Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 060454
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1054 PM MDT Fri Jun 5 2020

.SHORT TERM...This evening through Saturday night
Issued at 244 PM MDT Fri Jun 5 2020

A flat ridge will dominate weather for this afternoon resulting in
record or near record high temperatures today, given the 17C temps
at 700mb. This heat will be coupled with minimum relative humidity
of 11 to 15 percent across much of central and southern Wyoming
along with continued brisk winds. Our RFD is addressing this. There
could be enough instability and moisture over our northern tier
zones later this afternoon and evening for isolated thunderstorms,
especially over the northern mtns once the subsidence inversion
begins to vanish and the ridge exits east. However, moisture will be
lacking, along with sufficient buoyancy per SPC. Any storms that do
form across northern Johnson County later today could get rather
strong given the -6 lifted indices expected there with elevated
convection. Most of Johnson and northeast Natrona Counties are in a
marginal risk.

The well defined low well of the northern Baja California Coast will
make landfall this evening. Once the circulation is over land, it
will lose its identity and become a negatively tilted open wave. As
the open wave trough approaches the Rockies, clouds and showers will
spread northeast toward Wyoming, with southwest Wyoming expected to
experience showers by 06Z tonight. These showers will spread
northeast across most areas west of the Divide as far north as about
Jackson. This rainfall will be synoptic in scope with QG forcing and
PVA to work with. The individual rain showers will race north
northwest driven by ssw 45 knot winds at 700mb.

On Saturday, the timing of this batch of showers and stratiform
clouds will be crucial in where there will be sufficient solar
heating for meaningful convection Saturday afternoon and evening.
SPC has most of the CWA in a marginal risk for Saturday. Going with
the timing of the shortwave tracking northeast across Wyoming by 00Z
Sunday, it appears now that, as so often as is the case, northeast
Johnson County stands the best chance of developing a strong
thunderstorm before this mass of clouds interferes with heating.
Lapse rates will be around 8C/KM. Do not see much in the way of
directional shear during daytime heating. PW values are progged to
be 1.22 inches in eastern Johnson County. The GFS also spits out a
half inch of precip southeast of Casper from 18 to 21Z Saturday.

Central zones like the Wind River Basin could spend much of the day
attempting to rid the considerable high and mid clouds. The
southwest zones could get an earlier start on the convection. The
far west could see -3.5 lifted indices. The best capes appear to
surround our CWA as a reflection of the clouds and cooler temps over
Central WY.

The models seem to be siding with the Euro that originally went with
this slower solution of the trough passage, which is often the case.
If the timing of the main area of forecasted clouds and showers is
off, all bets are off with the expected placement of the best
convection Saturday which now appears to be in the periphery of the
CWA. This will have to be watched. High temperatures will be
cooler due to the clouds and showers.

On Saturday night, the initial cold front will sweep through with
the shower activity continuing, especially in the west, closer to
the energy of the upstream kicker off the Pacific Northwest coast,
that will sweep this shortwave feature northeast across WY. It
appears that by 12Z Sunday, snow levels could actually descend to 8K
in the far west and 9.5K farther east as much colder air filters in
from the west behind the sharp baroclinic zone. There will be some
left front quad dynamics in western Wyoming when the precip
changeover occurs in the mountains. Winds will pick up in many areas
later Saturday when the front begins to slide through.

.LONG TERM...Sunday through Friday
Issued at 244 PM MDT Fri Jun 5 2020

The wet weekend will continue across most of the area on Sunday as
the large trough and associated surface low positioned to our west
drifts east. This will be aided by favorable jet dynamics as it
does, especially across western Wyoming where the most QPF is
expected. Additionally, a stronger cold front will move east across
the area Sunday night. This will considerably lower snow levels
across the west with snow likely reaching the valleys by Monday
morning with half an inch or so possible. One to three inches look
likely in the mountains, with locally greater amounts at higher
elevations.

Monday`s activity will likely be more limited to the west and should
wane across the area during the evening as the trough moves away.
Precipitation totals west of the Continental Divide for the event
from Saturday through Monday will likely be in the 0.5 to 1 inch
range, with most of that falling during the weekend. In the lower
basins east of the Divide, generally less than a 0.10 inches is
expected for the same period.

Mainly dry northwest flow on Tuesday will give way to ridging
Wednesday through Friday, with shower activity mainly limited to
northwest Wyoming. Temperatures will be cooler behind the weekend
system, but should warm each day throughout the week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1050 PM MDT Fri Jun 5 2020

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

VFR conditions are expected to continue through the period. Gusty
winds will return early Saturday morning as a weather disturbance
enters the area. Expect the gusty south to southwest winds to
continue, occasionally getting strong, through the day Saturday.
The upper level trough will bring some vicinity showers to most
terminal areas Saturday afternoon, with some stronger
thunderstormsnot out of the question.

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

Expect mostly VFR conditions through the period. An upper level
trough will move in from the southwest tonight. Lighter showers
are already spreading to the northeast along the leading edge in
the vicinity of KRKS and KJAC. Showers will remain light tonight,
as the atmosphere has slightly stabilized with loss of heating and
churning from passing thunderstorms earlier this evening. Showers
will become more widespread west of the divide tomorrow morning.
Expect a further increase in coverage and thunderstorm development
by Saturday afternoon. Showers and thunderstorms will continue
into the overnight hours Saturday night, with southerly winds
further increasing, especially at KRKS.

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 244 PM MDT Fri Jun 5 2020

Critical fire weather thresholds will continue this afternoon in the
lower elevations of central and southern fire weather zones. Near
record heat, minimum relative humidity between 10 and 15 percent and
breezy southwest wind will all combine for elevated fire weather
behavior. Improving conditions are expected this weekend as chances
of precipitation increase and a cold front pulls east across the
area Saturday night into Sunday night. We will see snow levels drop
to the valley floors by Sunday night in the west.

&&

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

$$

SHORT TERM...Lipson
LONG TERM...Hulme/Myers
AVIATION...Straub
FIRE WEATHER...Lipson


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