819
FXUS65 KRIW 080405
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1005 PM MDT Tue May 7 2024

.KEY MESSAGES...

- An upper low will continue to lead to cool, wet, and windy
  conditions across much of the region over the next few days.

- Snow showers persist across the west and southwest this
  afternoon, then, as the low progresses westward, a cold front
  will bring rain and mountain snow to most areas east of the
  Divide Tonight through Wednesday.

- Lingering upslope enhanced showers and snow showers will persist
  through the day Thursday.

- The upper low weakens and moves out of our region by Friday, leading
  to warmer and drier weather through the weekend. Dirty
  ridging looks to keep the pattern warmer but active next week.

&&

.DISCUSSION...
Issued at 231 PM MDT Tue May 7 2024

The broad region of low pressure that has been impacting the region
for the past several days will continue to do so. As of 1 PM, the
low is circulating over the ND/SD state line. Over the next 24
hours, the low will slowly shift westward and back into WY. With it,
a cold front will drop southward and bring rain and mountain snow to
much of the region tonight through Wednesday night. In the meantime,
northwest flow is keeping persistent show showers across the far
west through the afternoon today and into tonight. Snowfall rates
look to increase overnight across the northwest mountains and
Yellowstone Park as the main precip band associated with the low
rotates southward into the region. Throughout the evening and
overnight, northwest flow will also persist, enhancing the
orographics along the Tetons. Given these trends, in conjunction
with webcams that show very snowy conditions across those areas,
have made the decision to extend both the Winter Weather Advisory
for Yellowstone Park and the Winter Storm Warning for the Tetons
through midday Wednesday.

In addition to the snow across the west, snow moves into the
northern portion of the state east of the Divide tonight as well.
The Bighorn Mountains are of particular note. QPF has trended upward
significantly across the northern portion of the Range, as well as
across portions of Sheridan County and points north across Montana.
The latest QPF forecast seems high, however, hi-res models also have
trended QPF higher compared to previous forecasts. Flow will also
begin to turn northerly and then eventually gain a bit of an
easterly component as well, which is favorable for a period of
upslope along the northeast slopes of the Bighorns. This will mainly
impact the Sheridan County and MT side of the mountains, however,
given the forecast and latest trends, have gone with a Winter Storm
Warning for the Bighorns, which will cover the threat through
Thursday morning. Upslope showery snow may continue through much of
the day Thursday, however, additional accumulations after about 5 to
6 AM Thursday should be minimal. The east slopes of the Wind River
Mountains, as well as Casper Mountain also need to be monitored, as
upslope snow could bring more than anticipated snowfall amounts to
those locations should QPF continue to trend higher.

Elsewhere east of the Divide, lower elevations will mostly be rain
with this event. There is some cooler air that will filter down with
the cold front this evening and tonight, however, given cloud cover,
precipitation occurring, among other factors, temps across most
lower elevation locations should remain warm enough for
precipitation to remain rain, rather than snow (85% chance of temps
>33F). QPF amounts will be generally in the 0.25 to 0.5" range across
most lower elevations east of the Divide, with highest amounts
across Johnson County (mainly along the east slopes of the Bighorns)
and also across the Cody Foothills and Lander Foothills due mainly
in part to upslope effects.

Besides precipitation, you would have to have been living under a
large rock to not notice that it has been especially windy over the
past few days. Persistent northwest flow as a result of a tight
pressure gradient and associated low pressure, has kept winds quite
gusty across most areas for several days. The good news is that this
will finally begin to diminish overnight. As the cold front turns
winds northerly and eventually northeasterly, wind speeds will
diminish. Gusty post-frontal winds will persist across the Bighorn
Basin and favored Johnson County through much of the day Wednesday,
but across other areas, we will see a significant decrease in wind
speeds and gusts tonight through tomorrow morning.

The low finally weakens and rotates out of our area (to the south)
by late Thursday night. Precipitation will end and we will see a
warmer and drier trend through the weekend. Also, as the low begins
to weaken Thursday, and especially Friday, winds will become mostly
light and variable. Winds will stay light through the weekend, and
with temps returning to seasonal norms and even above normal temps
by the end of the weekend, we are in for a very pleasant Spring
weekend. Weak "dirty" ridging looks to set up for next week, so
temps will remain warm, but there does look to be chances of showery
activity returning Monday, continuing through much of next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1005 PM MDT Tue May 7 2024

Snow showers continue to build in from the north around the low to
the east, and will to gradually spread southward overnight. KJAC
will continue with shower through the night, with borderline
VFR/MVFR cigs and vis. Precip spreads southward during the day
Wednesday, continuing with the lowered cigs and vis for all
locations. While colder temperatures are expected with this system,
most areas will return to rain during the morning, except KJAC
likely remaining light snow through the day. Some breaks in the
precipitation are noted for KJAC after 00Z/09, but system remains
close enough to keep lower clouds in the valley through tomorrow
night.

Winds in computer models have been overforecasting local winds the
past couple nights, so have undercut wind forecasts for most
locations. Gusty winds will continue, but will stay under 25kts for
most locations. Except being here at KRIW, where gusts of 40 to 45
kts continue to howl across the airport.

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 noon MDT Wednesday for WYZ001.

Winter Storm Warning until 6 AM MDT Thursday for WYZ008-009.

Winter Storm Warning until noon MDT Wednesday for WYZ012.

&&

$$

DISCUSSION...Hensley
AVIATION...Straub