000
FXUS65 KRIW 070525
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1125 PM MDT Tue Jun 6 2023

.DISCUSSION...(This evening through Tuesday)
Issued at 105 PM MDT Tue Jun 6 2023

The large upper-level low continues to spin over central California,
broadly reaching into the Intermountain West and central Rockies to
bring moisture and instability northward over Wyoming.  As of 1030
AM, thunderstorms already kicked off over the Bighorn Mountains, and
thunderstorms will increase throughout the afternoon.  Models are
focusing initiation today over the higher terrain and mountain
foothills, as well as southwest WY.  Weak steering flow will again
be from the SSE to NNW as storms move off the higher terrain and
impact the lower elevations. Some areas may even see storms stay
nearly stationary or backbuild toward the south.  The NAM and NAM
3km continue to show increased chances for showers and
thunderstorms over parts of the Wind River Basin and western
Bighorn Basin during the evening. As is the story of the week with
precipitable water values approaching 150% of normal and a weak
steering flow, some areas could see over 0.50" of rain in a short
time to cause excessive rainfall issues. Lingering showers should
remain through around midnight before dissipating. As the upper-
level flow migrates from central CA to just west of Las Vegas, the
moist unstable flow moves up from the southern into central
Rockies will get more concentrated from central CO toward western
MT. Models are indicating much better chances of showers and
thunderstorms on Thursday, although there are differences in
locations when comparing the CAMs with the coarser NAM/GFS.
Easterly surface flow from eastern into central WY should work
nicely with increasing SE to NW mid- and upper-level flow to
better organize storms and areas of precipitation. SPC even has a
marginal threat of severe storms across far western WY for Wed
afternoon. Moderate to heavy precipitation amounts will possibly
cause localized flooding issues and add to already full rivers and
creeks. Areas of showers are expected to continue well into Wed
evening and early Thursday morning across SW and central WY, then
move northward into northern WY during the afternoon with
increasing southerly flow.

The upper-level flow is expected to move into central Nevada on
Thursday, keeping moist southerly flow over WY.  Late morning into
afternoon showers and thunderstorms should concentrate from central
into northern WY, and again along the western WY border.  Both the
GFS and NAM are showing something of a dry slot moving from UT/CO
into south-central WY with drier air and stronger southerly
winds. With the low moving to OR and western ID on Friday,
showers and thunderstorms will not be as widespread, though
convection will persist over the western mountains and parts of
central WY. Saturday through Tuesday (yes, the entire four days!)
look like a return to widespread showers and thunderstorms each
and every day. Afternoon high temperatures should remain near or
just below normal for mid-June, ranging from the upper 60s in the
western valleys to upper 70s in some of the basins east of the
divide.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1122 PM MDT Tue Jun 6 2023

A cluster of thunderstorms lingers south of KWRL,but is likely to
dissipate in the next couple hours. Other storms have dissipated,
with VFR conditions expected at terminals overnight.

Winds will remain light and variable Wednesday, except in the
vicinity of showers and thunderstorms, where gusts to 35 kt are
possible. Models show an increase in thunderstorm coverage
tomorrow afternoon, as a weather system moves into southwestern
Wyoming. TAF sites west of the Divide will have a good chance of
rain showers Wednesday evening into early Thursday morning as
this circulation moves through the region.

&&

.FIRE WEATHER...
Issued at 113 PM MDT Tue Jun 6 2023

Mostly clear skies this morning are quickly giving way to convection
with lightning over the higher terrain by noon.  With the Bighorns
kicking off first today, storms have also developed over the
other mountain ranges of western WY. Moisture content in the
atmosphere remains above normal and mid-level flow is weak, so
moderate to heavy rain is likely with any storms. Convection will
move off the terrain over the lower elevations through the
afternoon, especially far SW WY, and the Dubois to Cody areas.
Evening convection should continue over the Wind River Basin and
western Bighorn Basin. Gusty winds of 30-40 mph as well as some
small hail can be expected too. Temps this afternoon will be a
couple of degrees warmer than Monday. Wednesday is expected to
have more widespread showers and thunderstorms, with storm
movement from SE to NW. Also, there is a higher risk of heavy
rainfall, with stronger storms across far western WY. Afternoon
temps will decrease by a couple of degrees, and overall RH values
should be more moist. Scattered showers will continue Wed evening
into early Thu morning. A similar pattern can be expected on
Thursday but focus more more in an arc from western into northern
WY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

DISCUSSION...McDonald
AVIATION...VandenBoogart
FIRE WEATHER...McDonald