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000
FXUS65 KCYS 120949
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
349 AM MDT Fri Apr 12 2024

.KEY MESSAGES...

- Above normal temperatures expected across much of the region
  Friday and through the weekend. Afternoon highs in the upper
  60s and 70s across the area.

- Elevated to locally critical fire weather conditions possible
  in the afternoon Friday through Sunday.

- Potential for a storm system to impact the region next week,
  but confidence on any expected impacts are low at this time.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 310 AM MDT Fri Apr 12 2024

Warm, above average temperatures are expected Friday through
this weekend across southeast WY and western NE. Latest GOES WV
imagery shows an upper level ridge across the western CONUS that
will continue to slide east today. Looking at afternoon highs in
the upper 60s and 70s across the area potentially reaching the
low 80s in portions of the North Platte River valley, especially
Saturday and Sunday.

A weak shortwave currently approaching the Baja will eject
towards the four corners later today ahead of a larger scale
upper level low off the Pacific Coast. This will lead to light shower
development late this afternoon over much of the central
Rockies, mainly confined to the higher terrain. Little to no
precipitation is expected. However, deterministic models have
been a little stronger with this shortwaves progression closer
to our area Saturday potential leading to increasing cloud cover
through midday Saturday. This could limit daytime heating
dropping temperatures a few degrees from the current forecast,
but above average temperatures are still expected with
climatological 90th percentile 700mb temps. Additionally, height
gradients along the Laramie Range tighten as this wave
progresses to the east, so could see elevated winds gusting up
to 40 mph Saturday morning.

With these above average temperatures, there are elevated fire
weather concerns through the weekend. Overall, parameters are
marginal but we could see afternoon RHs approach 15 percent this
afternoon in western Carbon Co with winds gusting up to 25 mph
out of the southwest. For Saturday, strongest winds are more
likely earlier in the day in addition to morning cloud cover
limiting afternoon RHs from dropping closer to critical
thresholds. Sunday could pose the greatest threat for critical
fire weather conditions with stronger flow aloft arriving just
ahead of the next upper level low slowly moving across the
western CONUS.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
Issued at 310 AM MDT Fri Apr 12 2024

A series of troughs will move through the area next week,
breaking the warm spell and ushering in cooler and more
unsettled weather. Monday looks like the last day of warmth as a
potent closed low traverses across the southwest. Expect
increasing cloud cover through the day, but with good agreement
on 700-mb temperatures still hovering around +4 to +6C we should
have another day of above normal temperatures. Expect to lose
about 5-10F off of Sunday`s highs though. As diffluent flow
aloft moves over the eastern slopes of the Rockies, this will
support surface cyclogenesis over Colorado through the day. This
will lead to increasing winds over our area, but the exact
direction will depend on exactly where the low ends up
developing. Things get more active later in the day as the
surface low wraps in the cold front moving across from the west.
Fairly substantial frontogenesis in a modestly unstable
atmosphere will kick off partially convective showers and
possibly thunderstorms Monday afternoon. With temperatures
staying quite warm and little to no access to fresh cold air
(this will be trapped behind the northern branch trough that
will lag behind this initial disturbance), expect fairly high
snow levels for this time of year. While the mountains should be
all snow, even valleys west of the Laramie range will be warm
enough to see rain Monday afternoon to start, but mixing with
snow as cooler air works in behind the front. Guidance continues
to waffle back and forth on QPF with this first round of
precipitation. The latest suite of ensembles has increased
slowed down the northern branch trough, which increases
separation between that and the ejecting closed low. The result
is slightly better wrap-around moisture and higher QPF Monday PM
and overnight. We are still watching the potential for strong
winds Monday night into Tuesday, mainly for the I-80 summit
eastward along the Cheyenne ridge in the northwest flow behind
the departing trough. Ensemble median 700-mb winds are now
around 50-knots over Cheyenne Tuesday morning, increasing
confidence in gusty winds. However, this looks like a more
marginal wind event especially compared to the winds last
weekend.

While a slower, weaker, and further north northern branch trough
would lead to increased precipitation in the first round, the flip
side is reduced precipitation in the second, colder round mid week.
The ECMWF ensemble and GEFS have swapped places over the last 24
hours, with the former now showing the drier, more progressive
solution, and the latter stalling the trough close enough to allow
for a series of vorticity maximums traversing over a stalled frontal
boundary. The latter solution would lead to several days with
precipitation chances in place of much lower snow levels. The former
solution would have chances for light snow around the area, but most
likely very little appreciable precipitation. Trended both PoPs and
QPF downward throughout the long term period by blending towards the
NBM 50th percentile. Ensemble spread in temperature for the
Wednesday to Friday period is quite large. While median 700-mb
temperatures plummet to -8 to -10C for Thursday and Friday, about 15
to 25% of members still have values closer to 0C. There is also
potential for something considerably colder, with a few members
pulling in 700-mb temperatures as low as -18C into our northern row
of counties. Overall, the main takeaway is confidence is high that
temperatures will be much cooler than the weekend, but how much
cooler is still uncertain.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 904 PM MDT Thu Apr 11 2024

A ridge aloft will build across the terminals overnight and on
Friday, with the flow aloft turning from northwest to west.

Scattered clouds near 15000 feet will occur. Winds will gust
from 20 to 27 knots from 15Z to 00Z at the Wyoming terminals.

&&

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

&&

$$

SHORT TERM...MB
LONG TERM...MN
AVIATION...RUBIN