555
FXUS65 KCYS 182333
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
533 PM MDT Thu Jul 18 2024

.KEY MESSAGES...

- Widely scattered thunderstorms will continue through the
  evening hours. A few isolated storms may become strong to
  severe.

- Another round of scattered to numerous thunderstorms is
  expected on Friday late morning through the afternoon. A few
  isolated storms may become strong to severe.

- Cooler weather expected this weekend with daily chances of
  storms through early next week.

&&

.SHORT TERM /THROUGH SATURDAY NIGHT/...
Issued at 300 PM MDT Thu Jul 18 2024

Our area remains on the northeast fringe of a potent upper level
ridge centered nearly exactly over the Four Corners. Decent monsoon
moisture is trapped underneath this ridge, while weakly increasing
pressure to the east is keeping dewpoints in the upper 40s to 50s
across the High Plains. Between these two components, above average
precipitable water is present across the area today, ranging from
around 0.6 inches over the Sierra Madre to just over 1 inch in the
nebraska panhandle. The environment is fairly unstable today with
700 to 500-mb lapse rates exceeding 8 C/km for most of Wyoming.
Shear is fairly weak, but present over the High Plains with weak low
level southeast winds underneath northwest flow aloft. There is not
much synoptic lift nearby today, so the higher terrain and a weak
convergence boundary along the Laramie range will provide the main
axis for storm development today. Scattered showers and storms have
already developed over Carbon and Albany counties right now, but
have remained weak so far. Modest MLCIN is still present east of the
Laramie range, but this is expected to break down over the next few
hours and allow for a few storms to tap into the better environment
further east. No major changes to the outlook for this afternoon,
but expect isolated strong to severe storms with the potential to
produce hail and gusty winds. Locally heavy rain will also be a
concern with weak steering flow continuing.

Friday will have a similar overall setup, but it will be a bit more
dynamic than today. A modest shortwave currently visible on
water vapor satellite imagery along the Canada/Montana border
will rotate over the top of the ridge and slide down the eastern
side on Friday. A surface cold front will also add some modest
frontogenesis over the plains. Additional lift from these
elements will help to increase shower and thunderstorm coverage
compared today. The shortwave will also nudge stronger flow
aloft back into our area, which will lead to slightly higher
shear values than today. Mid to upper level moisture will remain
fairly plentiful Friday, so we`ll have another threat for
isolated large hail, locally heavy rainfall, and gusty winds
with thunderstorms through the afternoon and evening hours.
Expect an early start to convection, possibly kicking off even
before noon.

Expect cooler temperatures on Saturday across the area as the ridge
retrogrades westward and cooler air is allowed down into the plains.
Look for another chance for showers and thunderstorms in the
afternoon hours with the next in a series of vort-maxes moves over
the top of the ridge. At least right now, the lapse rates look more
modest on Saturday, which may reduce the severe weather chances, but
cannot rule out some isolated severe.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
Issued at 316 AM MDT Thu Jul 18 2024

Overall, looking at cooler weather this weekend with afternoon highs
around 80 on Saturday and potentially dropping into the mid-70s for
Sunday. This will be all while much of the western CONUS is
expecting above average temperatures developing under the vast upper
level ridge, however much of our CWA will be positioned well east of
the ridge axis and more under the influence of the large scale
trough across the central and eastern CONUS allowing for cooler
weather. Daily chances for showers and thunderstorms continue with
multiple disturbances passing over and along the eastern periphery
of the ridge. Saturday and Sunday look to have the best chance for
more widespread coverage as stronger upper level support will be
present associated with the upper low over the central CONUS mixed
with post-frontal east/northeasterly flow up against the Laramie
Range.

Looking ahead to the middle of next week, a large scale trough
approaches the PacNW attempting to breakdown this stale upper level
pattern. Latest cluster analysis shows growing uncertainty with the
timing of the trough passage to the north. One cluster mainly
comprised of GEFS members shows a quicker trough progression across
the Canadian border that shifts the thermal ridge overhead while
other clusters more comprised of EC members are on the slower side.
Will need to continue to monitor model trends as this will impact
how quickly we return to above normal temperatures for the later
portions of next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 528 PM MDT Thu Jul 18 2024

Northwest flow aloft will continue.

Wyoming TAFS...Scattered to broken clouds from 7000 to
12000 feet will prevail. Occasional thunderstorms will occur at
Laramie and Cheyenne until 01Z, producing wind gusts to
35 knots and visibilities to 4 miles. Outside of thunderstorms,
winds will gust to 22 knots at all terminals until 02Z, and to
23 knots at Rawlins and Cheyenne after 15Z Friday.

Nebraska TAFS...Scattered to broken clouds from 8000 to
15000 feet will prevail. Winds will gust to 25 knots until 02Z,
except gusting to 24 knots at Scottsbluff until 08Z.

&&

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

&&

$$

SHORT TERM...MN
LONG TERM...BW
AVIATION...RUBIN