019
FXUS65 KCYS 211716
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1116 AM MDT Sat Sep 21 2024

.KEY MESSAGES...

- Well below normal temperatures expected this weekend as we see
  the autumnal equinox occur, with some areas seeing a higher
  chance of rain showers along the I-80 corridor.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 318 AM MDT Sat Sep 21 2024

Water vapor satellite imagery from the GOES-16 Channel 9 spectrum
depicts the surface cold front quickly advancing southward towards
the CO/WY state border as of 9Z this morning. The cold front aloft
is still somewhat staggered in the lower boundary layer, as surface
temperatures are slow to respond to the wind shift behind the FROPA.
However, surface winds are responding much more quickly as wind
gusts of 35-45mph along and behind the surface frontal boundary are
being realized on mesonets across the cwa. Hi-res model guidance
continues to ping the cold front being through the cwa southern
border by 12-13Z today, with gusty winds behind it. These winds will
subside in the subsequent hours after daybreak, with the majority of
the strongest winds further south by 18Z today. 700mb temperatures
are progged to be at approximately +5C to +10C at 12Z this morning,
and adiabatically cool aloft from the incoming push of colder air
advection to 0C to +5C by 0Z this evening. The dry cool front will
not bring any rain showers, but we will see daytime highs on the
order of 15-25 degrees cooler outdoors compared to yesterday`s high
temperature. The lower elevations will struggle to reach the 60s for
a majority of the day, with the North Platte River Valley in western
NE seeing the highest likelihood to eclipse the 70 degree mark later
today. Areas in the highest elevations will remain in the mid 40s to
mid 50s, despite there being sunshine for the first half of the day.

By this afternoon and evening, a quickly advancing upper level low
(ULL) will eject out of the Four Corners toward the Central Rockies.
Lee cyclogenesis on the eastern flank of the Continental
Divide/Front Range of CO will support a surface low intensification.
Pressure tendencies will fall, and a slightly increased pressure
gradient will cause our mountain zones/gap areas to see slightly
elevated wind gusts post-frontal passage. However, the most recent
model data supports a further south trajectory of the surface low,
and its deformation zone/precipitation TROWAL. What this means for
our region is that the chances for rain showers will decrease. It is
a good example of continually monitoring weather trends as the
northern gradient of precipitation will have a steep cutoff, leaving
the majority of our cwa dry. Nonetheless, we will have a 30-40%
chance of rain showers along the I-80 corridor from the Summit to
Sidney early Sunday morning. The pesky cloud cover, upslope flow,
and chance of rain showers along this area will keep temperatures
well below average. A light dusting of snowfall in elevations above
10,000 feet for Sunday morning remains for the Snowy/Sierra Madre
Mtns. Daytime highs closest to the WY/NE/CO border will struggle to
achieve a mid-50s F, but areas further to the north where clearing
skies will become more likely towards late Sunday should see the
highest likelihood of mid to upper 60s.

The further south trajectory of the surface low will cause any
lingering cloud cover to nudge out of our cwa by 6Z Monday for the
most part. The surface high pressure that is progged at 1020mb by 6Z
Monday will scoot towards the Central Plains overnight, leaving
mostly clear skies in its wake. Lee-surface troughing is favored by
daybreak Monday, so there will be an uptick in wind gusts on the
foothills adjacent to the Central and South Laramie Range. This
should allow areas due east to stay in the upper 30s to near 40
degrees for an overnight low. Western NE will see the mostly clear
skies and lack of overnight mixing from winds, so morning lows are
likely to be much cooler. Have nudged the morning lows another
degree or two lower in the valley areas and far southern portions of
the Nebraska Panhandle. Low to mid 30s for those regions and valley
areas in southeast WY should be achieved, which will equate to some
patchy frost in isolated areas. Weak upper level ridging is likely
for Monday across our cwa, so we will see a nice uptick in daytime
highs compared to Sunday. Overall, expect MaxT`s to hover in the
mid to upper 70s for areas along and east of the Laramie Range, with
slowly cooler temperatures in the mid to upper 60s for locations
west of the Laramie Range.
|

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
Issued at 318 AM MDT Sat Sep 21 2024

Headed into the middle of next week, the upper level pattern changes
with 500mb height rises across much of the western CONUS as the
upper low dives to our east. However, model solutions quickly begin
to diverge with a portion of the upper low becoming cutoff from the
flow setting up a rex block over the Great Lakes and Ohio River
valley. This pattern will support above average temperatures across
the area with 700mb temps climbing 12-14C and mostly dry conditions
as any notable upper level systems lift over the Canadian border. At
this time, Thursday looks to be the warmest day with afternoon highs
in the mid-70s and 80s, but windy across the Laramie Range with a
shortwave passing to the north. In-house guidance is not suggesting
high winds, but could still see downsloping winds gusting up to 40
mph. The ridge over the western CONUS looks to stick around headed
into next weekend and may be difficult to breakdown. Latest cluster
analysis shows a solution of a stronger upper level trough digging
through the northern Rockies late in the weekend with the potential
return of precipitation and fall temperatures, but this is ~30% of
the Grand Ensemble (mainly Canadian and EC members).

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1103 AM MDT Sat Sep 21 2024

Latest observations have seen the cold front, which moved across
the zone in the early morning hours, has continued digging
southward into Colorado, leaving behind much cooler temperatures
and some gusty winds from the north. Remainder of the forecast
period will see primarily VFR to MVFR conditions prevailing for
all terminals. Some concerns for showery activity with the upper
level low pushing from the Four Corners to the High Plains.
Latest HiRes model guidance keeps much of the showers further
south into Colorado, but could see a few storms pushing a bit
further north into the most southern terminals, primarily after
23z. Otherwise, mostly VCSH is expected.

&&

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

&&

$$

SHORT TERM...BW
LONG TERM...MB
AVIATION...MRD