568
FXUS65 KCYS 131645
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1045 AM MDT Sat Jul 13 2024

.KEY MESSAGES...

- Very hot temperatures are likely across all of southeast
  Wyoming and the western Nebraska Panhandle this weekend
  through early next week. A Heat Advisory has been issued for
  portions of southeast Wyoming Friday afternoon, and for all
  areas east of the Laramie Range for this weekend.

- Widespread precipitation possible Monday and Tuesday with a
  brief cooldown Monday through Wednesday. Warmer, drier
  conditions return Thursday onward.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 413 AM MDT Sat Jul 13 2024

The short term forecast continues to be active as multiple
weather hazards are at play the next 48-60 hours. Earlier this
morning, organized thunderstorm activity propagated through the
Nebraska Panhandle, warranting a couple of severe thunderstorm
headlines. Those thunderstorms have since moved to the east.
For those that have an eye for curious surface weather
observations, the rain showers that moved through KCDR after
midnight interacted with the boundary layer where the nighttime
inversion had set up. Temperatures dropped 20+ degrees from
evaporational cooling during the heavy rainfall from the passing
thunderstorm, and then additional light rain showers caused
mixing from the inversion aloft and a wind shift, to cause
temperatures to surge back to where they were previously, which
were the middle 80s. A good example of a possible heat burst in
the northern NE Panhandle. The remainder of the areas east of
the Laramie Range for southeast WY have slowly cooled, but
temperatures are still in the 60s and 70s. Similar temperatures
are present west of the Laramie Range, and while taking a peek
at the relative humidity readings along the I-80 corridor where
we have more available weather observations, relative humidity
has been very poor overnight. The thermal belt is displayed
easily when looking at the relative humidity there, as values
are in the teens and 20s. Very dry air indeed for locations west
of the Laramie Range.

The remainder of today will be hot. Heat Advisory remains on
track as we have several locations expected to be on the fringe
of tying or breaking their daily record highs. Cheyenne,
Laramie, Rawlins, Scottsbluff, Alliance, Chadron are just a few
to potentially see a daytime high record be tied or broken. Have
bumped up the winds east of the Laramie Range, due to hi-res
model data depicting a likely increase in sustained winds. The
highest wind gusts are favored for areas along and north of the
North Platte River Valley in Converse, Niobrara, Platte, and
Goshen County. Have gone with another short-fused Red Flag
Warning for those areas. The remainder of the cwa is highly
likely to see very low relative humidity values near or below 15
percent today, but the winds and wind gusts elsewhere will be
more light, particularly in the NE Panhandle. However, we are in
a Marginal Risk for severe wx on the eastern fringe of the NE
Panhandle, so gusty and erratic winds are possible with an
isolated strong thunderstorm that may develop after 0Z this
evening.

Another hot day is in store for the region on Sunday. We will
have another shot at locations in the cwa potentially tying or
breaking their daily record high temperatures. Isolated showers
and thunder is possible late in the afternoon. However, the
cloud bases will be elevated. With such a large region of very
dry air still being present, there is the possibility of dry
thunderstorms, which would present the opportunity for gusty and
erratic winds alongside lightning strikes. We will have to
monitor this in the event that another short-fused Red Flag
Warning for Dry Thunder is more favored. The warm temperatures
will continue on Monday, but we will have increasing cloud cover
thanks in part to monsoon moisture finally making its arrival to
our area. We can expect a higher possibility for a wetting
rainfall of 0.10 inches in some of the showers and thunderstorms
that develop by late Monday afternoon. Daytime highs will be in
the 80s and 90s, so some of the Heat Advisory headlines remain
in effect for the NE Panhandle through Monday evening. Make sure
to check back with forecast updates as the active weather
headline period persists for our area through mid-July.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
Issued at 413 AM MDT Sat Jul 13 2024

Deterministic and Ensemble model guidance is in very good agreement
on the large scale pattern evolution Tuesday through Friday. The
persistent ridge of high pressure will be centered over the 4-
Corners at the start of the period with a broad upper low centered
over central Ontario. As the upper low accelerates east into Quebec
by Thursday, the ridge will build further north and slightly west,
its axis oriented just west of WY. As a result, upper level flow
will become northwesterly for the week with marginal speed  of ~30
knots at 500 mb and occasional embedded disturbances. Moisture
wrapping around the high in the mid-upper levels combined with
persistent southerly/southeasterly flow in the low-levels will drive
PW values to 1-1.25 inches on Tuesday, decreasing slightly for Wed-
Fri. Values over 1" is significant for our region (90th percentile PW
is around 1" for both DEN and UNR).

Periodic disturbances in the mid-upper level flow rounding the peak
of the ridge, and their associated fronts, along with diurnal
heating and abundant moisture, will help aid daily thunderstorm
development over SE WY and the NE PH through the week. Precipitation
chances look to be best and most widespread (with greatest
forecaster confidence given model consensus) Tuesday
afternoon/evening, and given the abundant moisture in place, there
is potential for these storms to produce heavy rainfall. Showers and
storms should redevelop each afternoon Wed - Fri, with perhaps
slightly less coverage/precip intensity compared to Tuesday given a
slight decrease in moisture. Some storms during the week could be on
the strong side, especially in the NE panhandle, considering
adequate instability (dew points in the 50s beneath steep ML LRs)
and wind shear (southeasterly to easterly surface flow beneath the
~30 knot northwesterly mid-level jet). Confidence in the convective
scenario each day, including severe potential, remains low as we
are still a ways out.

Accompanying the increased precipitation chances next week will be
cooler temperatures as a result of the increase in atmospheric
moisture and positional shift of the upper ridge. Highs on Tues-
Thurs will generally stay in the 80s across the lower elevations of
se WY and the NE PH, with Wed looking to be the coolest day.
Temperatures are expected to increase again Friday into the weekend,
though not to the extent as currently, with lower precipitation
chances. Confidence in the temperature pattern is high given model
agreement in the large scale pattern evolution.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1035 AM MDT Sat Jul 13 2024

VFR conditions likely through the forecast period for all
terminals. Overall, no major concerns for the aviation forecast,
with the upper level high remaining firmly in place stretching
across the Intermountain West. Some minor disturbances pulsing
within the main flow could produce some vicinity showers across
KRWL and gusty winds across the remaining terminals.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Heat Advisory until 8 AM MDT Monday for WYZ101-102-107-108-118-
     119.
     Red Flag Warning until 8 PM MDT this evening for WYZ417-418.
NE...Heat Advisory until 8 PM MDT Monday for NEZ002-003-019>021-054-
     055-095-096.

&&

$$

SHORT TERM...BW
LONG TERM...BL
AVIATION...MRD