Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 141750 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1150 AM MDT Thu Sep 14 2017

.SHORT TERM...Thursday through Saturday night

It still looks like we will be making a hard turn into a more autumn
like pattern across the area. Today still looks to be fairly mild
East of the Divide however. The models have come in a but slower
with the arrival of the system. All models are fairly consistent. As
a result we have raised temperatures East of the Divide and also
removed POPS during the morning hours. Showers will continue across
the west though. It may turn out to be a decent day East of the
Divide, although a bit breezy at times ahead of the front. As for
precipitation type, most areas will stay rain today. There will also
be some thunderstorms as well and some could have heavy rain with
precipitable water values rising to around an inch in some areas.

Cooler air will begin to work in tonight following a cold frontal
passage. The NAM is more aggressive than the GFS with the cold air
with 700 millibar temperatures dropping as low as minus 2 across the
Absarokas by Friday morning which would lower snow levels to around
7500 feet. The GFS is slightly warmer. However, snow will likely be
flying across the peaks of the northern mountains by Friday morning.
There is still some uncertainty on whether to have advisories or
warnings in some areas, so at this point we will likely punt the
final decision back to the day shift. All the models show good
moisture moving over much of the area with over a half an inch of
QPF for many areas. There are still some differences in placement of
the heaviest QPF though. So this morning, we made only cosmetic
changes.

Periods of precipitation will continue on Friday as a 700 millibar
low moves across the state. At this point, the heaviest
precipitation would be north and west of a line from Buffalo to Rock
Springs. The NAM even shows a dry slot working into southeastern
portions of the area. Snow will continue across the mountains with
snow levels lowering as rather cool air drains into the state. The
NAM again is the coldest with the zero Celsius line dropping as low
as Riverton by the end of the day, pushing the snow levels down to
around 8500 feet and as low as 6500 feet across the northwest.
Another batch of heavier precipitation will then likely move into
the southwest late Friday afternoon and evening with some lift
associated with the left front quadrant of a jet streak. All in all,
a cloudy, cool and cruddy day across the area with even some of the
lower elevations possibly not rising much above 40.

By later Friday night, the heaviest precipitation should be shifting
northeast as the best dynamics exit and the trough axis swings
through. There will be some leftover precipitation however. And this
will be critical if any snow can make it down to the Valley floors.
At this point, the models show 700 millibar temperatures falling to
around minus 3 to minus 4 which would put levels around 6500 feet.
This can change however. The best chance of this happening would be
across northeastern portions of the county warning area where
precipitation will linger a bit longer. With cool temperatures,
freezing temperatures many become a concern as well.

So all in all for the storm, the most snow will likely fall across
the Big Horns and northern Absarokas. Amounts should decrease
somewhat further south with the cooler air taking longer to move in
and steadier precipitation quicker to exit. Most of the mountains
should see accumulating snow however with the highest elevations of
the Big Horn range possibly seeing over a foot.

Saturday should feature improving conditions as shortwave ridging
builds across the area. Any showers should end from southwest to
northeast with almost all areas dry by Saturday night. High
temperatures will be remain cool but could be tricky at times since
any areas that get some sunshine will likely get warmer than
predicted highs.

.LONG TERM...Sunday through Thursday

Mean longwave trough becomes well developed over the western U.S.
during this period lending strong support to what the models are
showing developing next week. Current cold system tracking
between Siberia and just east of Kamchatka will stay at a high
latitude as it tracks east across the nrn Pacific/Gulf of Alaska
region in the coming days. Further west in the Sea of Okhotsk, our
next storm system is brewing. This cold system will move eastward
towards the central Pacific and help to pump up a ridge over the
eastern Pacific early next week that will allow our future Gulf of
Alaska system to dig into the Pacific NW around Tuesday of next
week and then into the Great Basin Wednesday into Thursday of next
week. GFS is more fragmented and much farther north with this
system(s) and not preferred compared to a more consolidated Euro
and GEM upper low (which fits the large scale pattern of a mean
trough developing over us). With that said, we start the period
quiet but cold behind our current developing system. After some
freezing temps to start the period, we recover and end up with a
dry day with only a slight chance of mountain showers. On Monday,
the new upper low moves into the Pacific Northwest with the
Pacific front moving into central ID. We could start to see some
moisture and showers/isold storms start to move into the nw mtns
but the Euro and GEM hold off. Odds start to increase for
showers/storms in the west Tuesday with a few possible in the
north and sern zones but probably not as generous as the faster,
farther north GFS shows. As the system digs into the Great Basin
Tuesday night and Wednesday, widespread showers will likely move
into the west with possible band of showers lifting nwd across at
least the central zones. Odds increase with time as this dynamic
system strengthens just to our west and trough dynamics increase
later Wednesday into Thursday. Pops will need to be increased with
time but for now, can`t go much higher. Another round of snow
will likely occur in the mountains with this system and depending
on the exact track some lower elevations west of the divide could
see another period of rain changing to snow. Still a little too
early to see how cold it might get east of the divide, especially
up north but another period of cold rain and foothill snow is
quite possible/expected. So a couple days of warming temperatures
to start this period before we start cooling dramatically,
especially Wednesday into Thursday.

&&

.AVIATION...18Z Issuance

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

Isolated showers will exit Northeast Wyoming through 1830Z this
afternoon from the initial prefrontal surface trough. This trough
has shifted the winds to northerly, but the main cold front will
push southward across much of the area between 21Z Thursday and
03Z Friday. The winds will back to the southwest once again at
KCPR by 23Z ahead of main approaching cold front before shifting
back to the north northeast by 02Z Friday. An area of showers and
thunderstorms are currently over far Western Wyoming, Idaho, and
Utah and will be spreading into Central Wyoming during the course
of this afternoon. Widespread MVFR/IFR conditions are expected to
develop in wake of this main cold front with valley rain/mountain
snow becoming more widespread with time Thursday night and Friday.
Mountains will become obscured from north to south this afternoon
and evening. Northerly winds will increase during the night
tonight and continue through 18z Friday, especially east of the
divide.

Please see the Aviation Weather Center for the latest information on
icing and turbulence forecasts.

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

Widely scattered showers with even possibly one or two thunderstorms
will continue this morning with brief MVFR/IFR conditions possible
in the mountains. By late morning into early afternoon coverage of
showers and thunderstorms will increase, but still will only have
VCTS at terminals. However during the evening hours, dynamics from
an approaching trough is expected to result in widespread valley
rain/mountain snowfall breaking out with increasing chances of MVFR
conditions. Mountains will be obscured at times this morning,
becoming mainly obscured by this evening.
&&

.FIRE WEATHER...

We will begin a shift to a colder and wetter pattern today. A cold
front and upper level system will move into the area today and
spread showers and isolated thunderstorms across much of the area
from west to east, although much of the area East of the Divide
should be dry for much of the day. A gusty west to southwest breeze
will occur at times. However, relative humidity should remain above
critical levels for all areas. Mixing and smoke dispersal will range
from poor in the north to very good in the south. There is a good
chance for a wetting rain tonight through Friday night for all areas
with snow likely for the higher elevations Friday into Friday night.
Well below temperatures will be found for most areas Friday into
Saturday.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late tonight through Saturday morning
for WYZ001-002-008-009-012-014-015-024.

&&

$$

SHORT TERM...Hattings
LONG TERM...Skrbac
AVIATION...lipson
FIRE WEATHER...Hattings



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