Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 101740 AAA
AFDRIW

Area Forecast Discussion...Update
National Weather Service Riverton WY
1140 AM MDT Thu Aug 10 2017

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

Imagery continues to show longwave trof across most of the CONUS
except for intruding high pressure ridge (over weak low) across the
EPAC, west coast, to ridge center in srn NM. WY is still under
partial NW flow with a weak shortwave moving through the flow over
cntrl/ern WY and another upstream shortwave embedded from nrn/cntrl
ID through wrn/swrn MT and into the nrn Plains/extreme upper
midwest. A ridge topping vort-max/shortwave is now rotating
into/over wrn/swrn/cntrl WY. SFC very similar to that of 24 hrs ago
with low pressure across much of the wrn CONUS and a deep (heat) low
centered ovr sern CA/AZ/nrn Baja. High pressure current in place
over WY and the nrn/Central/High Plains with a weak nearly
stationary back door front through ne WY. The swrn quad of the FA,
including portions of central WY, is active with moderate convection
early this morning as the combination of the two main upper
disturbances begin to merge giving an elongated area of
precipitation stretching from the wcntrl border with ID to the east
and along the Wind River Mountains into the Wind River Basin...all
headed to the east.

This morning/today/tonight: As the two disturbances meet in the
current split flow regime over wcntrl/scntrl WY to the Wind River
Mountains/Basin early this morning...precipitation will continue
moving east through Sweetwater/Fremont/Natrona Counties. The frontal
boundary mentioned above should also sag into cntrl/srn WY with
isolated rainfall also moving along/near this boundary. Otherwise,
general high pressure and a somewhat drier atmosphere will follow
this system into WY while low(er) pressure and a weak SFC trof
builds into swrn/cntrl portions of the FA through the
afternoon/evening. Another weak upper wave moving through the nrn
branch of the split flow continues to look to impact the region by
late afternoon/evening. This will put best chances/areas for
precipitation across the mountains in addition to swrn/srn WY and
portions of cntrl WY near a weak front/trof boundary. Once again,
small hail, locally gusty winds and brief heavy rain will be
possible.

Friday, split flow continues with disturbed flow around the ridge to
the south and the next two minor embedded shortwaves in the nrn
branch of the flow come together beginning Friday morning,
initiating showers/storms first across srn WY...then widely
scattered over the rest of the FA by afternoon and continuing into
the evening. Brief heavy rain, small hail and gusty winds all
possible.

Saturday begins with lower pressure across the FA and a SFC trof up
through the nern corner of WY and a front to the north in MT poised
to move into/through the nrn half of the state by mid-day. The front
will be pushed swd by yet another upper shortwave with best chances
for precip over nrn/cntrl WY...particularly the mountains...in the
afternoon and early evening.

.LONG TERM...(Sunday through Thursday)

Overview...Slight chances of showers and thunderstorms will remain
over the north and west Sunday through Tuesday, with temperatures
near or slightly below normal across the area.  A drier westerly
flow will prevail across the area Wednesday and Thursday.

Discussion...Synoptic upper level pattern on Sunday will feature
upper ridge across the Rockies into central Canada flanked by an
upper low along the British Columbia coast and broad trough
downstream across the northern Plains to the NE U.S.  At the
surface, high pressure over the upper midwest will bring a return SE
flow and higher CAPES into eastern Wyoming.  Johnson County and NE
Natrona County look to be on the edge of moderate instability and
potential for some stronger/severe thunderstorms Sunday
afternoon/evening.

ECMWF has trended toward GFS solution of shearing upper low along
the B.C. coast over/through the downstream ridge rather than cutting
it off.  Most energy with this system is expected to shear north of
the area Monday and Tuesday.  Monday is expected to be the warmest
day of the period with Wyoming in the warm sector, lee side surface
trough across eastern Wyoming, cold front moving across western
Montana.  Broad, flat trough is expected to linger over the Notrhern
Rockies on Wednesday, but airmass is expected to be relatively dry,
and stable, so only very isolated storms expected in and near the
northern mountains.  A more zonal flow with storm track along the
U.S.-Canadian border sets up Thursday and Friday, with dry westerly
flow across Wyoming.  Some indications that this will continue to be
the prevailing pattern into August 21st, Eclipse Day.  We should
begin to see at least some measure of predictability emerge 10 days
out, or with this 12z Fri-00z Sat extended model output.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday morning)

The low deck of stratus that formed over KRKS and KJAC has dissipated.
Showers will be exiting KCPR to the east by 19Z. New showers and storms
will form after 19Z across the region and continue through 04Z
Friday. Local MVFR visibilities are possible due to heavy with some
storms. Mountains will be obscured at times. Showers will
become isolated around 06Z Friday.

&&

.FIRE WEATHER...

Fuels at critical levels for most of the low elevation forecast
zones in addition to a few mountain zones above 8000 feet across
western Wyoming. (Please see the Fire Weather Forecast product for
more information). An upper level disturbance early this morning
will extend areas of significant precipitation across portions of
the central mountains and perhaps the Wind River basin east through
Natrona County. The rest of today will be characterized by continued
unseasonably high RH values with relatively light wind. Good
precipitation chances also continue today across much of the
forecast area along and west of the Divide...more isolated east of
the Divide...with the approach and passing of another upper level
disturbance. Some localized heavy mountain rains along with small
hail and gusty winds will be typical. Friday will see similar, but a
little drier, conditions to those of today.

&&

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

$$

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



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