Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 220857
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
257 AM MDT Fri Sep 22 2017

.SHORT TERM...(Today through Sunday night)
Issued at 250 AM MDT Fri Sep 22 2017

Imagery shows amplified trof over the wrn CONUS with embedded closed
low centered ovr ID. An amplifying/broadening high pressure ridge is
currently across the ern CONUS (with the exception of the embedded
TS JOSE still east of Long Island, NY). Strong upper jet loops
around the wrn upper trof with the the most intense section
located from srn California nosing over wrn WY. The exit region of
this embedded jet streak now translating ewd into/through wrn WY.
SFC has low pressure around most of the wrn CONUS with a cold
front draped from the nrn Plains through srn WY and then down into
and out of srn California. Widespread precipitation...both rain
and snow...stretches from nrn/cntrl UT through ern ID/wrn WY and
into cntrl MT.

Copious precipitation continues this morning in earnest west of the
Divide...just starting to move east of the Divide across ncntrl WY.
The hard part is determining/forecasting where it will rain/ mix/or
snow with such an elongated north/south orientation on the east side
of trof. Further displacement of the deep upper trof ewd this will
provide the impetus to shift precipitation and colder air more
fully eastward and eventually east of the Divide by this
afternoon. With precipitation intensifying again this morning
coinciding with the arrival of slightly colder air, expecting much
of the area across the west to turn to snow or rain/snow mix this
morning even in the valleys. Most significant accumulating
snowfall will, however, be confined to elevations above 7500 feet
west of the Divide with heaviest precipitation - both rain and
snow -occurring from now into mid morning before tapering off.
Heaviest precip and coldest air this morning will be located
across Teton and Park counties...which is where to expect heaviest
snow above 7500 feet. Advisories are currently in place for most
of the mountainous region over nwrn/cntrl WY into/through this
afternoon. Thunder will also be possible this afternoon across
Sweetwater and srn Fremont counties...coincident within the warm
sector along and south of the front.

Beyond this period, the upper high pressure ridge across the ern
CONUS continues to strengthen and expand while the upper trof over
the western CONUS slows/stalls and digs. The upper trof and
associated jet will spawn lee cyclogenesis across nern CO which in
turn will pull the front down to near the srn border region with
CO. All the while, the base/axis of the upper trof/closed low will
only translate as far east as WY (from the ID starting point)
through the forecast period. This will keep a nearly stationary
front draped backdoor style from the Front Range of Colorado
up/along the Divide through WY while another front/boundary
lays/stays across srn WY. Periodic bouts of precipitation will
also continue throughout the forecast period with rain/rain-snow
mixes and light snow all possible even east of the Divide across
the lower elevations from Saturday night through Sunday night.
From now through tonight...precipitation amounts from 0.3 inch to
as much as 1 inch of liquid will be common west of the Divide
with from a few hundredths to as much as a half inch possible east
of the Divide. From later tonight through the day
Saturday...heavier precip will shift east of the Divide...ranging
from a few hundredths to around a quarter of an inch for most
locations. Sunday through Sunday night, moisture return and both
low and upper level forcing will increase, bringing precip back
across the FA...especially ern/cntrl/and srn WY with precip
amounts ranging from less than a tenth across the north...to as
much as 0.5 - 1.0 inches over srn and cntrl WY. Unseasonably cold
throughout with little in the way of fire danger.

.LONG TERM...(Monday through Friday)
Issued at 250 AM MDT Fri Sep 22 2017

Unsettled weather continues Monday as the upper level low lifts
northeast through WY. Thus a chance of rain and mountain snow
over the area. High temps will remain cold due to the clouds and
precipitation. Tuesday looks drier over the region. The ECMWF
model keeps the area dry Tue and Tue night as it takes energy well
west of WY and forms a circulation over southern Nevada by Wed
morning. The GFS model wants to bring another weather system into
the region Tue afternoon through Tue night with scattered rain
and snow showers. Leaned toward the GFS model and will have low
chance of precipitation Tue afternoon through Tue night. High
temps still on the cool side Tue. On Wed, the GFS keeps isolated
showers around the area while the ECMWF model is dry. Left
isolated showers in the mountains for now for Wed into Wed
evening. A little milder Wed. Next Thurs and Fri appear dry and
warmer. The ECMWF model wants to bring a weather system through
next Fri and Fri night. This seems too fast and will not buy into
this scenario at the present time.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday)
Issued at 250 AM MDT Fri Sep 22 2017

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

Current satellite and radar trends along with high-resolution models
continuing to support precipitation decreasing across the far west
early this morning. Jet dynamics with the nose of 100kt+ jet have
been a driver in enhancing precipitation since Thursday evening.
This band of precipitation will continue to push east with drier
conditions in its wake Friday morning. Another round of
precipitation will reach the west around midday through Friday
afternoon as a shortwave pinwheels north along the western border.
KJAC should see some improvement after 12Z/Fri before this second
round of showers impacts KJAC during the afternoon. Could again see
MVFR or local IFR at KJAC between 20Z/Fri and 02Z/Sat. Best dynamics
and moisture will slide east after 00Z/Sat with improving conditions
Friday night. KBPI, KPNA, and KRKS terminals should remain VFR
through the forecast period. Band of precipitation sweeps across
these terminals between about 11Z and 16Z/Fri, so there could be
some brief MVFR ceilings. Best bet would be at KBPI and KPNA.
Another round of precipitation looks to arrive between 00Z-06Z/Sat,
representing the next best opportunity for MVFR. Winds will stay
west or southwest at these three terminals, before cold front drops
south between 03Z-06Z/Sat and winds begin to veer to the northwest
and eventually north to northeast after 06Z/Sat. Mountain top
obscurations will be widespread throughout the period, with frequent
mountain obscurations over the far west mountains until 00Z/Sat.

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

Current satellite and radar trends along with high-resolution models
continuing to support band of precipitation trudging east through
mid-morning. There could still be a brief period of rain showers at
all but KCPR from 11Z-15Z/Fri. Jet dynamics the main driver in
sustaining this band of precipitation. KCOD will continue to see
IFR/MVFR through most of the forecast period. Best chance of heavier
precipitation at KCOD will come with the eastward-moving band
between 11Z-15Z/Fri, and then again after 00Z/Sat. Other terminals
will be VFR until moisture in southwest flow aloft overruns dome of
cooler air in place east of the Divide after 00Z/Sat. This scenario
could allow for a better chance of MVFR conditions after 03Z/Sat
through the end of the period. KRIW appears to be the favored
terminal in this setting. Northerly surface flow around 10kts to
remain in place Friday. Another northerly surge will push south
across the terminals between 00Z/Sat and 06Z/Sat with 10-20kt winds
anticipated. Best chance for mountain obscurations will be along the
east slopes of the Absaroka and Bighorn ranges during the period.
Mountain top obscurations will be widespread throughout.

&&

.FIRE WEATHER...
Issued AT 250 AM MDT Fri Sep 22 2017

Fuels at critical levels for most of the low elevation forecast
zones east of the Divide and across southern Wyoming. (Please see
the Fire Weather Forecast product for more information).

Today through the weekend will be cool/seasonally cold most of the
forecast area. Additionally, copious moisture/rainfall/mountain
snowfall will expand from the west across most of Wyoming
today...and continue through Monday. Significant precipitation
values can be expected in one form or another...with everyone having
a chance to get from a half inch to as much as an inch and a half of
liquid. While winds will periodically gust 20 to 30 mph at
time...high minimum relative humidity values and precipitation will
keep fire danger low.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT this evening for WYZ001-
002.

Winter Weather Advisory until noon MDT today for WYZ014-015.

Winter Weather Advisory until 3 PM MDT this afternoon for WYZ012.

&&

$$

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



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