565
FXUS65 KRIW 180250
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
850 PM MDT Fri May 17 2024

.KEY MESSAGES...

- Elevated to critical fire weather conditions today with
  humidity below 20 percent and widespread wind gusts of 35 to
  50 mph.

- Isolated thunderstorms are possible (10% to 20% chance) across
  northern Wyoming this afternoon, a few of which could be
  strong with gusty outflow winds.

- Elevated fire weather conditions continue on Saturday and
  Sunday.

- A large and wet weather system will bring rain and mountain
  snow on Monday and Tuesday. Mountain passes are expected to
  have winter driving conditions.

&&

.DISCUSSION...
Issued at 1255 PM MDT Fri May 17 2024

A hearty open shortwave trough is quickly moving through the
Northern Rockies this afternoon and evening, bringing active
spring weather. The tight pressure gradient on the south side
of this system has been the cause of the already gusty winds
that have already begun to mix down into the lower elevations.
Still expecting gusts in the 35 to 50 mph range for much of the
afternoon and evening today, quickly decreasing after sunset.

In addition to winds, unseasonably warm temperatures will be
felt this afternoon, with high temperatures about 15 degrees
above normal. The warm surface air, along with cooler air aloft
will lead to 200 to 400 J/kg of CAPE across the Bighorn
Mountains and the adjoining high plains of Johnson County and
the larger Powder River Basin. A few strong thunderstorms
featuring hail and strong outflow winds are possible here due to
enhanced shear aloft with the approaching shortwave. A majority
of the strong storms will stay further north across southern
Montana, but can`t rule out a rogue strong cell over Johnson
County this afternoon.

A third threat this afternoon and evening is the elevated fire
weather danger. The strong winds along with a dry air mass will
be conducive for fire growth. That being said, fuels are quite
moist this time of year during spring greenup, which will be a
mitigating factor. A Rangeland Fire Danger Statement covers this
threat. Fire danger decrease quickly after sunset.

The rest of the next 7 days remains fairly active as well. The
jet stream will remain overhead on Saturday, causing breezy
winds to continue. These will not be as strong as those felt
today and not quite as widespread, but gusts in the 25 to 35
mph range will be common. On Sunday, the jet stream sags
southward and a subtle embedded wave will enhance surface winds
to levels higher than Saturday, but not quite as windy as today.
There is modest lift with this wave, so scattered showers are
in the forecast, focused over the mountainous terrain.

Monday into Tuesday brings the next period of widespread
precipitation as a slow-moving cut-off low rotates through the
larger trough. Model trends have been to slow this system down,
and produce more rain with Gulf of Mexico-sourced inflow into
the system. Snow levels on Tuesday will be fairly low, around
7000 ft in the morning, so expect snow over mountain passes as
well as potential winter weather highlights for some areas.

Wednesday through Friday looks to feature another upper low,
this time over the Pacific Northwest, pumping moisture into the
western Wyoming mountain ranges. All these weather systems will
continue to add high elevation snowpack, and increase the
potential for river flooding issues later this spring.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 845 PM MDT Fri May 17 2024

VFR conditions will prevail through the entirety of
the TAF period. Winds have weakened over the past few hours,
with breezy conditions lingering across some terminals to start
the period. Skies will continue to clear overnight, with some
high clouds over northern WY gradually decreasing by the
Saturday morning. Winds return Saturday afternoon across all
terminals but will be less impactful. Gusts will range from 20
to 30 knots through the afternoon/evening Saturday for the
majority of terminals. Light and variable winds return around
sunset and continue through the end of the period. Skies remain
mostly SKC for all terminals Saturday, with mid level clouds
developing by the end of the period.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued at 1255 PM MDT Fri May 17 2024

Widespread elevated to critical fire weather conditions are
occurring this afternoon as a surface pressure gradient
causes strong winds ahead of a weak cold front. Sustained west
winds of 25 to 35 mph with gusts of 40 to 50 mph will continue
through sunset. Humidity values are 20 to 30 percent currently,
and likely (90% chance) to drop to 15 to 20 percent across most
of the area. A slightly drier airmass moves in for Saturday,
bringing humidity values down a few more percent. Wind will not
be as strong as today, but will still be breezy. Warm and dry
conditions continue Sunday before cooler and wetter conditions
arrive Monday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...VandenBoogart
AVIATION...Dziewaltowski
FIRE WEATHER...VandenBoogart