Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 220829
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
229 AM MDT Sat Jul 22 2017

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

Imagery continues to show upper flat ridging across the CONUS with a
broad high "centered" across the entire srn CONUS. Monsoon is in
full flow tonight/this morning with good moisture stretching from
MX/AZ/NM through the srn Rockies and across the sern Portion of WY
then into the High/Central Plains. SFC has general heat induced low
pressure across the swrn CONUS...post frontal high pressure across
the PAC NW with relatively strong a meso-high centered ovr CO. A
slow moving front is located across much of the CONUS...trailing off
through portions of the Central Plains and across srn WY just north
of the meso-high. Heavy areas of precipitation are occurring under
the monsoonal moisture plume with CO receiving the heavier
rainfall...especially over nern CO where a weak upper vorticity max
and locally strong LL convergence along a outflow from storms
originally over sern WY. No precip currently falling anywhere in the
CWA.

Today and Sunday, upper lvl ridging continues over the fcst area
with the general low to mid lvl circulation pattern associated
partially with the monsoonal flow being held further swd and out of
most of WY as the front slowly pushes swd into the southern Central
Plains. The drier airmass, warming aloft associated active ridging,
and light flow through the vertical will keep it stable across the
FA and without much of a trigger for storms/precip over the next
couple of days (isolated higher mountain locations excepted mainly
on Sunday). Otherwise, lots of sun and a warming trend with
seasonally light winds (occasional late afternoon mixed out gusts 15
to 20 mph).

Monday/Monday night, upper ridging flattens with the approach of an
upstream SW trof moving across the nrn CONUS. this feature will
generate increase flow aloft along with frontogenesis at the SFC.
Additionally, it now looks like at least weak monsoonal flow will
begin to slowly return to the area ahead of the front. All this
taken together will give slight chance for storms mainly across the
mountains ahead of the front...and slight to chance POPs with both
the front and terrain Monday night. Better monsoonal moisture will
then start to increase back into the wrn FA early late Monday night/
early Tuesday morning...as the mid/upper pattern adjusts to the
passage of the SW trof and rather amplified ridging just begins to
overtake the wrn CONUS and rotate clockwise over the Intermountain
West.


.LONG TERM...(Tuesday through Saturday

Overview...The most active days for thunderstorms under the
monsoonal plume will likely be Tuesday and Wednesday during the
extended period. Thunderstorm coverage will then be mainly
isolated Thursday through Saturday. Temperatures are expected to
be near normal with some cooler daytime readings Tuesday and
Wednesday due to cloud cover and shower activity.

Discussion...West and Central Wyoming enter into the
climatological peak of monsoon season during the last week of
July. The forecast shows the SW monsoon coming into full bloom
across the area Tuesday and Wednesday with upper high centered
over the southern Rockies-southern High Plains, PWAT values of 1
inch or greater rotating northward across the Great Basin on
Monday and into the area Tuesday and Wednesday. Surface high
pressure over the northern plains with easterly upslope flow will
also bring in low level moisture from the plains. Both GFS and
ECMWF bring initial perturbations or "ridge riders" up into SW
Wyoming Tuesday morning, rotating eastward across the area during
the day. Scattered coverage is expected across most of the area
Tuesday afternoon/evening during max heating, subsiding but not
completely extinguished Wednesday morning. A repeat pretty much
Wednesday afternoon/evening. Most thunderstorms in this pattern
are likely to be relatively low top, heavy rainers with some
localized flooding possible, especially on Wednesday, when some
soils may be more saturated. Upper low along the west coast is
expected to eject over the ridge across the northern Rockies
Wednesday night - Thursday, setting up a sharper lee side surface
trough across eastern Wyoming Thursday afternoon. Airmass will be
somewhat drier and more stable west of this boundary with stronger
and more organized thunderstorms expected along and east of
Interstate 25. GFS brings a tropical system off the Baja Coast
into the Intermountain West next weekend while the ECMWF shows a
more benign pattern over the area, most monsoonal moisture
suppressed to the south Friday and Saturday. GFS scenario is
unlikely, and forecast shows an overall drier trend, or the
typical isolated afternoon/evening summer convection Friday and
Saturday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)

VFR conditions will continue through the forecast period. Have
made some slight adjustments to winds at several of the TAF sites
during this afternoon and have lowered wind speeds slightly for
tonight with the gradient relaxing somewhat.

&&

.FIRE WEATHER...

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 through Monday, fire danger low to moderate for
most mountain locations...elevated for all other lower elevations
due to very warm to hot daytime temperatures and low humidity
levels. Winds will continue seasonally light across the entire
forecast area through Monday with typical diurnal slightly gusty
afternoon winds found around the region when the atmosphere mixes
out. Brief exceptions will occur, of course, near any stronger
shower/storm when winds could briefly gust 25 to 35 mph...especially
Monday over the mountains. Today and Sunday outside isolated chance
for a shower/storm over/near Casper Mountain. Slightly better
isolated to widely scattered chances over the mountains and adjacent
foothills on Monday.

&&

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

$$

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



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