Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 270807
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
207 AM MDT Thu Jul 27 2017

.SHORT TERM...(Today through Saturday night)
Issued at 1259 AM MDT Thu Jul 27 2017

Imagery shows broad slowly amplifying ridge across the CONUS with
the high still centered across the srn Rockies into the srn Plains.
Weak embedded shortwave moving through WY...with a stronger embedded
sw trof ovr nrn California/OR/nwrn NV headed this way. Subtropical
monsoonal moisture rotating around the upper high and being directed
nwd through the Desert SW/Great Basin and into portions of the
cntrl/nrn Rockies by heat induced SFC low pressure over AZ and high
pressure over the cntrl Plains. Otherwise the SFC has general
low(er) pressure across the wrn CONUS with a front moving
into/through the PAC NW and high pressure closely following. High
pressure also exists from the nrn/cntrl Plains to the west and over
WY with a weak lee side trof down through the Big Horn Basin.
Isolated shower/isolated thunder activity west of the Divide with
exiting isolated showers/thunder also over/near the ern borders of
Johnson and Natrona Counties.

While there were isolated areas of heavy rain Wednesday, the overall
convective situation with upper level forcing not arriving until
late in the day as the diurnal forcing was waning limited the extent
of precipitation even over mountains and a SFC boundary east of the
Divide. Best rainfall was noted across srn Fremont County, srn
Johnson County, and Natrona County.

Today moisture remains across the FA for another day while the upper
ridge continues to translate ewd toward the ern border of WY while
what`s left of the embedded wrn coastal low moves through the
increasingly southwest flow aloft later this afternoon and evening,
also pushing a weak front through the area, giving another day of
localized significant rainfall potential (but probably less flooding
potential as storm movement will be somewhat quicker). There should
also be a little better chance  for a few more storms on the
strong/marginally severe side found across WY...especially the ern
portion of WY especially over Natrona and Johnson counties in the
late afternoon/evening...and to a lesser extent over wcntrl WY
beginning mid to late afternoon. ML CAPE will range from 500 to 1100
J/Kg with effective shear from 20 to 30 kts. Although hail should
not get to 1 inch values, upper lvl temps will decrease some from
what they were on Wednesday...so at least some small hail will be a
possibility (whereas Wednesday it really wasn`t). Storm winds will
probably max out in the 25 to 35 kt west 35 to 50 kts over the
southern and ecntrl portions of WY. PWs will still be on the
statistically high side ranging from 0.75 to 1 inch...so an isolated
case of areal/flash flooding will be possible...especially over the
same locations that received some heavy rain on Wednesday. This
would point to swrn Johnson County near Barnum and across the srn
part of the Bighorn Mountains over nwrn Natrona County.

Friday, the ridge axis continues to move eastward and the FA begins
to dry out some from the west. Another weak upper lvl disturbance
will move across the nrn FA by late afternoon and evening giving
mainly small isolated chances for showers/storms over nrn and ecntrl
WY including Johnson and Natrona counties. There will also be low
chances for precip across the srn FA along/near a weak SFC
trof/boundary. Strong to severe storms look to remain east of the FA
over ern WY and into the cntrl Plains.

Saturday will be even drier with fewer chances for showers/thunder
as the monsoonal moisture gets cut-off from the forecast area and is
routed across the srn Rockies and into the cntrl Plains. Most
chances for convection will occur over the higher terrain of the FA
or (to an even lesser extent) along/near another SFC
trof/boundary.

.LONG TERM...(Sunday through Thursday)
Issued at 1259 AM MDT Thu Jul 27 2017

Through much of the extended the subtropical ridge will generally
be centered over the Great Basin, but will be not as well defined
or as strong as in previous weeks. The bulk of the monsoonal
moisture looks to remain south of the area, but plenty of moisture
remains with daytime heating for daily afternoon/evening
convection lingering late into the night. Currently models are
showing a disturbance or two impacting the area Sunday and/or
Monday which would help enhance activity across the forecast
areas those days. The slow steering currents Sunday and Monday
could also pose a heavy rain risk, but should limit the strength
of the convection.

Temperatures are expected to be very warm, but still near
seasonal levels for this time of year.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 1259 AM MDT Thu Jul 27 2017

Some patchy fog will likely form vicinity KJAC until around 14z.

An upper low will shear across northern Wyoming Thursday evening.
Widely scattered thunderstorms will develop primarily vicinity and
north of a KJAC-KCPR line with activity more isolated south of this
line, primarily from 21z Thursday until 03z Friday.  A few
marginally severe thunderstorms with hail and wind gusts near 40
knots will be possible in the vicinity and east of Interstate 25, or
KBYG-KCPR.  TSRA/SHRA activity should become isolated across the
region after 03z Friday.

&&

.FIRE WEATHER...
Issued AT 1259 AM MDT Thu Jul 27 2017

Fuels remain at critical levels for most forecast zones east of the
Divide (Please see the Fire Weather Forecast product for more
information). Today, fire danger low for most mountain
locations...low to moderate for all other lower elevations as
monsoonal moisture and precipitation chances continue across the
forecast area. Winds will continue seasonally light across the
forecast area today with the day`s gustiest winds found near
stronger showers or thunderstorms (25 to 35 mph across the wester
forecast area...35 to 55 mph east of the Divide and over southern
Wyoming). Today there will be isolated to widely scattered
showers/storms across the lower elevations...and widely scattered to
scattered chances over the mountains. Expect periods of of localized
heavy rain, small hail and gusty winds with stronger showers and
storms. Drier weather and less chances for precipitation return
Friday.

&&

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

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...Meunier
FIRE WEATHER...Braun



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