566
FXUS65 KCYS 172338
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
538 PM MDT Sat Aug 17 2024

.KEY MESSAGES...

- Hot temperatures will persist through the weekend with
  widespread daytime highs in the 90s for areas along and east
  of I-25.

- Daily chances for showers and thunderstorms will exist across
  the region over the next several days as several shots of
  monsoonal moisture ride along the western periphery of the
  upper level ridge.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 300 PM MDT Sat Aug 17 2024

Quiet weather today with hot temperatures across most of the
region with readings in the low to mid 90s along and east of
I-25, and mid to upper 80s west of I-25. Visible satellite loop
shows mostly sunny skies with some fair weather cumulus clouds
and intervals of midlevel cloudiness moving to the east.
Satellite also shows the next potent upper level shortwave
moving northeast across Utah this afternoon. Models show this
shortwave continuing to lift northeast into western Wyoming
tonight as some cooler air aloft works it`s way into south
central Wyoming. Thankfully, the timing of this shortwave will
be too late in the day to trigger widespread thunderstorm
activity across southeast Wyoming as we lose instability around
sunset. Kept POP below 30 percent across the lower elevations
for scattered showers and high-based thunderstorms. Current CAMs
and high res guidance are showing isolated to widely scattered
coverage at best...mainly west of the I-25 corridor through
midnight tonight. The main concern will be gusty outflow winds
with a few of these cells. Otherwise, should be pretty quiet and
mild tonight.

Weather will be similar on Sunday (compared to today) even as
the upper level shortwave moves east across the area. Current
model timing suggests this feature will be dissipating while
eject east into the central plains just in time for Sunday
afternoon peak heating. The upper level ridge axis is forecast
to quickly restrengthen behind this shortwave with increasing
upper level subsidence through the afternoon. Expect the dry air
and lower lapse rates aloft to suppress most convection through
the afternoon hours. Continued the trend of lowering POP Sunday
afternoon and left probability of thunder below 20 percent for
most locations...with a slight trend upward towards the evening
hours as low level moisture really starts to ramp up.

For Monday, SREF guidance is beginning to show higher
probabilities of low stratus and fog in the morning. Surface
winds are forecast to veer into the east and southeast due to a
stronger than average surface high pressure pushing into the
Great Plains. With increasing low level moisture through the
day, starting to get concerned with the potential for heavy
rainfall and flooding concerns as NAEFS is showing 95th to 99th
percentile PWATs across the eastern plains and even around 90th
percentile for all of central Wyoming. Storm motions look pretty
weak, especially through the mid afternoon hours with winds
aloft increasing a bit into Monday evening. Kept POP around 55
to 75 percent for most of the area for numerous showers and
thunderstorms. May need to watch for strong to severe
thunderstorms, but with the lack of 0-6km shear through much of
the day...confidence is pretty low at this time. Slightly cooler
temperatures are expected on Monday due to increasing cloud
cover and upslope east to southeast winds, but highs should stay
close to average for this time of the year (80s to near 90).

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 210 PM MDT Sat Aug 17 2024

A fairly typical late summer weather pattern will carry us through
most of the long term forecast period. Overall, it looks like a
fairly active monsoon pattern, but with a few break periods in
between days of active monsoon surges.

Picking up on Tuesday, most model guidance shows a break period in
the monsoonal flow. The strong upper level ridge centered over New
Mexico will broaden westward into southern California, which will
deflect the monsoon moisture into the eastern Pacific and instead
draw dry subtropical air into our area. Precipitable water drops to
near or slightly below average for this time of year over our area.
Meanwhile, the shortwave passing through Monday will have passed to
the area, and we will see some subtle synoptic scale subsidence. All
together, this should lead to a hotter and drier day on Tuesday with
highs running perhaps 5-10F above average and just a few isolated
showers and thunderstorms, mainly confined to the vicinity of the
higher terrain.

This break period looks short-lived though, as most models indicate
the western flank of the upper level ridge over Arizona breaking
down on Wednesday, allowing for the monsoon moisture to push through
once again. Some subtle vort-maxes will also be pulled out of the
southwest, helping to enhance shower and thunderstorm activity. This
overall pattern looks to mainly hold through the second half of next
week with a series of weak shortwaves locally enhancing lift for
thunderstorms. While the timing of those features is uncertain at
this lead time, there is moderate confidence in a fairly active
pattern of afternoon and evening showers and thunderstorms beginning
again on Wednesday (possibly late) and continuing into Friday. Most
model guidance shows the monsoon pattern at least weakening by
Saturday, though it`s a little early to say if we`ll completely dry
out or just see lower storm coverage.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 532 PM MDT Sat Aug 17 2024

VFR expected at all terminals through the forecast period. VCTS
and VCSH will cause gusty winds through 3Z this evening for
southeast WY terminals. Wind gusts of 20-40 knots this evening
can be expected before all terminals see wind gusts of 12 knots
or less overnight. Sunday will bring another round of afternoon
wind gusts of 15-25mph at times.

&&

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

&&

$$

SHORT TERM...TJT
LONG TERM...MN
AVIATION...BW