Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 122103
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
203 PM MST Fri Jan 12 2018

.SHORT TERM...(This evening through Monday)
Issued at 118 PM MST Fri Jan 12 2018

Cold north northwest flow behind the main low
pressure trough will keep areas of light snow in the
north and west, mainly in the mountains tonight.
Temperatures will generally cool especially in the north.
Some light snow will linger over the northwest through
Sunday with partial clearing.  Monday the first wave
of colder modified Arctic air will bring light snow
to the north and east with temperatures falling across
the area.  Snowfall is expected to be light with the
best accumulations in the northern Bighorn mountains
and Northern Absaroka mountains.

.LONG TERM...(Monday night through Friday)
Issued at 118 PM MST Fri Jan 12 2018

Overall, the MR models with respect to the extended forecast are in
fair agreement through the forecast period - with a few low impact
differences. After an Arctic cold front does its business Monday,
leaving clearing/cold skies/atmosphere in its wake, shortwave
ridging immediately begins Tuesday. While Tuesday morning
temperatures have been lowered some over those given (and compared
to our eastern neighbors a bit), the strength of the front and later
proximity to a post frontal 1050 mb high over the northern/high
Plains will likely clear skies and allow for even colder morning
temperatures than are currently in place. This will probably be a
daily chore over the next few days. As the shortwave ridge
translates through WY Tuesday and part of Wednesday, a shortwave
trof follows it into wrn WY with EPAC moisture and precipitation
chances increasing west of the Divide. Another small difference
between models that may mean something later on arises with this
trof as the EC slows and plunges a bit further south and pinches off
a closed low in the process produces. This would allow for higher
QPF across the southwestern FA (EC) while the slightly faster GFS
keeps the system split to some extent with the wetter portion racing
off in the northern stream while the southern portion gets ripped up
and disappears over the CO mountains. Differences ATtM are minimal.
Then, the models return to some common symmetry with respect to time
and place with another period of brief ridging taking place on
Thursday. The slightly faster GFS then follows the passing ridge,
under transitioning southwest flow, with renewed EPAC moisture and
precip chances go up about 6 to 8 hrs ahead of the EC. Timing
differences aside (not bad for 7-8 days out), the EC is probably
more bullish with respect to moisture and the location of the mean
trof position. The GFS expresses better the pattern(s) the FA has
been seeing lately while the EC seems slightly anomalous and heavy
handed. One thing is for sure, none of this really matters in an
absolute sense and will likely change face several times over the
next week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday)
Issued at 118 PM MST Fri Jan 12 2018

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

Mountain/valley stratus clouds in place, widespread, and not likely
to go away soon across the far western mountains and
valleys...including terminal sites KJAC and KAFO. Along with
frequent mountain obscuration through the end of the forecast
period, sky bases will vary at times between VFR and MVFR in lower
CIGS and reduced visibility this evening. Then later this evening,
as temperatures drop, clouds will lower and MVFR (occasional IFR)
conditions in lower CIGS and some reduced visibility in occasional
-SN will occur through at least Saturday morning (possibly longer).

Sites KBPI, KPNA and KRKS will see mostly VFR conditions through the
forecast period with gusty west winds at KRKS into early evening.
Both KPNA and KBPI may experience brief periods of MVFR conditions
in lower CIGS and possible visibility reduction in -
BR/flurries...especially after 06Z Saturday.

Please see individual terminal sites for more details.

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

VFR conditions will prevail through the forecast period. Fog may
develop near the KRIW and KWRL airports after 05Z.

Please see individual terminal sites for more details. Additionally,
the Aviation Weather Center will have the latest information on
icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 118 PM MST Fri Jan 12 2018

Cold north northwest flow behind the main low
pressure trough will keep areas of light snow in the
north and west, mainly in the mountains tonight.
Temperatures will generally cool especially in the north.
Some light snow will linger over the northwest through
Sunday with partial clearing.  Monday the first wave
of colder modified Arctic air will bring light snow
to the north and east with temperatures falling across
the area.  Snowfall is expected to be light with the
best accumulations in the northern Bighorn mountains
and Northern Absaroka mountains.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST this afternoon for WYZ001-
002-014-015-024-025.

Winter Storm Warning until 5 PM MST this afternoon for WYZ012-
013-023.

&&

$$

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



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