152
FXUS65 KRIW 210853
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
253 AM MDT Tue May 21 2024

.KEY MESSAGES...

- Rain and mountain snow continue today, ending from west to
  east this afternoon and evening.

- Temperatures will average below normal though most of the
  week.

- An active pattern will continue with additional chances for
  showers, mainly on Thursday and later in the holiday weekend.

&&

.DISCUSSION...
Issued at 250 AM MDT Tue May 21 2024

It has been a rather wet overnight shift at our office tonight, with
around a quarter of an inch of rain so far as I write this. Rain
continues to be ongoing East of the Divide as well and should
continue into the morning hours. By afternoon, the best forcing
moves away to the east and things become more scattered and showery.
All in all, the previous forecast looked fairly reasonable so we
made few changes to it. We did think about taking down the advisory
for the Absarokas, but will let it ride for now. As for travel
impacts, the main impacts today still look to be the Bighorn Range,
where most locations about 8500 feet have at least a 1 in 2 chance
of seeing and additional 6 inches or more of snow. Even here,
impacts should decrease through the day as the snow becomes lighter
and the high May sun angle melts snow off of the roads. As for
potential for flooding, rainfall rates have been mainly light to
moderate. This combined with several days of dry weather proceeding
yesterday and no snowmelt in the higher elevations should keep that
threat to a minimum. Not zero, but small. And there could be a
few snowflakes in the lower elevations. We had a few at the
office earlier tonight. This precipitation will end from west to
east, with all areas dry later this evening. Temperatures will
remain chilly, averaging 10 to 20 degrees below normal. And this
brings another concern, the potential for a freeze. There is still
some question since some clouds may hang around and keep
temperatures up. For now, we will keep the watch up and punt to the
day shift for the final decision.

Transitory ridging will bring a milder and dry day for many areas
Wednesday, the exception being the northwest which could see a
few showers and storms in the afternoon. Breezes will also
increase for the usual areas pre frontal, mainly from Rock
Springs to Casper. The culprit for this, an upper level low
moving in from the Pacific northwest will move near the Montana
and Wyoming border from Thursday into Thursday night. This will
bring another chance of precipitation, mainly to northern
Wyoming although many areas could see a shower or thunderstorm.
Wednesday and Thursday is looking a little more interesting as
well. Models are indicating 700 millibar temperatures falling as
low as minus 8 to minus 9 across the west, which could drop
snow levels to the valley floors. Amounts are still in question
though. The best forcing and moisture will likely remain over
Montana, with most of the area only seeing a glancing blow. Some
locations may see advisory level amounts though. Especially
those favored by westerly flow, mainly the Tetons and western
Bighorns. It is too early for highlights but something to watch.

Flat ridging should then bring drier weather for Friday and much of
Saturday as well. Then, a third in the series of lows will approach
the area for Sunday with another chance for showers from Sunday and
possibly into Memorial Day as well. There is more model spread on
this so details are impossible to thresh out at this time. Ridging
may then finally bring a few days of dry weather early next week.
But emphasis on may, not will.


&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 959 PM MDT Mon May 20 2024

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

A slow moving weather system will pivot through the region over the
next 24 hours, which will bring rain chances to KRKS/KPNA Tuesday
afternoon. At KJAC/KBPI mostly dry conditions are expected, but an
isolated rain shower is possible (20-30%) Tuesday afternoon.
Westerly/northwesterly wind will increase Tuesday afternoon and
gusts will be 20-25 knots through the afternoon hours, quickly
fading Tuesday evening. Overall conditions will remain VFR, but a
drop to MVFR is likely (80%) if a heavier shower moves directly over
a terminal. Mountaintop obscuration will be prevalent through
tuesday afternoon.

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

A slow moving weather system will pivot through the region over the
next 24 hours, which will bring rain to all terminals tonight into
Tuesday morning. Steady rain tonight will give way to rain showers
late Tuesday morning through Tuesday afternoon at KRIW/KLND. A dry
period is likely (70%) at KCOD from 15Z-20Z/Tues before more rain
showers develop in the afternoon. A steady rain will hang on at KCPR
the longest, until 23Z, and then rain showers hang on for a few
hours before rain ends Tuesday evening. Wind increases at KCPR late
in the afternoon and will remain breezy for a few hours. At KWRL
rain ends at 20Z/Tues. As rain occurs MVFR/IFR conditions will
prevail at terminals. Mountain obscuration will develop and
persist through Tuesday afternoon.

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 6 PM MDT this evening for WYZ002-
015.

Freeze Watch from late tonight through Wednesday morning for
WYZ003>006.

Winter Storm Warning until 6 PM MDT this evening for WYZ008-
009.

&&

$$

DISCUSSION...Hattings
AVIATION...Rowe