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000
FXUS65 KRIW 210736
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
136 AM MDT Sun Apr 21 2024

.KEY MESSAGES...

- Temperatures climb to above normal today. A passing weather
  system brings scattered precipitation (20% to 60% chances)
  for northern Wyoming, mainly the mountains.

- Elevated fire weather concerns this afternoon for portions of central
  and southern Wyoming. Elevated fire weather returns for
  southern Wyoming Monday.

- A wet weather pattern looks in store for this weekend, though
  details are lacking for now.

&&

.DISCUSSION...
Issued at 135 AM MDT Sun Apr 21 2024

A brief day of ridging occurs today, with a trough sweeping through
later today. Starting with the ridge, the main outcome of this is
warmer temperatures. Most of the area has been below normal the past
few days, but the ridge brings both warmer temperatures and wind to
mix the warmer air down (more on the wind later). High temperatures
are forecast to be 10 to 20 degrees warmer than Saturday, bringing
the area about 10 degrees above normal. Temperatures have gone down
a bit from yesterday`s model runs, sitting in the mid-to-upper 60s
for much of the area. Warmest places will be the Bighorn Basin and
Johnson County, where there is a 20% chance to hit the 70 degree
mark.

The second half of the day is when it gets more interesting. An
upper low centered in Saskatchewan extends down into the area,
sweeping through starting this afternoon. This brings two main
weather elements, precipitation chances and elevated fire weather
concerns. Will discuss these in the next few paragraphs.

Starting with precipitation, there still doesn`t look to be much, if
any, impacts. Shower chances begin around 20Z across the northwest
mountains. These chances spread eastward through the afternoon,
reaching the Bighorn Mountains around 00Z/Monday. Chances remain
mainly over northern Wyoming, and should be confined to the
mountains. About a 40% chance of a shower for the northern mountains
and lower elevation locations north of a Cody to Greybull line. The
northern Bighorn Mountains are looking closer to 60% chances. This
system lacks moisture, with total liquid equivalent precipitation
under half an inch everywhere, and under a tenth of an inch for most
locations, including the lower elevations. As snow in the mountains,
this equates to less than inch, to maybe 3 or so inches for the
higher elevations of the northern Bighorn Mountains. In summary, not
expecting impacts from any precipitation.

The other, more impactful element, will be fire weather concerns.
Starting with relative humidities (RH), the lowest values occur from
about the Rock Springs to Casper area. There is about an 80% to 90%
chance minimum RHs go below 20% in the Natrona County area. The
chances drop to around 40% near Rock Springs. There is about an 80%
chance that minimum RHs between 12% and 22% for Natrona County this
afternoon. RHs begin to improve after sunset.

The second aspect to fire weather is wind. Looking at 700mb winds,
still not seeing indication of any high wind. However, gusts likely
(80%) between 15 and 30mph occur over the Wind Corridor this
afternoon and evening. Gusts to 40mph are possible (20%). The focus
here was on the Wind Corridor (where elevated fire weather concerns
are highest), but many other locations are also windy today. Western
and central mountains are favored to see gusts near 50mph. The Green
River Basin sees winds near 30mph. Elsewhere across the area, gusts
to around 20mph are possible. Though much of the area does not have
critical fuels, elevated fire weather concerns occur for much of the
CWA today due to this combination of wind and low RHs.

Elevated fire weather concerns return for Monday, this time mainly
across southern Wyoming (Green River Basin, Sweetwater County,
southern Fremont County). Models bring RHs lower for these locations
compared to today. Winds also increase for the afternoon, with gusts
30 to 40mph (25th to 75th percentile). Monday is also a few degrees
cooler, as a more northwest flow occurs after the previous day`s
trough passes.

Tuesday into the first half of Wednesday are controlled by a flat
ridge. Isolated (30% or less) mountain showers through this period,
with no impacts expected. Temperatures also warm these days, with a
possibility of 70s for Wednesday. Wind doesn`t look like anything
out of the ordinary either, with the main jet energy well to our
east and southwest.

Moisture begins to stream into the area Wednesday, that by the
afternoon, precipitation chances are on the rise. Best chances look
to remain for the mountains, where the best lift occurs. By
Thursday, however, an upper low moves into the Las Veges to Colorado
area (depending on what model you subscribe to). This increases
precipitation chances, including for the lower elevations. A wet and
messy pattern looks to follow, lasting through at least the weekend
as multiple weather system pass through and develop. This far out,
details are still lacking, but models are in consensus of this wet
weekend pattern. The Climate Prediction Center has above normal
precipitation favored for the area in both the 6-10 and 8-14 day
outlooks.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 955 PM MDT Sat Apr 20 2024

VFR conditions are expected to prevail through the period. Wind
will generally be light through Sunday morning. Southwest wind
will then increase for most terminals through the afternoon with
gusts of 20 to 25 knots. A passing shortwave will bring shower
chances to western and northern Wyoming late Sunday afternoon
through the evening, but low confidence on coverage precludes
mentioning impacts at terminals for now. This will bring in some
scattered to broken mid cloud decks late in the period.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

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

&&

$$

DISCUSSION...Wittmann
AVIATION...Myers