Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 231738
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1135 AM MDT Tue May 23 2017

.SHORT TERM...Tuesday through Thursday night
Issued at 150 AM MDT Tue May 23 2017

Almost all of the showers out there have ended or will come to an
end early this morning. Following the ending of the showers ridging
will build across the area and bring a pleasant and mild day with a
good deal of sunshine for all areas by afternoon. Temperatures will
be warmer than previous days but likely still below normal for most
areas. We did go above MOS guidance for most areas however with the
sunshine expected. We also can`t rule out some fog in some areas
that got rain this morning, but whether or not to include it will be
a game time decision. Winds should remain light to moderate to most
areas today. However, there could be some increase in wind tonight
across some of the higher elevations and also in vicinity of Clark
as mid level winds increase as a cold front approaches from the
west.

Wednesday will start off dry for all areas and continue into early
afternoon. However, as the front approaches, there will be a chance
of a few thunderstorms developing across northern and western
Wyoming. The models continue to have differences on coverage with
the NAM the wetter model and the GFS drier. For the most part, we
made few changes to continuity. The models do show some decent winds
at 700 millibars of 40 to 45 knots across much of the area. As a
result, windy conditions are likely across much of western Wyoming
as well as the southwestern Wind Corridor. It will also warm things
significantly with the west to southwest flow. Highs in the 80s will
be likely in many areas East of the Divide. As a result, this could
increase snow melt and lead to some rises in the rivers. There are
some differences in the models at night however. The NAM shows some
showers and thunderstorms breaking out across central Wyoming
although this was the first run to show that. The GFS, meanwhile,
has been more consistent in keeping precipitation mainly across
the south with some jet energy riding across the front as it drops
southward. For now, we like the GFS idea.

AS for Thursday, it will be much cooler behind the front, about 15
to 20 degrees cooler in many spots. With low heights over the area,
there will also likely be some showers and thunderstorms around. The
models do disagree on placement however. So for now, we left
continuity alone for the most part with the best chance across the
northwest and southern portions of the state.


.LONG TERM...(Wednesday through Monday)
Issued at 150 AM MDT Tue May 23 2017

Medium range period starts out quite unsettled as fairly strong
shortwave dives southward down larger scale trough/upper low over
southern Saskatchewan/Manitoba. Models have been reasonable
consistent in tracking this low through ern ID or far wrn WY late
Friday into Friday night. This track is very favorable for a period
of lower elevation rain and higher mountain snow over the sw half of
the area over into the upslope areas along the Wind River Mountains.
Right entrance region of a jet streak may also help the far south
during the day Friday. Bumped up pops well above the model blend as
they were once again woefully underdone. Snow levels will probably
be above 9K during the day Friday and 7.5-8K Friday night. Some
significant higher elevation snow is possible especially around the
Wind Rivers where we could see .50 to .75 inches of liquid with at a
tenth to a third across the nearby lower elevations with locally
higher amounts. Cyclonic nw flow looks like it may actually last
into Saturday for scattered mountain rain/snow showers and a few
showers/storms around the lower elevations. Higher concentration of
showers/storms may exist along convergence of old frontal boundary
in the southwestern zones where nw and ne winds converge. We should
start to see building heights Sunday although now the GFS keeps the
cyclonic nw flow through Sunday and even the GEM and Euro have some
moisture in the nw flow which generates afternoon convection. Will
confine any showers to just the mountains for now with temps
starting to moderate. Even Monday we may see a weak ripple in the nw
flow combining with good daytime heating to get some showers and
thunderstorms forming off the mtns and then moving s-se late in the
day, especially along and east of the divide. Heights build in
Tuesday but ridge is still not strong enough to keep weak
disturbances in the flow away and these combined with strong daytime
heating leads to at least building mountain showers/isolated storms
with a few trying to come off the mountains late in the day. Temps
do gradually warm through the period after a fairly cool, unsettled
start Friday where many lower elevations will only be in the 50s. By
the end of this period, we`ll be well up into the upper 60s to upper
70s for the lower elevations.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1135 AM MDT Tue May 23 2017

VFR conditions expected for the next 24-30 hours across the area and
at all terminals.  No precipitation expected with only a few high
clouds and some mid-level cumulus developing mostly over the
mountains and from KBYG to KCPR this afternoon and early evening.
Some gusts of 15-20 kts possible this afternoon in north to
northwest flow, though generally light wind with weak surface
pressure gradient. Upper level low pressure approaches from the
PacNW Wednesday, increasing the surface pressure gradient and
creating breezy conditions before 18Z/Wed. Scattered cumulus will
redevelop over NW WY Wednesday morning, with stronger west wind and
showers/thunderstorms Wed afternoon and into the evening hours
across the area.

&&

.FIRE WEATHER...
Issued at 150 AM MDT Tue May 23 2017

High pressure will bring dry and mild weather for all areas today.
Wind should remain light to moderate for the most part. Relative
humidity will fall to around 20 percent in some areas but should
remain above critical levels. Mixing and smoke dispersal will range
from poor in the northwest to good in south and east. Windy
conditions will develop in many areas late tonight and into
Wednesday with isolated thunderstorms possible in northern and
western Wyoming later in the day Wednesday.&&

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

&&

$$

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



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