Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 290727
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
127 AM MDT Fri May 29 2020

.SHORT TERM...Today through Sunday night

Most everything is fairly quiet this morning across most of western
and central Wyoming. This won`t last today though, as we appear
to be in a ring of fire pattern with the top of the ridge laying
fairly close to Wyoming. This will allow a weak shortwave to move
over the top of the ridge and bring a few showers and
thunderstorms to mainly the northern half of the area this
afternoon and evening. Most models indicate this being more of a
late show, with the most coverage later in the afternoon and into
the evening. There are some decent instability parameters with
the GFS showing as much as 1500 J/KG of surface CAPE and lifted
indices as low as minus 3. As a result, a few storms could become
strong. The Storm Prediction Center has the area in a marginal
risk as well. As for the main threat, it looks like strong wind
gusts given the pattern although some small hail will also be
possible. Storms could linger into the evening, before coming to
an end shortly after midnight as the shortwave shifts further to
the east. Further south, with less moisture to work, the chance
will be less. However, with larger dew point depressions, there
could be a few "Little Green Blobs" with little precipitation and
no lightning but strong wind gusts.

The most active day at this point looks to be Saturday. A stronger
shortwave will move into the area in the afternoon. At this same
time, with some lee cyclogenesis over southern Colorado some east to
southeasterly upslope flow will develop across portions of the area,
especially west of the Bighorn Range and into the northern
Bighorn Basin. The jet energy is not the best. However, soundings
are showing a decent amount of shear across the area. Instability
parameters also look decent as well, especially in Johnson and
Natrona Counties, with CAPE values approaching 2000 J/KG in some
locations and lifted indices as low as minus 3 to minus 4. As a
result, there could be some strong to possibly severe
thunderstorms, especially from the Bighorn Range and east. The
Storm Prediction Center has this area in a marginal risk at this
time, but I would not be surprised to see this increased by
tomorrow morning. These storms again should end by around midnight
Saturday night.

Most of the models have trended drier for Sunday. The NAM still has
some isolated convection in eastern areas, but both the GFS and
European are dry and keep convection east of our area. This is
probably due to drier air moving in, as southeast flow increases.
This will mean downsloping flow, but also lower humidity and a
gusty breeze in many locations. This could result in increased
fire behavior across some areas, especially the more wind prone
areas like Sweetwater County. For now, we have gone with a dry
forecast with low humidity.

.LONG TERM...Monday through Friday

The pattern will not change much to start the work week next week. A
broad, upper level ridge will maintain its hold over the Plains
Monday, with a deep longwave trough off the CA coast. The base of
this trough will be as far south as Baja California. The predominant
flow over the Cowboy state will be from the southwest through the
period, due to this pattern. Record temperatures are expected once
again Monday, with above normal temperatures expected through the
week. 700mb temperatures will average between 11C to 15C through
much of the week.

Showers and thunderstorms will be possible over far southern
portions late Monday afternoon into the evening. A weak wave
rotating northward from NM/CO will tap into the abundant Pacific
moisture during this time. However, most of this activity looks to
stay over CO at this time.

The jet will be situated over southern Canada/US northern tier
states during this time. A shortwave moving through the jet will
pass over MT/WY Tuesday, initiating showers and thunderstorms over
Johnson/Natrona counties late in the afternoon into the evening once
again.

A shortwave, ejecting from the upper low off the southern CA coast,
will move over the Great Basin Wednesday. This will lead to
scattered showers and thunderstorms west of the Divide Wednesday
afternoon. This activity could spill over east of the Divide as
showers through the overnight hours, as the wave continues off to
the northeast. There could be a repeat of this activity Thursday, as
the upper low begins to move onshore. Another ejecting shortwave and
a more diffluent flow aloft could lead to afternoon and evening
showers and thunderstorms to be more widespread across the forecast
area. By Friday, the remnants of the upper low will move over the
Cowboy State. This could lead to widespread showers and
thunderstorms through the afternoon and evening.

&&

.AVIATION...For the 12Z TAFS through 12Z Saturday

VFR conditions will occur at the terminal sites through the period.
Isolated showers and thunderstorms will occur after 20Z and continue
through 06Z. Outflow wind gusts to 30 knots will be possible near
storms.

&&

.FIRE WEATHER...

An upper level disturbance will cross the area today and bring
isolated to widely scattered afternoon and evening showers and
thunderstorms. A few thunderstorms could turn strong, especially in
northern Wyoming, with strong wind gusts being the main threat.
Otherwise, expect a warm day. Relative humidity will fall into the
teens at times. However, winds should remain light to moderate
except in and around any shower or thunderstorm. Mixing and smoke
dispersal will range from fair to poor in some of the northern
basins, to good to excellent in the west and south.

&&

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

&&

$$

SHORT TERM...Hattings
LONG TERM...LaVoie
AVIATION...Ross
FIRE WEATHER...Hattings


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