Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 232239
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
439 PM MDT Thu Mar 23 2017

.SHORT TERM...(This evening through Sunday)

A well defined circulation is evident over far Southwest Wyoming
along with an extensive thick elongated cloud mass oriented
northeast to southwest aligned with the asct frontogenesis.
Southeast Fremont, Eastern Sweetwater, and Southeast Natrona
Counties have been relatively mild ahead of the main area of
moisture with Casper Mountain well above freezing all day today.
The coldest air appears to have been closely linked to the mid
level cold core low over Southwest Wyoming today where 1 to 3
inches of snow has fallen today across the Upper Green River Basin
and the surrounding foothills. Many areas have received over one
half of an inch of precipitation along the aforementioned
northeast southwest oriented stream of moisture along with added
lift from the jet couplet/left front quad dynamics from the
southern stream jet, as well as QG forcing. Snowfall did occur
briefly in Lander thanks to adiabatic cooling from low level
upslope flow, otherwise rain fell all day there today. The trend
for tonight will be clearing from the northwest as cyclogenesis
takes place over Southeast Colorado with the energy quickly
propagating to the south as the trough splits with the blocking
mean ridge over the High Plains still in place. Friday will be a
dry day as a transitory positively tilted ridge builds into the
area. Then on Saturday, the next Pacific trough now out at 140W
will deliver more snowfall to far Western Wyoming beginning late
Friday night and continuing and increasing in intensity Saturday
with perhaps 3 to 7 inches of snow in the Western Mountains and 1
to 3 inches in the valleys along with at least a half of an inch
of precip over much of the area. There may be a little rain mixed
in with the snow in the valleys, but should be more snow than
rain. Then this trough will split, just like this evenings trough,
with cyclogenesis taking place once again over Southeast Co. As
this system also splits, precip will spread east of the divide as
an asct cold front gets pulled south over the area and the
northern stream energy tracks east. Areas east of the divide were
left with high pops, with the most significant qpf in the northern
zones east of the Divide Saturday night. Precip will exit east on
Sunday.

The flood warning remains in effect for Lincoln County with more
precip expected but at least temps will be cooling off.

.LONG TERM...(Sunday night through Thursday)

Active weather pattern continues into and through the medium-
range forecast period. Departing Sunday trough will leave
gradually improving and drier conditions Sunday night into Monday
morning with transient ridge sweeping east across the forecast
area. The next trough will already be moving into the Pacific
Northwest Monday with moisture spreading east across the
Intermountain West. As a result, expect scattered showers across
the far west by late Monday afternoon. Eastward push of this
moisture continues Monday night into Tuesday morning. Upper low
will form in the base of this trough and continue to dig into the
Desert Southwest Tuesday. Pieces of energy and associated moisture
are likely to hand around Tuesday with general troughiness
remaining across the forecast area. At this time, believe the best
chance for precipitation will be in areas east of the Continental
Divide, and especially the central basins and Johnson/Natrona.
This scenario could be similar to what we have seen in central
Wyoming today (Thursday). Split flow will again be the rule as we
head into Wednesday with upper low in the Desert Southwest and
northern stream kicker moving the precipitation east of Wyoming
with drier westerly flow aloft. Wednesday looks to be mainly dry
on both sides of the Continental Divide as the upper low sets up
shop in the southern Rockies and the next trough moves into the
Pacific Northwest. This next trough will serve to nudge the upper
low onto the southern Plains and the pattern may repeat itself all
over again. Moisture returns to the west Thursday as the Pacific
Northwest trough moves onshore. Temperatures will yo-yo during the
week with Tuesday and Thursday the coolest days, albeit still
close to normal. More sunshine and generally dry conditions in
warmer west to southwest flow will enable temperatures to be 5 to
8 degrees above normal Monday and Wednesday. Gusty southwest wind
will be found Monday in the Wind Corridor from Sweetwater to
Natrona County. Otherwise, nothing more than a typical Wyoming
breeze anticipated through the period. |

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Mid-level circulation over Sublette County Thursday morning is
gradually becoming less defined as stronger low to our south begins
to dominate. Model forecast soundings, HRRR, and satellite trends
all point to improvement in IFR/LIFR conditions at KPNA and KBPI
between about 23Z/Thu and 02Z/Fri. Snow will be on the wane and
conditions will gradually improve to VFR by 03Z/Fri at both
terminals. Low-topped convection across Sweetwater County and in the
vicinity of KRKS will continue into the evening hours ahead of
strengthening upper low to our south. MVFR conditions and convection
should diminish around KRKS between 02Z-04Z/Fri. KJAC will remain VFR
through the entire forecast period. These VFR conditions will
gradually spread northwest-to-southeast from 06Z-12Z/Fri at all
terminals. Rich boundary layer moisture, especially around KPNA and
KBPI, could lead to some late night and early morning VCFG. This will
depend on decreasing north to northwest wind and how much mid-cloud
cover remains by 12Z/Fri. For now, have kept VCFG between 10Z-16Z/Fri
at both terminals. Mountain obscurations will be widespread through
about 03Z/Fri with mountain top obscurations continuing until about
10Z/Fri. Friday afternoon will be VFR with increasing mid and high
clouds ahead of the next weather system approaching from the west.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

MVFR/IFR conditions will prevail through the 02Z-05Z/Fri timeframe,
mainly due to persistent lower ceilings at the central terminals of
KRIW, KLND, and KCPR. Colder air has filtered into this area from
the northwest as the large-scale upper low forms to our south. However,
the decrease in precipitation intensity has enabled temperatures to
rebound enough at KLND and hold steady at KRIW that precipitation may
remain only rain. It will be a close call at these two terminals, with
the best chance for snow coming between 00Z-02Z/Fri. KCPR has warmed
through the afternoon hours. This will likely lead to any
precipitation remaining as rain at KCPR as it may take too long
for the atmosphere to moisten up and cool down. Precipitation will
gradually shift south of these areas by 06Z/Fri. KCOD and KWRL
will see MVFR conditions wane late Thursday afternoon and early
evening. Conditions will gradually become VFR after 06Z/Fri with
KCPR the last to hold on to any MVFR clouds and precipitation.
Would not be surprised to see some sunrise fog with high boundary
layer moisture and clearing sky. Most likely locations would be
KLND and KRIW. Any fog that does form will diminish by 16Z/Fri.
VFR conditions will be found at all terminals through all of
Friday. Mountain obscurations will be widespread until 06Z/Fri
with mountain top obscurations lingering until 12Z/Fri.

&&

.FIRE WEATHER...

Fire danger will be low through the weekend. Increasing moisture
will continue to spread across WY along with measurable
precipitation at many locations...both liquid and frozen. Friday
night through Saturday night the next spring/winter system will
move through the region with rain and snow most likely west of the
Divide...isolated to widely scattered east. Smoke dispersion will
be fair to good most afternoons.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory until 6 PM MDT this evening for WYZ025.

Winter Weather Advisory until midnight MDT tonight for WYZ008-
009-014-015.

&&

$$

SHORT TERM...Lipson
LONG TERM...Jones
AVIATION...Jones
FIRE WEATHER...Lipson


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