Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 241735
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1135 AM MDT Wed May 24 2017

.SHORT TERM...Wednesday through Friday night

After a quiet day yesterday, things look to turn more active once
again today and continuing into the end of the week.

For today, we have two main concerns. The first is the chance of
thunderstorms. The models during the day have come into better
agreement and backed off on the chance of many showers or storms
through most of the day with some rather dry air in place. The main
chance would be across portions of the western mountains and Bighorn
range but even here activity would be isolated and mainly late.
However, the chance will increase this evening as a cold front drops
across the area and interacts with some right rear quadrant jet
energy to increase convection. At this point, this would mainly be
across southern portions of the area, roughly south of a Casper to
Riverton to Afton line. There could be some small hail, but the main
threat will be wind given the large temperature-dew point spreads
and inverted V model soundings. And speaking of wind...

It will likely become quite windy across portions of the area this
afternoon. The models show 700 millibar winds increasing to 40 to 50
knots across much of the area and with a front dropping into the
area there will be a good mechanism to mix the wind to the surface.
The strongest winds would be east of the Absarokas as well as other
areas favored by west flow such as the Lander Foothills and the
Wind River Basin as well as portions of the west including the
Jackson Valley.I had a thought of possibly a high wind warning,
but this would be the exception rather than the rule. For now, we
will issue a special weather statement for gusts over 40 mph. In
addition, it will be a warm day with 80s likely in many of the lower
elevations East of the Divide. With the wind, this could lead to
some decent snowmelt that could cause area rivers to rise. At this
point, since the warm up will be short lived, we do not anticipate a
lot of trouble with rivers but this will have to be watched. Wind
should decrease fairly quickly tonight following the passage of the
front and the decoupling of the wind though.

Thursday at this point looks much cooler with light wind. There will
also be a few showers and thunderstorms around with the low heights
around the area. The main threat would shift to small hail with the
light winds and low wet bulb zero levels though. As for the best
chance, it would be across the northwest as well as the far south
with the least chance in the middle. However, many areas should be
mainly dry most of the time. As for Friday, with cool northeasterly
flow across the area temperatures will again fall well below normal
with a good deal of cloud cover. There will also be some light
precipitation around but at this point it does not look heavy. And
again, it does look cool enough for some snow in the higher
elevations, although amounts look fairly light at this time.

.LONG TERM...(Saturday through Wednesday)

Through the long term, models are showing a longwave ridge over
the west and a longwave trough over the central/eastern U.S. The
ridge axis is expected to remain over the Intermountain West with
a northwest flow impacting the area through the extended. In this
flow, models are showing a few shortwave troughs diving southeast
or south out of Canada with a few distinct reinforcing cold
fronts this weekend into early next week. These shortwaves, and
reinforcing cold fronts, and some instability will keep slight
chances of at least daily afternoon/evening convection across
portions to much of the area through the extended.

Temperatures will be below average Saturday across the forecast
area. Temps are then expected to warm to seasonal to slightly
above average west, and seasonal to slightly below average east
early next week. The greatest impact from cold fronts this weekend
into early next week will be felt east of the Divide especially
across Johnson County.

&&

.AVIATION...18Z Issuance

As a cold front approaches from the northwest, the surface pressure
gradient has increased across WY, thus strengthening surface wind to
15-20 kts with gusts 25-35 kts.  Wind dir is mostly west, but some
local forcing is from the south (KGEY, KWRL) before the front pushes
through.  Expect a wind shift to the northwest from 01z-03z/Thu
across much of the area, with wind speeds also decreasing after
03z/Thu.  Isolated showers and thunderstorms are likely between
21z/Wed-00z/Thu across SW WY and NE WY - which is decreased from
yesterday`s forecast.  Expecting activity to continue south of line
from KBPI to KCPR through 06Z/Thu and some lingering showers during
the overnight hours.  Some showers will linger after 06Z Thursday,
with the NAM being rather strong with precipitation across southern
WY. While VFR conditions generally expected, some MVFR possible due
to reduced ceilings and visibility. Outflow from the convective
activity likely to create some gusts around 50 kts.  Brief clearing
area moves through from the northwest between 12z-18z/Thu, followed
by another shot of cool moist air from the northwest, increasing
clouds Thu afternoon and bringing some showers to northern WY Thu
afternoon.

&&

.FIRE WEATHER...

A cold front will approach the area and bring isolated late day and
evening showers and thunderstorms, mainly to northern mountains and
the southern half of Wyoming. Winds will increase today and become
strong at times in western Wyoming, the Lee of the Absarokas, the
Lander Foothills and Wind River Basin with windy conditions possible
in many areas. It will be a warm day with humidity possibly falling
into the teens at times in the lower elevations. Mixing and smoke
dispersal will be good to excellent. Much cooler weather will return
on Thursday.

&&

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

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...McDonald
FIRE WEATHER...Hattings


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