Area Forecast Discussion
Issued by NWS Riverton, WY

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879
FXUS65 KRIW 281730
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1130 AM MDT Wed Jun 28 2017

.SHORT TERM...(Today through Friday night)

Imagery shows flat ridge/trof pattern west to east across the CONUS
with a few embedded SW/jet in the nearly zonal flow across the west.
One of those embedded SWs is currently moving out of cntrl WY and
over the High/Central Plains with the next embedded SW trof just
upstream over CA/OR/WA. SFC has low pressure exiting into the Plains
with post frontal high pressure building into WY from the west. A
few isolated showers/storms currently moving across the nrn quarter
of WY from w to e.

After a few strong/severe storms (mainly wind) were inspired earlier
Tuesday across the nrn half of WY due to a passing upper shortwave,
only a few terrain induced showers/storms are persisting overnight
across the Yellowstone area. Today, post upper trof/sfc frontal
early with fast moving nearly zonal flow aloft and a seasonally
strong e/w jet across srn WY. As weak SFC high P begins to break
with lowering pressure over srn WY and mesoscale high P left over
the nrn FA through the morning/afternoon, LL flow will turn back out
of the sw to w. Brisk flow and cooling aloft along with with a
slightly colder approaching embedded SW/jet streak will keep good
instability, forcing and dynamics over the nrn half of WY. With some
low to mid level moisture still retained across nrn WY from the
Tuesday`s/night`s precipitation (particularly across nern WY),
showers and thunderstorms will have another chance for initiation by
mid-day for a few fast moving wind/hail producers across northern WY
spurred on by an approaching developing front. By mid-afternoon, the
Big Horn and possibly the Wind River Basin will open up to strong
convection as the front races through the FA from the northwest.
Isolated strong/severe storms will be possible from Teton/Park
County on the west across Big Horn County through Johnson County.
Fremont County may also see a strong storm or two. The storm
environment will be similar but a little different today than that
of Tuesday. Storms across the nrn half of WY will be privy to
slightly better moisture, ML CAPE of 800 to 1200 J/kg, 30 to 50 kt
of shear, a little less DCAPE remain under 1000 J/kg, and will be
slower moving than Tuesday`s storms by some 5 to 10 kts. All this
may add up to slightly better developed storms with a little less
wind and a little more hail than Tuesday`s storms. Teton/Park
Counties will tend toward near severe hail while the counties east
may see an even mix of small hail and near severe winds. By late
afternoon/early evening, Johnson County may see a couple of the
strongest storms of the day with hail, wind and even a funnel cloud
or two (tornado?). Later, showers and isolated thunder continue
through the overnight period until the SW trof axis moves east of
the FA near sunrise Thursday morning. Nwrn WY will continue
lingering showers/isold storms.

Thursday, will begin post frontal and under continued good nw flow
aloft with yet another SW trof on its way into/across the FA more
from the north by mid-day. Frontogenesis will be also be ongoing
across MT early before crossing through WY in the afternoon. Cooler
conditions from top to bottom will limit CAPE and overall
instability for much of the area. However, a few stronger storms
will be found along/near/just behind the front across portions of
central/nrn WY. Isolated to widely scattered storms are expected to
move across WY this area a bit slower than the previous two days
...but still pushing occasional strong wind gusts out ahead of the
thunderstorms along with some small hail within the storms.
Strongest storms of the day look to be across Fremont/Natrona and
srn Johnson county. Again, with the upper SW trof still moving
through the FA Thursday night along with cooling temps aloft,
showers/isolated storms will continue through at least midnight.

Friday/Friday night, continued nw flow aloft but post frontal
stable with modest high P in place across the SFC and little in
the way of disturbances at any level to change this. So, little to
no chance for showers/storms...except isolated over a few higher
peaks of the region mid to late afternoon.

.LONG TERM...(Saturday through Wednesday)

Overview...A warming trend will occur over the weekend through early
next week.  Isolated to widely scattered thunderstorms will blossom
across mainly the north half of Wyoming over the weekend.  Most
areas will be dry Monday through Wednesday. On Independence Day,
there will be a slight chance of thunderstorms across the northwest
mountains including Yellowstone and Grand Teton National Parks, with
other areas seeing lots of sunshine, and most highs in the mid 80s
to lower 90s across the lower elevations.

Discussion...No major changes seen over the last few runs of the GFS
and ECMWF.  At the outset of this period, Pacific shortwave is
forecast to move onto the Oregon coast Saturday morning, and across
Montana and northern Wyoming Saturday night-Sunday morning.  Some
spread remains between GFS and ECMWF with the timing of this
shortwave with trends favoring slower progression, the shortwave
translating eastward across Idaho Saturday night, and across
northern Wyoming on Sunday. This timing could result in mostly a
nocturnal event across northern Wyoming Saturday night with most of
the activity along and east of Interstate 25 Sunday afternoon.
For forecast purposes, not attempting to nail down the timing this
far out, so kept low PoPs broadbrushed across the north Saturday
afternoon through Sunday afternoon, drying things out Sunday
evening. Upper high strengthens over the southern/central Rockies
behind this feature on Monday, with dry and warmer conditions
across west and central Wyoming. A Pacific trough is expected to
move east along the U.S. / Canadian border, across the northern
Rockies Tuesday night, the ECMWF a tad slower and further north
than the GFS. This system will be fighting a strengthening high
over the Four Corners region, so not expecting any impacts beyond
a dry, shallow cold frontal intrusion into northern Wyoming
Tuesday night- Wednesday morning, high temperatures trending
slightly cooler on Wednesday.


&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)

This morning already saw a few showers with isolated thunderstorms
move through the northern half of the area, with wind gusts over 30
kts.  With instability of advancing shortwave trough and morning
heating, convection is starting again over NW WY and moving in from
eastern ID fairly quickly.  Expecting KJAC to be impacted first by
18Z, then activity advancing to KCOD,a nd SE toward other terminals.
 Likely yesterday, wind gusts over 40 kts can be expected along with
small hail and brief heavy rain.  VFR conditions generally expected,
with local MVFR/IFR possible under and around storms and showers.
General wind shift to the NW will occur starting at 22Z/Wed and
spread SE as outfloow and pressure gradient combine, making for
another windy afternoon and evening across the area.  Could see a
couple of waves of thunderstorms move west to east through the area
this afternoon.  Lingering shower activity expected after 06Z/Thu.
Another disturbance will move south out of Canada and MT early
Thursday, brining more showers to NW WY by 15Z/Thu.  A northerly
push will cool temperatures and shift winds, primarily along and
east of the divide.  The showers Thursday could also have isolated
thunderstorms.  This activity is expected to last through 06Z/Fri.

&&

.FIRE WEATHER...

All fuels are currently in green-up and below critical levels for
all locations. Fire danger remains low across all mountain
areas...but will continue somewhat elevated in the afternoons across
the lower elevations east of the Divide over portions of southern
and central WY. These areas with elevated fire concerns will
generally coincide with low afternoon RH values and/or gusty
winds...today through Friday. Winds west to northwest 15 to 25 mph
today...north to northwest 10 to 20 on Thursday...and north to west
5 to 15 on Friday...today and Thursday with higher gusts in the
afternoons and especially near stronger showers/storms. Showers and
isolated thunderstorms continue to move into/through the western
Wyoming early this morning, continuing across the area through
tonight. By this afternoon and evening, these isolated to scattered
storms will produce brief heavy rainfall, hail to 1 inch and strong
gusty outflow winds of 40 to 55 mph...especially across portions of
the northern half of Wyoming. This unstable pattern will continue
through Thursday night. Smoke dispersion will be good to excellent
each afternoon and evening.

&&

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

&&

$$

SHORT TERM...Braun
LONG TERM...Meunier
AVIATION...McDonald
FIRE WEATHER...Braun



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