Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 211752
Area Forecast Discussion
National Weather Service Riverton WY
1152 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
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.
East of the Continental Divide...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
Frontal boundary will push south into the Big Horn Basin this
afternoon. Low clouds with MVFR conditions will develop early this
evening at KCOD with favorable upslope component behind the front.
Mountain obscruations will be frequent around the east slopes of the
Absaroka Range and around the Bighorn Range. Gusty northerly wind of
10-20kts will be common behind the front through late this evening.
Some confidence in light showers drifting north across the region
late tonight. Expect ceilings to rise Thursday morning, setting the
stage for afternoon convection.
West of the Continental Divide...KBPI/KJAC/KPNA/KRKS ROUTES
Convection will increase close to the western Wyoming border this
afternoon, mainly impacting KJAC. Have continued VCTS at KJAC while
VCSH will suffice at KBPI and KPNA. Orientation of the precipitation
axis will leave KRKS without VCSH through the afternoon. Jet streak
and shortwave energy will combine to produce gradually deteriorating
conditions at all but KRKS by 06Z/Thu. Mountain obscurations will be
widespread from that time through the remainder of the foreast
period. Showers will be persistent from about 04Z/Thu through at
least 12Z/Thu. There could be a lull Thursday morning before the
precipitation picks up again by midday Thursday.
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