822
FXUS65 KRIW 200934
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
334 AM MDT Mon May 20 2024

.KEY MESSAGES...

- Cool and wet weather returns to the area today into Tuesday.

- Several inches of snow are likely in the northern mountains
  tonight and Tuesday.

- Below normal temperatures and continued chances of showers
  continue through the week.

&&

.DISCUSSION...
Issued at 330 AM MDT Mon May 20 2024

After a few days of typical May weather, the pattern will take a
step back in the time machine and make things look more like April.
Colder air has filtered into the area this morning. The main driver
of the weather over the next couple of days will be a deep trough
now over the Pacific Northwest that will slowly drift eastward and
bring a period of wet weather. The steadiest precipitation today
will be focused along a right rear / left front jet couplet that
will move across the area. This will focus the heaviest
precipitation roughly from Sweetwater County through Fremont and into
Natrona Counties, but all areas will have a chance of some showers
and even an isolated thunderstorm. The thing many will notice are
the temperatures, which will average 15 degrees cooler then
yesterday and similar amounts below normal.

This will continue into tonight. The main player then becomes the
trough that will close off into an upper level low that will slowly
move across northern Wyoming. And, that will transition the
steadiest and heaviest precipitation to northern Wyoming, with
lesser amount further south, especially on the other side of the
divide. And that transitions another concern, the mountain snow, how
much of it and the impacts of it. Models have finally come into
somewhat better agreement in putting the greatest QPF across the
northern ranges, mainly the Bighorns, Absarokas and to a lesser
extent, the eastern Wind River Range with more favorable upslope
flow developing. Maximum impacts will travel from west to east, with
the heaviest in the Eastern Wind Rivers and Absarokas late tonight
into Tuesday morning, and the Bighorns on Tuesday.  As for
highlights, we have gone with advisories rather than warnings. Why
you ask? Well, amounts in the Wind Rivers are in the advisory range.
In the Absarokas, there are some locations with warning level
amounts, with a few areas in the southern Absarokas having a greater
than 2 in 3 chance of 12 inches or more. However, these are in areas
with few people, roads or impacts. The Bighorn range has the best
chance, with a decent area having around 1 in 2 chance of 12 inches
or more. The difference here is that the heaviest snow will occur
during the day on Tuesday. And the combination of the high sun angle
and warm ground temperatures will make it difficult for snow to
accumulate on roads during the day. So, we went with advisories here
as well. As for snow levels, the coldest most 700 millibar
temperatures get is around minus 4, which would put snow levels at
around 6500 feet. So, lower elevations look to have mainly rain. A
place like Dubois could see a coating though. And I can`t rule out
some flakes in the air at night if rates are heavy enough in the
lower elevations.

QPF amounts look to average between 0.25 and 0.90 inches across most
of the area. As for the threat of flooding, it looks low right now.
The rain should fall over a long period of time for one. We have
also had 4 days of dry weather so the soil can absorb some rain.
And, with the cold temperatures in the mountains, snowmelt will be
slim to non existent through the period. So, no flood highlights at
this time.

Most of the area will have a brief break on Wednesday with
transitory ridging over the area, although another approaching wave
will bring some additional showers to the west at this time. This
wave and low will cross the area on Thursday. Models have trended
somewhat further north with it this morning. This would keep the
heaviest rain over Montana but models have not been consistent with
it so uncertainty this higher. Another wave may approach for Friday
or Saturday but there is more spread on timing with this. The area
will remain in zonal to a weak trough though the period, so the
trend of below normal temperatures and above normal precipitation
should continue at least to the start of the upcoming holiday
weekend.


&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 938 PM MDT Sun May 19 2024

VFR conditions will light winds and some scattered mid to upper
level clouds throughout the CWA. Clouds will increase in
coverage with mid level ceilings around 6-8kft by around 15Z.
Increasing rain shower activity after this point reaching CPR
around 16Z, RKS by 17Z, and WRL by 20Z. Most locations will see
lower ceilings above VFR thresholds as shower activity decreases
after 00Z. WRL and CPR will see lingering shower activity for
point further north and east with likely MVFR ceilings as
suggested by LAMP/BUFKIT data. Quiet overnight before an active
Monday arises by mid morning Monday.

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 from 6 PM this evening to 6 PM MDT
Tuesday for WYZ002-008-009-015.

&&

$$

DISCUSSION...Wittmann
AVIATION...Lowe