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000
FXUS65 KCYS 151132
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
531 AM MDT Mon Apr 15 2024

.KEY MESSAGES...

- Critical fire weather conditions across the southern Nebraska
  Panhandle today where Red Flag Warnings are in effect from
  noon to 7 PM for gusty winds and low humidity.

- A strong storm system arriving today will bring the potential
  for high winds, showers and thunderstorms (could be strong to
  severe in the northern NE panhandle), and mountain snow this
  afternoon through 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 NIGHT/...
Issued at 500 AM MDT Mon Apr 15 2024

An active weather day is anticipated across the area today as a
vigorous closed low approaches from the west. Cloud cover and
elevated moisture associated with this system are creeping into our
western zones already this morning. Much of the day is expected to
be deceptively quiet, but rapidly becoming more active in the late
afternoon through the evening. Southerly flow in advance of the low
is leading to a potent low level jet over far eastern WY and the
Nebraska panhandle. This is helping to usher in better surface
moisture with dewpoints climbing close to 40F and wind gusts of 40
to 45 MPH at times. Expect this to come down for a few hours this
morning, but then resume in later in the morning when mixing takes
over. Hazards this afternoon and evening include fire weather, high
winds, thunderstorms, and accumulating snow. We`ll break down the
upcoming system by hazard type.

FIRE WEATHER: The surface trough is currently located just east of
the mountains, roughly along I-25 from Sheridan to Denver. This is
also acting as the dryline today, with moisture held to the west. As
the surface cyclone over Colorado strengthens today, models show it
rapidly deepening and shifting northward across our NE panhandle
counties. Much drier air behind this process should drop RH to below
critical thresholds with gusty southerlies continuing. The highest
confidence in reaching critical thresholds is in the southern
panhandle, with more uncertainty further north depending on how
quickly dry air pushes northward. There is also a question in how
long the window for critical fire weather will be open. The strong
front pushing in from the west will arrive this evening to the Red
Flag Warning area and bring an abrupt wind shift to the west, but
also most likely a boost in humidity.

THUNDERSTORMS: On the flip side of the dryline/surface trough, we`ll
have a thunderstorm threat. Thus, the uncertainty in how far north
the fire weather concerns will extend also impact the thunderstorm
threat. Hi-res model guidance remains split on this afternoon and
evening`s convective evolution. The HRRR continues to show the dry
slot of the system flooding across most of the High Plains this
afternoon, leading to deep inverted-v soundings by the time the
favorable dynamic lift arrives. This scenario would likely lead to
fairly little thunderstorm activity, with the primary threat of any
storms being gusty outflow winds. However, another scenario involves
the moisture plume holding strong further south. In this case, shown
by the NAM, has sufficient surface moisture in place later into the
afternoon and evening, leading to more widespread convection. Such
an outcome could lead to a few storms becoming strong to severe, and
putting a hail threat back on the table with deep shear, dry-ish mid-
levels, and plenty of instability. Lapse rates will be very steep
regardless of low-level moisture outcome. Will need to monitor short-
range model guidance over the next few cycles, but for now we have
the threat for storms in the official forecast, but it`s worth
noting the fairly substantial bust potential in the case of the
storm system`s dry slot pushing northward faster. There will also be
another window for thunderstorms, though likely sub-severe, in the
wrap-around flow overnight tonight and into Tuesday morning.

HIGH WINDS: This is a fairly low confidence wind forecast today,
with numerous compounding uncertainties, largely stemming from the
expectation that the surface low track will be directly through our
area today. The initial wind threat will be immediately along and
behind the cold front as it passes through this afternoon and early
evening. It`s a little unclear if this will manifest as a
thunderstorm wind threat, or non-thunderstorm. Right now, leaning
that this will come with some showers, but likely not enough
moisture for actual thunder, though certainly can`t rule it out.
Strong descending motion in the wake of the front will lead to an
abrupt round of strong westerly winds, but the question is if and
how widespread 60+ MPH winds will be. Decided to upgrade the High
Wind Watch to a Warning for the I-80 summit and foothills to include
this period. Will need to watch this front, as the start time of the
other warnings may need to be adjusted to include the frontal
passage. Elsewhere, northwest flow takes over this evening and it
certainly looks like a windy night over the High Plains. This looks
quite marginal in terms of actually reaching high wind criteria, but
had enough confidence to upgrade the southeast Wyoming zones to a
Warning. Strong winds continue into the day Tuesday as NNW flow
aloft mixes down to the surface, leading to a short window of more
widespread gusts of 55 to 60 MPH. A fairly narrow band of 700-mb
winds of 50 to 65 knots is expected to translate across the area
from west to east between roughly 9AM and 3PM Tuesday, as the storm
system quickly ejects eastward. While the NE panhandle could hit
briefly behind the cold front, the best window is not until the
daytime hours Tuesday. Therefore, will let the watch continue
another cycle. Also decided to leave Converse county as a watch, due
to uncertainty in the positioning of the strongest winds. While in-
house guidance is pretty consistent showing little threat for the
wind prone areas of Arlington/Elk Mountain and Bordeaux (the best
forcing is along and east of the Laramie range inhibiting the
former, and the NNW wind direction is not favorable for the latter),
it`s always a gamble going into a wind event without warnings for
these zones, so will also need to keep a close eye on those areas
through the day.

SNOW: The last piece of this storm system will be the wintry threat.
While this will be a fairly warm system for this time of year, our
higher elevations are still expected to see accumulating snow. Rapid
cold air advection is expected to come in behind the cold front with
the initial round of precipitation mainly impacting the highest
mountains. The better window for snow will come late this evening,
after about 9PM, when the upper level low clears the Rockies and
allows better moisture and lift to wrap back into the area from the
north. Isentropic lift, orographic lift, and some frontogenesis will
be the primary mechanisms with this event. Deep NNW flow in this
period will be best for the Snowy Range, and we could see fairly
heavy snowfall rates during a short window late tonight into Tuesday
morning. While some models are likely overdone with this, even the
more conservative HRRR and HREF (which are favored in the official
forecast) reach warning criteria in the higher elevations of the
Snowy range. Therefore, decided to upgrade to a warning. The wind
direction in the wrap-around flow is not as favorable for the Sierra
Madre range, so went with an advisory there. Lastly, the southern
Laramie range is also high enough that the area can expect some
snow. Accumulations will be more modest, but there was enough
confidence to issue a Winter Weather Advisory for a few inches of
slushy snow late tonight into Tuesday. The strong winds are likely
to compound impacts. Still need to watch the Laramie valley for a
potential boom scenario, but the most likely outcome right now is a
coating to an inch of wet snow around the city of Laramie. The lift
dynamics shift eastward over the High Plains Tuesday morning, so
have higher PoPs over this area then, but snow levels are expected
to be much too high here. Rain totals may be spotty, but most should
see at least some light rain by Tuesday afternoon. QPF was also
trended downward slightly, towards the HRRR and HREF, due to the
somewhat inconsistent lift and unfavorable downslope component to
the surface winds. Don`t think the downslope will completely
overcome the lift aloft though this time.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 500 AM MDT Mon Apr 15 2024


Active pattern will bring daily precipitation chances and much
colder temperatures to SE Wyoming and the Nebraska Panhandle.

On Wednesday our region will be on the back side of the departing
weak ridge axis that formed between low pressure systems across Iowa
and the second diving south out of western Canada. Strengthening
jet energy is expected to develop at the base of this large storm
system that will begin to impact our region for several days. Ahead
of this storm system a stout zonal flow develops as this incoming
storm system begins to elongate across much of western and central
Canada and the main trough begins to flatten out. With strengthening
jet energy and good upper level divergence expecting a large of
precipitation to develop along the leading edges of the surface and
mid-level cold front. The initial surge of precipitation are being
handle moderately well amongst the deterministic models but
orientation of this large trough continues to be the main difference
which ultimately will determine how far south both the surface cold
front and the CAA will go. As this elongated now large open trough
moves south on Wednesday, increasing heights across the south
central plains begin to extend northward and where these collide
will determine where an expected stationary boundary will develop
come Wednesday night. The concern is with a stationary boundary in
place across the region we should see enough CAA surge into most of
the region to allow for the precipitation to fall as all snow across
much of the region. With multiple surges of for verdict maxima`s
progged to keep a favorable cyclonic vorticity advection ongoing to
support life we would see a small swath of some modest snowfall
accumulations especially near this boundary. Current indications
look like the south Laramie Range and adjacent Foothills could see
the best chances for snowfall as models show the stalled boundary
snake up and drape itself along the spine of the Laramie Range. This
feature will need be watched as this will greatly affect the
snowfall forecast for the I-80 corridor between the Summit and Pine
Bluffs.

The highest confidence from Thursday into the first half of next
weekend is that temperatures will be much colder with area wide
highs looking to remain in the 40s for several days with overnight
lows in the mid to upper 20s to low 30s. We look to finally begin a
brief warming trend toward Sunday and heading into next week as an
upper level ridge moves into the region, but on its heels is another
potential weather maker that could bring back chances of
precipitation and cooler temperatures to the region. Deterministic
models are not in good agreement in this next system so will just
keep to NBM for the day 7 period for now.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 531 AM MDT Mon Apr 15 2024

Light and variable winds will become southwesterly across the
terminals this afternoon before transitioning to gusty westerly
winds, in which wind gusts could approach 30 to 35 knots. There is
an additional concern for some low level wind shear issues across
the Nebraska terminals between 12-14z across the Nebraska terminals.
With the arrival of a frontal boundary and precipitation expect MVFR
conditions to develop across the terminals from west to east
beginning around 03z.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from late tonight through Tuesday afternoon
     for WYZ101.
     High Wind Warning from 9 PM this evening to 3 PM MDT Tuesday
     for WYZ107-108.
     Winter Weather Advisory from 3 PM this afternoon to 3 PM MDT
     Tuesday for WYZ112.
     Winter Storm Warning from 3 PM this afternoon to 3 PM MDT
     Tuesday for WYZ114.
     High Wind Warning from 3 PM this afternoon to 6 PM MDT Tuesday
     for WYZ116-117.
     Winter Weather Advisory from 9 PM this evening to 3 PM MDT
     Tuesday for WYZ116.
     High Wind Warning from 9 PM this evening to 6 PM MDT Tuesday
     for WYZ118-119.
NE...Red Flag Warning from noon today to 7 PM MDT this evening for
     NEZ436-437.
     High Wind Watch from Tuesday morning through Tuesday evening
     for NEZ019>021-054-055.

&&

$$

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