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000
FXUS65 KRIW 142053
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
253 PM MDT Mon Aug 14 2017

.SHORT TERM...(This evening through Thursday) Issued at 240 PM
MDT Mon Aug 14 2017

Imagery shows longwave trof across the CONUS with two embedded
shortwave trofs west and east and a weak ridge between. The upstream
shortwave axis is located from sern BC into the Great Basin and on
through nrn California with a local jet max ahead of the axis
between nwrn WY and nrn California. This jet is producing quite an
elongated area of vorticity across srn ID and into nw WY with a nice
baroclinic leaf showing up in the imagery. SFC has general low
pressure across the High and Central Plains/Central and Southern
Rockies to the Desert Southwest. Post frontal high pressure now
across the PAC NW is following a cold/stationary front draped from
ern MT through ern WY...then across srn WY and on to NV/nrn
California. Isolated showers now across nwrn WY...spreading ewd.

The rest of today along with Tuesday will likely be the two most
critical days weather-wise of the forecast period...with a change
from the seasonally cool/wet weather of the last few days to a
somewhat warmer/drier pattern by week`s end. A transition to sw flow
aloft with the approach and passing of the aforementioned SW/jet
today will spawn frontogenesis across nwrn WY which will push
through in a robust manner through the state diagonally from nw to
se this afternoon and evening. Moisture will continue to feed in
from the south in a monsoonal-like fashion with additional (weak)
EPAC moisture accompanying the shortwave/front into and across the
nwrn/nrn FA. With mid/upper instability increasing with the
shortwave/jet and the SFC front helping to trigger/maintain
convection along and behind it...most of the FA will have at least a
small chance for precipitation today/tonight. Strong or locally
severe storms will be a possibility at times mainly across northern
WY this afternoon as MU CAPE ranges from 1000 to 2000 j/kg (ML CAPE
about 70 percent of that). Effective shear in response to the
shortwave is high for this area at above 30 kts (much higher
under/near the front)...which may be a bit too high for the amount
of usable CAPE. Still, strongly tilted rotating storms in some
cases will allow for persistence and some organization. However,
with mid/high level clouds keeping sun and warming down, a general
lack of a dry layer aloft, and (even with relatively low LCLs) a
mostly elevated convective threat - not surface based - the severe
threat should remain on the lower side. Best chances for
strong/marginally severe threat (marginally large hail and wind in
addition to heavy rain) will be across Johnson and Natrona
counties...perhaps ern Fremont...late this afternoon/evening. Areas
of heavy rainfall could continue this evening into the overnight
hours as support from the upper shortwave/jet continue from
Sweetwater county northeast through Natrona county (and of course
over ern WY).

Tuesday will begin post frontal but with only modest high pressure
building across WY due to the upper shortwave trof axis only then
moving over the region. It looks as though a second bit of
frontogenesis could occur, pushing through the FA Tuesday afternoon
with another (final) round of stronger showers/storms possible
(although, not as strong as those from today as both CAPE and shear
will be much lower this day).

Wednesday and Thursday: Upper trof axis and SFC front through the
area with NW flow aloft returning to the FA will combine with a
generally warming/drying atmosphere from the west as monsoonal flow
gets shunted ewd and over the srn Plains. Wednesday will see some
weak shortwave ridging and increasing stability...keeping any of the
day`s convection main over a few higher peaks of the FA. Thursday
will also be rather stable and dry...with perhaps slightly better
chances for precipitation just beyond this forecast period (Thursday
night) across northern WY. Nothing too special here.

.LONG TERM...(Thursday night through Monday) Issued at 240 PM MDT
Mon Aug 14 2017

Synopsis...A shortwave trough will clip NE WY through the
overnight hours Thursday, as a ridge of high pressure quickly
transitions over the Northern Rockies and Northern Plains Friday.
This ridge continues its eastward progression through the weekend,
as another longwave trough moves over western Canada starting 12Z
Sunday. At the same time, a shortwave trough will begin to move
over the Great Basin. The subtropical jet (STJ) will also
strengthen over southern CA and northern portions of the Four
Corners. The latter 2 features are expected to bring an increase
of clouds in time for the eclipse Monday morning.

Discussion...Isolated showers will be possible Thursday night
across northern and northeastern portions , as the cold front
associated with the shortwave moves through the area. Dry and
clear conditions will occur Friday and Saturday, as temperatures
return to more seasonal levels. Skies will become partly cloudy
late Saturday afternoon, with isolated showers possible across the
far west by Sunday morning. Isolated to widely scattered showers
and thunderstorms will be possible across western and northern
portions Sunday afternoon and evening. Some of this activity could
linger overnight and into Monday morning over NW portions. AT THIS
POINT...Skies are expected to be at least partly cloudy, with
mostly cloudy conditions over NW portions of the state. Isolated
to widely scattered thunderstorms are expected during the
afternoon across but southern portions.

&&

.AVIATION...00Z Issuance

Clouds and widespread rainshowers and thunderstorms continue to
stream over the area this afternoon. A cold front continues its
southward progression, passing KCOD-KWRL, but has yet to reach
KRIW-KCPR. The front is expected to pass through these areas
around 00Z Tue and eventually southern portions later tonight.
Winds will briefly stay gusty 10-20kt after 00Z, but will be
northerly as the front goes through. Some locations will begin to
switch to the southwest after 12Z Tue. Strong thunderstorms will
be possible along the east slopes of the Bighorns and into
portions of Natrona County. Main hazards will be brief heavy rain,
small hail, and wind gusts of 35-45kts. VFR conditions will
continue this afternoon, but will lower through the overnight
hours to marginal MVFR as rainshowers become widespread this
evening and through the overnight hours. These showers will
continue, especially east of the Divide, through 18Z Tuesday.
Mountain top obscuration will be common across northwest Wyoming,
spreading southward through the afternoon. These obscurations will
then spread east during the evening with many mountain peaks above
10K MSL obscured by sunrise Tuesday.

&&

.FIRE WEATHER...
Issued AT 240 PM MDT Mon Aug 14 2017

Fuels at critical levels for most of the low elevation forecast
zones in addition to a few mountain zones above 8000 feet across
western Wyoming. (Please see the Fire Weather Forecast product for
more information). An unseasonably strong upper level disturbance
along with an associated cold front will push through the forecast
area today giving at least some chance for precipitation to all
areas of WY this afternoon and overnight. Locally very heavy rain
possible across portions of central and eastern Wyoming.
Strong/locally severe storms possible over portions of
northern/cntrl/eastern WY. Tuesday will remain cool and relatively
moist for another day with another decent chance for
precipitation...especially across central and northern WY...with
pockets of heavy rain possible once again. Wednesday and Thursday
will see a generally warmer, drier and more stable atmosphere with
small precipitation chances mainly over the higher peaks of the
area. Winds will remain seasonably light with RH values only
bottoming out in the upper teens and 20s (percent).

&&

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

$$

SHORT TERM...Braun
LONG TERM...LaVoie
AVIATION...LaVoie
FIRE WEATHER...Braun