Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 240818
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
218 AM MDT Mon Jul 24 2017

.SHORT TERM...(Today through Wednesday night)
Issued at 112 AM MDT Mon Jul 24 2017

Imagery continues to show broad upper flat ridging across the CONUS
with bookend trofs over each coast and the high "centered" across
the srn/cntrl Rockies. Monsoon continues in full flow with the best
deep moisture stretching across nrn MX/AZ/wrn NM and into srn CO and
srn UT. SFC has general low pressure across the swrn/wrn CONUS
(partially heat induced)...post frontal high pressure now moving
over the nrn Plains and upper Midwest. A lee side trof exists from a
weak low centered over ncntrl MT into ncntrl and ern WY...providing
a focus for showers/storms Sunday night over Johnson County.
Otherwise, the heavy areas of precipitation are still trapped much
further south...mainly occurring under the monsoonal moisture plume
across AZ and wrn NM.

Today through the end of the forecast period is a transition period
with the first real effects of the monsoon being felt. First, the
upper ridging pattern flattens with the passing of a strong upstream
SW trof to the north across srn Canada/nrn CONUS. While this feature
will generate increased flow aloft along with frontogenesis at the
SFC, weak monsoonal moisture to begin the period (today) will have
only slowly begun to return across portions of srn/swrn/wrn WY.
Other weak moisture return of monsoonal moisture will take the long
way around through the Great Basin before getting wrapped up in the
faster upper flow offered from the nrn upper trof and possibly acted
on by the cool front generated by the upper trof this afternoon.
This will mean slight chances for storms mainly across the nrn
mountains this afternoon and evening. Better monsoonal moisture will
then start to increase back into the swrn/wrn FA tonight then across
the rest of the FA Tuesday...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 by Wednesday. Secondary frontogenesis and passage, also spurred
by the rather large trof to the north, will aid in LL convergence
and forcing by Tuesday afternoon...increasing the likelihood of
showers and storms Tuesday afternoon/night. PWs will "soar"
appropriately to around an inch (+/-) during this period. It looks
like a relatively cool(er)/very moist, low CAPE/high shear day with
heavy rain and localized flooding possible. Wednesday, the monsoonal
moisture remains across the FA with a growing ridge aloft. A warm
front looks to be extending across WY from southwest to northeast
with this boundary along with the terrain of the region providing
the forcing and focusing need for 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.


.LONG TERM...(Thursday through Monday)
Issued at 112 AM MDT Mon Jul 24 2017

A tight upper-low off the N. California Coast early this morning
will gradually fill, open up, and move east northeast across the
Northern Rockies Thursday. Subtropical ridge builds into the
region Friday and Saturday, and then the upper ridge looks to
flatten out a bit early next week. Disturbances in the flow in
combination with insolation will result in isolated to locally
scattered diurnal convection each day especially over the
mountains and adjacent foothills. Although there will be day to
day variability, looks like the best shot for convection right
now looks to be Thursday, and then again Sunday and Monday.

Temperatures are expected to be slightly above average through the
extended.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 112 AM MDT Mon Jul 24 2017

VFR conditions will occur through the period. Isolated showers and
thunderstorms will develop over and near the mountains after 20Z and
continue through 03Z Tue. Some of the activity will drift over the
foothill locations. For now no storms will be forecasted at the
terminal sites. Gusty wind to 35 knots will occur near showers and
storms. Smoke from wildfires will reduce long range visibility with
local MVFR conditions possible around KCOD airport.

&&

.FIRE WEATHER...
Issued AT 112 AM MDT Mon Jul 24 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 moderate for most mountain
locations...elevated for all other lower elevations due to very warm
to hot daytime temperatures and low humidity levels. Tuesday will
begin to see a stronger influx of monsoonal moisture into the
forecast area and fire danger levels should decrease. Winds will
continue seasonally light across the entire forecast area today with
typical diurnal slightly gusty afternoon winds found around the
region when the atmosphere mixes out. Slightly stronger frontal/post
frontal winds out of the north expected on Tuesday before winds
settle down once again Wednesday. Some exceptions will occur, of
course, near any stronger shower/storm when winds could briefly gust
25 to 45 mph...especially today over the northern mountains and
adjacent foothills. Today, slightly better isolated to widely
scattered chances for showers/storms will occur over the northern
mountains and adjacent foothills before much better chances for
wetting rainfall, even possible periods of of localized heavy rain,
on Tuesday and Wednesday.

&&

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

&&

$$

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


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