Area Forecast Discussion
Issued by NWS Riverton, WY

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356
FXUS65 KRIW 201730
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1130 AM MDT Sat May 20 2017

.SHORT TERM...Saturday through Monday night
Issued at 249 AM MDT Sat May 20 2017

Any showers out there early this morning should end before sunrise
and the balance of the morning should be mainly dry. However, the
dry period won`t last very long as a shortwave rotates into the
area. This will touch off some isolated to scattered showers and
thunderstorms across the area this afternoon and into the evening.
As for the main threat, with temperatures rather cool, in addition
to low wet bulb zero levels, small hail will be a possibility with
any thunderstorm. Mid level winds are not that strong though and
with temperatures / dew point spreads not that much, strong wind
should not be a threat, or any more than any other thunderstorm in
Wyoming. Temperatures will generally average around 10 degrees
warmer than yesterday, but still well below normal for the middle of
May. Some instability will linger tonight as will some showers. With
the troughiness and cyclonic curvature hanging around the area,
there will again be a chance of showers and thunderstorms on Sunday.
All in all, Sunday looks fairly similar to Saturday with most areas
having a chance of showers and thunderstorms but likely dry most of
the day. Temperatures will also be similar to Saturday. As for snow,
with the warmer temperatures, this would be restricted to above pass
level and accumulations would not be very heavy.

Yet another system will drop in from the north of Monday. At this
point, most of the forcing and moisture looks to miss the area to
the east. However, there will be enough for another round of showers
and thunderstorms. A few inches of snow will also be possible in the
higher elevations of the Bighorn Range with the favorable
northwesterly flow. In addition, this looks to be a windier event.
700 millibar winds climb to around 45 knots across Johnson County
during the day on Monday with MOS showing sustained winds around 30
knots with the passage. At this point, it does not look to quite
reach high wind criteria but it will have to be watched. Quiet
weather should return Monday night after the trough moves away and
ridging slowly builds across the area.

.LONG TERM...(Tuesday through Saturday)
Issued at 249 AM MDT Sat May 20 2017

Tuesday will be dry with some fair weather clouds around during
the day. Temps will warm up some on Tuesday with the coolest highs
in the far east zones. Wednesday looks dry with windy areas and
warm. This will be ahead of a cold front which arrives into far
northern WY at daybreak Thursday. The weather pattern next Thurs
through Saturday looks unsettled with a frontal boundary lying
across the region and southwest flow aloft. A series of weather
disturbances rotate through this southwest flow and pass through
WY. This sets the stage for daily shower and thunderstorm activity
over the region with more coverage over the north and central and
less in the south. Could see some strong storms on Thurs and Fri
afternoon and evening. Cooler highs Thurs and Fri all areas. Next
Saturday may see rain showers and mountain snow showers along
with even colder high temps.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday)
Issued at 1130 AM MDT Sat May 20 2017

West of the Continental Divide...KBPI/KJAC/KPNA/KRKS Terminals

Shortwave over northern Idaho will plunge south toward western
Wyoming today and tonight. Enough moisture and instability
associated with this disturbance to generate convection, the
beginnings of which can already be seen in satellite imagery late
Saturday morning. Lifted indices of -1C to -2C expected by 21Z/Sat,
and expect that small hail will be possible with this convection.
Expect VFR conditions to prevail through the forecast period except
in conjunction with the heavier showers/storms. Thunderstorms will
wane between 01Z-04Z/Sun with loss of solar heating, but the
expectation is that light showers will continue through about
08Z/Sun. Have extended VCSH and mid-cloud decks through 08Z/Sun as
shortwave shears and elongates across southwest Wyoming. Gusty
northwest wind 10-20kts expected at KBPI, KPNA, and KRKS this
afternoon before decreasing around 02Z/Sun. Mountain top
obscurations will be frequent this afternoon and evening due to the
convection.

East of the Continental Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

Cumulus already building over the higher terrain of central and
northern Wyoming late this morning. Most terminals to have VCSH
through at least this evening as shortwave over northern Idaho drops
south and eventually shears and elongates across southwest Wyoming.
KCPR will be the least likely location to see showers through the
afternoon.
Convection will weaken and decrease in coverage this evening with the
setting sun. The location of this weak wave later tonight could favor
some weak upslope at KLND, KRIW, and potentially KCPR. While VFR
conditions will generally prevail, have brought low-end VFR to KLND
and KRIW late tonight and Sunday morning given low-level moisture
availability and weak upslope. Mountain top obscurations will be
frequent this afternoon and evening due to the convection, and again
along the east slopes of the Wind River Range Sunday morning.

&&

.FIRE WEATHER...
Issued at 249 AM MDT Sat May 20 2017

Unsettled weather will continue into the weekend. An upper level
disturbance will move across the area this weekend and bring the
chance of mainly afternoon and evening showers and thunderstorms
both Saturday and Sunday. Wind should generally remain light to
moderate except around any thunderstorm. Relative humidity should
remain well above critical levels with temperatures continuing to
average below normal. Mixing and smoke dispersal will be generally
fair to good.

&&

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

&&

$$

SHORT TERM...Hattings
LONG TERM...AR
AVIATION...AEM
FIRE WEATHER...Hattings



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