529
FXUS65 KRIW 201057
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
457 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 readings 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 amounts 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 4C,
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 is higher. Another wave may approach for
Friday or Saturday, but there is more spread on timing with
this. The area will remain in zonal flow to a weak trough
through 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 /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 457 AM MDT Mon May 20 2024

Showers will continue from the southwest corner of WY to
Natrona/southern Johnson County to begin the TAF period,
skirting the KRIW/KLND and KCPR terminals. These showers will
continue through the rest of the morning, becoming widespread
across the rest of the CWA through the afternoon. MVFR
conditions are expected with these showers. Any shower activity
near KJAC will be late this afternoon into the early evening,
but confidence is too low to include in the forecast at this
time. Precipitation will become more focused east of the Divide
after 00Z, with rain and mountain snow and mountain
obscurations. IFR/MVFR conditions will occur with the rain, with
LIFR/IFR conditions more common after 06Z. Although temperatures
will be well below normal for mid-May, snow is not expected.

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...Hattings
AVIATION...LaVoie