in
enough instability for showers and thunderstorms, with 500b flow
increasing to around 30-40 knots by 0z. However, a primary
difference in the Thursday forecast compared to Wednesday looks to
be surface and lower-level flow, which seems light and muddled at
best lending to straight/shorter hodographs and thus more
clustered/outflow dominant showers and storms. Still, will have to
watch the Thursday thunderstorm forecast as several ingredients for
severe weather remain in the vicinity of the CWA.

On Friday, a frontal system will pass through the central High
Plains bringing a drier airmass with cooler temperatures for both
Friday and Saturday. Only a few isolated orographic showers look
possible on Saturday afternoon. By Sunday, expect temperatures to
rebound as height rise, however a dry airmass will remain in place
with only a few showers in the higher terrain of southeast
Wyoming.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
-- Changed Discussion --
Issued at 1136 AM MDT Mon Jun 24 2024

VFR prevails. Scattered to numerous showers & perhaps a few weak
high-based thunderstorms will impact a large portion of the area
this afternoon and early evening. Gusty and erratic surface wind
gusts of 50 to 60 MPH will be the primary hazards. Expect SCT to
BKN coverage of mid-level clouds around 10k feet AGL through 02z
with clearing skies expected overnight.
-- End Changed Discussion --

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
WY...Heat Advisory until 6 PM MDT this evening for WYZ108.
NE...Heat Advisory until 6 PM MDT this evening for NEZ003-019>021-
     096.
-- End Changed Discussion --

&&

$$

SHORT TERM...MN
LONG TERM...MAC
AVIATION...CLH