Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 250759
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
159 AM MDT Tue Jul 25 2017

.SHORT TERM...(Today through Thursday night)
Issued at 104 AM MDT Tue Jul 25 2017

Imagery continues to show broad upper flat ridging across the CONUS
with bookend trofs/lows over each coast and the high "centered"
across the srn/cntrl Rockies into the srn Plains. Strong upper level
trof located across srn Canada/nrn MT is mashing the ridge as it
moves ewd. Monsoon continues in full flow with the best deep
moisture stretching across nrn MX/AZ/wrn NM and into swrn CO and
cntrl UT. SFC has general low pressure across the swrn/wrn CONUS
(partially heat induced)...post frontal high pressure over the PAC
NW with a front being generated by the upper low/trof located from
ecntrl OR into ID and through the nrn half of WY where it meets a
n/s oriented lee trof. The lee side trof tonight is providing a
weak focus for a couple of showers/storms Monday night over nrn/ern
Johnson County. Convection is not look as strong as those Sunday
night. The heavy areas of precipitation are still "trapped" further
south...mainly occurring under the monsoonal moisture plume across
AZ/swrn CO and into cntrl UT.

Today through the end of the forecast period the FA will (finally)
feel the first real effects of the monsoon. First, the upper ridging
pattern continues to flatten with the passing of the strong upper
lvl trof to the north across srn Canada/nrn CONUS. This feature will
generate increased flow aloft along with frontogenesis (currently
happening over the NW CONUS into WY as mentioned above) at the SFC,
while monsoonal moisture begins to return across/into portions of
srn/swrn/wrn WY today as the mid/upper pattern adjusts to the
passage of the shortwave trof and rather amplified ridging begins to
overtake the wrn CONUS then rotate clockwise over the Intermountain
West through tonight and into Wednesday. Secondary and stronger
frontogenesis and passage, also spurred by the strong trof to the
northeast, will aid in LL convergence and forcing by this
afternoon...increasing the likelihood of showers and storms well
into the overnight hours. During this time, PWs will increase
rapidly to around an inch (+/-) a localized convective flood
potential will increase through nearly the end of the forecast
period...especially with stronger training showers/storms.
Otherwise, today looks like a relatively cool/increasingly moist,
low CAPE/low to moderate shear day with storm movement to the east
or northeast 15 to 25 kts across srn/cntrl WY and to the east to
southeast 5 to 15 kts over nrn WY. All this will lead to some
organization and clumping of precipitation with localized heavy rain
and flooding possible. Wednesday will be post frontal, however the
monsoonal moisture will remain across the FA with as the ridge
rotates over the region. A warm front looks to form by late in the
Day, extending across WY from southwest to northeast. This boundary,
along with the terrain of the region, will provide the forcing and
focusing needed for another day of locally heavy rain/showers and
thunder. Flooding potential will be heightened once again
(especially over the mountains) while a few stronger thunderstorms
will be possible across wrn/swrn WY in the later afternoon/early
evening period. Thursday, moisture remains across the FA for another
day while the upper ridge continues to translate ewd toward the ern
border of WY while what`s left of the wrn coastal low moves through
the increasingly southwest flow aloft Thursday afternoon and evening
giving one last day of localized significant rainfall potential
(less flooding potential however) with perhaps even a few more
storms on the strong side found over portions of central
WY...especially Natrona County where CAPE and effective shear will
be at their best.


.LONG TERM...(Friday through Tuesday)
Issued at 104 AM MDT Tue Jul 25 2017

Models show the subtropical ridge centered over the Four Corners
Region late this week, shifting west into Nevada by early next
week. Weak disturbances, weak fronts pushing south into the area
along with daytime heating will keep isolated to locally scattered
diurnal convection possible each day. The coverage and the over
strength of storms in the extended are not expected to be as much
as what is expected through Thursday.

Temperatures are expected to be seasonal to slightly above average
for late July into early August.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 104 AM MDT Tue Jul 25 2017

VFR conditions will prevail through 18Z. Isolated showers will occur
in southwest WY through 15Z. After 15Z, showers and thunderstorms
will be developing over south and southwest WY. This activity will
spread northeast through this afternoon into tonight. Locally heavy
rain will occur which will create local MVFR conditions due to
reduced visibility. Wind gusts to 30 knots are expected with
thunderstorms. Showers and storms will decrease toward 06Z Wed
with isolated showers through 12Z Wed.&&

.FIRE WEATHER...
Issued AT 104 AM MDT Tue Jul 25 2017

Fuels are now at critical levels for most forecast zones east of the
Divide (Please see the Fire Weather Forecast product for more
information). Today, fire danger becoming low for most mountain
locations...moderate to low for all other lower elevations as
moisture, clouds and cooler weather overtake the forecast area while
an upper level disturbance and associated front meet up with
monsoonal moisture infiltrating from the south/southwest. Winds will
continue seasonally light across the forecast area today with the
day`s gustiest winds found along and behind a frontal passage. Winds
settle down once again Wednesday. Exceptions locally will occur, of
course, near any stronger shower/storm when winds could briefly gust
25 to 35 mph. Today there will be increasing chances for isolated to
widely scattered showers/storms across the lower elevations...and
scattered to numerous chances over the mountains...including periods
of of localized heavy rain today and again Wednesday.

&&

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

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...Ross
FIRE WEATHER...Braun



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