873
FXUS65 KCYS 122010
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
210 PM MDT Mon Aug 12 2024

.KEY MESSAGES...

- Scattered to numerous showers and thunderstorms are expected
  to develop this afternoon. Storms will have the potential to
  produce torrential rainfall, gusty winds, and isolated large
  hail.

- An active pattern continues through the week ahead with near
  normal temperatures and daily chances for afternoon and
  evening showers and thunderstorms.

&&

.MESOSCALE...
Issued at 140 PM MDT Mon Aug 12 2024

Daytime insolation has allowed the atmosphere to begin to
percolate across portions of far southeast WY. 18Z RAP
mesoanalysis via SPC depicts steep lapse rates of 9 degrees C/km
in the H5-H7 lower levels, and effective bulk shear from 0-6km
between 30-45 knots along the stationary frontal boundary that
is presently draped on a north to south transect from the Front
Range to the North Laramie Range. Additionally, surface-based
CAPE of doing quite well in response to the daytime sunshine, as
integrated amounts of 1000-2500 J/kg from the I-25 corridor
toward the Sandhills of west-central NE exists. Unsurprisingly,
we have had Cu fields and agitated towering cumulus develop in
the past 1-2 hours along the South Laramie Range, with a couple
of strong thunderstorms initiating in the same area. PWATs are
in the 90-99th %ile so far today, with deep Theta-e moisture
advection being evidenced on the GOES-16 satellite imagery.
Southeast flow from the south and central Plains has kept the
low cloud cover near the CO/NE/KS border. In the next 1-2 hours,
expect more clearing across our cwa, with additional
thundershowers occurring across the I-80 corridor and east of
I-25. With the slow-moving convection being anticipated to
develop/continue soon, we will have increased chances for
isolated flooding ticking upwards this afternoon. Hi-res model
guidance does hint at multiple rounds of showers and
thunderstorms through this evening, and into the overnight
hours. Highest concern through the evening and overnight hours
is potential heavy rainfall accumulation in central and south NE
Panhandle.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 330 AM MDT Mon Aug 12 2024

After a brief break in the wet pattern yesterday, moisture is
returning to the area today with more widespread thunderstorm
activity expected this afternoon. GOES satellite water vapor channel
imagery shows that the shortwave that passed through the area
yesterday is now moving across the Dakotas. The subsident pocket of
dry air is shifting eastward also, with the driest air now in
central and eastern Nebraska. As the upper level ridge axis slowly
shifts east today, the flow aloft will shift from northwest to west
and subtly southwest, allowing for the monsoon moisture tap to open
up fully again. High cirrus and much better mid to upper level
moisture is already visible moving from southwest to northeast
across our area this morning. Meanwhile at the surface, we have a
weak surface high over the northern plains and a weak surface low
over southeast Colorado. East to southeast surface winds in between
these two features are pushing low level moisture back into the high
plains. Good radiative cooling ahead of the approaching cirrus deck
is resulting in fairly widespread fog forming along portions of the
I-80 corridor from the summit east towards Sidney. In the areas
where the fog coverage is highest (eastern Laramie, Kimball, and
Cheyenne counties), a Dense Fog Advisory has been issued through 9AM.

Heading into this afternoon, the return of moisture will set the
stage for a more active day of afternoon convection. By this
afternoon, both precipitable water and integrated water vapor
transport will be above the 90th percentile of climatology over much
of the area. The next vort-max aloft will arrive this afternoon
and kick off another round of modest isentropic lift which
should start to pop up convection on radar between 12PM and 2PM.
Nearly saturated soundings will indicate primarily a heavy
rainfall threat with fairly slow moving thunderstorms. However,
mid level lapse rates and vertical wind shear look to be more
than sufficient to get a few storms to become strong to severe.
While the plentiful moisture points more towards stronger storms
producing ample amounts of small hail, isolated large hail is
still possible along with the usual strong winds. The
environment for strong to severe storms will be most favorable
inside roughly a Laramie to Glendo to Bridgeport triangle.
Isentropic lift will continue all the way through the night and
into Tuesday morning, so some shower and isolated thunder
activity is expected to remain on the radar for most of the
short term forecast period. Some of this nocturnal activity
could still produce heavy rainfall and hail.

Tuesday will have a similar environment to Monday across the area.
Moisture looks even higher, pushing the 97.5 to 99th percentile in
some areas. Dewpoints in the 60s may cover most of the NE panhandle
and even into far eastern Wyoming, while nearly saturated air
remains in the mid to upper levels of the atmosphere. A more potent
shortwave aloft is expected to push through the area and force
another round of scattered to numerous thunderstorms. As a result,
we`ll be looking at another locally heavy rainfall threat. Most
model guidance shows weaker lapse rates Tuesday, which may reduce
the hail threat. However, an increase potential for upscale
organizing compared to Monday may lead to a slightly higher threat
for strong thunderstorm winds.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 209 PM MDT Mon Aug 12 2024

Upper-level troughing will be positioned over northern Idaho with a
strong ridge across the central CONUS on Wednesday. 500mb vorticity
maxima will eject out ahead of the trough, enhancing lift across the
region. Northwesterly flow at 700mb will continue through the day as
a 700mb trough moves off to the east of the region. Surface dewpoints
across western Nebraska will be in the mid-50s to low-60s with
dewpoints closer to the 40 to 50F mark across southeast Wyoming. The
PW values are progged to increase into the 1.1-1.3 inch range across
western Nebraska and closer to an inch across southeast Wyoming.
With upper-level support pushing across the area at this same time,
isolated to widely scattered showers and thunderstorms will be
possible Wednesday afternoon and evening. Forecast NAM and GFS
soundings suggest weak steering flow with minimal shear across
western Nebraska. With PW values well over 1.0 inches, weak steering
flow, and MLCAPE around 1000-1500 J/kg, slow moving, heavy rain
producers are possible Wednesday afternoon. SPC has a Marginal Risk
of severe weather just off to the east on Wednesday. Temperatures
will be warm, but more seasonable for this time of years. Highs will
range from the mid-70s to upper-80s across the Panhandle.

For Thursday, a secondary upper-level low attempts to move into the
region, but a ridge is progged to build over the Intermountain West.
This ridge will push the low further to the north. Despite this,
several 500mb vorticity maxima will eject out ahead of the low and
be absorbed into the building ridge. These maxima should enable
enough synoptic lift to get storms going again for Thursday.
Coverage does not look to be as good on Thursday, due to the less
concentrated synoptic lift over the region and the displacement of
the low to the north. Weak MLCAPE will be present across western
Nebraska, around 500 J/kg. Bulk shear will be weak as well Thursday,
so any showers and thunderstorms that do develop with the
disturbance aloft will likely not be severe and will struggle to
maintain themselves. Highs will be slightly cooler than Wednesday
due a delay in the warmest 700mb temperatures moving overhead. Highs
will be in the mid-70s to low-80s across the region.

The remaining long term forecast will be dominated by a building
upper-level ridge across the western CONUS. The ridge will still be
rather flat on Friday, so precipitation chances remain as a few
disturbances traverse through the ridge. However, Saturday onward,
the ridge will become amplified. Precipitation chances will decrease
significantly for the weekend, but could see some isolated
convection due to daytime heating and terrain-induced lift. Highs
for the weekend will be in the mid-80s to mid-90s with warm
temperatures continuing through the early half of the weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1136 AM MDT Mon Aug 12 2024

VFR conditions everywhere except SNY at this time. The low
clouds at SNY are expected to lift by 19Z and return to VFR
conditions. Next concern switches to afternoon showers and
thunderstorms near all terminals. The mostly likely terminals to
be impacted are CDR and BFF, but cannot rule out thunderstorms
impacting every terminal. Gusty winds are expected this
afternoon and through the overnight hours. SNY and AIA may go
down in low stratus and fog again overnight.

&&

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

&&

$$

MESOSCALE...BW
SHORT TERM...MN
LONG TERM...AM
AVIATION...AM