Leo, Wyoming Weather Forecast Discussion

289
FXUS65 KCYS 161725
AFDCYS
Area Forecast Discussion
National Weather Service Cheyenne WY
1125 AM MDT Tue Sep 16 2025
.KEY MESSAGES...
- Another cold front and potent Pacific storm system will push
  across Wyoming Tuesday and Wednesday, resulting in cooler
  temperatures, scattered thunderstorms, and more widespread
  rainfall through the end of the week.
- Temperatures are expected to warm Thursday through the
  weekend with some minor chances for precipitation during this
  time.
&&
.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 310 AM MDT Tue Sep 16 2025
Unsettled and active weather expected through Wednesday, but
also quite a bit of uncertainty remains with forecast
precipitation amounts and precip coverage. Pacific upper level
trough, which is currently slowly moving into western Wyoming
early this morning, is forecast to drift eastward over eastern
Wyoming and western Nebraska over the next 24 to 36 hours. Yes,
it is expected to take that long for the system to cross the
Continental Divide...setting up a prolonged periods of cloudy
skies, periods of rain, and a good chance for thunderstorms this
afternoon and into this evening. For today, as the upper level
trough slowly moves east, a potent disturbance is expected to
track east and then northeast around the base of the trough with
substantial upper level diffluence at jet stream level and some
low level convergence along the passing cold front. With a
pretty dynamic environment today, expect multiple lines of
thunderstorms to develop...with even some developing in the
morning hours based on most high res CAM guidance. I-25 corridor
may have a bit of a lull and could be the last area to see
thunderstorm development today as the disturbance aloft will
takes it`s sweet time moving northeast over the Front Range
later today. Everywhere else could see several lines of
thunderstorms pushing across the area with gusty winds and some
hail. The most favorable area for severe weather will be where
high boundary layer instability and forcing are
colocated...which at this time appears to be the southern and
central Nebraska panhandle. Agree with Slight Risk area proposed
by SPC. Large hail, strong damaging winds, and heavy rainfall
are the most likely hazards today. However, can`t rule out an
isolated tornado or two for Cheyenne County & Morrill County
and vicinity through the day today. High res guidance shows
decent 0-1km helicity in this region with sufficient veering
winds in the lowest levels of the boundary layer. This shows up
well in the Sidney hodograph, so will need to monitor this area
through this afternoon. Given the time of the year, won`t
messaging this threat too aggressively at this time.
What happens after this evening in terms of precip coverage and
evolution with the upper level system is resulting in lower than
average confidence tonight and Wednesday. Synoptic models favor
the upper level trough closing off and forming a midlevel low
across far northeast Colorado and southern Nebraska by late
tonight. High resolution guidance is struggling with the
placement and evolution of this system, and therefore struggling
with precip coverage, rainfall rates, and thunderstorm coverage.
The ECMWF continues to be the most aggressive, showing a 575
closed low forming across far northeast Colorado on Wednesday
with plentiful wrap-around moisture on the back side of the low
into most of southeast Wyoming and western Nebraska. The GFS and
Canadian are similar, but a bit further north with the placement
of the developing low. The NAM and several of the high res
models (including the HRRR) show the upper level low forming
well to the north into the Dakotas...leaving most of the
forecast area in the drier westerly flow south of the center of
the upper level low. Therefore, not totally confident about the
precip forecast and especially the rainfall amounts/rainfall
rates with this storm system through Thursday. For now, kept the
previous forecast and blended with the grand ensemble 25th to
75th percentile spread. Wednesday still looks like the coolest
day with highs possibly not reaching the low 60s for portions of
southeast Wyoming. Another round of snow is likely above 9500
feet as 700mb temperatures lower to 1c to 3c late Tuesday and
through Wednesday. Snowlevel do not appear as low as yesterday,
but expect dynamic cooling to be a factor behind the main storm
system through Wednesday. Can`t rule out the I-80 Summit seeing
their first snow flakes of the season, but no accumulation is
expected at this time. Although some thunder is expected on
Wednesday, it will be far more stable and much cooler. Lows
Wednesday night will drop into the 30s to mid 40s for most of
southeast Wyoming...with upper 20s to mid 30s for the
mountains.
&&
.LONG TERM /THURSDAY THROUGH MONDAY/...
Issued at 325 AM MDT Tue Sep 16 2025
Towards the end of the work week, a stagnant upper-level low will
slowly spin over the north central part of the country, impacting
the CWA with minor precipitation chances. By Thursday, this low will
start pushing its way eastward, leaving precipitation from this
system lingering over the CWA. Instability across the area will be
weak, suggesting stratiform-like precipitation with perhaps a few
embedded rumbles of thunder. With the upper-level low slowly
beginning to move out, Thursday will also mark the start of a warm
up as warmer 700 mb temperatures are ushered in. High temperatures
on Thursday will be about 5 degrees warmer than Wednesday, with
highs in the 60s and low 70s. Temperatures will continue warming
into the day Friday, with mostly sunny skies helping facilitate
highs in the 70s across most of the CWA.
Heading into the weekend, warming continues with high temperatures
on both Saturday and Sunday making it into the 70s and 80s. A weak
shortwave will make its way across the northern tier of the country
over the weekend, potentially providing enough lift and a bit of
moisture for showers and thunderstorms to develop. Any storms that
do develop look to be mainly confined to the high terrain with
minimal impacts. Aside from precipitation chances over the weekend,
another weak disturbance moving directly across the CWA could
potentially spark some strong to marginally high winds Sunday
afternoon. 700 mb winds over the usual southeast Wyoming wind prones
will reach 50 kts with decent subsidence. Gradients are elevated
which will lead to the increase in wind speeds, but continued model
runs will be needed to assess whether or not high winds will be
possible. Despite this, a windy day is still expected on Sunday.
Headed into the beginning of the work week, weak ridging/zonal flow
will lead to a mild and dry pattern. Mostly sunny skies and above
average temperatures can be expected for Monday and Tuesday.
&&
.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1116 AM MDT Tue Sep 16 2025
We have a complex aviation forecast with multiple hazards over
the current TAF period as a strong but very slow moving storm
system inches its way over our area through Wednesday. The first
hazard will be the line of showers and thunderstorms moving
through the Nebraska panhandle late this morning. Expect a few
more hours with nearby lightning at NE panhandle terminals. VIS
drops and gusty winds are likely at KAIA and KSNY. KCDR is
behind the main line, but another round is possible. The first
cluster of storms should depart by around 21z.
Then, attention shifts back to the west where another cluster of
showers and thunderstorms is expected to develop. This has the
potential to bring thunderstorm impacts to all terminals between
20z and 04z or so. Confidence in exact timing for an individual
terminal is fairly low. There is also a potential that this
second round fails to materialize due to some stability and dry
air present.
Lastly, the upper level low pressure center will shift to our
east overnight, allowing moisture to wrap around into the area
into Wednesday morning. In addition to shower activity
continuing across the area, low CIGs are possible at KLAR and
all High Plains terminals. However, confidence is again quite
low due to major discrepancies between model systems.
&&
.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&
$$
SHORT TERM...TJT
LONG TERM...MAC
AVIATION...MN

NWS CYS Office Area Forecast Discussion