Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 141138
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
538 AM MDT Sun Apr 14 2024

.KEY MESSAGES...

- Above normal temperatures expected again today with elevated
  to near critical fire weather conditions this afternoon.

- The next storm system will arrive Monday afternoon and bring
  showers and thunderstorms followed by strong winds to portions
  of the area. Accumulating snow is expected in the mountains,
  and snow may mix in for the higher valleys and plains of
  southeast Wyoming Monday night into Tuesday.

- Strong cold front associated with a large system across
  central Canada will bring much colder temperatures and the
  chance for valley snows Wednesday into Friday.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 440 AM MDT Sun Apr 14 2024

We have about 36 hours left of this warm spell before it comes to an
abrupt end Monday afternoon. Before that though, we are on track for
a beautiful day Sunday across the area. Satellite imagery shows
mainly clear skies aside from a few midlevel clouds filtering
through. Expect plenty of sunshine today with another round of mild
temperatures. To our west, the next weather maker is visible on
satellite as a closed low centered near San Francisco. As this digs
inland over the course of the day, the downstream ridge located over
our area will amplify slightly and shift eastward. Surface winds
will turn back southerly over the plains, and southwesterly west of
the Laramie range with warm air advection. 700-mb temperatures
around +8C this afternoon will support another day of very warm
temperatures. Expect temperatures to be similar to yesterday, except
likely a few degrees warmer for the southern portion of the forecast
area that saw more cloud cover yesterday. As the closed low inches
closer, models show pressure falls in the immediate lee of the
mountains, with a surface trough developing from roughly Sheridan to
Denver. With a surface high over the upper Midwest, this will set
the stage for a SSE low-level jet over the Nebraska panhandle and
far eastern Wyoming tonight into early Monday. Gusts of 40 to 50 MPH
could be possible, particularly along the Pine Ridge.

Monday will be a dynamic day as the storm system finally arrives.
The day will start off like the last few, with mostly sunny skies
and a quick warm up. However, cold air advection encroaching on
areas west of the Laramie range should mean highs about 10-15F
cooler than Sunday. Areas further east will still be solidly in the
warm sector, so our far eastern counties will have another chance at
80F. Southerly gusty winds may also lead to near critical fire
weather conditions in the southern panhandle. The trough axis of the
upper level low is expected to reach our area in the early
afternoon, which should lead to a quick increase in shower
coverage after about 2PM. Strong frontogenesis should lead to
precipitation along the leading edge of the front. Cold air
advection aloft will precede the low-level front, leading to
very steep lapse rates over the High Plains Monday afternoon.
Veering wind profiles will also create some deep layer shear,
and could support a few strong thunderstorms. The uncertainty at
this time is moisture availability. The drier HRRR shows
essentially nothing developing over the High Plains, while the
wet-biased 3km NAM has a defined squall line pushing through.
The reality will probably be somewhere in between this, with a
line of showers and thunderstorms developing along and ahead of
the front, with the potential to produce mainly gusty winds, but
also possibly some hail. One challenge will be differentiating
thunderstorm winds from the non-thunderstorm post frontal winds.
The front will clear the mountains and get wrapped up into the
surface low just as it rapidly deepens and slides northward. If
the low center manages to get north of I-80, we`ll have the
strong descending motion, leading to a potent shot of strong
winds quickly moving across the Plains. This has the potential
to easily clear 60+ MPH. Additionally, this system will again
feature a flow reversal aloft with a deeper layer of easterly
flow between about 250 and 500-mb. While the low-level flow is
much weaker than last weekend`s system, this will have the
potential to produce significant mountain wave breaking and low-
level amplification of the winds as mountain waves are reflected
by the flow reversal towards the ground. There may then be a
lull in the winds late Monday evening into the overnight, before
the northwest flow behind the low kicks in Tuesday and lead to
another round of strong winds. For now, decided to issue a High
Wind Watch for the areas where confidence in hitting 60+ MPH for
both of these time periods is highest, which includes the I-80
summit and foothills. Will continue to monitor for potential
expansion eastward, both for the initial round of post-frontal
winds, and then for the NNW wrap- around flow during the day on
Tuesday.

While the initial post-frontal air-mass is expected to promote
strong descent and winds, this system is expected to have a
narrow area of wrap-around lift on its backside. This should
push back into the area from the north late Monday evening,
filling precipitation back in mainly along and west of the
Laramie range. Isentropic lift and local upslope flow should
lead to a decent window for precip across these areas Monday
night into early Tuesday. The main uncertainty is how far east
this area of lift will stretch. There is fairly good confidence
that Albany county will be favored by this system, but more
uncertainty for the Rawlins area, and for the I-25 corridor on
the other side. Continue to think the NBM is a tad aggressive
for areas east of the Laramie range considering the fairly
strong westerly component to the low level winds. Overcoming
that would take very strong dynamic lift, which is not
consistently shown on guidance east of the Laramie range. In
terms of precipitation type, this will be a fairly warm system
for this time of year. Snow levels will initially be quite high,
mostly above all valley floors, but will crash quickly late
Monday afternoon into the evening. Now thinking a period of snow
is likely in Laramie, but accumulation is still up in the air.
These transitions tend to happen faster than modeled, so current
forecast blends towards the NBM 10th percentile for snow
levels. Since northwest flow is a favorable upslope direction
for Laramie, there is a pretty high ceiling for this event and
we will need to continue to monitor it closely. Snow impacts are
also expected over the I-80 summit and for the Arlington/Elk
Mountain area, but temperatures should be too warm for much
impact below about 6500 feet.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
Issued at 440 AM MDT Sun Apr 14 2024

Unsettled and cooler weather looks to impact the region through much
of the long term forecast period with breezy conditions possible
Tuesday night.

In the wake of the surface low pushing east of our region, a broad
trough will drop down from Canada over most of northern CONUS. This
will lead to continued windy conditions over much of the CWA as
pressure gradients tighten ahead of a cold front. Probabilities from
in-house guidance are increasing for the potential for an elevated
to marginal high wind event to occur primarily over the wind prone
areas. As we head into Wednesday, a strong cold front pushes through
the regions and will bring with it much colder temperatures with the
threat of for some precipitation ahead and along the main frontal
boundary. Models have been consistent from run to run with pushing
in 700 mb temperatures between -10C to -13C. The strong CAA will
allow for any precipitation that does occur to fall as snow across
the entire region.

Cold NW flow is expected to advect some strong CAA with 700mb
temperatures crashing into the -11 to -14C range Thursday and we
stay under this cold regime into Friday. Moisture will be ample
enough to initiate some upslope/orographic snow showers across the
higher terrain of the Laramie Range the Snowy Mountains. Some of
this moisture will also be pooled along the frontal boundary so some
precipitation will be possible along and ahead of the frontal
boundary. The post frontal setup will remain in place as persistent
cold northwesterly flow will continue into the first half of next
weekend with the potential of pinched of shortwave moving through
the region that could kick-start another round of light snow showers
across the area. Confidence in valley snow accumulations remain low
at this time. The highest confidence aspect of the forecast will be
the much colder temperatures expected across the region beginning
Wednesday night with temperatures expected to stay at below average
throughout the rest off the week before we begin to modify slightly
for next weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 531 AM MDT Sun Apr 14 2024

VFR conditions are expected across all terminals through the
valid TAF period. The main aviation concern will be the gusty
westerly winds that develop after 00z in which wind gusts could
approach 25 knots. There is an additional concern for some low
level wind shear issues across the Nebraska terminals between
09-12z.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from Monday afternoon through Tuesday
     afternoon for WYZ116-117.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...AW
AVIATION...AW


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