Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 281730

Area Forecast Discussion
National Weather Service Riverton WY
1130 AM MDT TUE JUN 28 2016

.SHORT TERM...Tuesday through Thursday night

We are seeing a change this morning as we transition from a quiet
pattern of the past few days into a more active one for the next
three. Today looks to be the most interesting. The ridge that has
brought the hot and dry weather of the past few days has sagged
south a bit with heights lowering a bit. In addition, a weak
shortwave will ride through the area and help to destabilize the
atmosphere a bit. There are still some questions as for how strong
the thunderstorms will get. Factors for the storms include a good
amount of shear that could bring tilted updrafts and some stronger
storms. There is also plenty of CAPE in the east with the models
showing as much as 1500 J/KG in Johnson and eastern Natrona Counties
with lifted indices down to minus 6. However, there are some
inhibiting factors. Although 500 millibar heights will be a bit
lower, they will still be over 5900 meters so it may take a while to
break. The biggest limiting factor though is moisture though. The
GFS keeps the 50 degree dew points out of the entire area except for
eastern Johnson County. The NAM backs them in a bit further but only
to around a Buffalo to Midwest line. In addition, the Storm
Prediction Center has removed the slight risk from the area. As a
result, we trimmed the severe area a bit. We followed to NAM 50
degree dew point line through most of Johnson County and eastern
Natrona Counties. Elsewhere, with drier air and inverted V
soundings, the main threat will be downburst winds and small hail
With the timing changed to a bit later, we also raised temperatures
a bit. Most of this activity should end by midnight as the wave
moves away to the east.

On Wednesday there will be more moisture over areas with better east
to southeast flow. However, there will be less shear. The Storm
Prediction Center only has general thunderstorms in the area as
well. So for now, we went with small hail and gusty winds. The
models also differ on the best coverage of the thunderstorms. The
NAM shows a surge of moisture from the south. The NAM is the driest
and has the best coverage in the north. The European has the best
coverage in the east. So as a result we kept isolated over all areas
with a better chance over Johnson County where the deeper moisture
will be.

A deeper plume of monsoonal moisture will move in for Wednesday
night into Thursday. As a result we increased some showers across
the area later Wednesday night into Thursday. This will also bring
more cloud cover which will keep temperatures a bit lower. This
looks to be the day with the best coverage of thunderstorms,
although possibly the least chance of severe storms.

.LONG TERM...Friday Through Tuesday

Warm ridge axis over the area to start the period gradually
flattens over the weekend leading to a potential shortwave passing
to the north around Monday night which may push a decent front
across the area. There looks like there will be quite a few
`ripples` in the flow but with limited moisture to work with
resulting in mainly diurnally driven convection centered on the
mountains during the afternoon with a few trying to move onto the
lower elevations by the evening hours. Airmass will then dry out
even more Monday into Tuesday with increasing sw flow Monday and a
generally dry wly flow next Tuesday. Quite warm each day,
especially Saturday through Monday when it looks like each day
will be in the 90s for most areas east of the divide with upper
70s and 80s west. The high country will be in the 70s to lower 80s
with 60s above 9500 to 10000 feet. Dry to very dry from the
weekend onward with min rh`s in the teens for most areas east of
the divide with some increase in wind on the 4th leading to
increased fire danger as combo of hot temps, breezy to windy conds
and low rh`s combine.


.AVIATION.../18Z Issuance/

Conditions will remain VFR through the forecast period. Shortwave
topping the ridge and moving across Montana will help to initiate
convection today, with the best coverage mainly north and east of
a line from KCOD-KCPR. Small hail and localized MVFR conditions
will be possible with the strongest storms. Best instability
will be along and east of the Bighorn Range across Johnson County
stretching into northeast Wyoming. Weaker convection will form
over the east slopes of the western/central mountains and have
left VCSH in those terminals. Showers and storms across this
region are more likely to produce gusty outflow wind to 40kts with
large temperature-dew point spreads. Convection will wane after
01Z-02Z/Wednesday as shortwave tracks east away from the forecast
area. Could be a few lingering weak showers across the far north
through 04Z with gusty outflow wind the main hazard. Gusty
west-northwest surface wind 10-20kts anticipated after 20Z at
KBPI and KPNA, while other terminals remain less than 12kts.



...Elevated fire conditions over southern Wyoming today...

An upper level disturbance will cross the area today and bring a
chance of mainly afternoon showers and thunderstorms, most numerous
in northern and eastern sections. A gusty westerly wind and relative
humidity falling into the teens will bring elevated fire danger
across the south today. Near critical fire weather is possible in
western Natrona County but red flag conditions are not expected.
Strong to possible severe thunderstorms are possible in Natrona and
Johnson Counties this afternoon and evening. Smoke dispersal and
mixing will be fair to good east of the Divide and good to excellent
across western and southern Wyoming.

.RIW Watches/Warnings/Advisories...None.



SHORT TERM...Hattings
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