Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 101737
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1037 AM MST Tue Jan 10 2017

.SHORT TERM...Tuesday through Thursday night

Given the contrast in the conditions between the regions...we will
again divide the discussion between West and East of the Divide.

West of the Divide...As I write this around midnight, much of the
area is in a lull between systems as the first system moves out of
the area. There still appear to be two more main shots of snow. We
did make a few tweaks to the snowfall, but the overall result
remains the same. The models have backed off on the intensity of the
piece today so we did lower amounts somewhat during the day.
However, the final piece of energy for the main event moving into
the area appears a bit stronger so we increased QPF and snowfall
amounts from Tuesday night into Wednesday morning when the bulk of
the snow will fall...especially tonight when there could likely be 1
to 2 inch snowfall rates at times. At this time, it appears the worst
of the snow would be over sometime during the afternoon on Wednesday
although light to occasionally moderate snow will continue into
Wednesday night as well. Some light snowfall will continue into
Thursday as well, especially for areas favored by northwesterly
flow. However, this will likely be more banded and difficult to
pinpoint exactly where things will set up. Temperatures may actually
fall during the day on Thursday as Arctic air drains into the area
from East of the Divide. The snow looks to finally come to an end
Thursday night as high pressure builds across the area.

East of the Divide...Even here we have multiple concerns. The main
concern today will be wind. Across the Cody foothills, the wind has
decreased and we will cancel the high wind warning in that vicinity.
There is even some question further south. MOS guidance has dropped
below high wind criteria. However, 700 millibar winds still look to
increase again to 60 to 65 knots and with another trough moving
across the area to mix the atmosphere, we will continue the high
wind warning although the strongest winds would be in the afternoon
and at night. As for temperatures, many areas will be above freezing
again with gusty breezes mixing the atmosphere. As for the chance of
precipitation, we do have some POPS but anything that falls during
the day would be mainly light.

However, things may change tonight, especially across the northern
border. Another cold front will drop southward into the area. A
couple of the models are showing some overrunning precipitation
breaking out as it drops south. There will be some decent lift and
some decent jet support. However, there is not a tremendous amount
of moisture and this could be a limited factor. We have been burned
in this area with snow before however. As of right now, I would lean
more toward an advisory but for now, we will punt back to the day
crew and let them make the final decision. The GFS has also been
consistent in bringing some snow down into Johnson and Natrona
County as well later tonight into Wednesday morning. For now, we
raised POPS a bit but advisories may be needed here as well,
especially with the timing around rush hour. Some snow will be
possible everywhere during this time. High temperatures will be
tricky through depending on the timing of the front. As a result, we
made few changes.

By Thursday, dry and cold weather will return to the area. The core
of the coldest air should not move into the area. As a result,
temperatures should not be as cold as the last cold outbreak.
However, we did go cooler than guidance, especially in the basins
where inversions may set up.

.LONG TERM...Friday through Tuesday

Synopsis...After a stormy period in the short-term, split flow will
bring a more benign weather pattern across the area through the
extended period with mainly dry conditions; near normal
temperatures returning over the weekend.

Discussion...On Friday, a southwest upper low off the southern
California coast will dig south into the Baja region, with
deformation area near the southern Wyoming border. Mostly clear,
cool and dry conditions will prevail across all but the far south
through Saturday. Inversions over the lower central basins and
western valleys will keep highs mostly in the teens, with 20s to
near 30 across the wind corridors and near the foothills above the
inversions.

10/00z GFS is a stronger and further south than the 10/00z ECMWF
with a northern stream shortwave crossing the northern Rockies on
Sunday as SW low lifts out of the southern Rockies.  Forecast puts
low PoPs in Yellowstone National Park and the Bighorns to cover this
scenario.

GFS and ECMWF continue to show significant spread in the northern
stream Monday and Tuesday of next week, but individual shortwaves in
this flow look progressive and fairly weak.  Forecast keeps some low
PoPs in the northern mountains Monday and Tuesday to cover the
uncertainty in timing.  This relatively tranquil Winter weather
pattern looks like it will come to an end just beyond the extended
period with another series of powerful Pacific storms pushing onto
the West Coast mid to late next week.


&&

.AVIATION.../18Z ISSUANCE/

West of the divide...KBPI/KJAC/KPNA/KRKS Terminals

Light to moderate snow to continue across the western mountains
through this afternoon. Then increased widespread moderate to heavy
snow will develop with MVFR to LIFR conditions through 18Z
Wednesday. KRKS will take until after 06z to reach MVFR conditions
or worse. Mountains will be obscured for all of the period. Gusty
southwest wind will also occur at most terminal sites. Please see
the individual terminal forecasts for more details.

East of the divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

VFR conditions will prevail through 04z Wednesday before MVFR to IFR
in lower CIGS and or VIS develop across northern WY including KCOD
as a cold front pushes into the region from the north. These
conditions will slowly work to the east and south across the
region...reaching KCPR by around 12z Wednesday. Otherwise, strong
gusty wind can be expected periodically near all foothill locations
through the period. At KLND, occasional strong wind gusts are
expected between 04Z and 10z Wed. LLWS will occur at all terminal
sites through 18Z Wednesday. Mountain snow will continue along the
continental divide with some obscuration also expected through the
period. Please see the individual terminal forecasts for more
details.
&&

.FIRE WEATHER...

Snow will fall across the west for the next couple days with some
areas measuring the amounts in feet. Meanwhile, East of the Divide
high winds will be likely from the Red Desert to Natrona County
today.
Most areas should have near normal temperatures with a bit of a
breeze outside of high wind area. A cold front will drop southward
across the area from late tonight through Wednesday and bring a
return to colder temperatures, light snow East of the Divide as well
as basin and valley inversions. &&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 8 AM MST Wednesday for WYZ018>020.

Winter Storm Warning until 5 AM MST Thursday for WYZ001-002-012>015-
023>027.

Winter Storm Watch from this afternoon through late Wednesday night
for WYZ003-004.

&&

$$

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



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