714
FXUS65 KCYS 302101
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
301 PM MDT Sun Jun 30 2024

.KEY MESSAGES...

- A few strong to severe thunderstorms this afternoon and
  evening.

- Cooler week ahead before strong ridge and toasty temperatures
  return for next weekend.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 300 PM MDT Sun Jun 30 2024

Current KCYS radar loop shows Scattered showers and widely
scattered thunderstorms developing across the mountains and
pushing east-northeast into the high valleys and high plains
along the I-25 corridor this afternoon. Thermodynamic profiles
and current mesoscale analysis are not nearly as impressive
compared to what models were showing yesterday. Although 0-6km
shear is currently around 40 to 45 knots, MLCAPE is still around
500 to 800 j/kg this afternoon with the best 1-6km lapse rates
far north of I-80. Overall, dynamic forcing is limited and
nearly nonexistent at this time with the only notable upper
level shortwave lifting off to the northeast earlier today. With
all this said, low level moisture is still impressive today
with dewpoints in the 50s to low 60s, CIN has been slowly
eroding early this afternoon, and a frontal boundary is located
just west of the I-25 corridor, providing some low level lift
and convergence. Therefore, didn`t change the forecast all that
much with scattered showers and thunderstorms forecast through
late this evening...gradually pushing east into western
Nebraska. Expect a few of these storms to become strong to
marginally severe. Strong gusty winds appear the most likely
mode of severe weather into this evening. Expect some of the
activity to linger after sunset, and possibly after midnight, as
the Pacific upper level trough slides east with increasing jet
energy aloft.

For Monday and Tuesday, models remain in good agreement with the
next Pacific cold front, and associated broad upper level
trough, rapidly progressing eastward across the northern Front
Range Monday and Monday night. Most of Monday should be a pretty
nice day (outside of the wind) with highs in the 80s...to
around 90 for areas below 5000 feet. It should start out pretty
mild due to persistent cloud cover and low temperatures likely
in the mid 50s to mid 60s...possibly as high as 70 degrees
across the northern Nebraska Panhandle due to the persistent low
level jet. Winds are forecast to shift into the west and
increasing through the afternoon with gusts as high as 50 MPH
possible. Not expecting gusts over 58 MPH at this time, but the
last few wind events were a last minute surprise...so will
continue to monitor these trends for now. These winds will also
start bringing in some much drier air with a dryline-like
feature develop just east of the Laramie Range and progressing
rapidly eastward into western Nebraska through early to mid
afternoon. This should greatly limit convection across the area,
even with better forcing compared to Sunday. However, this
drier air will have a hard time moving into east central Wyoming
and the northern Nebraska Panhandle as a surface low develops
and keeps winds relatively light or light out of the northeast
for places such as Chadron, Harrison, and Lusk. This generally
area (east central Wyoming and the northern Nebraska panhandle)
will have the best chance to see severe thunderstorms Monday
afternoon where SPC continues the Slight Risk for Severe
Thunderstorms on Day 2. Coverage of thunderstorms will decrease
even further Tuesday as the much drier airmass enters the
forecast area with dewpoints dropping into the 20s and 30s with
west to northwest winds. Isolated thunderstorms are still
possible across western Nebraska and over east central Wyoming
as the trough axis and upper level vort max slowly moves
eastward over central Wyoming where surface dewpoints will still
be in the 40s through the afternoon. It will be slightly cooler
on Tuesday with highs in the mid 70s to low 80s.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 116 PM MDT Sun Jun 30 2024

The overall weather pattern surrounding the Independence Day holiday
looks generally cool-ish and dry-ish for early July. On the synoptic
scale, this will be driven by the strong ridge aloft developing near
or just offshore of the west coast this week. The resulting
northwest flow pattern over the northern Rockies/northern Plains
will suppress the monsoon moisture back to its core region in Mexico
and southern AZ/NM, while frequent shortwave troughs swing through
the area keeping the brunt of the summer heat at bay.

On Wednesday, our area will be in between shortwave troughs. WSW
flow aloft ahead of the second trough will support decent warm air
advection across the area, pushing highs near to a few degrees above
normal. Model guidance is still quite consistent in showing a sharp
dryline developing somewhere across our area, but there its exact
position ranges from the eastern border of the forecast area to
just west of the WY/NE border. Expect some thunderstorm development
in the afternoon along the dryline, an there will be the potential
to become strong to severe. The main uncertainty remains dryline
position though, as a solution on the eastern end of guidance would
keep most of the storms and precipitation to our east, while the
western end of guidance would result in a more active day for our
area. The shortwave trough axis will push through around the
Wednesday night time frame, bringing with it a fairly strong surface
cold front. This is expected to lead to a cooler day on Thursday
(July 4th) across our area, with some gusty northwest to northerly
winds possible. Precipitation chances look more limited, but a few
showers and storms are still possible mainly over and near the
higher terrain, and across the northern portion of the forecast
area.

The trough starts to move out of the area on Friday, which should
leave a pretty nice day in place across the area. Temperatures
should remain a few degrees cooler than normal with lighter winds.
There will still be a chance for afternoon/evening shower activity,
but it looks quite limited with precipitable water below normal for
this time of year. Temperatures will warm back up to near or
slightly above average for Saturday. While model guidance is still
somewhat split for next weekend, more members have been coming on
board with continuing to delay the return of the heat. Rather,
recent trends have been towards another shortwave trough coming
through around the Sunday time period, which would continue to keep
temperatures on the cool side. Given the lead time though, there is
more uncertainty for the weekend temperatures and precipitation
chicanes.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1148 AM MDT Sun Jun 30 2024

The main aviation concern today will be scattered showers and
thunderstorms currently developing in Wyoming. Multiple rounds
of showers/storms will be possible this afternoon, spreading
into Nebraska in the late afternoon and evening. Storms will
probably struggle after moving east of the WY/NE border, so
confidence is lower that they will make it to KCDR, KAIA, and
KSNY. Where confidence is higher, added TEMPO groups for VIS
reductions and gusty/erratic winds during the most likely time
period for impacts. Storms may also bring hail. Activity should
last well into the evening hours, and possibly after midnight in
the NE panhandle and near KRWL.

Low clouds may creep into the eastern panhandle late tonight
into early Monday morning. MVFR to IFR CIGs are possible for the
High Plains terminals, but confidence is highest for this
occurring at KAIA and KSNY.

&&

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

&&

$$

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