Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210847
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
247 AM MDT WED SEP 21 2016

.SHORT TERM...Wednesday through Friday night

Quite an active pattern is setting up along with a change to some
fall like conditions over the next few days. Today at this point
looks to be the quietest of the three days. All is quiet right now.
However, some moisture moving in ahead of a deep upper level low
will begin flowing into western Wyoming and bring an increase in
coverage of showers and thunderstorms this afternoon West of the
Divide. Meanwhile, most areas East of the Divide should be able to
squeak out one more dry day. With a bit more cloud cover and
northeast flow, temperatures will be a bit cooler than yesterday but
still warm for this time of year. There will be a few showers and
storms spreading East of the Divide tonight but most areas should
remain mainly dry until late Wednesday night when deeper moisture
will move into the region. Rain will become steadier West of the
Divide late tonight as well.

Deeper moisture will spread across the area Thursday with an assist
from moisture moving up from the remnants of Hurricane Paine. With
upper level divergence increasing and a Pacific cold front
approaching and enhancing lift. The real kicker looks to be a jet
streak rounding the base of the upper level low. At this point, the
heaviest precipitation looks to occur on Thursday afternoon for most
areas, although there are some differences on the placement of the
heaviest QPF. Thunderstorms will be possible everywhere as well. The
SPC has placed southwestern portions of the area under a marginal
risk with the favorable jet dynamics and timing of the cold front so
a few storms could become strong. There could also be a few stronger
storms in eastern portions of the area. There is more uncertainty
there however. Shear does not look as good as yesterday with the
best east of the area. Also, a lot depends on how warm it gets ahead
of the front. For now given the uncertainty, we left high
temperatures alone. Some cooler air will also work into the west,
and snow will likely develop across the higher elevations but
through Thursday night should remain above pass level. The other
concern is the chance of flooding. It has been dry so many areas can
absorb the moisture. The best chance of this would be in areas with
recent fire activity where burn scars have developed. We though
about a Flash Flood Watch this morning but for now we will punt to
the day shift for future consideration.

Friday at this point looks like a cooler and unsettled day. The
initial round of rain should shift east of the area with both the
NAM and GFS showing a dry punch of air moving into portions of
Central Wyoming. However, another jet streak rounding the upper
level low could bring another round of precipitation to the west.
And with cooler air in place, snow levels could drop down to pass
level. Some of the models are indicating some clearing possible on
Friday. With lapse rates over the area, more showers are likely. We
thought about adding thunder but held off for now. With the low wet
bulb zero levels, any showers or thunderstorms could have some hail
with them or maybe some graupel. Some uncertainty with Friday night
as well. The NAM shows some moisture wrapping back into eastern
portions of the County Warning Area with a jet max. The GFS keeps
things drier and further east. Again, with the uncertainty few
changes were made.

.LONG TERM...Saturday THROUGH Wednesday

The low pressure system and the axis of its associated wave will
be in the process of swinging through the state Saturday morning,
with the potential for continued widespread shower activity.
Models are in slightly better agreement this morning regarding the
path of this system, with the core being located over eastern
Montana/western North Dakota...and the axis being over central
Wyoming.

By Saturday evening, much of the system will have moved through
the state, with weakening shower coverage. Building northwest
flow will be found over western Wyoming as an area of high
pressure moves closer to the Pacific coast. Models are in
disagreement with the potential for residual moisture and
associated showers over eastern Wyoming early Sunday morning.
Continued the trend of keeping some shower activity over the
Bighorn mountains, with widespread drying elsewhere.

This lingering moisture disappears by late Sunday evening, with
widespread dry conditions. The axis of the ridge will be advancing
towards the intermountain west, with dry and warming conditions
continuing through Wednesday.

&&

.AVIATION.../12Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

The main concern will be MVFR ceilings and mountain obscurations
developing this evening in wake of a cold front pushing south this
afternoon and evening. The best chance of low clouds will be across
the Cody Foothills, and Johnson County with less certainty farther
south. Also some convection is possible late this afternoon into the
overnight hours, but only brief restrictions are expected with this
activity. Breezy northerly winds of 15-25kt will occur across
northern Wyoming late this afternoon through late evening in wake of
the front, but are expected to weaken in most areas by around
midnight.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Marginal LLWS form the south is expected at the KJAC terminal this
morning. Attention then turns to increasing chances of convection
this afternoon into the overnight hours ahead of a slow moving upper
low that will impact the area for a few days. Instability is limited
with a gradient across the far west. Thus will only have VCTS at
KJAC terminal, but only VCSH at other terminal. Brief MFVR/IFR
restrictions could occur this afternoon and evening with convection.
After 03-06Z there is better shot at persistent rain showers with
MVFR or low VFR ceilings and mountain obscurations. KJAC terminal
has the best shot at seeing persistent MVFR ceilings with rainfall
after 06Z.
&&

.FIRE WEATHER...

Fire concerns should remain low to moderate for the next few days.
Although it will be another warm day for most areas East of the
Divide, with less wind and higher humidity fire concerns will ease.
Across the west, a surge of moisture will bring an increasing chance
of showers and thunderstorms through the day and into to tonight.
East of the Divide most areas will be mainly dry most of the time.
Mixing and smoke dispersal will be generally fair east of the divide
and good West of the Divide. An approaching low will bring a good
chance of a wetting rain for many areas from tonight through
Friday.&&

.RIW Watches/Warnings/Advisories...
None.
&&

$$

SHORT TERM...Hattings
LONG TERM...Branham
AVIATION...WM
FIRE WEATHER...Hattings



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