Area Forecast Discussion
Issued by NWS Riverton, WY

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556
FXUS65 KRIW 170950
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
350 AM MDT Sun Sep 17 2017

.SHORT TERM...(Today through Tuesday night)
Issued at 3 AM MDT Sun Sep 17 2017

Imagery shows relatively deep amplified trof across the western
CONUS with a ridge over the eastern CONUS. Two trof axes associated
with the central low located over scntrl Canada extend to the east
and through the ern Northern Plains...and to the west down the
coastal states. The SFC has strong post frontal high pressure across
the nrn/cntrl Rockies and High/Northern/Central Plains including
Wyoming. No precipitation is falling in Wyoming.

Today, brief shortwave ridging aloft and high pressure at the
surface will allow for relatively stable, sunny to partly cloudy
skies with virtually no chance for precipitation and warming/drying
surface conditions. Tonight will see a transition to near zonal flow
aloft before repositioning to southwest flow on Monday ahead of the
next rather strong upper level trof. With weak lee side troffing,
(dry) warming at the surface and relatively cool temperatures aloft -
dry adiabatic lapse rates will be strong enough to allow
strengthening winds above to get near the SFC...increasing SFC winds
both today and Monday. Today will see west to southwest winds across
the western FA - in addition to the Wind Corridor - gusting 20 to 30
kts...while on Monday winds will increase a bit more and gust 25 to
45 kts out of the southwest across much of the area. Isolated
showers/thunder (possible) across the western mountains Monday
afternoon will give way to the effects of a large trof and attending
cold front Monday night. This system will approach the FA from the
west...moving into/through northwestern/western Wyoming late Monday
evening and overnight. Precipitation chances increase rapidly with
what now looks like another period of significant EPAC
precipitation. Snow levels will begin rather high at well above
10.5k ft Monday evening...lowering to around 6.5k ft around sun-up
Tuesday. Accumulating snowfall looks to begin sometime early Tuesday
morning, continuing into Tuesday evening with what looks to be sub-
Advisory snow amounts ATTM.

.LONG TERM...(Wednesday through Saturday)
Issued at 3 AM MDT Sun Sep 17 2017

The flow on Wednesday is expected to back to the southwest, and
remain strong ahead of a digging longwave trough across the
western U.S. A distinct shortwave trough in the flow is expected
to rapidly push east northeast from the Pacific Northwest into
Montana Wednesday. As all of this occurs a strong SW-NE 700mb
baroclinic zone will develop across western Wyoming with 700mb
temps going from near 0C at KJAC to +8C at KRIW. The far
west/northwest sections of the area will see increasing lift with
this first wave in addition some favorable upper jet dynamics
spreading over the area. Farther south, and areas east of the
Divide look to be mainly dry with downsloping. The main issue east
of the Divide will be the wind. The wind corridor between RKS-
CPR could approach high wind criteria due to mixing, but a
relatively weak surface gradient will limit the extent if any high
wind.

Wednesday night/Thursday morning, precipitation chances spread to
include all of the area as the baroclinic zone pushes east across
the area and some upper jet dynamics overspread the area. However
overall forcing looks to be in a bit of a lull.

This should change, based on current guidance Late Thursday/Thursday
night into Friday morning as the brunt of the main longwave trough
impacts the region. Models are showing the potential for the
forecast area to be influenced by both the right entrance region
of a departing jet streak, and the left exit region of the main
jet rounding the bottom of the trough. Models show the baroclinic
zone tightening up again late Thursday into Thursday evening
across central/southeast Wyoming with the possibility of strong
isentropic lift up and over over the baroclinic zone. This could
bring significant precipitation to parts of the area with an
emphasis along and east of the Divide. Temperature profiles look
to be mainly valley rain/mountain snow.

By Friday afternoon, the main dynamics shift away from the area,
but the overall trough will remain through the weekend with cool
and possibly inclement weather continuing through the rest of the
long-term.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 3 AM MDT Sun Sep 17 2017

VFR conditions will occur at the terminal sites through 12Z
Monday. Isolated showers will occur this afternoon into early
evening across far southern WY.

&&

.FIRE WEATHER...
Issued AT 3 AM MDT Sun Sep 17 2017

Fuels at critical levels for most of the low elevation forecast
zones in addition to many mountain zones below 8500 feet. (Please
see the Fire Weather Forecast product for more information).

Today will be somewhat warmer and drier than Saturday with no
precipitation expected across the forecast area. Winds will be
rather gusty along and west of the Divide in addition to the Wind
Corridor...gusting 20 to 25 mph out of the west to southwest at
times. Monday will be slightly warmer and drier yet with winds
gusting out of the southwest 25 to 45 mph at times over much of the
forecast area. Minimum relative humidity today and Monday, however,
will generally range near or above 20 percent. Smoke dispersion in
the afternoons will tend from fair to very good today...then very
good to excellent Monday. The next period of significant
precipitation will begin Monday night, lasting into Tuesday evening.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Freeze Warning until 9 AM MDT this morning for WYZ003>006-010-
011-013-016>020-023-025>030.

&&

$$

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



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