825
FXUS65 KCYS 211136
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
536 AM MDT Tue May 21 2024

.KEY MESSAGES...

- Much cooler temperatures with more widespread showers and
  thunderstorms today. Accumulating snow likely above 8000 feet,
  with a rain and snow mix possible down to 6000 feet late
  Tuesday night into early Wednesday morning.

- Patchy frost is expected on Wednesday morning due to
  temperatures hovering near or below the freezing mark.
  Recently planted vegetation may need to be covered to prevent
  any frost/freeze damage.

- Daily shower and thunderstorm chances continue through the
  long term. Could see some elevated winds in the wind prone
  regions throughout the week and weekend.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 330 AM MDT Tue May 21 2024

Chilly temperatures, scattered rain showers, and patchy dense
fog are present as of 9Z this morning across southeast WY and
western NE. Observations earlier this morning around midnight
had several locations reporting less than 1 mile for visibility
as upslope flow and the rain showers created an abundantly moist
column to contend with east of the Laramie Range. Went with a
Dense Fog Advisory headline to reflect this, with model guidance
showing clear signs that we should begin to erode the patchy,
dense fog by daybreak for most locations. However, the rain
showers and mostly cloudy conditions will be sticking around for
most of the day. The rain showers present on radar as of 9Z this
morning will slowly trek to the northeast as time goes on.

We will have a temporary break with respect to active weather
before the next shortwave disturbance arrives on the heels of
the previous weather disturbance, which will push out toward the
Central Plains and Great Lakes region by later today. There
will be a short timeframe where partly cloudy skies are present
in the Nebraska Panhandle, but the pesky cloud cover will stick
around for southeast Wyoming thanks in part to continual
impulses of cold air advection. There will be orographic
enhanced snow shower activity in the higher terrain today. Due
to some of the snow falling throughout the day, despite
temperatures being near or below freezing in the higher
elevations, have shaved off the total snowfall accumulations.
Nonetheless, the North Laramie Range, Snowy & Sierra Madre
mountains at elevations above 8000 feet will pick up light
snowfall accumulations between today and late tonight. Daytime
highs in the lower elevations will range from the middle 40s to
upper 50s, which is well below normal.

Scattered rain showers and isolated thunderstorms are
anticipated from this afternoon into the early evening hours.
However, severe weather isn`t anticipated compared to yesterday
where we had a few stronger thunderstorms that propagated
through the region. Rain/snow showers are anticipated between
elevations of 6500 to 8000 feet, but accumulations are not
anticipated. The shortwave disturbance will have propagated
through most of the cwa by 6Z Wednesday. Surface high pressure
is expected to quickly propagate toward central CO by 12Z
Wednesday. As this occurs, the nighttime radiation inversion and
clearing skies will cause temperatures to rapidly decrease. Have
nudged temperatures down a degree or two, especially for valley
areas across the cwa in southeast WY. Patchy frost is certainly
plausible for several areas, and there is concern for vegetation
to be covered because of overnight lows hovering near the
freezing mark. Have decided against any frost/freeze headlines
at this time due to the climatological average last
frost/freeze dates happening around this time of spring for
most of the high plains. If model guidance signals that our
temperatures decrease well below freezing for areas east of the
Laramie Range in the coming 24 hours, this decision may need
to be revisited. Locations west of the Laramie Range typically
have their last freeze in June, per our in-house climate
statistics.

We will have a quiet Wednesday, as seasonal temperatures make a
well-deserved return to the region. Mostly sunny skies and weak
upper level ridging will cause surface temperatures to rebound
nicely. Daytime highs will range from the lower 60s to middle
70s, with the warmest temperatures residing in the NE Panhandle.
Cooler temperatures will remain in the southeast WY mountain
zones due to increasing clouds by later in the afternoon. Breezy
winds will also make a return as we have an increasing surface pressure
gradient. 700mb winds will ramp up to 40-50 knots between 6Z
and 12Z Thursday. While we may not be looking at any high wind
headlines for Thursday morning, there could be a few wind gusts
of 50-55mph in the wind prone and gap regions of southeast WY.
The warmer temperatures look to stick around for the majority of
Thursday, especially for areas east of the Laramie Range because
of weak downslope air compression effects. Daytime highs in the
60s and 70s are anticipated, with cooler temperatures further
to the west as increasing clouds once again make a return ahead
of our next approaching weather disturbance by Thursday evening.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
Issued at 330 AM MDT Tue May 21 2024

Fairly strong shortwave trough moving through the CWA Thursday
evening. Cold front looks to move well south of the CWA with
gusty north to northwest winds behind the front Thursday
evening. GFS and ECMWF in good agreement with this feature along
with timing. Did increase PoPs slightly over NBM guidance for
areas east of the Laramie Range Thursday evening before drying
out after 06Z.

May see some light afternoon showers Friday as a weak shortwave
moves through. ECMWF painting some light QPF over our western
mountains Friday afternoon while GFS shows dry conditions. Did
opt to go for some scattered showers (30-40 percent) for Friday
afternoon for the I-80 Corridor from Sidney to Rawlins.

Fairly flat shortwave riding for Friday night and most of
Saturday before next low approaches from the west. GFS and ECMWF
both showing rain developing over our western mountains Saturday
morning...spreading east by the afternoon.

Upper low tracks across the northern CWA Sunday with showers
continuing through the day. going to be quite cool Sunday as GFS
700mb temperatures fall to -2 to -3C Sunday afternoon. Finally
some drier weather for Monday as upper low tracks east into
eastern Nebraska.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 533 AM MDT Tue May 21 2024

Winds have shifted more westerly this morning...creating
downsloping conditions...which has broken up the low clouds for
most areas east of the Laramie Range. Will see this trend
continue the rest of the day with showers and thunderstorms this
afternoon expected. Drier tonight with VFR conditions expected.

&&

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

&&

$$

SHORT TERM...BW
LONG TERM...GCC
AVIATION...GCC