Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 250447
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1047 PM MDT Tue Apr 24 2018

.SHORT TERM...(This evening through Friday)
Issued at 144 PM MDT Tue Apr 24 2018

Cold stable and drying high pressure will be over western
and central Wyoming tonight.  Already a lee side low
pressure trough is developing in the lee of the Canadian
Rockies and will spread south across eastern Wyoming on
Wednesday.  This will increase down slope west southwest
winds across central Wyoming. Temperatures will warm
significantly into the upper 60s to lower 70s with minimum
relative humidity dropping into the lower teens across
southern fremont and Natrona counties.  Winds will increase
to 15 to 20 mph with gusts to 30 mph.  These conditions will
combine with Critical dry fuels warranting a red flag warning
for the area Wednesday afternoon and early evening.  A cold
front from Montana will sweep south Wednesday night with colder
temperatures, much higher relative humidity, lower clouds
and showers across the north and east.  Through Thursday
morning.  Then high pressure will once again produce drying
with clearing conditions through Friday.


.LONG TERM...(Friday night through Tuesday)
Issued at 144 PM MDT Tue Apr 24 2018

Beginning of forecast: Ridging over the Rockie Mountain states
continues. Upstream, a large closed/nearly cut-off low (COL)
continues (and continues) to spin off the west coast over the EPAC.
Post frontal high pressure at the surface will be slowly falling
across WY with warm dry weather in place.

Overall, the MR models continue to diverge from the beginning of the
extended forecast, becoming well out of phase on Saturday. The cause
continues to be due to the persistent (nearly) COL sitting over the
EPAC. Problems/differences with the evolution of the COL system
largely stem from what will be going on (or not) upstream of the
COL. There have been multiple varying intra-solutions concerning
each individual MR model over the past week. And then there are the
more obvious inter-differences that continue to show between the
models themselves. The most recent thinking is that what was(is) to
be the upstream kicker will now pinch-off a COL well to the west
over the Pacific. The GFS however keeps the portion north of the new
COL south enough to still act on the west coast/EPAC COL without
being too nudged out of the way by a ridge between the two COLs. The
EC, however, is much less robust in building a strong ridge between
the two areas in question than it once was. So, (some) eastward
movement of the west coast COL is expected this weekend (where that
may have been somewhat more questionable a few days ago according to
the Euro model). Bottom line, the GFS opens and kicks east sooner
and faster than the EC, but at least the EC is now moving. When all
model evolutions (trajectories and dispersion) of the COL/trough
system are blended, an extended period of modest precip
(showers/diurnal T-storms) are spread from Saturday into Monday
mainly west of the Divide.

Then, the models diverge widely again with the development of
another closed low system within the expanding leftover trough over
the southwest CONUS later Monday (the one that finally brought the
COL through the region). Again, the GFS continues faster in
developing and moving the (new) system. This will keep the
atmosphere across much of the western CONUS, including WY, remaining
unstable with continuing chances for precipitation (almost
seamlessly from one system to another). At the moment none of this
activity looks particularly moist with temperatures, after the
warmest day of the year on Saturday, moderating through the first
half of next week...slowing snow/stream melt/run-off some. With
low/modest QPF and temperatures moderating beyond Saturday,
projected stream/river rises should remain below critical at least
through mid-week next.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1046 PM MDT Tue Apr 24 2018

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

VFR with increasing high cloudiness through 00Z Thursday. A lee
trough developing will result in windy southwest to west wind
developing from South Pass to Casper, and lee side of mountain
ranges. A cold front is expected to push into northern Wyoming
between 21Z Wednesday and 00Z Thursday. This cold front will sweep
to the Continental Divide by 03Z Thursday with breezy to windy
northerly winds developing in wake of the cold front. MVFR ceilings
are expected to develop in some areas a few hours after frontal
passage with a window of 3 to 6 hours where some valley
rain/mountain snow showers could occur. The best shot for low
ceilings at area terminals will be KCOD and KCPR. Improving
conditions are expected from north to south after 09Z Thursday.

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

VFR conditions with some increasing high cloudiness is expected
today. Winds are expected to be mainly light this morning, with a
west to northwest breezy developing late in the day. A cold front is
expected to sweep west/southwest across Sweetwater County between
03Z and 06Z with windy northeast to east winds in wake of the front.
There is a slight chance of some low ceilings tying to develop at
KRKS after 06Z Thursday, but the overall dryness of the system
should limit the chances of low ceilings. Otherwise VFR conditions
are expected with any precipitation remaining east of the Divide.

&&

.FIRE WEATHER...
Issued AT 144 PM MDT Tue Apr 24 2018

Cold stable and drying high pressure will be over western
and central Wyoming tonight.  Already a lee side low
pressure trough is developing in the lee of the Canadian
Rockies and will spread south across eastern Wyoming on
Wednesday.  This will increase down slope west southwest
winds across central Wyoming. Temperatures will warm
significantly into the upper 60s to lower 70s with minimum
relative humidity dropping into the lower teens across
southern fremont and Natrona counties.  Winds will increase
to 15 to 20 mph with gusts to 30 mph.  These conditions will
combine with Critical dry fuels warranting a red flag warning
for the area Wednesday afternoon and early evening.  A cold
front from Montana will sweep south Wednesday night with colder
temperatures, much higher relative humidity, lower clouds
and showers across the north and east.  Through Thursday
morning.  Then high pressure will once again produce drying
with clearing conditions through Friday.  With sunny conditions,
warming and increased winds most areas expected good or better
smoke dispersal wednesday afternoon.  Cooler temperatures
will likely drop smoke dispersal to fair across the north and
east on Thursday, with good smoke dispersal remaining west and
south.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon to 7 PM MDT Wednesday for WYZ280-289.

&&

$$

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



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