Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 181643
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1042 AM MDT Mon Jun 18 2018

.SHORT TERM...(Today through Wednesday night

The upper low remains over NW/NC NV this morning, with the trough
axis neutrally-tilted. A broad area of divergence over E ID/W WY has
allowed for showers and thunderstorms to continue over W WY from
Sunday night. PW values are 150 to 175% above normal over the
region, which is why models continue to resolve 1"+ values occurring
from 03Z through 09Z Monday. Recorded rainfall amounts range between
0.30 to 0.60" since 21Z Sunday. Radar trends Sunday evening have
been mainly discrete cells that pulse up and collapse within 30
minutes, but continue to develop over NE UT and move northward.
Rainfall rates from PIH and SLC radars are generally 0.10"/hr, with
locally higher rates up to 0.50"/hr. These higher rates have been
relatively short-lived. An area of heavier rain developed over
western portions of the Wind River Basin between 03Z-06Z Monday
and has since lifted northward. Readings from FFMP show 1.5 to
2.5" fell along the foothills of the Wind River Mountains toward
Dubois, while readings from nearby RAWS/HADS sites show 0.50 to
0.80". The exception is Crowheart, which apparently received 1.6"
from 00Z to 06Z Monday. Issued a Flash Flood Warning for creeks
draining toward Ethete and will keep an eye on river gauges on the
Wind River from Dubois through Crowheart for now.

By 12Z Monday, most of the heaviest rain will lift north out of the
forecast area, with light showers continuing across western and
northern portions. The front that was stationary across southern
portions Sunday will lift northward as a warm front through the
morning hours today. Shower and thunderstorm activity will remain
confined to central and western portions, as the jet sets up from
Uinta County to the Bighorn Basin. Heavier showers and thunderstorms
could be possible over the Cody foothills this afternoon and
evening, as the left exit region of the jet moves over the area.
Temperatures east of the Divide today will be 5 to 10 degrees warmer
than Sunday, but still be below normal for mid-June.

Precipitation will be very isolated across the area by 12Z Tuesday,
as the core of the jet continues to shift eastward. The upper low
will also be over E ID and be much weaker, as it begins to fill
during the day Monday. This weakening trend will continue through
the day Tuesday, but will also bring enough lift to generate showers
and storms across western and northern portions by late Tuesday
morning and continue into the evening hours. This will spread
eastward through the day and impact eastern portions by late
afternoon/early evening. PW values will remain 100 to 150 percent
above normal, so locally heavy rain will remain a threat.

An upper level ridge will develop over CA/OR by Wednesday, as the
low moves over WY and continues to weaken and get sheared apart. As
elements of the subtropical jet move over SW WY, most of the
shower/thunderstorm activity will be confined to areas east of the
Divide (mainly east of a Cody to Casper line). Precipitation will
end from northwest to southeast through the overnight hours
Wednesday night. PW`s will remain well above normal Wednesday.

.LONG TERM...Thursday through Monday

Large model differences tonight, however there is agreement that
a couple systems will likely increase the shower/tstm chances
across the area Thu night into the weekend. The first arrives
around Thu night and swings across the north into Friday. The GFS
is then more aggressive with the second system eventually forming
a closed circulation over nern WY Sat. The ECMWF keeps this
system much further north in MT. The GEM is similar to the GFS,
just a day slower. We have some pops across the nrn two-thirds for
starters, which gradually increase into Sat but if the GFS/GEM
idea is more correct, we will need to increase these significantly
with time. Other than the slower GEM, conditions improve next
Monday with shortwave ridging developing.

&&

.AVIATION...18Z Issuance

Moisture continuing to surge northward will keep the chance of
showers and thunderstorms in the forecast through 00Z Tuesday,
although coverage may decrease early in the afternoon. The biggest
concentration of MVFR conditions from moderate to heavy rain showers
will be found west of a KLND to KWRL line this afternoon, although
all areas will have a chance. There will be occasional mountain
obscurations with low ceilings and reduced visibilities. Showers and
thunderstorms should decrease in coverage around sunset, although
showers may linger in spots through the night. Expect further
showers and thunderstorms Tuesday although most will hold off until
after 18Z Tuesday. Ceilings and visibilities should also improve on
Tuesday.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 127 AM MDT Mon Jun 18 2018

Warmer conditions across all but far western portions (which will be
cooler). Scattered to numerous showers and thunderstorms will occur
again today, with locally heavy rain continuing to be the main
threat. Mixing and smoke dispersal will range from poor to good,
with excellent conditions over portions of zones 277, 278, 279 and
289.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this evening for WYZ001>003-012-013.

&&

$$

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


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