Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181013

Area Forecast Discussion
National Weather Service Riverton WY
313 AM MST Wed Jan 18 2017

.SHORT TERM...Today through Friday night

Imagery shows split flow around the FA continuing with very weak
flow through a deep layer. SFC has high pressure over the Great
Basin and into WY west of the Divide with marginally lower pressure
to the east. No precipitation anywhere around WY this morning.

Split flow today gives way to increasing southwest flow aloft on
Thursday ahead of a digging and increasingly negatively tilted trof
across the western CONUS. Accumulating precip with this developing
trof wont`t arrive over the far western FA until Thursday afternoon/
evening. In the meantime, the main focus today will be for some
possible patchy morning dense fog...particularly in the western
valleys and over the southern Green River Basin into portions of
Sweetwater county near/along/north of I80. Another weather element
to watch for today through Thursday morning will be for gusty
southwest winds across the usual areas in the Cody foothills and
wind corridor from southeast Fremont county east across Natrona
county due to a strong surface PGRAD. While there may be a few high
wind gusts for a few locations, widespread high wind does not in the
offing as winds aloft are quite light and will not not add any speed
component to to the surface flow until Thursday morning for a short
period. An SPS currently describes the wind situation.

Thursday afternoon through the end of the forecast, digging trof
head into the intermountain west with the main jet and upper level
forcing also digging well to the south over the Desert Southwest. A
weak embedded shortwave will then lift across Wyoming Friday
bringing some low end chances for precip east of the Divide with
the better chance over the northern/northeastern FA under the tail
end of an exiting weak jet to the north. Heaviest accumulating
snowfall out west will occur Thursday night and Friday with what
looks to be primarily AOB Advisory level snowfall for the most part
(could have localized Advisory amounts in the Tetons/Gros Ventre
mountains). Snowfall east of the Divide will arrive Friday and
Friday evening with less than an inch for most lower elevations and
less than 3 inches over the Big Horn mountains. Winds should not be
an issue with this event and no strong (or any) frontal passage is
really expected. However, there is some hope that there is enough
upward movement to remove the high pressure that has been entrenched
across the region and allow some of the colder trapped valley/basin
air to moderate from frigid to just cold even though H7 temps will
fall once again from near +5 deg C today to -7 deg C or so later
on Friday. Hope springs eternal.

.LONG TERM...Saturday through Wednesday

Scattered light snow in the west and southwest Saturday into
Saturday night. Mostly dry weather elsewhere. On Sunday, scattered
light snow in the far west will occur with dry weather over the
rest of the region. The next weather system will spread more snow
into the west Sunday night through Monday. Snow will develop in
the south on Monday as well. Will have to watch areas east of the
divide for some snow Monday night into Tuesday morning. The GFS
and ECMWF indicate an upslope scenario from Lander to Casper as
700 MB wind direction goes north northeast. The dynamics are fair
for the event. Will have to watch how this plays out over the
coming days. Will have a chance of snow in these areas. Improving
conditions Tuesday afternoon east of the divide with isolated snow
showers in the west. Next Wednesday looks mostly dry for now.
Temperatures will be fairly seasonal through the period with
colder highs next Tue and Wed provided the storm occurs.
Valley/basin inversions should not be as strong through the period
allowing some moderation to highs and lows.



Low confidence in fog development impacting terminals this morning
across the basins and valleys as well as Sweetwater County as
boundary layer moisture remains limited. As of early this morning
the satellite fog product showed little development. Will monitor
satellite/observations trends to decide at 12Z issuance on how hard
to hit the fog. Otherwise VFR conditions are expected with windy
conditions in SE Fremont into Natrona County impacting KCPR...and
low-level wind shear impacting KCOD through the period. Also an
approaching storm system will bring in some lower clouds over the
far west after 00Z Thursday. This would primarily impact the KJAC
terminal, but still remain VFR with ceilings between 040-070 KT AGL
at KJAC. There could also be some snow showers developing over the
western mountains mainly after 06Z Thursday, but coverage is
expected to quite limited through 12Z.



Fire Danger low for all of WY through, at least, the weekend. Cold
to below seasonal temperatures and a good existing snow pack with
light snow to begin west of the Divide by Thursday morning and
continue through the day Friday. Light scattered snow showers
expected east of the Divide on Friday. There will be a period of
gusty southwest wind over the usual areas of the Cody Foothills and
the Wind Corridor later today through Thursday morning. Smoke
dispersion will remain poor east of the Divide through Friday
morning before seeing improvement...until Thursday afternoon west
of the Divide.





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