548
FXUS65 KCYS 162011
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
211 PM MDT Fri Aug 16 2024

.KEY MESSAGES...

- A few severe thunderstorms possible this afternoon across
  eastern zones where moisture will be better for storm
  development.

- Seasonable temperatures will continue through the period.
  Showers and thunderstorms will be most likely on Monday,
  Wednesday and Thursday, with less coverage for Tuesday.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 210 PM MDT Fri Aug 16 2024

Current Visible Satellite loop shows mostly sunny skies across
southeast Wyoming and western Nebraska early this afternoon with
surface observations indicating temperatures in the 80s to near
90 across the area. One lone thunder shower is currently
northwest of Cheyenne, but is struggling to intensify as it
moves into the I-25 corridor. This is mainly due to drier air
aloft, especially above 8000 feet as indicated by recent
observations with dewpoints falling in the higher valleys and
midslopes. Further east, dewpoints have remained in the 50s to
near 60 and have even increase a few degrees since this morning
indicating some weak moisture advection across the eastern
plains. Models all show a dryline boundary forming along the
I-25 corridor shortly and progressing eastward through this
afternoon and into this evening. With a dewpoint of 34 at
Vedauwoo, and 53 at Cheyenne...believe this boundary is
developing as we speak. Along and to the east of this boundary
is where isolated showers and thunderstorms will form. Expect
the coverage to be limited today due to the drier airmass aloft,
which will take it`s time mixing down to the surface. CAMs and
high res guidance shows only a few thunderstorm cells developing
this afternoon and mainly impacting western Nebraska and far
eastern Wyoming. Can`t rule out one or two strong to marginally
severe cells, but not terribly confident either due to limited
coverage and forcing. Kept POP between 15 to 25 percent into
early this evening, with showers and thunderstorms dissipating
around sunset.

For Saturday, drier airmass will remain over the whole forecast
area with the potential for thunderstorms less than 5 percent
through Saturday afternoon. 700mb temperatures will continue to
increase between 16c to 19c, resulting in highs in the upper 80s
to upper 90s...hottest below 5000 feet. Models show a fast
moving upper level disturbance lifting north across western
Colorado and southern Wyoming later Saturday. This feature may
trigger widely scattered late evening thunderstorms across south
central Wyoming as some cooler air aloft moves into Carbon and
Albany counties. Confidence is limited at this time, so kept POP
around 20 to 30 percent for now. This will be the start of the
next monsoon push as PWATs increase above 0.75 to around 1.00
inch across the forecast area on Sunday and early next week.

Similar weather on Sunday with highs in the mid 80s to mid 90s
across the area. Upper level disturbance is forecast to shift
eastward during the day with a more favorable environment for
showers and thunderstorms. Expect most of this activity to be
in the late afternoon and evening hours as moisture will be slow
to increase through the day.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
Issued at 116 PM MDT Fri Aug 16 2024

A persistent ridge of high pressure over the southwestern CONUS will
remain in place for the entirety of the long-term period from Sunday
night through Friday of next week. With this ridge in place, expect
afternoon high temperatures to remain above average by around 5 to
10 degrees. Temperatures would probably be higher through the long-
term period if not for the prominent monsoon pattern expected to
continue underneath the ridge through the week ahead. PWAT
normalized anomalies from both the GEFS and ECMWF ensemble guidance
suggest running around 1-2 sigma through the period, with peak
deterministic PWAT values themselves reaching in to the 1.25"
category across southeast Wyoming and western Nebraska by Thursday
and Friday. As one would expect, daily monsoon showers and
thunderstorms will be possible each day, first originating in
orographically favored areas in the high terrain and then moving
downvalley by the later afternoon and evening hours each day.
Tuesday appears to be the driest day of the week given a slight push
of dry air from the north, with recovering chances of thunderstorm
activity each day through Friday. On Wednesday and Thursday, a piece
of the subtropical jet is progged to drift northward around the
ridge, increasing flow in the mid and upper levels. This increased
wind shear will help support a better chance of organized convection
as some guidance supports the idea of one or two thunderstorm
clusters/outflows pushing well east of the mountains and into the
WY/NE high plains in the evening hours each day. Overall, total
precipitation through the long-term has at least a 60% chance of
running above average given the favorable monsoon pattern. There is
high confidence (above 60%) of seeing above-normal temperatures as
well through the period.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1104 AM MDT Fri Aug 16 2024

Isolated showers and thunderstorms will develop this afternoon
mainly from 20z to 23z near CYS, and perhaps an isolated shower
at BFF and CDR in the 0z timeframe. Winds should remain easterly
upslope to the east of I-25 with dry westerly flow at LAR and
RWL. Expect VFR conditions throughout the day at all terminals
and a mainly clear overnight at all terminals.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...

WY...Red Flag Warning until 8 PM MDT this evening for WYZ420>423-
     425-427-428.

NE...None.

&&

$$

SHORT TERM...TJT
LONG TERM...RUBIN
AVIATION...MAC