Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 261752
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1152 AM MDT THU MAY 26 2016

.Short Term...Thursday through Saturday night

All in all it looks like more of the same for the next few days. For
the most part the radar is quiet except for a stray shower here or
there. Thursday looks fairly similar to Wednesday with a mix of
clouds and sunshine along with isolated to scattered showers and
thunderstorms mainly of the afternoon variety. Once again, the best
chance will be across the mountains. With heights slightly higher
than yesterday and jet dynamics a bit weaker, coverage will
likely be a bit less than yesterday. However, everyone will have
at least a small chance of shower or thunderstorm. But much like
yesterday, many areas will see nothing and even places that do see
something will be dry a majority of the time. With the higher
thicknesses, high temperatures should be a couple of degrees
warmer than on Wednesday. Any storms that do develop will likely
have slow movement since steering winds are fairly weak.

Convection could decrease a bit more on Friday. Both the NAM and GFS
show some shortwave ridging over the area that could cap the
atmosphere a bit more. In addition, there will little upper level
forcing from the jet. But with the still fairly low heights over the
area there will still be at least an isolated shower or thunderstorm
possible for all areas in the afternoon and evening. It is the same
story for Saturday as well. However, similar to today, most areas
outside of the mountains should be dry a majority of the day.
Temperatures will slowly rise into the weekend but remain
below seasonal normals.

.LONG TERM...Sunday through Thursday

Synopsis...Sunday will be the least active and mildest day over
the holiday weekend with mainly isolated afternoon and evening
thunderstorms across the south and west. A cold front will push
across the area on Memorial Day spreading more numerous showers
and thunderstorms across the area with isolated strong to severe
thunderstorms possible across the north and central. A cool and
unsettled northwest flow will follow this system on Tuesday with
several inches of snow possible across the northern mountains and
Yellowstone National Park Monday night and Tuesday morning. A
ridge of high pressure will bring drier and warmer weather to the
area by Thursday.

Discussion...Omega-type block with upper high over Alaska and
trough across western Canada and the northwest U.S. is expected to
breakdown Sunday and Memorial Day, dislodging the trough across
the northern Rockies on Memorial Day. GFS and ECMWF show much more
spread with this transition than 24 hours ago. Between the two
models, ECMWF shows better continuity progressing the upper
trough into the northern plains midweek. There was little weight
given to GFS cutting off upper low over the area Tuesday and
Wednesday.

After a relatively pleasant day Sunday, upper trough is dislodged
from western Canada and translates the northern Rockies Memorial
Day.  Showers and thunderstorms are likely to spread into the west
during the morning.  Depending on timing, if this strong upper
trough crosses the area during maximum heating Monday afternoon, we
could see some strong to severe thunderstorms develop, especially
across north and central Wyoming.

ECMWF has trended colder behind the trough passage Monday night and
Tuesday with H7 -3C isotherm dipping across the north.  This would
bring snow levels down to near 7000 feet with several inches of snow
possible in the northern mountains, the ECMWF targeting mainly the
Beartooth Pass area and the Bighorn Range.

Upper ridge is expected to slowly progress into the area Wednesday
and Thursday bringing more seasonal, even summer-like weather by
Thursday with some mid 80s possible across the lower Bighorn
Basin.

&&

.Aviation.../18Z Issuance/

West of the Continental Divide...KJAC/KRKS/KPNA/KBPI Routes

Cyclonic flow around upper low in western South Dakota will keep
west-northwest flow across the region Thursday afternoon and tonight.
While the best moisture and instability will be east of the Divide,
there is still enough residual moisture to generate scattered
showers and thunderstorms through 03Z/Friday. Convection will be
primarily diurnal in nature. VFR conditions will prevaily through at
least 06Z/Friday. Other than surface wind gusts in excess of 30kts
with convection, surface winds will be less than 12kts from the west-
to-northwest. Weak frontal boundary will slide southwest across KRKS
this evening. This will produce lowering ceilings, still VFR, and a
chance of light showers. Expect areas of IFR/MVFR ceilings in the
far west valleys again tonight between 10Z-15Z/Friday. This will
produce localized obscurations of the foothills at KJAC and in the
Star Valley. All terminals will be VFR after 15Z/Friday.

East of the Continental Divide...KCPR/KCOD/KRIW/KWRL/KLND Routes

Cyclonic flow around upper low in western South Dakota will keep
west-northwest flow across the region Thursday afternoon and tonight.
Already ongoing convection north of a KLND-KCPR with the bulk over
the Big Horn Basin. Weak frontal boundary has also pushed south to
around this KLND-KCPR line with light north wind in its wake. Expect
most robust and widespread convection within this region through
03Z/Friday. Best instiability will be along an axis from KCOD-KWRL-
KCPR. Convection will weaken but continue through 06Z-09Z/Friday,
particularly across northern Wyoming. Terminals are expected to
remain VFR, with the best chance of MVFR visbiility with convection
at KCOD-KWRL-KCPR. Occasional mountain top obscurations are likely
later Thursday afternoon into the evening hours.

&&

.FIRE WEATHER...

It will be more of the same today with a mix of clouds and
sunshine along with isolated to scattered thunderstorms, mainly
of the afternoon and evening variety. Relative humidity should
remain above critical levels. Winds should remain light to
moderate outside of any thunderstorms, where gusty and erratic
winds are possible. Mixing and smoke dispersal should be fair to
good outside of any precipitation.
&&

.RIW Watches/Warnings/Advisories...
None.
&&

$$

SHORT TERM...Hattings
LONG TERM...AEM
AVIATION...CNJ
FIRE WEATHER...Hattings



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