566
FXUS65 KRIW 031739
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1139 AM MDT Fri May 3 2024

.KEY MESSAGES...

- Snow and rain showers today, ending from West to East.

- Saturday looks dry and mild. Snow returns to the west Sunday,
  with windy and warm conditions East of the Divide.

- Cool, blustery and unsettled much of next week. The heaviest
  precipitation is expected in northern Wyoming. Details past
  Monday remain uncertain.

&&

.DISCUSSION...
Issued at 334 AM MDT Fri May 3 2024

The forecast today will have two prominent words beginning with W.
The first is the state we are in Wyoming. The second is the season
that seems to last longer than any other here, winter. Winter is
reminding us that it is not done yet. Snow is currently ongoing
across much of the west, courtesy of a trough and cold front moving
into western Wyoming, with a circulation noted over eastern Idaho.
As I write this around 2:30 am, areas East of the Divide are mainly
dry. There is rather dry air at the surface, this evening`s sounding
showed it and the tell tale donut hole in the imagery over the radar
site is showing the snow is having trouble reaching the ground. Dew
point depressions are still rather large, but it should be snowing
by the time most people wake up. If you don`t like snow, this will
be good since the longer it takes to start, the less that can
accumulate before the sun comes up. After around 8 or 9 am, the
strong May sun should keep roads mainly wet and limit accumulation.
Even now, only some of the passes are slick. Reasoning on the
timing of the precipitation remains the same, with the steadiest in
the morning, then gradually tapering off in the afternoon as the
system moves to the east. All showers should end by around 9 pm
tonight. Many locations will see some accumulation of the grass,
but this will be elevation dependent. For a rough guide, the 1 in 2
chance of an inch or more of snow lies mainly above or below 6000
feet in areas of central Wyoming.Far northern areas will see
the least chance of snow. With the clouds and precipitation,
temperatures will average below normal.

Much of the weekend looks dry and warm, especially East of the
Divide. Saturday looks to be the nicest day with high pressure
over the area, bringing sunshine, near to somewhat above normal
temperatures and mainly light wind. As for Sunday, the
approaching, more potent Pacific system appears a bit slower, so
POPs and QPF were reduced across the west during Saturday night
and Saturday morning. We did maintain a small chance of
convection East of the divide late, but this is looking less
likely with less than a 1 in 5 chance in any location. Most
locations East of the Divide should be dry through sunset with
downsloping, southwest flow continuing. This will bring in
additional concerns though. One, the potential for strong to
potentially high wind. I would put the chance at 1 out of 3 at
this point. This will especially be case in areas prone in
southwest flow, like Casper, the Green and Rattlesnake Range and
possibly Lander. The strongest winds may end up being across
southern Wyoming, closer to the jet energy, courtesy of the
right front quadrant of an 100 knot jet streak. Another clue is
700 millibar winds, with some 50 to even 60 knot barbs showing
up in southern Wyoming. There could also be elevated fire
weather, although increasing dew points may mitigate this.
Temperatures will peak Sunday East of the Divide, with mainly
locations rising into the 70s.

Heavier precipitation will move into the west in the afternoon and
especially in the evening as the low moves toward and area and the
best lift from the left front quadrant of the aforementioned jet
streak approaches the area. Snow levels look to remain above 7000
feet through the day as 700 millibar temperatures hold at minus 3 or
warmer. Colder air will move in at night though and drop snow levels
to the floor across the west. Favorable, moist westerly flow will
keep snow going in the mountains into Monday as well. There is
greater than a 3 in 4 chance of over a foot of snow across the
Tetons and over 6 inches across the remainder of the western
mountains. Some highlights will likely be needed over this period.
Meanwhile, in areas East of the Divide, rain and snow will
overspread the area. With the expected track of the low to be across
central Wyoming, the main impacts would be in northern Wyoming. This
would especially be the case in Johnson County and the Bighorns,
where upslope will be favorable and wrap around precipitation may
continue as the low moves slowly to the east. There is more
uncertainty for amounts here with more model differences. It also
looks warmer here, with 700 millibar temperatures falling to around
minus 5 or so. This would keep snow levels around 6000 feet or
higher, keeping the populated areas mainly rain. Several inches of
snow will be possible in the Bighorns though, with a greater than 1
in 2 chance of advisory level amounts.

Reasoning remains the same through much of the week, as a blocking
ridge sets up across the Great Lakes and the upper level low can
only move slowly away and long wave troughing hangs over the
Rockies. No major storms are expected. The main concern will be wrap
around precipitation and if it can move into northern Wyoming.
Uncertainty is high on details here through. A tight pressure
gradient will also stay across the area, keeping gusty to strong
wind going across many areas possibly through Wednesday before
finally weakening. So, to sum up the extended, we have high
confidence (greater than 4 in 5) of a cool, blustery and damp
pattern through much of the next workweek. Confidence on the details
past Monday remains very low, however.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1139 AM MDT Fri May 3 2024

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals.

An upper-level low is rotating through southwest Wyoming late Friday
morning. Cold core and added lift will generate scattered snow
showers and potentially brief MVFR conditions until around 21Z.
In general, terminals remain VFR through the period. Gusty west
to northwest wind 15-25kts becomes widespread by mid-afternoon
on the backside of the upper low. Wind speeds diminish early this
evening and lingering mid-cloud decks give way to a clearing
sky. Mountain tops frequently obscured above 10K feet MSL until
01Z/Saturday.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals.

Moisture axis streaming across central Wyoming coupled with jet
energy will support IFR/MVFR at KCPR until 20Z-21Z Friday.
Improvement is already being noted at KRIW and KLND where the
snow ends and VFR returns between 18Z-20Z Friday. KCOD sees VFR
conditions develop during that same time period. Mid-cloud
decks will clear between 23Z-02Z Friday evening. A clearing sky
and sufficient boundary layer moisture may allow for fog
development at KCPR and KRIW between 10Z-14Z/Saturday. This will
need to be monitored for inclusion with the next routine TAF
issuance. Mountains frequently obscured until 21Z, with tops
obscured until 01Z/Saturday.

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...
Winter Weather Advisory until 3 PM MDT this afternoon for
WYZ015.

&&

$$

DISCUSSION...Hattings
AVIATION...CNJ