Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 110550
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1050 PM MST Sat Feb 10 2018

.SHORT TERM...(This evening through Tuesday)
Issued at 300 PM MST Sat Feb 10 2018

Cold and dry high pressure with north northwest flow aloft
will be over the area tonight and Sunday with some lingering
light snow in the far south tonight clearing on Sunday.
Temperatures will be coldest in the north with a lee side trough
beginning to develop with some warming central areas. Temperatures
will be quite difficult tonight and Sunday.  Sunday night and Monday
an upper level low will develop in the great basin with frontogenesis
over Wyoming with east low level flow with developing light snow with
very light accumulations. mainly an inch or two at best in the lower
elevations and 2 to 3 inches in the east slopes of the mountains.
The upper level low will split with the main low digging southwest
away from Wyoming with drying on Monday night and Tuesday keeping
colder air in the north.

.LONG TERM...(Tuesday night through Saturday)
Issued at 300 PM MST Sat Feb 10 2018

Interesting extended forecast period. Thankfully the MR models,
while diverging somewhat in their output, are keeping the events
that each model does generate low impact for now. That the EC and
GFS come back together spatially and temporally by the very end of
the forecast period is good, but doesn`t help in the meantime. This
"meantime period" is characterized by the different handling of the
storm track with respect to path, amplitude and the timing of
approaching embedded shortwave troughs (Wed night), along with the
potential for kicking out a cut-off low (COL) over the southwestern
CONUS. Just looking at differences between the EC and GFS, EC seems
slightly more amplified with respect to the embedded shortwave
arriving Wed night. This creates differences in temperatures aloft
and subsequent (embedded) convective potential/strength between
models. The EC is colder aloft overall and is "broad-brushing" an
extended period of light precipitation across much of the
northern/high Plains and Intermountain West through Thursday night.
The GFS on the other hand is warmer aloft and more selective in
producing precipitation mainly Wednesday night with the embedded
wave and associated surface front doing the forcing. Then, then GFS
keeps a lingering period of light precip confined mainly to
northwest WY under northwest flow aloft Thursday night (which seems
the more likely of the two scenarios and falls in well with
climatology). Have gone with essentially a 70/30 (GFS/EC) split
through Saturday. As stated before, both models return to to similar
patterns by Saturday night, allowing enough confidence for some
Chance wording in the place of Slight Chance.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1049 PM MST Sat Feb 10 2018

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

VFR conditions will prevail through 00z Monday.  Patchy fog may
partially obscure terrain vicinity kJAC, KBPI and KPNA through
18z, but is not expected to prevail at any TAF site.

A cold front will spread areas of light snow, increasing mountain
obscurations south to a vicinity KJAC-KDUB line between 03z and
12z Monday.

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

VFR conditions will prevail through 21z Sunday. Some patchy fog
may partially obscure terrain vicinity KRIW, KWRL and KLND until
17z. Areas of LLWS will occur along the east slopes of the
Absaroka Mountains, vicinity KCOD-KDUB until 18z.

A cold front will push south to a vicinity KCOD-KCPR 21z Sunday-
00z Monday, and through KRIW-KLND around 03z Monday. A wind shift
from the N-NE and areas of IFR/MVFR ceilings with mountains
becoming obscured will follow front. Light snow will then develop
a few hours behind the front through 12z Monday.

&&

.FIRE WEATHER...
Issued AT 300 PM MST Sat Feb 10 2018

Cold and dry high pressure with north northwest flow aloft
will be over the area tonight and Sunday with some lingering
light snow in the far south tonight clearing on Sunday.
Temperatures will be coldest in the north with a lee side trough
beginning to develop with some warming central areas. Temperatures
will be quite difficult tonight and Sunday.  Sunday night and Monday
an upper level low will develop in the great basin with frontogenesis
over Wyoming with east low level flow with developing light snow with
very light accumulations. mainly an inch or two at best in the lower
elevations and 2 to 3 inches in the east slopes of the mountains.
The upper level low will split with the main low digging southwest
away from Wyoming with drying on Monday night and Tuesday keeping
colder air in the north.  Inversions should be strong enough to limit
mixing producing general poor smoke dispersal.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

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



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