Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 150922
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
222 AM MST Wed Nov 15 2017

.SHORT TERM...Wednesday through Friday night

With the contrast in weather, we will split the discussion into West
and East of the Divide.

West of the Divide...We will not talk about today that much since it
will be mild and dry with increasing clouds. Things then get more
interesting tonight as the next Pacific front and trough
approaches. However, there are still some uncertainties and
concerns. At this point, it looks like precipitation will break
out tonight, with the steadier precipitation moving in later at
night. There are some concerns however. First, the flow ahead of
the system is southwest. This not tremendously favorable for heavy
precipitation, and it lasts through Thursday. Also, the best
upper level forcing, core of the left front quadrant of the jet,
will be over Montana. The other concern are temperatures. With the
southwest flow, this will be a warm system ahead of the front.
Temperatures are expected to rise into the 40s in the valleys on
Thursday. As for snow levels, with 700 millibar temperatures
expected to range anywhere from minus 3.5 Celsius in the north to
plus 2 in the south, this could put snow levels anywhere from 7000
feet to as high as 9500 feet in the south. So as a result, most
precipitation will be rain or a rain/snow mix in the valleys
through Thursday and possibly into Thursday evening. As for
highlights, we will  keep the watch for the Tetons and let the
day shift make the final call on any upgrade.

The cold front will swing through Thursday night and lower snow
levels down to the valley floors. At this point, this time looks to
have the steadiest and heaviest snow with the combination of the
frontal and trough passage as well as some good jet dynamics. As for
impacts, this would definitely be the worst time to travel.

The best forcing then shifts southward into southern Wyoming for
Friday. It will still be unstable with northwesterly flow in the
northwest and there could be some snow bands and locally heavy
snow, but good luck pinpointing those this far out. The concern on
this day may shift to the Interstate 80. With the combination of
the front moving through and very good jet dynamics from an 140
knot jet passing to the south, it could get interesting Friday
morning. Temperatures will be the factor as when they coincide
with the precipitation, as they will likely be falling through the
day. By Friday night, conditions should improve as the system
moves away to the east.

East of the Divide...Here, once again the concern shifts to wind.
Not too bad today though. However, across the Cody Foothills we will
likely see another high wind event Wednesday night into Thursday
morning. The models again indicate tighter packing of the isobars in
the Lee of the Absarokas. In addition, 700 millibar winds look very
impressive. And there will be some downward forcing with the
right front quadrant of a jet streak. Even the more conservative
GFS has 60 knots. The NAM has up to 75 knots. As a result, we will
upgrade the high wind watch to a warning this morning.

On Thursday, the threat of higher winds will shift to the
southern Wind Corridor. The ingredients will be there in the form
of a tightening pressure gradient across the area. 700 millibar
winds also respond, increasing to 50 to 55 knots. With good mixing
with the front moving in, we will hoist a high wind watch from
eastern Sweetwater County through Natrona County. Expect this
time, there will be a better chance of winds impacting travel
routes, not just the tops of ridges and mountains where impacts
are minimal. As for precipitation, strong downsloping will likely
keep things dry. It will also be a mild day, with 50s fairly
widespread and the warmer and windier areas likely seeing high
temperatures rise into the lower 60s.

The front will then move across the area Thursday night and into
Friday. Cooler temperatures will follow the front with temperatures
possibly falling through the day with the cooler air moving in. With
the Pacific origin of the air, temperatures will not be that
cold. As for the chance of precipitation, there will be some but
the models are all over the place as to placement and it could be
banded and hard to nail down. It will feel a lot cooler though
with a brisk northwest wind behind the front, however. And here,
like out west, conditions should improve as the system moves away
to the east.

.LONG TERM...Saturday through Wednesday

Overview...A ridge of high pressure will prevail over the western
U.S. this weekend into Thanksgiving Day; and after a cool Saturday,
temperatures will return to above normal Sunday through Wednesday.
Slight chances of precipitation return to the Far West Monday and
Tuesday, otherwise dry conditions are expected.

Discussion...The large-scale upper air pattern featuring a ridge
over the central Pacific, trough across the eastern Pacific and flat
ridge across the western U.S. is expected to persist through the
extended period and into Thanksgiving weekend.  The only possible
weather maker upstream is a system currently moving across the
Bering Sea early this morning.  This system is expected to be much
more elongated/weaker than previous systems as it moves down the
British Columbia coast over the weekend and shears inland Sunday
night-Monday.  The ECMWF shows a much more split system with
northern stream outrunning southern stream as it moves into western
U.S. ridge than GFS`s slower, more intact system.  ECMWF has been
verifying much better than GFS so far with this pattern, so more
weight was given to its forecast, with chances for precipitation
kept on the low side as this system translates the area Monday-
Tuesday.  A weak, dry cold Canadian cold front should push south
across the area Monday night, bringing slightly cooler
temperatures on Tuesday. Wednesday and Thanksgiving Day are
expected to be dry and mild, low impacts for travelers, with a
progressive storm track expected to remain along/north of the
Canadian border, flat ridge prevailing across Wyoming.

&&

.AVIATION...12Z Issuance

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

Conditions will be VFR through the forecast period over the lower
elevations.  Gusty west to southwest wind 15 - 30kts will develop
between 18Z-21Z/Wed over and near the east slopes of the mountains
and from KRKS to KCPR.  Over and near the Continental Divide
areas of MVFR to IFR ceilings and Visibility reduced in snow
and fog will obscure the higher mountains becoming widespread
above 9000 feet from 03z - 06z Thu.

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

Conditions will be VFR through 00z Thu over the lower elevations.
MVFr to IFR ceilings and visibility reduced in snow and fog will
increase across the west from 00z - 03z Thu with mountains becoming
obscured as deep pacific moisture in southwest flow aloft spreads
east from Idaho and  northern Utah.  Gusty west to southwest wind
15 - 30kts will develop between 20Z - 23Z/Wed.
&&

.FIRE WEATHER...

Today will be mild and breezy and largely dry. Relative humidity
will remain above critical levels. A Pacific front will approach
the west tonight and Thursday and spread mountains snow and lower
elevation rain, with snow levels falling to the valley floors
Thursday night. Meanwhile, east of the divide, high winds are likely
in the Lee of the Absarokas tonight and from the Red Desert through
Natrona County Thursday. Expect mild temperatures Thursday followed
by a cool down on Friday. Mixing and smoke dispersal will be
generally fair to poor.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Thursday morning through Thursday afternoon
for WYZ019-020-030.

Winter Storm Watch from late tonight through Friday morning for
WYZ012.

High Wind Warning from 8 PM this evening to 11 AM MST Thursday
for WYZ003.

&&

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Baker
FIRE WEATHER...Hattings


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