Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181925
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
125 PM MDT Tue Jul 18 2017

.SHORT TERM...(This evening through Friday) Issued at 122 PM MDT
Tue Jul 18 2017

Imagery shows flat ridging across the CONUS with an embedded
shortwave moving through the flow over the ncntrl CONUS. Center of
flat ridge extends from the cntrl/srn Plains wwd across the
srn/scntrl Rockies. Wyoming is currently under modest slight sw to
nearly zonal flow aloft. SFC has general low(er) pressure across the
wrn CONUS with high(er) pressure to the east and a cool front now
located to the east and south of WY. Monsoonal flow conditions
continue to be maintained from the Desert SW region to the north or
northeast daily. The most current pattern has the best moisture
headed to the northeast and into the cntrl Plains with portions of
AZ/NM/CO caught in the cross-hairs. ATTM...no precip is falling over
the FA.

Rest of today/tonight, upper level flow becomes nearly zonal over WY
while the mid/upper atmosphere slowly dries out from west to east
across the FA as the monsoon gets held back over the srn/scntrl
Rockies and Desert Southwest with lee side low pressure (trof)
forming over ern CO. Low lvl moisture will be present but mostly
capped to convection. The generally mid lvl flow/feed into WY under
these conditions will limit moisture return to far ern WY and
perhaps portions of nrn WY where a small chance for an isolated
afternoon/evening storm may be possible... with Johnson County this
evening being the most likely locations (if the cap can be
overcome). TS initiation, if it occurs, will be elevated and
primarily a rain/sm hail/wind producer. As DCAPE values will likely
be well above 1000 J/Kg, severe wind gusts would not be out of the
question...again, if it happens.

Wednesday, slightly better but modest ridging returns to the region
with monsoonal flow looking for another surge nwd through the
srn/cntrl Rockies and into portions of srn WY Wednesday afternoon
and evening. The lee side low over CO will slowly migrate
vicinity/sern WY/nern CO/wrn NE by Wed night which may also aid in
precip production across srn and scntrl WY including ern Sweetwater
and srn Natrona Counties as a very complex LL flow pattern evolves
under the area of weak forcing aloft. Heavy orographically forced
and possible MCV induced rainfall expected across the mountains of
UT and CO will limit the amount of moisture that is able to get into
this FA...so hard to pinpoint possible localized heavy rainfall in
this FA ATTM. Weak frontogenesis may occur over a SFC low pressure
trof extending into cntrl/srn WY from the low pressure system to the
east and this may provide a mesoscale LL convergence zone to help
focus showers/storms for a period of time.

Thursday, as early morning activity exits into the Plains along with
the stronger SFC/LL forcing while the monsoon will begin to pump a
little broader area of moisture into the FA into/through swrn/scntrl
WY. Heavy rain with slow shower/storm movement will be possible with
what looks like possible isolated flooding situation over portions
of southern WY late Thursday afternoon (and into the evening) as PWs
soar to an inch or more. This will tend to be a (regionally) high
CAPE/low(er) shear day. Small hail possible...and maybe a wet
microburst. All this a portent of coming attractions over the next
month or so as the monsoon settles in. Another MCV heads off into
the High Plains overnight.

Friday, looks to dry out significantly from top to bottom as a rela-
tively dry SW trof aloft and SFC front move through WY while the
deep circulation pattern draws dry air in from the swrn CONUS over
the top of the subtropical flow in the Desert SW...keeping the
monsoon at bay this day. The only chance for isolated precip will
occur across far srn portions (border huggers) of WY in the
afternoon.

.LONG TERM...(Friday night through Tuesday) Issued at 122 PM MDT
Tue Jul 18 2017

Medium range period continues to be dominated by upper high/ridge to
the south with very warm/hot temperatures continuing. The GFS seems
overdone it how far south the nrn stream is as mean ridge holds over
the area through the period. Lean more towards a Euro/GEM type
solution. There could still be a shortwave passing off to the north
Monday with a weaker one coming through our area which may be enough
to fire a few storms in the nrn border area and around the mtns. On
Tuesday, a better wave may across from the w/sw with a better
coverage of mountain storms and a few across some of the lower
elevations. Lower elevations east of the divide will remain mostly
in the 90s with 70s and 80s west. The GFS with its closer nrn stream
has much cooler conds in the far north from the weekend on but I
think the Euro and GEM idea with the ridge predominating will be
more right with hotter temps continuing. Clouds and possible
showers/storms on Tuesday could cool some areas similar to
yesterday`s system. But overall, continued quite warm to hot through
the period.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 122 PM MDT Tue Jul 18 2017

VFR conditions will continue through Wednesday. There will be very
little if any convection/storms around this afternoon and evening.
If any form, they will likely stay over the mountains and perhaps
over Johnson County. On Wednesday, a batch of moisture will move up
from the southwest giving the southeast half of the area isolated to
widely scattered gusty showers and thunderstorms in the afternoon
and evening.

&&

.FIRE WEATHER...
Issued AT 122 PM MDT Tue Jul 18 2017

Fuels are now at critical levels for most forecast zones east of the
Divide (Please see the Fire Weather Forecast product for more
information). Today through Friday, fire danger low to moderate for
most mountain locations...elevated across Yellowstone due to low RH
values. Elevated for all other lower elevations due to very warm to
hot daytime temperatures and low humidity levels. Winds, however,
will continue relatively light across the entire forecast area today
through Friday with typical diurnal somewhat gusty afternoon winds
found along and west of the Divide. Exceptions will occur near any
shower/storm today where winds could briefly gust 25 to 45 mph or
more...especially over north central WY. Only an isolated
shower/storm will be found around the forecast area...with the best
chance for an isolated storm over Johnson County this evening.
Otherwise, expect isolated to widely scattered showers/storms over
southern Wyoming on Wednesday followed by possible localized heavy
rain across southern Wyoming on Thursday. Warm/hot/dry on Friday
with the only outside chance for precipitation occurring near the
CO/WY border in the afternoon/evening.

&&

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

$$

SHORT TERM...Braun
LONG TERM...Skrbac
AVIATION...Skrbac
FIRE WEATHER...Braun


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