Area Forecast Discussion
Issued by NWS Riverton, WY

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388
FXUS65 KRIW 240511
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1111 PM MDT Mon Apr 23 2018

.SHORT TERM...(Tonight through Thursday)
Issued at 149 PM MDT Mon Apr 23 2018

Imagery shows longwave trough across North America with two embedded
troughs within the flow...one over the southeast CONUS and the other
passing through the central/northern Rockies, including WY,
stretching into the northern Plains. Weak ridge building along the
West Coast. The surface has high pressure over most northern tier
states, bisected by a cold front lying from the northern Plains of
northeast ND back through WY and into the Great Basin of UT/NV. Most
precipitation, scattered rain and snow showers, is falling from
eastern MT and down into/through northern and eastern WY with
isolated showers occurring across west central and northwest WY.

Tonight, southward moving front continues through WY and the CWA
while the trough aloft stalls to the west, stretching out over the
Plains. This will effectively keep precip chances around the
forecast area to some extent tonight and Tuesday morning
...especially across the mountains and the lower elevations of
Johnson and Natrona Counties. Perhaps an additional inch of snow
possible somewhere over the lower elevations with maybe a couple of
inches atop the Bighorn or Casper mountains.

Tuesday will start off post frontal with below seasonal temperatures
but with the trough still lagging behind overhead. This will allow
for low chance POPs, mainly lingering over the central mountains and
east central WY. By Tuesday afternoon, most shower activity should
be finished while the upper trough exits into the Central Plains and
ridging begins to overtake the region from the west. Wednesday will
see a nice warm-up back to seasonal temperatures ahead of an upper
trough and associated cool front that will make their way
over/through the region east of the Divide Wednesday night. Elevated
fire concerns will be possible from southern Fremont County east and
over Natrona county with low RH and occasionally gusty (20 to 25
mph) winds. Otherwise, while upper level dynamics are OK with this
system, there is really no good moisture source or return to work
on. Therefore the region will have spotty precipitation chances east
of the Divide, at best, through Thursday morning. Thursday, while
somewhat cooler than the day before will also see partly to mostly
sunny skies by afternoon with no additional precipitation. Winds
will gust, with frontal passage, out of the north/northwest across
Johnson county Thursday morning from 25 to 35 mph.

.LONG TERM...(Thursday night through Monday)
Issued at 149 PM MDT Mon Apr 23 2018

Mean longwave ridge over the western U.S. will gradually be replaced
by a developing modest longwave trough by next Monday. Shortwave
ridge moves overhead to start this period producing a mild/warm
Friday (and Sat). OP models are struggling with the pattern change.
In particular, what happens with the trough currently out near 140W.
The GFS wants to move it much faster zipping it across us late Sat
into Sat ngt. The new Euro has a strong upper low over NV Sunday
which gradually weakens with the remnant trough coming through
Monday into Monday night. The GEM is further south with the main low
putting it over nwrn AZ midday Monday before it gradually lifts nw
producing a very strong closed low over ne CO Tuesday morning. A big
part of the issue is the next trough west of the dateline. The GFS
shows a piece of energy breaking off of this trough which becomes
the kicker and pushes the upper low across us much sooner. The Euro
takes this piece of energy up and over the ridge first so it takes
longer and does not really become a kicker for the western U.S. low.
Tough call with the big changes to the pattern evolving. Would
probably lean closer to the Euro but we`ll have to watch it.
Overall, some moisture may be drawn up ahead of the low into the far
west Saturday for some isold showers/tstms with more on Sunday
across most of the area as the trough approaches. Depending on the
timing, fire wx concerns will increase Friday into Saturday.
Probably not significant wind Friday but very dry east of the
divide. Saturday, potentially could have more sw wind if the faster
GFS is more right (and quite warm). Dry either way east of the
divide with elevated fire concerns and potentially more. Sunday
would be the worse day potentially on the Euro with wind and isold
tstms. Unsettled Monday with general troughiness and mid level
cooling. Rain showers for the lower elevations and foothills with
some higher elevation snow showers and isold thunder. The coming
days will likely see adjustments to this forecast.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1106 PM MDT Mon Apr 23 2018

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

MVFR Ceilings are likely with isolated snow showers or occasional
snow flurries across the Lander Foothills, and much of Johnson and
Natrona counties this morning. MVFR ceilings in the Cody
Foothills are expected to improve to VFR by 12Z.

By around 18Z...VFR conditions are expected at all terminals with
skies becoming mainly SKC by sunset. Not out of the question for a
few snow showers coming off the Bighorn Mountains in the
afternoon. Breezy northerly winds are expected to weaken by
sunrise, but remain light northerly through the day Tuesday. Winds
are then expected to light and variable Tuesday night.

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

Areas with MVFR ceilings are expected to develop across the Upper
Green River Basin, as well as Sweetwater County late tonight with
most confidence at the KRKS terminal. There could also be some
snow showers across the Upper Green River Basin. Far western
valleys have a chance for low clouds and/or fog Tuesday morning,
but confidence is not high enough to include those conditions at
KJAC terminal. Either way, ceilings are expected to lift to VFR or
scatter out between 16Z and 19Z. Skies are then expected to
become mainly SKC by early evening. Breezy easterly winds will
occur today across Sweetwater County.

&&

.FIRE WEATHER...
Issued AT 149 PM MDT Mon Apr 23 2018

A decent upper level disturbance and associated seasonally strong
cold front continue south across Wyoming today/tonight. This will
keep post frontal gusty north to northwest winds (gusting 25 to 35
mph at times) around much of the area east of the Divide...at least
into the early evening period. Light to occasionally moderate rain
or snow showers with the stray chance for an isolated thunderstorm
or two will occur through this afternoon into the early evening
period too. By Tuesday afternoon all precipitation will be out of
the region with very dry and warmer conditions projected for
Wednesday. Portions of the lower elevations of central Wyoming will
certainly see elevated fire weather conditions on Wednesday. At the
moment, it does not appear to be Red Flag level except for a couple
isolated OBs...but will be worth watching.

&&

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

$$

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



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