Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 180601 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1201 AM MDT THU AUG 18 2016

.UPDATE...Added 06z aviation discussion.


&&

.AVIATION.../06Z ISSUANCE/

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

SCT-BKN FL150-180 with isolated showers will linger overnight.  An
upper level low pressure system will lift northeast out of Utah
across the Continental Divide Thursday morning and combine with a
southward moving cold front to produce scattered showers and
thunderstorms across the area in the late morning and afternoon.
Brief MVFR conditions in heavy rain, small hail and wind gusts 25-
35kts will be possible with the stronger storms. Drier air,
decreasing showers and storms will occur from north to south late
Thursday afternoon and evening.  LCL IFR ceilings may linger with
this clearing. A strong cold front will push south across
Yellowstone National Park to VCNTY KJAC 09z-12z Friday spreading
areas of MVFR -showers and gusty northeast winds into northwest
Wyoming.

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

SCT-BKN FL150-170 and isolated showers will linger over the area
through 12z.  A cold front will push south across the area this
morning, with scattered showers and isolated thunderstorms expected
to spread south from the Montana border to a KLND-KCPR line 14z-18z.
Scattered showers and thunderstorms will continue through the
afternoon with a few strong storms producing brief MVFR/IFR
conditions with heavy rain, small hail and wind gusts 30-40kts.  A
brief window of clearing will occur from north to south Thursday
evening into early Friday morning with some lingering areas of
MVFR/IFR ceilings and patchy fog.  A stronger cold front will push south
across the Montana border around 09z Friday, spreading numerous MVFR
showers and isolated thunderstorms south to a VCNTY KWRL-KBYG line
by 12z Friday.

&&

.PREV DISCUSSION... /issued 252 PM MDT WED AUG 17 2016/

SHORT TERM...Tonight through Thursday

Imagery shows flat ridging across the central and northern Rockies
with a decent trough, for this time of year, moving through the
Great Basin and towards WY. Weak embedded disturbance leading out
the trough in modest southwest flow coming out of northern UT and
eventually into/across southwestern/western WY. The SFC has weak
low pressure across northern WY into eastern MT with a lee side
trough starting to build over eastern WY.

As the embedded wave moves over the western portion of WY this
afternoon/evening, isolated to widely scattered showers/thunder will
blossom through the afternoon west of the Divide...with more
isolated chances east of the Divide. The atmosphere today will be
somewhat moisture challenged with lower CAPE (< 700 j/kg) and modest
to low shear (< 25 kts). However, DCAPE looks to range up to 1000-
1200 j/kg by late this afternoon east of the Divide...so gusty
outflow winds 30 to 50 mph will once again be a possibility with
these showers/storms along with brief rain and small hail.

Tonight as the main western trough starts to impinge on the
western portion of WY, a cold front will push into/through the
northern FA by morning. With cold air aloft overflowing WY,
widespread instability showers and some thunderstorms will both
along and behind the front followed by seasonally deep upslope
flow into the northern and central mountains and adjacent
foothills by late afternoon Thursday. Weak instability with this
almost winter-like system and backing winds aloft will limit storm
strength with the strongest thunderstorms likely found near the
front (max shear of 25 - 40 kts and early CAPE from 700 to 1100
j/kg) as it plows south through the day as well as some post
frontal across Johnson, Natrona and eastern Fremont counties in
the afternoon as the upper wave moves through. The northern Big
Horn Basin may see a few strong morning storms when the front
first pushes in. Hail and wind will be the main threats from these
storms with a few approaching strong to severe limits.

LONG TERM...Thursday Night through Wednesday

Thursday evening and overnight will see the first northerly push
move through central and eastern WY, bringing considerably cooler
air into the state. The main push should bring a decent amount of
precipitation to the eastern half of the state, including Johnson
and Natrona counties in the RIW CWA. With the N/NE flow, a good
boundary will establish along the continental divide from YNP
southwestward through the Wind River mountains, allowing from
stronger precipitation to fall along the east slopes of the
mountains. Models are indicating that by 09z Friday, the second
colder push from Montana will start moving into NW WY from the
northern Absarokas into the Big Horn Basin. 700mb temps are
looking to drop to -2C over north central WY by 18Z Friday,
bringing the snow level down to about 9500 feet. Widespread
precipitation is expected Friday morning in the Big Horn Basin and
spreading into the Wind River Basin, Johnson and Natrona Counties,
and the surrounding mountains through the day. Breezy and cold
north wind should keep the afternoon high temperatures in the
upper 50s and low 60s in the lower elevations. By Friday late
afternoon, expecting only showers in northern WY with
precipitation concentrating across central WY from South Pass
eastward to Casper. Precipitation will diminish during the evening
hours Friday with 700mb temps ranging from 0C to +6C along the
ID/WY border. The path of the systems on Friday will largely leave
southwest WY precipitation free as the core of the disturbances
move east of the divide. Saturday morning will see some locations
in the western valleys and mountains drop below freezing, with
most locations in the upper 30s into the low 40s - the coldest
morning of the next 7 days.

Upper level northwest flow remains on Saturday with 700mb temps
from around 5C in NE WY to 11C across SW WY. Afternoon temps will
be warmer than Friday, but only jump up to the upper 60s and low
70s. Not really expecting post-frontal showers on Saturday, but a
couple may develop over the higher terrain. Weak high pressure
moves over the state on Sunday, again warming afternoon
temperatures by another 10 degrees or so, with isolated showers
and thunderstorms possible in the afternoon and evening. Tuesday
will again see seasonal temperatures with isolated showers and
thunderstorms as a larger system in southern Canada drapes weak
energy across the northern Rockies. Wednesday will be fairly
similar to Tuesday with a broad/weak dirty trough over the western
US. Medium range models are indicating an active week next week
with the GFS typically producing more precipitation over WY. This
is a certain change from the hot dry pattern of most of the
summer.

AVIATION.../00Z ISSUANCE/

VFR to prevail through the forecast period except for brief periods
of MVFR in lower ceilings and possible reduced visibility in heavier
showers/storms. A weak upper level disturbance leading out from an
approaching upper level trough will continue to influence western
and central Wyoming this evening. This system will provide widely
scattered showers and thunderstorms through the evening period west
of the Divide while east of the Divide, showers and storms will be
more isolated. The main hazard will be gusty outflow winds 30-45kts,
with thunderstorm outflow winds likely dominating the wind field in
the early evening. Overnight and through the morning hours, a cold
front will push south to vcnty the Montana/Wyoming border by around
06z Thursday and on to vcnty KRIW-KBYG line by 12z Thursday,
accompanied by a strong northerly wind shift. Ceilings near FL150
and isolated showers will linger 6z-12z across most of the area.
More showers and thunderstorms will arrive early morning, developing
from north to south across WY with a few strong storms expected
early in the Big Horn Basin then later in the afternoon across
portions of central/northern WY as the main disturbance passes by.
Some of the stronger storms will contain small hail and be able to
produce strong wind gusts greater than 45 mph along with lowering
visibility for intermittent periods.

FIRE WEATHER...

Fire weather conditions moderate to elevated today across the entire
forecast area as RH values drop back into the mid to upper teens
(percent) for portions of the lower elevations today and 20s  in the
mountain zones. At the same time, winds will remain mostly on the
light side at less than 15 kts unless influenced in the presence of
outflow wind from nearby showers/storms which could gust 30 to 45
mph at times even well away from the parent shower or storm.
Precipitation chances will come from instability over the mountains
and later this afternoon and evening from the passage of a weak
disturbance aloft. Overnight tonight and through the day Thursday
and Friday will see a drastic change in the weather as an early fall
type weather system impacts Wyoming, as a large upper level trough
pushes through a couple of cold fronts, shifting the winds out of
the north to northeast, greatly decreasing temperatures and
increasing precipitation chances across the entire forecast area.
Many northern mountain locations will pick up between a third of an
inch and an inch of precipitation through Friday with accumulating
snow possible overnight Thursday through Friday morning above 9000
feet. Warming and drying expected once again over the weekend.

&&

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

$$

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



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