Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 160501
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1101 PM MDT Fri Sep 15 2017

.SHORT TERM...This evening through Monday
Issued at 338 PM MDT Fri Sep 15 2017

The cold wet synoptic scale weather event is well underway. The
main trough axis is tracking east across the Great Basin attm with
an stream of Pacific moisture rounding the base of the trough and
feeding into Wyoming. Yesterday, the models were hinting at a
weak circulation forming east of the divide today but this
circulation has become quite prominent and visible both on radar
and satellite. This circulation has already tracking across
Northeast Johnson County. Several hours of stratiform precip has
taken place across much of Northern and Central Wyoming overnight
and this morning on the back side of the cold Canadian surface
cold front. Anywhere from 1 to 2 inches of precipitation has
fallen over much of Northern and Northwest Wyoming thus far. Many
of these areas have already seen the majority of the precip they
will receive from this 3 day event. One last heavier band of
precip followed directly behind the aforementioned mid level
circulation before precip diminished over central Wyoming. Farther
north in Northern wyoming, closer to where the large swath of
isentropic lift has been, there will not be as of a break in
todays precip. Freezing levels are now as low as about 7500 feet
in Northern Wyoming and down to about 9500- 1000 feet over Central
Wyoming according to area Snotel and RAWS sites. In fact, at the
time of this writing, Beartooth Pass is reported to be closed due
to snow. In the meantime, the main mid level low over Eastern
Idaho will be mixing the ACT mid level trough across WY this
afternoon for round two of significant precip. A cluster of
thunderstorms are evident just east of SLC and will be tracking
northeast along this trough across central and southern Wyoming
during the remainder of this afternoon and evening in the form of
precip as this particular batch of precip encounters the cooler
air along with just sold mtg in Sweetwater and Lincoln Counties
through this evening. The left front quad of the jet max will
likely encourage the precip to continue through tonight for a
while in at least southern Sweetwater County. The precip should
end in the south half of Wyoming for the most part after 06z
tonight but continue over the north half from QG forcing ACT with
the Idaho low and ACT wrap around as the low tracks slowly
northeast across Southwest Montana through Saturday. The remaining
precip over Central and Northern Wyoming will end from south to
north Saturday as the h5 trough axis tracks to the east of us by
Saturday afternoon. Will keep the advisories and warnings going
for the Northern and Western Mountains through 18z Saturday with
the bulk of the snowfall falling in the mtns east of the divide
where the added nne sfc upslope component will aid in snowfall.
Even areas like Dubois and Cody could see 1 to 3 inches of snow by
Saturday morning, or at least over the surrounding nearby hilly
areas.

After the storm system exits the region, clearing will take place
for pronounced radiational cooling Saturday night with many areas,
especially east of the divide experiencing their first frost
for Sunday morning. In the west, many protected areas of the lower
elevations will see low 20s for a low Sunday morning. With the newly
fallen abundant moisture on the ground, fog will likely be prevalent
in the valleys and basins Sunday morning as well. After any
valley fog dissipates, Sunday will turn out to be an ultra crisp
Autumn like day with smokeless skies and pleasant high temperatures,
mainly in the 60s at the lower elevations.

Then on Monday, the next main trough approaches the area within the
progressive weather pattern. As it does so, the pressure gradient
will increase. Sfc winds will likely increase as we get underneath
the right front quad of the jet for a brief period of time, but most
of the wind that we experience will simply be from the enhanced
winds aloft mixing down from the increasing instability ahead of the
trough. However, h7 wind speeds do not look impressive attm but it
will at least be considered to be a windy day overall in many areas
with h7 winds 30 knots. A band of convection from the ACT cold
front will be entering NW Wyoming by late in the day Monday. Ahead
of the front Monday, temps will increase by 5 degrees east of the
divide and about the same west.

.LONG TERM...Monday night through Friday night
Issued at 338 PM MDT Fri Sep 15 2017

Synopsis...The longwave trough will continue to dig southward over
the NW CONUS to start the period. A tight height and temp gradient
will be in place, as a strong cold front blasts through during the
day Tuesday. This will be associated with the first of a couple of
shortwaves that could impact the region through the forecast period.
The mean longwave trough will be situated over the Northern and
Central Rockies, with another shortwave moving trough on Friday.
Models continue to bounce back and forth whether to dig the longwave
trough southward into the southern portions Great Basin or to keep
the base of the trough near northern/central portions of the Basin.
This system will be supported by a 120-140kt jet, that will pass
directly over the forecast area at various times Tuesday through
Friday. This system also brings much colder air to the region, with
-4 to -6C 700mb temps Tuesday night/Wednesday morning across the
 state. This will fluctuate, again, due to where the jet and
 associated shortwaves set up, but either way will stay below 0C.

Discussion...Rain showers will change over to snow across northwest
portions after 00Z Tuesday, as the cold front moves through. Any
thunderstorms that develop over the Wind Corridor Monday afternoon
should be weakening after sunset.  Precipitation will mainly be
along and ahead of the front east of the Divide, as the front
progresses. Areas of precipitation will occur throughout the rest of
the period, with snow levels dropping to 7k or 8k feet at times.
Western portions will be mainly impacted by this, but again,
depending on how far south the trough digs will influence the
location of the jet and the associated dynamics.

&&

.AVIATION...For the 06Z TAFS through 06Z Sunday
Issued at 1100 PM MDT Fri Sep 15 2017

Areas of rain and mountain snow will occur over the north through
18Z Saturday. Rain and snow showers will occur in the northern WY
mountains through 00Z Sunday before ending by 03Z Sunday. Areas of
MVFR to IFR conditions will occur in the north along with frequent
mountain obscuration.

In the central and south, isolated rain showers will occur
through 15Z Saturday. Areas of MVFR to IFR conditions are expected
from KRIW to KCPR until 15Z. Then VFR conditions will prevail the
rest of Saturday into Saturday night.

Please see terminal forecasts for more details.

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

&&

.FIRE WEATHER...
Issued AT 338 PM MDT Fri Sep 15 2017

Expect cold and wet conditions for the rest of this afternoon,
tonight and tomorrow morning as low pressure moves across the
state. Rain will fall across many locations with several inches of
snow likely for the mountains. All areas have a good chance of
receiving a wetting rain. A gusty north wind will blow at times.
The precipitation will end from south to north Saturday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until noon MDT Saturday for WYZ002-008-009-
015.

Winter Weather Advisory until noon MDT Saturday for WYZ001-012-
014.

&&

$$

SHORT TERM...Lipson
LONG TERM...LaVoie
AVIATION...AR
FIRE WEATHER...Lipson



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