Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 230807
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
107 AM MST Fri Feb 23 2018

.SHORT TERM...(Today through Sunday night)
Issued at 105 AM MST Fri Feb 23 2018

Imagery continues to show a large broad trough across the western
CONUS with mature and flattening ridging east of the Mississippi.
One weak embedded shortwave is moving across WY with the stronger
upstream, nearly closed, shortwave trough located from the northern
Rockies into the Great Basin. 100 kt jet located on the western side
of the primary stronger shortwave trough down through the western
coastal states with the downstream portion of the jet (110-150 kts)
over CO/NM to the Great Lakes. Large area of + vorticity set over
the intermountain west/Great Basin. The surface has an expanding
area of low pressure across the Great Basin and Desert SW with high
pressure over MT. A developing nearly stationary front currently
separates these two areas from northern CO, through southwest WY and
into central/northern ID. Isolated to widely scattered snow showers
are occasionally present from west central through southwestern WY,
while more widespread precipitation is occurring from central CA
through UT and into western CO then northwest across ID.

Cold (but not brutally cold) temperatures in place this morning.
However, with some modest warming and mixing aloft, moderation of
the cold temperatures will continue again today. Low temps this
morning will generally range from 0 to 5 above zero to as low as 15
below zero. Otherwise, the approach of the embedded strong shortwave
trough will allow for the development a surface low over
southeastern CO that will help pull down a weak clipper type front
east of the Divide and into central and southern Wyoming. Meanwhile,
the main upper shortwave will continue to induce an area of low
pressure across the Great Basin, pushing southerly boundary layer
winds and a possible weak warm front into/through southern Wyoming
this morning. These conditions will provide for modest boundary
layer convergence across portions of southern/central Wyoming - and
the impetus for the development and maintenance of areas of
light/occasionally moderate snow over this area today (in addition
to light snow out west). Generally the mountains of the west will
see less than 3 inches of snow fall through tonight with a dusting
to 3 inches possible across Sweetwater and portions of Natrona
Counties. There will also be some potential for gusty west to
southwest winds to return to the mountains and foothills along and
east of the Divide later tonight/Saturday morning. The Clark area
may get occasional high wind gusts late tonight/Saturday
morning...but really no one else (just the usual gusty).
Temperatures will continue below seasonal.

Saturday will see a brief period of flat shortwave ridging with a
few showers across the west in the morning leading up to the
approach and passage of the next embedded shortwave trough later
Saturday afternoon through Sunday morning. Precipitation/snow
chances will accordingly increase rapidly across the western
mountains with perhaps localized Advisory level snowfall by mid-day
Sunday. Later Sunday afternoon and night, an even strong and more
moist upper level trough will arrive across the west with much
better chances for significant snowfall possible.

.LONG TERM...(Monday through Thursday)
Issued at 105 AM MST Fri Feb 23 2018

A longwave trough will remain in place across the western U.S. for
the extended. Temperatures are expected to below average to much
below average as a result. On Monday into Tuesday the northern part
of a splitting trough across the western U.S. will push across the
Northern Rockies. A moist isentropic upglide flow across the west in
addition to synoptic lift with the approaching trough should result
in widespread snowfall over the west with possibly some significant
snowfall over the far west. By Monday evening, the energy begins to
shear out over the forecast area as the trough splits, with snowfall
chances and intensity come down pretty quickly. East of the Divide
will be mainly dry, but still could see some spill over late Monday
into the Day Tuesday. However any amounts east of the Divide will be
on the light side.

In wake of this trough Monday and Tuesday, global models have
significant differences on the next trough mid to late next week.
The 00Z ECMWF digs a large upper low/trough into the West Coast
Thursday/Friday, while the GFS shows more of a strong northwest
flow. Either way models show the best shot of snowfall will be across
the west with mainly dry conditions east of the Divide.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 105 AM MST Fri Feb 23 2018

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

Occasional MVFR and brief IFR conditions in -SN will occur at KJAC-
KPNA=KBPI, along with frequent mountain until around 15z Friday. An
upper level disturbance will lift out of Colorado into southern
Wyoming Friday afternoon, spreading areas of MVFR, local IFR
conditions north to KRKS and points east along Interstate 80 from
18z Friday through 03z Saturday.  Drier air and increasing surface
west winds will prevail behind this system 06z-12z Saturday.


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

VFR conditions will prevail through 18z Friday with higher peaks
along the Continental Divide occasionally obscured in light snow. An
upper level disturbance will lift north out of Colorado across
southern Wyoming Friday afternoon and evening, spreading areas of
MVFR light snow vicinity and south of a KLND-KCPR line.  These
conditions will be most widespread vicinity and south of a KCPR-KRKS
line 21z Friday-06z Saturday with brief IFR conditions possible at
KCPR Friday evening.  Clearing skies and areas of sfc west-northwest
wind 10-20kts will follow the system 06z-12z Saturday.

&&

.FIRE WEATHER...
Issued AT 105 AM MST Fri Feb 23 2018

Very cold conditions continue across the region this morning. Most
locations will see quite cold morning lows ranging from 5 above zero
to 15 below zero. While some warming is expected over the next few
days, temperatures will still be below seasonal through the weekend.
Winds will also remain relatively quiet today, not increasing again
across the usual locations until this evening. Areas of light snow
expected across western, southern and portions of central Wyoming
today. Smoke dispersion will be poor to fair east of the Divide and
fair to good west of the Divide today...poor to good east of the
Divide and fair to very good west of the Divide on Saturday.

&&

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

&&

$$

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



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