Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 260902
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
302 AM MDT Mon Jun 26 2017

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

Imagery shows upper high P centered over the Desert SW and nrn Baja
with a ridge axis extending to the n through the Mojave, Great
Basin, and ID. Best moisture is being diverted across the swrn
CONUS. Broad trof lies across the ern CONUS. WY is still east of the
ridge under nw flow. The SFC has high P across the nrn High Plains
including most of WY with n to ne SFC flow towards a heat derived
low P over the Desert SW. No precip across WY.

Most rivers across the FA, including the Big and Little Wind Rivers,
are responding positively (decreasing in flow and gauge height) to
the cooler post frontal temperatures the past couple of days. The
Green River near La Barge in nern Lincoln County is, however, still
high and steady but should begin to decrease today.

Today and tonight, the FA continues east of the Ridge under NW flow
aloft and mostly seasonally cool dry high P and nrly flow at the
SFC. As the day progresses, the upper ridge begins to translate ewd
over the Central Plains with lower P pushing into wrn WY,
redirecting SFC flow into the FA from the e and se. Moisture return
into the FA, however, will have to wait until later tonight and into
Tuesday as the ridge axis passes through the CWA and into the cntrl
High Plains while lee side low P develops across the SFC over cntrl
and wrn WY. Low to mid lvl moisture will invade WY from the se with
mid lvl moisture moving in from the nw.

By Tuesday morning an embedded upper level SW will ride through the
flow w of the ridge axis and across nwrn WY along with a SFC front.
Scattered showers and thunderstorms will quickly move into/through
the Yellowstone area (more isolated to the south) at 30 to 40 kts
relatively light rainfall...small hail and very gusty winds that
could border on severe at times from mid-day on. With seasonally
warm/hot temperatures continuing Tuesday, dpt depressions will be in
the 50 to 60 deg range early in the day with DCAPE running 1000-1500
in the afternoon across nrn WY...supporting strong to severe wind
gusts from the convection...particularly across the Big Horn Basin
and adjacent foothills/mountains. Late afternoon Tuesday and through
the evening, increasing moisture and post frontal decreasing temps
and over ncntrl and nwrn WY will increase RH values and lower LCLs
with deeper available moisture in place for a period of time. Heavy
rain and hail will become more and more likely as evening nears and
MLCAPE ranges from 800 to 1200 J/KG. Somewhat less wind threat can
be expected from Yellowstone through the Big Horn Basin at this
time. Heaviest rain of the day/night, with a few possible severe
storms, will occur in the evening across nrn WY...with perhaps the
storm environment over Johnson County providing conditions conducive
to marginally large hail and strong wet downburst winds. Possible
moderate to heavy rain to continue with the passage of the upper SW
trof through at least midnight.

Wednesday, post trof/frontal with nearly quick moving zonal flow
aloft and a seasonally strong e/w jet across srn WY will keep
instability and wind forcing and dynamics elevated over the nrn half
of the state. Upstream embedded SW trof and associated jet streak
will also add to the pattern by evening. With some low to mid level
moisture still retained across nrn WY from the previous day/night`s
precipitation, showers and thunderstorms will have another chance
for initiation by mid-day with a few strong wing producers across
the Big Horn Basin...and possible isolated severe over Johnson
County once again.

.LONG TERM...(Thursday through Monday)

Overview...Cool and unsettled conditions will prevail behind a cold
front across north and central Wyoming on Thursday into Friday with
chances of showers and thunderstorms.  The better coverage will be
on Thursday.  Dry and breezy conditions will prevail across the
southwest during this time.  Warmer conditions will prevail over the
weekend through early next week with chances of afternoon/evening
thunderstorms across mainly the northern third of Wyoming.

Discussion...Synoptic pattern on Thursday morning features upper low
(along the British Columbia Coast early this morning) over
Saskatchewan with ridge along the west coast. Both GFS and ECMWF
show shortwave rotating through broad cyclonic flow across the
northern Rockies/northern Plains, the shortwave into NW Wyoming
Thursday morning, pushing out into the northern High Plains by 00z
Friday. Attendant cold front with this shortwave should push
south to near a Lander-Casper line by 18z with best chances of
showers and thunderstorms along and north of the front. Thursday
will be the coolest day of the upcoming week with lower elevation
highs expected mostly in the 70s. Spread between GFS and ECMWF
increase Friday through Monday. The GFS pulls upper low down
further south and west into the northern Plains on Friday than the
ECMWF, so the GFS is wetter and cooler east of the Continental
Divide. ECMWF also shows better consistency in transition to a
more zonal flow over the weekend into early next week, so forecast
was weighted in favor of Ecmwf through Monday. This will result
in some ripples in westerly flow bringing some late day
thunderstorms across the north over the weekend with dry and
breezy conditions across southern Wyoming. A drier airmass will
prevail over the area Monday, with few if any thunderstorms. The
outlook for the Fourth shows some backing of upper flow to a more
SW direction, brining mainly dry conditions with above normal
temperatures, breezy conditions through the southern wind
corridor.

&&

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

VFR conditions will occur through 12Z Tuesday. Isolated showers or
thunderstorms will occur in the western mountains after 03Z. Showers
and thunderstorms will increase in coverage over NW WY after 06Z
Tuesday. Expect gusty outflow wind to 30 knots near any storms.
Isolated showers or storms may occur in southern WY after 06Z
Tuesday.

&&

.FIRE WEATHER...

All fuels are currently in green-up and below critical levels for
all locations. Fire danger remains low across all mountain
areas...but will continue somewhat elevated in the afternoons across
the lower elevations east of the Divide and over portions of
southern WY. These areas with elevated fire concerns will generally
coincide with low afternoon RH values and/or gusty winds...today
through Wednesday.  Winds will become south to southwest 5 to 20 mph
today...west to northwest 15 to 30 on Tuesday...and west to
northwest 5 to 25 on Wednesday...all days with higher gusts in the
afternoons and near stronger showers/storms. No appreciable
precipitation expected until later overnight tonight through Tuesday
night as fast moving isolated to scattered showers and storms cross
the northern half of Wyoming quickly from west to east. These
showers and storms will produce little rainfall through the mid-day
hours with strong gusty outflow winds of 40 to 60 mph possible at
times. Then, by late Tuesday afternoon and through the evening,
potential heavy rainfall and large hail will be possible
possible...especially across portions of north central and
northeastern Wyoming. Smoke dispersion will be good to excellent
each afternoon and evening.

&&

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

$$

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



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