Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 282007
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
207 PM MDT Tue Mar 28 2017

.SHORT TERM...(This evening through Friday) Issued at 155 PM MDT
Tue Mar 28 2017

Imagery shows general LW trof across the CONUS with an embedded deep
trof with closed low centered in the base of the trof over NM. Just
upstream of this trof is a SW ridge across the wrn CONUS with the
next further upstream trof brewing out over EPAC offshore WA/OR. SFC
has the main low P centered well to the south in NM...under the
upper low and nearly occluded at this time. Weak high P currently
starting to infiltrate WY from the west. Light to moderate
precipitation, both rain and snow, is spread across much of the area
east of the Divide as well as over srn WY from Green River, east to
the WY/NE border.

Today, the intermountain trof continues to make its way east of the
Divide and into the Plains tonight while the closed portion of the
low develops into the southern Plains with possible severe wx for
TX/OK. A decent return of moisture continues around the main low to
the south up through eastern CO and into/across
eastern/central/northern WY. Post frontal and with H7 temps range
from -4 to -6, temperatures continue to be borderline for snow below
6500 feet. However, Rock Springs has been snow hard at times through
the morning hours. Moderate to heavy wet snow was also occurring
across Sand Draw/Beaver Rim and South Pass...but only Rain or a
rain/snow mix in Jeffery City. Moderate wet snow at time also
continues over the Bighorns where 3 to 10 inches of snow has already
occurred generally above 8000 feet. Winter Wx Advisory highlights
continue today for zones 8, 9, 19, 28, 29 and 30 have been cancelled
this afternoon as the system weakens and drifts eastward.

Then, the next shortwave/jet heads inland from the EPAC with
moisture feeding in ahead of the trof and into wrn WY. Through the
mid-levels, upstream frontogenesis will be proceeding through the
day Thursday...arriving just west of WY over eastern ID by Thursday
evening. While there may be a few late morning snow showers out west
down to valley levels, as the better moisture arrive in the
afternoon snow levels will rise to near/above 7000 feet before
dropping back down below 6000 feet in the evening as a strong front
pushes into/through wrn WY while new frontogenesis takes place over
MT.

The bulk of moisture/precip/snow west of the Divide will occur
Thursday and into Thursday evening before the Wx system`s emphasis
shifts east of the Divide. Variable diurnal temperatures around the
freezing mark in addition to the FA`s position relative to the
developing/digging strongly baroclinic upper low will cause another
very tricky period of forecasting both QPF and snowfall
(accumulation). Thursday night east of the Divide and over southern
WY, the best forcing looks to be located near/ahead of the first
front across central and southern WY. By Friday morning, the second
front (the one that developed over MT) will push into WY from the
north with relatively good moisture return into the area ahead of
the front from out of the Plains. Rain showers expected for most
locations below 7500 feet early Friday while awaiting the arrival of
colder mid-level and SFC temps Friday afternoon as the second front
plows southward.

Although widespread Highlights are not expected at this time, a few
impact based or elevation dependent Advisories may be necessary for
Friday...similar to those of today.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 155 PM MDT Tue Mar 28 2017

Significant upper low is expected to be near the 4-corners area to
start the medium range with potential for significant precipitation
continuing for the south half of our forecast area. Lean towards a
farther south or slightly weaker solution closer to the GEM but
we`ll see. The bigger picture (longwave) is still showing a mean
ridge building into the inter-mountain west Friday and holding
through Saturday. Hard to discount the strong upper low and
significant pcpn shown on the op GFS/Euro but I`ve seen many a time
where the overlying big picture will aid or hamper the individual
systems. On top of that, temps are borderline in parts of the area
for snow Friday leading into this period. From Beaver Rim south to
Sweetwater County could have blizzard conditions continuing due to
very strong ne wind and potential for snow continuing (or changing
to snow). The far north will improve after a period of rain
(basin) or rain and snow mixed (Cody Foothills and areas above the
basin floor) on Friday. By 12z Saturday most of the Bighorn Basin
will be dry with decreasing wind and just some lingering morning
lower clouds. Temps will warm into the 50s to near 60 across the
north Saturday afternoon with dry conds and then this drying and
gradual improvement will spread south Saturday morning into
Saturday afternoon. Most of the west will be dry to start the
period with just some snow in the sw Friday evening. Saturday
night and most of Sunday day will dry ahead of a new shortwave.
Most of the medium range guidance has morphed into two shortwaves
Sunday night through Monday night without a closed low look (Euro
only has one main shortwave with a faster timing). Bigger picture
still supports potential for strengthening so we`ll have to watch
these closely with time. For now, unsettled beginning Sunday night
in the west (with snow), spreading east of the divide later
Sunday night or Monday morning. Cold enough for snow east of the
divide Monday with H7 temps from -7C to -10C depending on model
with -26 to -30c H5 temps. Lowered highs but potentially not
enough if this pattern works out or leans toward the Euro. GFS and
GEM have another system fast on this ones heels to keep it
unsettled into Tuesday while the new Euro is building in a
shortwave ridge. Lean towards some sort of a trough remaining but
confidence moderate at best with changing model solutions with
each run.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 155 PM MDT Tue Mar 28 2017

Scattered areas of MVFR/IFR cigs/vsbys from KRKS to KCPR through
02z. Otherwise, most areas will see VFR conds tonight through
Wednesday. However, patchy fog and low clouds will develop (or
remain) where recent rain and snow has fallen mainly near and
south of a line from KLND to KCPR south to KRKS. Sweetwater County
and the KRKS area will likely have the most potential for IFR
cigs/vsbys in fog and very low cigs later tonight and Wednesday
morning.

&&

.FIRE WEATHER...
Issued AT 155 PM MDT Tue Mar 28 2017

Fire danger will remain low through much of the week. A cool and
very progressive unstable pattern will overtake the forecast area
and will remain in place over the next week or so. Periods of
precipitation will move across WY every other day or so along with
several frontal passages. Temperatures will be seasonal to slightly
below seasonal for much of the time with gustiest winds accompanying
frontal passages. Smoke dispersion will be fair to good most
afternoons.

&&

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

&&

$$

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


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