Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 210540
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1040 PM MST Sat Jan 20 2018

.SHORT TERM...(This evening through Tuesday)
Issued at 227 PM MST Sat Jan 20 2018

...Winter Storm Warnings in place for much of southern and central
WY with assorted Winter Weather Advisories adjacent to the Warnings
to the northwest all continuing through tonight...

Imagery shows broad flat low amplitude ridging across the eastern
2/3rds of the CONUS with a digging and developing trof moving
into/through the Great Basin and Desert SW. A long strong jet
extends from southern California coast through the Great Basin
region of UT/WY and into sern MT, ND and through northern MN
to/north of the Great Lakes. A strong baroclinic signature is in
place over UT/CO/WY and into the Plains with an associated WCB
flowing across the region from Baja. The SFC has strong
frontogenesis ongoing across central/southern WY and into nern CO
with also strong cyclogenesis proceeding over ern/sern CO. Light to
moderate precipitation continues to develop across srn/cntrl/sern
WY...most as snow.

A strong period of transition continues today, albeit a bit slower
than originally thought, with the approach and intensification of
the western CONUS trof/jet and WCB. The main emphasis has now
shifted east of the Divide across cntrl WY in addition to southern
WY. Embedded transverse waves within the flow aloft are showing in
WV Imagery across CO and into srn/cntrl WY. Strong areas of
isentropic lift aloft are combining with low level upslope
flow/convergence, continuing frontogenesis, and the and now strong
cyclogenesis over ern/sern Colorado Plains. All these aforementioned
details are now rapidly coming together and starting to phase. Best
conditions will max out this afternoon and evening expanding and
strengthening areas of snowfall spreading out across the developing
cold front and other areas of LL convergence
(southwest/south/central WY). Meanwhile, strong cyclogenesis will
continue to proceed across eastern/southeastern CO with a secondary
WCB starting to come into play Sunday morning with likely instant
occlusion processes taking over by that time...and finally pulling
the front and snow out of the FA. Additionally, areas of maximum low
level convergence will occur along/near the front and near local
terrain induced convergence zones from the southern Bighorn
Mountains, across western Natrona/eastern/southern Fremont County
and into the Lander Foothills. For current total snow amounts
through tonight, please see the current Winter Storm Product or
associated Graphical product on the NWS Riverton web page.

Again, through Sunday morning, this mess of a system gets pulled out
of the FA and into the High Plains of CO/Central Plains of KS/NE as
strong instant occlusion cyclogenesis takes place. Post frontal cold
temperatures will lay in the wake of the system. Monday morning low
temperatures may have to be lowered more than they already have
depending on departing/returning cloud cover within a brief period
of shortwave ridging. Then Monday, northwest flow aloft returns
while an embedded shortwave trof moves through the flow and into the
west and precip chances once again increase. However, dynamic and
moisture with this system will be significantly less than with the
current system and only light snowfall accumulations. This system
will quickly exit the area overnight Monday leaving most of the FA
without precipitation through the day Tuesday.

.LONG TERM...(Tuesday night through Saturday)
Issued at 227 PM MST Sat Jan 20 2018

Overview...The weather pattern will continue to be more active as we
head into the latter part of the extended forecast period. The
weather impacts are expected to still be snow that is expected to
move east of the Divide from Thursday night into Saturday. Mainly
dry weather is expected otherwise, except for the western mountains.
Temperatures will trend milder ahead of this system Wednesday and
Thursday, before turning colder Friday and Saturday.

Discussion...Between Tuesday night and Wednesday, an upper ridge
axis will move across the region, with dry and warmer temps expected
along with southwest winds expected to increase across southwest
Wyoming extending northeast into Natrona county. The next storm
system to affect the region will originate out of an upper trough
that will move east-southeast out of the eastern Pacific onto the
west coast on Thursday and into the Rocky mountain region by Friday.

Both ECMWF and GFS progressively move a cold front east-southeast
through Wyoming by Friday, with snow expected to move into the west
by Thursday. The models spread the snow eastward along and on the
heels of the cold frontal passage and will continue with chance pops
across the forecast area from west to east from Thursday night
through Friday.

Timing has not changed much involving the placement of the strongest
baroclinic zone and upper jet support across the area by late
Thursday night/Friday morning, when snowfall should be at its peak.
The models are still in good agreement involving progressively
moving the upper trough and surface cold front east-southeast of the
region after Friday in the progressive pattern. At the end of the
extended forecast period, the trailing upper ridge will build across
the region over next weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday)
Issued at 1030 PM MST Sat Jan 20 2018

West of the Continental Divide...KJAC, KBPI, KPNA and KRKS

VFR conditions will prevail through the period. Local MVFR
conditions will occur through 12Z with low clouds and light snow.
This will mainly occur around KRKS until 13Z.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

Northern WY will see VFR conditions through the period. MVFR to IFR
conditions will prevail through 18Z in central WY from KLND to KCPR.
VFR conditions will prevail after 18Z in this area.

Please see individual terminal sites for more details. Additionally,
the Aviation Weather Center will have the latest information on
icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 227 PM MST Sat Jan 20 2018

A strong winter weather system  continues to develop across southern
and central Wyoming today through tonight with significant areas of
snowfall possible. Please see the Winter Weather Product product for
more details. Smoke dispersion will generally be fair to good
today and poor to fair Sunday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 5 AM MST Sunday for WYZ018>020-022-
027>030.

Winter Weather Advisory until 5 AM MST Sunday for WYZ008-009-011-
014-015-017-026.

&&

$$

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


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