Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 170756
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
156 AM MDT Thu May 17 2018

.SHORT TERM...Today through Saturday night

Unsettled weather looks to continue through most of the next three
days. At this point, today looks to be the driest day. We still have
some isolated showers and even a stray thunderstorm but these are
becoming less numerous and should be just about gone by sunrise.
With the low still moving slowly across Montana, we have another
round of isolated to widely scattered thunderstorms today into
tonight. The most likely area looks fairly similar to Wednesday and
mainly across western and northern Wyoming. However, with a bit more
moisture there could be a bit more coverage than yesterday. Threats
look similar as well with gusty winds and possibly small hail. As
for temperatures, with more cloud cover temperatures will be a few
degrees cooler but still rather mild. All in all, most areas should
be dry most of the time.

However, this will change for Friday, especially in the afternoon
and evening. Low pressure will move into northern Utah late tonight
and into Friday. This will turn flow to the northeast and bring
upslope flow across much of the area. The result will be a lot of
clouds along with an increasing chance of showers, especially in the
afternoon and into Friday night. The set up is actually somewhat
familiar to last weekend. There may not be quite as much QPF though,
since convection and possible MCS development to across Nebraska
could rob some of the moisture. Nevertheless, it looks rather damp
Friday afternoon into Friday night with the best lift and dynamics
across the area. In addition, there will be much cooler temperatures
and just make for a cloudy, cool and crappy day. It could even get
cool enough for some mountain snow, but snow levels would be rather
high and at this point and travel problems minimal.

The models do split more for Saturday. They all have a mostly cloudy
and cool day for many areas. However, the NAM is rather pessimistic
and keeps a lot of showers in eastern portions of the county warning
area. This is especially so in Johnson and Natrona Counties. The
other models are drier though. My gut feeling is that it would be
like last Saturday, with lots of clouds and cool temperatures but
only light showers. Still some uncertainty though, we will have to
see how things shake out.

.LONG TERM...Sunday through Thursday

The area will be between storm systems Sunday and Monday with one
system exiting east across the Plains, and the next Pacific upper
low digging down the California coast. Some weaker disturbances in
southwest flow aloft will interact with slight instability and some
moisture provided by return flow around surface high pressure over
the Plains, resulting in isolate to scattered afternoon/evening
thunderstorms across mainly the south and west on Sunday, and across
most of the area on Monday. A warming trend will follow Saturday`s
unseasonably cool start to the weekend, with most highs from 65 to
75 across the basins and valleys on Sunday, and 70s to near 80 on
Monday.

The Pacific upper low will lift northeast across the Great Basin
Tuesday and Wednesday, and across Wyoming Wednesday night and
Thursday.  Low level easterly flow will bring in more abundant
moisture from the Plains to the east slopes of the Divide both days,
so we will likely see some stronger storms with heavy rain potential
across the area.  This system mostly be north and east of the area
by Thursday evening; surface trough axis should be over the High
Plains Thursday afternoon-evening with stronger, more organized
convection along and east of this axis.

&&

.AVIATION...For the 12Z TAFS through 12Z Friday morning

VFR will prevail at all terminals through 12Z/Fri. Convection slowly
decreasing early this morning but shortwave and jet streak kept
things alive much of the night. Southwest upper-level flow persists
through Thursday ahead of the next upper low heading across the
Great Basin today. Convection will increase again around 21Z/Thu
with the best coverage across the north and central. KJAC and KCOD
the most likely terminals for thunderstorms on Thursday with KRKS
the least likely. Temperature-dew point spreads favor no more than
30-40 mph outflow wind gusts east of the Divide. Thunderstorms
should diminish by 04Z/Fri, but lingering showers are anticipated
through at least 08Z/Fri across the northwest.
&&

.FIRE WEATHER...

A slow moving weather system will move across Montana today and
bring another round of afternoon showers and thunderstorms. The best
concentration of showers and storms will be in the north and west,
but more areas will have a chance today and coverage could be a bit
greater than on Wednesday. Wind will be light to moderate in most
areas except in Sweetwater where a gusty southwest breeze will
develop this afternoon. Gusty winds are possible around any shower
or thunderstorm however. Humidity will fall below 15 percent in
southern Wyoming, but should stay higher elsewhere. Mixing and smoke
dispersal will range from poor in some of the basins to excellent in
eastern Sweetwater County.
&&

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

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Jones
FIRE WEATHER...Hattings



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