113
FXUS65 KRIW 281742
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1142 AM MDT Sun Apr 28 2024

.KEY MESSAGES...

- Remaining unsettled today and Monday with a scattering of
  showers and thunderstorms.

- Windy conditions likely Monday and especially Tuesday with
  elevated fire weather possible.

- Remaining unsettled through much of the work week. Potential
  for some accumulating snow Wednesday night and Thursday
  morning.

&&

.DISCUSSION...
Issued at 227 AM MDT Sun Apr 28 2024

Steady precipitation has ended across the area as the low that was
moving across Colorado has pulled into the Plains states, the
trigger for the severe weather outbreak across the central and
southern Plains states. The weather looks to remain unsettled
through most of the period, but some details are still in flux,
especially further out in the forecast period. To sum the
pattern up, I will use a Sesame Street reference, as I heard
the song Rubber Ducky earlier today, sparking some memories from
my young childhood many years ago when big collars and bell
bottoms were in style. It also got the creative juices flowing.
So, this forecast is brought to you by the letters Z (Zonal
flow) and U (Unsettled) and the number 4 (the number of waves
that will likely impact the area through the week).

We continue to have some showers across western Wyoming as a
shortwave moves in from Idaho and slowly crosses the state, bringing
a chance of showers and thunderstorms, anytime in the west and
mainly afternoon East of the Divide. Many places won`t see one
though in the lower elevations, the chance is generally 1 in 3 or
less at any point and less than 1 in 5 for a thunderstorm. Gusty
wind will be possible with any storm, and with low wet bulb zero
levels, small hail will also be a possibility. This could also be a
day of self-destructive sunshine, when a clear sky will lead to more
clouds through the day with the unstable atmosphere (lifted indices
fall to around minus 1). Temperatures will be warmer than yesterday,
but still averaging around 5 degrees below normal. Showers should
end around midnight as the trough moves away.

After a brief break from late tonight through Monday morning, the
next Pacific shortwave and weak cold front will swing across the
state. With the later arrival, temperatures should be warmer than
today, rising to near to slightly above normals levels. We have two
concerns for Monday. The first is for the potential of elevated fire
weather. The tightening pressure gradient ahead of the system will
increase southwest flow ahead of the front. This will lead to
some gusty wind. The 700 millibar wind does not look sufficient
for high wind though (only around 40 knots), there is a greater
than 1 in 2 chance of a few wind gusts past 40 mph. Relative
humidity will be rather low as well, falling to around 20
percent. The main area of concern would be Johnson and Natrona
Counties, which did not receive the soaking rain of further
west. The second concern is convective wind gusts. There will be
some dry air ahead of the front. This will likely lead to some
high based convection. Model soundings are showing the tell tale
inverted V signature that leads to dry downbursts. With dew
point depressions approaching 50 degrees in the afternoon, there
is a 1 in 2 chance of a wind gust of around 50 mph with any
thunderstorms or shower. This could be our first "Little Green
Blob" day of the season.

Tuesday looks drier as we will be in between systems with zonal
flow with somewhat cooler temperatures behind the front.
Coverage of showers should be less than 20 percent. The main
concern will again be fire weather and possibly strong to high
wind. A rather potent 120 knot jet will be crossing the area,
with the core coinciding with maximum heating and mixing. The
main area of concern will be across the southern two thirds of
the state, which has the best possibility to be in the right
front quadrant of the jet, helping to enhance downward momentum.
There is greater than a 1 in 2 chance of wind gusts past 45 mph
in these areas. And with this, the air will be very dry, with
dew points falling in the single digits, pushing relative
humidity into the teens. This could, again, bring a day of
elevated to near critical fire weather.

Models continue to trend southward with the Wednesday system,
bringing a better chance of precipitation to the area. There
are still large differences in placement of the eventually
developing upper level low though, so confidence in details
remains low. Of particular interest is Wednesday night, as we
could have a north to northeast upslope pattern developing that
could bring some mountain snow to areas like the Absarokas, Wind
Rivers and Casper Mountain during this timeframe. And, with low
temperatures falling to around freezing and 700 millibar
temperatures falling to around minus 8, snow levels could lower
to around 4500 feet, bringing the chance of accumulating snow to
the populated areas as well. This is still uncertain though, as
it all depends on the eventual track of the low pressure area,
but we will have to watch this carefully. Transitory ridging
should bring mainly dry weather for Thursday before another wave
possibly approaches for Friday with another chance of showers.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1137 AM MDT Sun Apr 28 2024

Scattered showers will continue to bubble across the state
today, with the best focus on KJAC, KCOD, and KCPR. Any showers
may bring a brief threat for thunder, but this will be very
isolated in nature. Cigs with showers will be briefly MVFR, but
should be staying above 2500 through the day. Northwest winds
will gust 20 to 25kts west of the divide this afternoon, before
decreasing after sunset. Next round of showers follows quickly
Monday morning, with likely rain at KJAC and KCOD through the
day. This precip is not expected to reach east of the divide
before Monday evening.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Hattings
AVIATION...Straub