Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190502
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1102 PM MDT THU AUG 18 2016

.SHORT TERM..Tonight through Friday (Issued at 123 PM MDT)

Imagery shows large broad trough cross the western CONUS from
east to west with the axis located from northeastern MT through
central WY and southwest into central CA. Best upper dynamics with
this trough currently located from eastern MT and across WY.
Surface has lee surface trough across eastern WY with cold front
sagging into northern WY.

As the main western trough continues to move across the FA and
the cold front pushes southward through northern WY and into
central WY, colder air aloft overflowing WY will allow widespread
modest instability showers and some thunderstorms to form both
along and behind the front. This will be followed by seasonally
deep (to H7) upslope flow into the area later this afternoon.
Relatively weak instability with this fall-like system and backing
winds aloft will limit storm strength with the strongest
thunderstorms likely found near and behind the front (max shear of
25 - 35 kts and CAPE from 700 to 1100 j/kg) as it plows south
through the day. As it looks now, stronger storms will be possible
from southern Johnson county and south and west across Natrona,
Fremont, Washakie and Hot Springs counties this afternoon. Hail,
wind and brief heavy rain will be the main threats from these
storms with a few approaching strong to severe limits.

Tonight, some brief drying and breaking of skies ahead of the next
and stronger cold front being pushed into the area by a strong upper
level shortwave moving down the backside of the main upper trough.
The front should enter the northwest corner of WY after midnight
with increasing precip/showers following it into the FA. By 12z
the front will begin to cross into central WY with rather
widespread precipitation and embedded showers east of the Divide
through the morning. By afternoon, showers with a few embedded
thunderstorms will be located mainly from eastern Fremont and
across Natrona county. Small hail and gusty winds in addition to
periods of heavy rain will be the main threat.

.LONG TERM...Friday night through Thursday

The showers and thunderstorms from the strong cold front that
moved through on Friday should be ending on Friday evening. All of
the models have trended drier so we reduced POPS in the evening
and removed them for later Friday night. Saturday at this point
looks like a dry day so we dried things out for that time period
as well. With more sunshine expected for Saturday as well, we have
raised high temperatures a few degrees and they may need to be
raised some more if trends continue. Ridging will dominate the
weather for Sunday and bring a pleasant day with seasonable
temperatures along with a good deal of sunshine with little or no
chance of rain.

A plume of monsoonal moisture will then move northward into areas
west of the Divide for Monday and bring a return to mainly higher
elevation showers and thunderstorms. Mainly dry conditions are
expected to continue East of the Divide. There could be a rise in
Fire Danger for Monday as well. The relative humidity will likely be
low enough, but wind right now is borderline.

The next cold front will then swing through Tuesday into Tuesday
night. This front has little moisture to work and not a lot of
energy so the forecast looks to be mainly dry. Further out, we have
similar issues that we raised by the overnight shift with the GFS
having mainly diurnal higher elevation convection with zonal flow
and the European bringing down another system for Friday. Like
before, with the uncertainty we made few changes to continuity.&&

.AVIATION.../06Z ISSUANCE

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

Isolated showers and storms will end overnight. Scattered rain
and snow showers will occur in the mountains along the continental
divide from after 12z through 21z. Isolated rain showers could
impact KJAC and KPNA terminal sites until 18z due to the close
proximity to the mountains. Areas of MVFR/IFR will occur over
NW WY from Yellowstone National Park southeast through the
southeast end of the Wind River mountains. Conditions will improve
over NW WY after 21z and these improving conditions will spread
southeast along the continental divide through 03z Sat. Most of
Sweetwater county will see a gusty northeast wind Friday afternoon
into the evening hours as the cold front backs westward into
southwest WY.

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

A weather system and cold front will push south out of Montana
overnight and through Central WY by 18z. Scattered rain showers
will move into Northern WY overnight and spread southward through
Friday morning. MVFR conditions will develop in many locations with
local IFR conditions at times. Snow will fall in the higher mountain
elevations. Conditions will be improving over the north after 18z
with vfr conditions prevailing by 00z Sat. In Central WY conditions
will improve toward 00z Sat with vfr conditions prevailing by 03z
Saturday. The exception may be KCPR where lower clouds hang on until
06z Sat.

&&

.FIRE WEATHER...

Fire weather conditions moderate most locations with localized
elevated across portions of southwest WY. RH values will range from
the mid 20s to 60 percent for most of the area except the far
southwest which will still drop into teens. Through the day today
and again Friday the area will see a drastic change in the weather
as an early fall type weather system impacts Wyoming. A large upper
level trough will push across WY along with a couple of cold fronts,
shifting the winds out of the north to northeast, greatly decreasing
temperatures and increasing precipitation chances across the entire
forecast area. Many northern mountain locations will pick up between
a quarter of an inch and a half an inch of precipitation through
Friday with accumulating snow possible overnight tonight through
Friday morning mainly above 8500 feet. Otherwise warming and drying
expected once again over the weekend with possible elevated fire
weather conditions across southern WY on Monday.

&&

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

&&

$$

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



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