484
FXUS65 KRIW 211119
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
519 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 through 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 through 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 an 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 8C to minus 9C 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 flesh 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 /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 0515 AM MDT Tue May 21 2024

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

A slow moving weather system continues to impact the region
through the next 18 hours. Low clouds, and possible LIFR
ceilings over far western portions this morning has led to light
snow near KJAC. SKC skies have developed over KBPI/KPNA and
KRKS. Most of the precipitation will stay over the mountains
today, with a chance for rain east of KRKS later this afternoon.
Westerly/northwesterly wind will increase this afternoon and
gusts will be 20-25 knots through the afternoon hours, quickly
fading in the 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 the afternoon.

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

A slow moving weather system will continue to pivot over the
region over the next 24 hours. Rain will be more focused over
Johnson and Natrona counties through the morning, with
additional showers developing off the mountains this afternoon.
VLIFR/LIFR ceilings are expected at KCPR through 18Z, improving
to MVFR afterwards. Similar conditions will be possible at KLND
through 14Z, as the rain shifts eastward this morning. MVFR to
VFR conditions will be place elsewhere this morning, with VFR
conditions more widespread this afternoon. SKC skies will be
possible at KRIW/KLND after 06Z, which could allow for fog
development toward 12Z. Mountain obscuration will persist
through the afternoon, with improving conditions toward 06Z.

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...LaVoie