Area Forecast Discussion
Issued by NWS Riverton, WY

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970
FXUS65 KRIW 280749
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
149 AM MDT Fri Jul 28 2017

.SHORT TERM...(Today through Sunday night)
Issued at 100 AM MDT Fri Jul 28 2017

Imagery continues to show a broad upper lvl ridge across the wrn
2/3rds of the CONUS with an upstream trof ovr the wcst. The "center"
of this broad high is still located across the srn Rockies into the
srn Plains, a location quite good for pushing monsoonal flow up
north. An embedded SW continues moving through the overhead skies of
WY this morning. The SFC has seasonal general low pressure across
the swrn CONUS w/ the deepest heat induced low ovr sern AZ providing
additional direction to the monsoonal moisture flow. A weak SFC
trof/front is located meridionally down through ern WY with a meso-
high building ovr wrn WY. Best moisture across the wrn CONUS lay
from nrn MX, through AZ and NM and into UT and CO. A couple of
lingering showers/iso thunder ovr Johnson/Natrona counties near the
SFC trof/front is all that is left of the strong convection Thursday
evening.

Widely scattered showers/storms were present west of the Divide
Thursday afternoon/evening with isolated strong showers and storms
east of the Divide. With PWATs ranging between 0.75 and 1 inch, any
storm could produce brief periods of very heavy rain along its path.
Even with the relatively moist airmass in place Thursday, collapsing
storms or those with heavy rain cores where dew pt depressions
approached 40 deg F and moving > 30 kts were able to get a isolated
wind gusts between 35 and 50 kts at times. Hail size of marginal
consequence was produced over portions of cntrl/ern  WY (mainly
Johnson County) Thursday evening...but was limited in the warm
environment to generally less than a half inch with the large hail
well east going into Nebraska.

This morning the upper embedded SW trof moves out of Wyoming while
the ridge axis continues to migrate slowly ewd. The FA then begins
somewhat of a drying process from west to east as monsoonal moisture
gets held to the south behind the passage of a weak SFC front formed
by the passing SW trof. A weak upper lvl will move through the ridge
across the nrn FA by late afternoon and evening giving mainly small
isolated chances for showers/storms over nrn and ecntrl WY including
Johnson County and perhaps (to a much lesser extent) Natrona County.
ATTM, it looks like the front will clear the srn border of WY this
afternoon/evening...taking low chances for precip across the srn FA
along/near a weak SFC trof/boundary with it. If it hangs form some
reason, then the forecast may have to be revised slightly for this
possibility later. Severe convection across ern WY Thursday evening
will leave rather stable conditions across this area...keeping
strong to severe storms well east of the FA and into the cntrl
Plains.

Saturday will be even drier across the wrn FA...but more moist east
as strong lee troffing occurs east of the srn/scntrl Rockies feeding
moisture back towards ern WY. However, it also looks like an
seasonally strong MCV will be acting on this
moisture...precipitating much of it out and cutting much of it off
before it gets back into central WY. This will give much few chances
for significant vigorous convection across the FA even as another
weak upper level induced front moves s through WY...leaving terrain
features with best upslope flow aspects or convergence zones around
these features to be the only locations to see a storm or two. CAPE
and shear will both be marginal with generally slow moving and
possibly terrain/boundary tied storms with heavy rain at those
locations...gusty outflow winds elsewhere.

Sunday will generally have the ridge axis ovr the wrn half of WY
with little in the way of upper level forcing. Heat and terrain
features will be the primary areas of TS initiation and focus with a
secondary possibility for initiation occurring over/near left over
convective boundaries from the previous night or a warm type
front/boundary lifting into srn/sern WY in the evening.

.LONG TERM...(Monday through Friday)
Issued at 100 AM MDT Fri Jul 28 2017

The Subtropical Ridge becomes centered over the Great Basin early
next week, and remains in place for much of the week, before
models show the center of ridge shifts east into the Rockies late
in the week. The best combo of moisture/instability in the
extended looks to be Monday with the mean RH declining a bit into
the mid to late next week. Thus will keep widespread slight valley
PoPs/some mountain low chance PoPs Monday, but will introduce a
drying trend as the week progresses with only isolated mountain
pops by Friday as the ridge center pushes into the area.

Temperatures will be seasonal through Wednesday, but look to
return to above average by 5 to possibly as much as 10 degrees by
Friday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 100 AM MDT Fri Jul 28 2017

VFR conditions will prevail through 12z Saturday. Some patchy fog
will likely form vicinity KJAC through 14z this morning. West and
central Wyoming will have much less moisture and instability to work
with today compared to Thursday. Isolated thunderstorms are expected
to initiate over and near the Absaroka and Bighorn Ranges early this
afternoon, affecting primarily vicinity KCOD.   A few storms will
also be found near the WY/CO/UT border, but not make it near KRKS.
This thunderstorm activity will be mostly out of the area after 06z
Saturday. Winds will be light and primarily of a N-NE direction with
surface high pressure to the north, low pressure to the south.

&&

.FIRE WEATHER...
Issued AT 100 AM MDT Fri Jul 28 2017

Fuels remain at critical levels for most forecast zones east of the
Divide (Please see the Fire Weather Forecast product for more
information), below critical west of the Divide. Today, fire danger
low for most mountain locations (except Casper mountain and the
southern Bighorn Mountains. Moderate fire danger for all other lower
elevations as weak monsoonal moisture keep the lower atmosphere from
drying out too much across the forecast area. Winds will continue
seasonally light across the forecast area today. Today there will be
isolated (lower elevations) to widely scattered (mountains)
showers/storms primarily across the northern and eastern zones.
Expect areas of of localized heavy rain in the mountains and gusty
outflow winds elsewhere. Saturday, chances for isolated showers and
thunderstorms will shift to mainly along or east of the
Divide...with everyone having at least an isolated chance for
precipitation on Sunday.

&&

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

&&

$$

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



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