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000
FXUS65 KCYS 130958 RRA
AFDCYS

Area Forecast Discussion...DELAYED
National Weather Service Cheyenne WY
358 AM MDT Sat Apr 13 2024

.KEY MESSAGES...

- Above normal temperatures expected again today and Sunday.

- Elevated to locally critical fire weather conditions both
  Saturday and Sunday afternoons. Red Flag Warnings and Fire
  Weather Watches are in effect.

- The next storm system will arrive Monday afternoon and bring
  showers and thunderstorms to the valleys and snow to the
  mountains. Snow may also mix in for the higher valleys and
  plains of Southeast Wyoming Tuesday morning.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 400 AM MDT Sat Apr 13 2024

It`s a pleasant spring morning across the area today as we open up a
warm weekend. GOES water vapor imagery shows some mid to high level
cloud cover and increased moisture moving into the area from the
southwest this morning as a weak shortwave approaches. This feature
will pass over the area this morning, but won`t have much of an
impact aside from the cloud cover and possibly a few showers/rumbles
of thunder this afternoon. Additionally, the flow will turn slightly
more WNW in its wake, leading to some breezy conditions this
afternoon. Combined with marginally critical RH expected, this lead
to the issuance of a Red Flag Warning for portions of SE Wyoming.
Expect highs fairly similar to yesterday, reaching the upper 60s
west of the Laramie range and 70s to the east. There is a better
chance of cracking 80F in the Nebraska panhandle this afternoon.

Heading into Sunday, the next weather maker will start to approach
the area. This feature is a vigorous closed low currently visible on
satellite approaching the California coast. As it moves inland, it
will amplify the downstream ridge over our area, leading to more
southerly flow and stronger warm air advection over the area. Thus,
looking at the warmest temperatures of the warm spell and the best
chance at 80F for the Nebraska panhandle (and Torrington/Wheatland)
if this isn`t met today.

The recent trend towards a later arrival of the closed low has
resulted in higher confidence in a warm forecast again for Monday.
While increased clouds and the arrival of the cold front will knock
temperatures down by 10-15F west of the Laramie range, areas to the
east will still be quite mild, particularly in Nebraska. Recent
guidance has 700-mb temperatures still around +8C in Nebraska even
as cooler air moves in from the west. Beyond the temperatures
though, Monday looks like a fairly complicated setup across the
area. As the upper level low swings across the Rockies, diffluent
flow over the lee will support surface cyclogenesis in eastern
Colorado through the day Monday. This will increased low level winds
over the High Plains.

The surface cyclone will really take off once the cold air is
wrapped in sometime Monday afternoon or evening. As this front moves
across the area, expect fairly potent frontogenesis to set up,
likely somewhere in the vicinity of the Laramie range. This will
kick off shower activity Monday afternoon that will carry into the
overnight hours. Additionally, cold air advection aloft will precede
the front, leading to rather steep lapse rates Monday, and thus some
convective instability. Deep shear profiles may be enough to lead to
a few strong storms. In fact, the latest SPC guidance has pulled a
Marginal risk for severe thunderstorms into our far northeast zones.
Initially, snow levels will be quite high, around 8500 feet, leading
to rain for most populated areas to start. However, crashing
temperatures behind the front will begin to drop snow levels. Most
ensembles have at least snow mixing in for Laramie, but are more
split on accumulation. About 25% of ensemble members are cold enough
to drop snow levels to Cheyenne and Rawlins also. Below about 6000
feet, the chance of snow is pretty low with this round. There is
also still some uncertainty in precipitation totals. Guidance has
consolidated around a solution that brings in a good lift window
overnight, with isentropic lift in northerly flow behind the low.
This should impact areas along and west of the Laramie range, but
there is uncertainty on the peripheries (Rawlins on the west side,
and Cheyenne eastward on the east side). The last piece of the
system to discuss will be the wind. Behind the departing surface
cyclone, we`ll have another round of strong NW 700-mb winds. The
main upper level low will traverse to our south over Kansas, which
should lead to a more of a northerly component than last weekend`s
strong wind event. The window is also fairly short, but ensemble
median 700-mb winds exceed 50 knots near KCYS around 12z Tuesday.
While GFS based in-house guidance has fairly low probabilities for
High Wind, it`s worth noting that overall parameters on the
deterministic GFS are quite a bit lower than ensemble consensus, and
other deterministic models. While we`re too far out now, will need
to watch the next several model cycles for the potential for high
wind headlines near the I-80 summit and in Laramie county, plus
winter headlines possibly for the mountains, I-80 summit, and
Laramie Valley.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Issued at 400 AM MDT Sat Apr 13 2024

Quick transition day expected on Tuesday as the region sits under a
weak ridge between a closed low pressure storm system to the east
and elongated long wave trough diving south across central Montanta.
Much colder temperatures and unsettled weather return to the
forecast beginning Wednesday but trending cooler still with the
arrival of a strong cold front on Thursday.

As mentioned above most locations east of the Laramie Range and much
of the Nebraska Panhandle will be dealing with some wrap around
moisture associated with a deepening low pressure system progged to
be sitting across central Nebraska come Tuesday. Some light snow
showers will be possible across the Laramie Range before tapering
off by Tuesday evening. As this system moves east and the next
elongated longwave trough begins to move out of the Pacific NW, a
weak, flat ridge will briefly build across the region and bring
mostly quiet conditions, but as mentioned, the next system will be
moving into western Wyoming by Tuesday evening.

Models are still divergent on handling this longwave trough and what
potential impacts this next storm system may bring to the region.
Tend to favor the GFS with these mid April storm systems, which has
this elongated system stretching form central Montana and the upper
level closed low in eastern Canada. This system is progged to drag a
strong cold front out of Canada and into the northern high plains
Wednesday night into Thursday. This cold front will come racing
south across our area throughout the day on Thursday. 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.
Forecast approach was to trend down both Max and Min temperatures
below NBM guidance by about 2-3 degrees across the board.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1033 PM MDT Fri Apr 12 2024

VFR for TAFs through the forecast period. Winds will be mostly
VRB overnight for terminals, before rapidly picking up midday Saturday
morning. Wind gusts of 20-30 knots will be expected after 16z
for all terminals.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Red Flag Warning from noon today to 7 PM MDT this evening for
     WYZ418-432.
     Fire Weather Watch from Sunday afternoon through Sunday
     evening for WYZ430-431.
NE...None.

&&

$$

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