Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 231742
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1142 AM MDT FRI SEP 23 2016

.SHORT TERM...Today through Sunday night

Imagery shows large deep trof/closed low slowing becoming negatively
tilted across the western CONUS. The tropical stream has now
shifted more to the east and over the high plains of CO and into
the Central Plains. However, their is still plenty of EPAC moisture
associated with the trof to take its place. SFC has low pressure to
the south across UT and CO with a nearly stationary front draped
across southern WY and a cold front to the southwest. Showers and
embedded isolated thunder across the western half of WY.

Today/tonight/Saturday, the upper trof and closed low will bottom out
across UT before making the turn to the northeast and over WY.
Rapid cyclogenesis will then take place over
northern/northeastern CO/southeastern WY through the day,
maturing by evening with. With this location for the surface low,
rapid moisture return can be expected across eastern WY and into
portions of central and northern WY including parts of Natrona and
Johnson counties...particularly northern Johnson county and the
Bighorn mountains tonight. Heavy rain will be the primary
name of the game across the lower elevations of this area with a
chance of small hail with some stronger thunder. Winds will also gust
to 40 mph or so near heavier downpours of the stronger showers
and storms. Before all this occurs however, it looks like the FA
will be under somewhat of a dry slot due to the position
of the upper trof/sfc low during the day. Heavier rain this
morning likely to be located around the Wind River and Absaroka
mountains and Big Horn Basin. Flash Flood Watch continues for all of
the western mountains through 12Z...then over northwestern WY
through 12Z Saturday. Have issued a Winter Weather Advisory for the
Bighorn Mountains beginning this evening and continuing through the
day Saturday...mainly for the northern portion of the mountains as
snow levels drop from 10k ft or so to around 8k ft by Saturday
morning. Although moisture won`t be quite as copious across the
western/northwestern mountains by this time, still wondering if an
WW Advisory might be more well suited across this area than the
Flash Flood Watch. On the fence here. Will let the day crew take a
look at the possibility. Otherwise, by Saturday morning the trof
should be exiting to the east of the FA with the surface cyclone
rapidly moving off into the northern plains. Heaviest precip across
the CWA should be located across northern WY before finally exiting
the area Saturday evening.

Sunday, the upper trof is off over the Plains as relatively strong
high pressure builds across the surface with clouds dissipating and
the sun returning with slight warming. No precip expected under this
very stable scenario.

.LONG TERM...Monday through Friday

Monday and Tuesday will be dry with high pressure over the area.
Most of the area will be dry Wednesday. However, isolated showers
or storms may develop over southwest WY late in the day Wednesday
into Wed night. On Thursday, more moisture moves into the region
as a weak weather system passes through the area. Isolated showers
and storms will suffice for now. Southwest flow aloft on Friday
with some moisture in the flow. Scattered showers and isolated
storms for the mountains with isolated activity for the lower
elevations. Mild temperatures Mon through Wed, then slightly
cooler Thurs and Fri due to more clouds around and the chance of
rain.

&&

.AVIATION.../18Z Issuance/

Upper low along the west-central Wyoming border late this morning
will gradually move east tonight and into northeast Wyoming by
midday Saturday. South-to-north oriented jet streak of 90+kts is
already enhancing precipitation from KRKS to around KPNA, KLND, and
KRIW. HRRR has timing of precipitation correct but placement is too
far east. Southerly wind will increase in this same general area
this afternoon with speeds 10-20kts, and more like 15-25kts at KRKS.
Could be some MVFR visibilities with the strongest convection after
21Z/Fri in the general area from KRKS to KRIW. Trend early this
evening would be for a quick lull in the action as upper low tracks
east almost directly through the center of the forecast area. Rain
will increase late this evening and continue overnight at KJAC,
KBPI, and KPNA. There are likely to be occasional MVFR conditions
from around 04Z-08Z/Sat, if not until 12Z/Sat, at these three
terminals. Low-end VFR expected across the basins east of the Divide
until 16Z-18Z/Sat. The exception will be KCOD where IFR or lower
conditions will persist through late Saturday morning due to low-
level upslope flow. To the west, should be improving conditions
during the same 16Z-18Z/Sat time period as less favorable northwest
flow kicks-in. This will lead to increasing northwest wind 15-25kts
at KBPI, KPNA, and KRKS during the morning.

&&

.FIRE WEATHER...

Fire danger low all areas today...lasting through at least the
weekend. A large upper low is slowly progressing across the western
US, including Wyoming, and will spin up a vigorous surface cyclone
by this afternoon. Copious moisture has made its way into the the
forecast area and will remain around the region through the day
Saturday. From Thursday through Saturday, 1 to 3 inches of rainfall
may be found around much of the region...especially the mountains.
Snow levels will drop from around 10000 feet to near 8000 tonight
through Saturday morning with some mountain locations seeing between
2 and 12 inches tonight through the day Saturday. Heaviest snowfall
potential looks to be found over the western Wind River
mountains...Tetons...southern Absarokas...and especially the Big
Horns.

&&

.RIW Watches/Warnings/Advisories...
Flash Flood Watch through late tonight for WYZ001-002-012.

Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Saturday for WYZ008-009.

Flash Flood Watch until 6 AM MDT early this morning for WYZ014-
015-024.

&&

$$

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



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