933
FXUS65 KRIW 021711
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1111 AM MDT Thu May 2 2024

.KEY MESSAGES...

- Mainly dry today with critical fire weather for Natrona County
  this afternoon.

- A weather system will spread showers across the much of the
  area tonight and tomorrow. Small accumulations of snow are
  possible late tonight and Friday morning, including in the
  lower elevations.

- Unsettled weather returns Sunday night and continues much of
  next week with below normal temperatures, gusty to strong wind
  and rounds of showers. Details of the placement in intensity
  of the rounds of precipitation remains highly uncertain.

&&

.DISCUSSION...
Issued at 338 AM MDT Thu May 2 2024

Lets play a game this morning. Let us see if you can guess the word
that will describe the over the next week or so. It is a nine letter
word. Still don`t know? OK, it starts with a U. Still don`t
know? It means that the weather will be active but some up and
downs. If you guessed unsettled, you are correct. No prizes will
be given though. We are a government agency after all.

Today will be somewhat more settled. Other than a few mountain
showers, most of the day will be dry, with only a 1 in 5 chance of
showers through the daylight hours. The main concern will be fire
weather; more specifically, in Natrona County. Some areas had
Red Flag Conditions for several hours today. And with a gusty
wind expected to develop and dew points expected to drop into
the single digits, we have issued a Red Flag Warning for this
location. In other zones, fuels are not critical and one that
is, Johnson County, wind will be less and humidity only has a 1
in 5 chance of reaching criteria.

The first shortwave and cold front will move into the area tonight
and move across through Friday. It could get interesting in a few
locations. Models have come into decent agreement with the main
impacts across the southern two thirds of the state. The system does
not have a ton of moisture to work with though. I do have some
concerns though. There will be some left front quadrant jet energy
for the system to tap into. In addition, temperatures will be cold
enough for snow across most areas, including the lower elevations.
The problem is that, given the dynamic nature of the system,
snow will be of the showery and banded variety and models all
have different solutions in placement of the bands. Impacts
would not be widespread though. In addition, with the warm
ground snow would have trouble sticking to the roads, especially
anything that falls after around 8 am or so given the high May
sun angle. So to sum it up, we have high confidence that some
locations will wake up to a coating of snow Friday morning (more
than 1 in 2). However, pinpointing locally higher amounts has
low confidence (less than 1 in 4). As for specific amounts,
probabilistic guidance gives the western valleys less than 2 in
5 chance of an inch or more of snow, and the chance is less
than 1 in 10 in almost all locations East of the Divide. Friday
looks rather chilly as well, with temperatures averaging 10 to
15 degrees below normal.

If there is some good news, it is that at least part of weekend
looks fairly nice for most locations. This will especially be the
case for Saturday as ridging moves over the area, bringing the
nicest day of the period with near to somewhat above
temperatures and dry conditions. This will not last though as
the next Pacific cold front and trough approaches. This will
spread snow showers into the west much of the day. In areas East
of the Divide, most areas will be dry most of the day courtesy
of southwest, downsloping flow. This will bring gusty to strong
wind to many areas though. And there are some 50 knot barbs at
700 millibars appearing as well. This brings the possibility of
some high winds into the picture. The chance is less than 1 in 4
at this point, but we will have to watch it carefully.

Much of next week looks rather unsettled with below normal
temperatures and rounds of precipitation. An upper level low will
move towards Wyoming on Monday and bring an unsettled day with rain
and snow across portions of the area. This first storm looks warmer
for Monday, with 700 millibar temperatures of minus 5 or warmer,
which would keep snow levels about 6000 feet so most populated areas
would have mainly rain, with snow at the higher elevations. With a
tight pressure gradient, strong to high is possible as well.

Uncertainty then increases starting on Tuesday, some models are
showing a blocking ridge developing over the central portions of the
country, which would keep the upper level low and general
troughiness of the area at least through Thursday and possibly into
Friday. With tight pressure gradient remaining, wind will likely
remain strong to high for much of the time as well. This will be
especially so in areas favored by west to northwesterly flow,
like Buffalo, the northern Big Horn Basin and Sweetwater
County. However, slow moving and meandering upper level lows
make details of the forecast like precipitation amounts, timing
and intensity very hard to predict this far out. So, to sum
things up starting Monday, we have fairly high confidence
(around a 3 in 4 chance) of a cool, blustery and unsettled
pattern continuing through much of the week. However, confidence
in the details of each day remains very low.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1106 AM MDT Thu May 2 2024

VFR conditions through the day today. A shortwave passes through the
area beginning this evening, with impacts tonight through early
Friday afternoon. Precipitation chances (which will most likely be
in the form of snow for all sites) begin at KJAC around 00Z/Fri,
with the best chances occurring after 06Z/Fri. Models are showing
KJAC on the edge of the best snow chances, so timing and categories
are hard to narrow down; VFR to IFR is possible. The recent NBM run
has peak chances for the worst flight category at about 10Z/Mon (50-
60% chance of at least MVFR). For KBPI and KPNA, snow chances look
to be more banded, making it very difficult to pin down a timing and
duration of snow lasting.

Snow bands pass through the KRKS area around 10Z-14Z/Mon. NBM has
the peak chances for the worst flight category at about 12Z/Mon,
which lines up well with other high-resolution models. If a snow
band does pass, expect a quick drop in visibilities. Snow bands are
difficult for models to place accurately in time, so a change in this
timing is very possible.

Snow impacts begin at KLND and KRIW around 08Z/Mon. The best snow
chances continue through most of the morning and should be ending by
18Z/Mon. Though MVFR currently looks to be the most probable
category, IFR is possible (20%). Snow looks to last longer at KCPR,
potentially continuing a little after the TAF period.

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

&&

.FIRE WEATHER...
Issued at 338 AM MDT Thu May 2 2024

A combination of relative humidity falling toward 10 percent and
wind gusting to 30 to 40 mph will bring critical fire weather to
Natrona County this afternoon. Elsewhere, local elevated fire
weather is possible but wind or humidity is not expected to
reach critical levels. Concerns will ease tomorrow as showers
and higher humidity spreads across the area.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 8 PM MDT this evening for WYZ280.

&&

$$

DISCUSSION...Hattings
AVIATION...Wittmann
FIRE WEATHER...Hattings