Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 230836
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
236 AM MDT Sun Jul 23 2017

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

Imagery continues to show upper flat ridging across the CONUS with a
broad high "centered" across the entire srn CONUS. Monsoon also
continues in full flow tonight/this morning with good moisture
stretching from MX/AZ/NM through the srn Rockies and into the
High/Central Plains of Kansas. SFC has general heat induced low
pressure across the swrn CONUS...post frontal high pressure across
the PAC NW and into portions of the nrn/cntrl Rockies with a
somewhat weaker (than yesterday) meso-high centered ovr CO. A slow
moving front is located across much of the CONUS...trailing off
through portions of the south Central Plains and across CO just
north of the meso-high. Heavy areas of precipitation have been held
much further south today...mainly occurring under the monsoonal
moisture plume across AZ and NM into KS (along the front).

Overall, it`s quite hard in predicting accurate locations for
rainfall potential too far out when there is no good forcing to go
along with the increased moisture other than heat and terrain
convergence. That said, today the upper lvl ridging continues over
the fcst area with the general low to mid lvl circulation pattern
associated with the monsoonal flow being held further swd and out of
most of WY for a second day. A drier airmass and warming and
relatively light flow aloft associated active ridging will keep it
stable and precipitation free across the FA without much of a
trigger for storms/precip (except low chances over very isolated
higher mountain locations). Otherwise, lots of sun and a warming
trend with seasonally light winds (occasional late afternoon mixed
out gusts 15 to 20 mph).

Monday through Tuesday night is a transition period as upper ridging
flattens with the passing of a strong upstream SW trof to the north
across srn Canada/nrn CONUS. While this feature will generate
increase flow aloft along with frontogenesis at the SFC, weak
monsoonal moisture will have only slowly begun to return across
portions of ern WY as a "warm front" by the time of cool frontal
passage Monday afternoon. This will mean slight chances for storms
mainly across the nrn mountains Monday afternoon and night. Much
better monsoonal moisture will then start to increase back into the
swrn/wrn FA early late Monday night then across the rest of the CWA
Tuesday...as the mid/upper pattern adjusts to the passage of the SW
trof and rather amplified ridging begins to overtake the wrn CONUS
and rotate clockwise over the Intermountain West. Secondary
frontogenesis and passage also spurred by the trof to the north will
aid in LL convergence and forcing by later Tuesday, increasing the
likelihood of showers and storms Tuesday afternoon/night. PWs will
rise appropriately to around an inch (+-) during this period. It
looks like a relatively cool/very moist, low CAPE/high shear day
with heavy rain and localized flooding possible.

.LONG TERM...(Wednesday through Sunday)

Overview...An active monsoonal pattern will remain over the area on
Wednesday with scattered showers and thunderstorms along with
relatively cool conditions. Monsoonal moisture will be
increasingly suppressed Thursday through the weekend, with
thunderstorm activity becoming more isolated, temperatures heating
up.

Discussion...Synoptic pattern on Wednesday morning features
upper high over the southern Rockies/southern High Plains and a
weak upper low moving across northern California. Monsoonal plume
with PWs around an inch remain over Wyoming along with low level
east-southeast flow across the area. ECMWF and GFS have both sped
up the progression of upper low, moving it from northern
California northeast, shearing out across Montana Thursday
morning. Monsoonal plume will blossom with thunderstorms across
the area Wednesday afternoon and evening, and some soils may
become more saturated on day 2 of this pattern with an elevated
threat of localized flooding. Upper low shearing across Montana
will keep some activity going well into Wednesday night-Thursday
morning. Faster movement of this feature may push lee surface
trough further east into the High Plains Thursday afternoon and
evening with drier air to the west. However; kept scattered
coverage of thunderstorms roughly along and east of Interstate 25
where higher CAPE values may remain.

Upper high is expected to retrograde into the desert SW Friday
through the weekend which will block or shift the monsoonal plume
further westward, away from the area. Upper flow is expected to
become more NW with best chances of thunderstorms across the north
and east, drier and more stable conditions across the southwest
one-half of Wyoming. H7 temperatures averaging 16-18c Friday
through Sunday will push central basin highs well into the 90s
with possibly a few triple digit readings in the lower Bighorn
Basin.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning) Issued
at 106 AM MDT Sun Jul 23 2017

VFR conditions will occur over the region through 12Z Monday. Very
isolated showers or thunderstorms will occur over the mountains from
20Z until 02Z. Local wind gusts to 35 knots will occur near storms.
Smoke from wildfires will reduce long range visibility with local
MVFR conditions possible in places.

&&

.FIRE WEATHER...
Issued AT 106 AM MDT Sun Jul 23 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 through Tuesday, fire danger low to moderate for
most mountain locations...elevated for all other lower elevations
through Monday due to very warm to hot daytime temperatures and low
humidity levels. Tuesday will begin to see a stronger influx of
monsoonal moisture back into the area and fire danger levels should
decrease. Winds will continue seasonally light across the entire
forecast area through Tuesday 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 45 mph...especially
Monday over the mountains and adjacent foothills. Today, outside
isolated chance for a shower/storm near Casper Mountain. Slightly
better isolated to widely scattered chances over the northern
mountains and adjacent foothills on Monday then better chances for
rain, even possible periods of of localized heavy rain, on
Tuesday.

&&

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

$$

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



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