Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 131727
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1127 AM MDT Sun May 13 2018

.SHORT TERM...Sunday through Tuesday night

Over the forecast period, the pattern reminds me of a toy that was
popular when I was little kid back in the 70s. That toy is the Sit-N-
Spin, where you spun around until you were dizzy and then got up,
fell down and hit your head. It this case the Sit-N-Spin is an upper
level low over the Great Basin that continues to spin and slowly
fill as we head through the forecast period. And as is the case with
these slow and no moving lows, determining details of the forecast
is sketchy at best.

As for this morning, we have what looks the last of the steady rain
now shifting toward central Wyoming with most of the far west
drying out. As a result, we will cancel the winter weather
advisories for the west. We will also take down the warning for
the Absarokas since places that see any additional snow will be
areas with few impacts. With snow still occurring on South Pass,
we will likely keep the Wind River Range up until expiration. Both
the NAM and GFS show the steady precipitation shutting off toward
5 am as the jet dynamics shift to the east. The other problem
this morning will be the fog monster, as we have a lot of small
dew point/temperature spreads. For now, we kept it patchy.

As for today, there will be some improvement. With the dynamics
moving away, coverage of showers should be much less than the previous
couple of days. At this point, the best chance would be in the
west but all areas will have some chance of a shower or
thunderstorm given low heights. And with the continued low wet
bulb zero levels, there could be some small hail with any
thunderstorm. Temperatures should be somewhat warmer but still
below normal. There could also be some sunshine at times, but
could be the self destructive variety with instability still over
the area. As for the flood watch, we will keep it up for now but
the chance of any decent flooding continues to decrease late
tonight.

It looks to be the same story for Monday and Tuesday, although
temperatures should warm more with more sunshine. The coverage
of convection will also continue to decrease each day as the
low slowly fills and heights slowly rise.  However, pinpointing
the location of any disturbances riding around it is very
difficult. As a result, we will keep pops in the isolated to
scattered variety and not try to nail things down until the
details become clearer.

.LONG TERM...Wednesday through Sunday

The pesky upper low in the short-term is progged to push northeast
and shear out across E. Idaho/S. Montana Wednesday/Wednesday night.
This feature combined with diurnal heating should result in isolated
to scattered convection especially over the northwest and north.

On Thursday, attention shifts to an upper low across the Great
Basin/California area, and a northern stream trough digging into the
extreme northern Plains, and Upper Midwest. The associated cold
front with the northern stream trough is expected to push south
across the forecast area in the Thursday/Thursday night time frame.
The compact upper low/low-amplitude trough over the Great
Basin/California area Thursday is progged to push east into the
central/southern Rockies Friday and Friday night. The easterly
upslope flow combined with this feature should give a decent shot of
precipitation across much of the area Friday/Friday night with an
emphasis across the central areas. This system is a bit warmer than
the system this weekend with snow levels dipping down to around 7000-
8000 feet Friday night with the lowest snow levels east of the
Divide. The Gulf of Mexico moisture looks to be cut off, and upper-
level jet dynamics appear to be minimal with this system which would
be limiting factors for significant precip. Having said that, the
easterly upslope conditions could still squeeze out a decent amount
of precipitation, but is expected to be much less than what occurred
with this past system.

One system then exits on Saturday with yet another storm system
possibly impacting the area Sunday.

Well above temperatures are expected Wednesday, and Thursday in most
areas. Below average temperatures are expected Friday and Saturday,
before seasonal to slightly above average temperatures return Sunday.

The combination of rapid snowmelt from above average temperatures
Tuesday through Thursday, followed by precipitation Friday could
cause some significant rises on some area rivers and streams late
this week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1125 AM MDT Sun May 13 2018

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

Showers and isolated thunderstorms will develop across the area aft
19Z and last through 02Z, before decreasing in coverage. Local MVFR
conditions will occur with storms due to lower cigs and vsbys.
Mountains will be obscured at times. Patchy fog with lcl IFR conds
are possible again Monday morning, especially in the wrn valleys
including KJAC. Sct showers and isold tstms will develop across the
area again aft 19z Mon.

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

Sct showers and isolated thunderstorms (aft 20z) will last through
03Z, then decrease in coverage by 06Z. Local MVFR conditions will
occur with storms due to lower cigs and vsbys. Mountains will be
obscured at times. Low clouds will increase again aft 06z tonight
through 16z Mon with MVFR conds at most TAF sites. Sct showers
will return Monday with isold aft tstms.

&&

.FIRE WEATHER...

A weather system to the west will keep the chance of showers and
isolated thunderstorms in the forecast for most areas. However,
today looks to be a much drier day then Saturday with most areas
rain free most of the time. Temperatures will be a bit warmer, but
still below normal. Relative humidity will remain above critical
levels with winds generally light to moderate. Mixing and smoke
dispersal will range from poor over some of the basins to very good
across Sweetwater County.

&&

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

&&

$$

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



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