Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 240900
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
300 AM MDT FRI JUN 24 2016

.SHORT TERM...Today through Sunday night

...Primary threat during this period concerns increasing fire danger
each afternoon and evening...

Imagery continues to show broad ridge across the CONUS with an
amplified upstream trof moving into the PAC NW. SFC has weak high
pressure across the Rockies with low pressure and lee side trof from
south central MT down through eastern WY. Weak cold front just west
of Yellowstone NP.

Today, fire weather concerns and Red Flag Warnings continue
across portions of central and southwest Wyoming. Fire danger
conditions across these zones where fuels are considered
critically dry will be more extreme today as a stronger lee side
trof/low forms across eastern Wyoming...tightening the P GRAD and
greatly increasing wind speeds across the forecast area come ahead
of and with FROPA...with gusts 30 to 40 mph common. This combined
with an already a very dry boundary layer, HAINES instability
indices of 6 and some potential for more dry thunder across Natrona
county will create extreme fire weather conditions that will last
through early Saturday evening. As zones 414 and 416 just went fuel
critical, will add them into the Red Flag mix for Today mainly for
FROPA, and very gusty winds although RH values will only be marginal
at best.

Saturday, Red Flag conditions continue. Troffing aloft and post
frontal at the surface, it will be cooler...but still quite dry
through the boundary layer with west winds increasing to 15 to 25
mph with gusts 25 to 35 in the afternoon. No precipitation expected
anywhere in the CWA. Red Flag conditions should abate Saturday
evening under mild upper level ridging and weak high pressure
building across the FA that will lessen wind speeds...keeping them
within check through Sunday.

Sunday, fire weather concerns decrease below extreme but will still
remain enhanced for most of the lower elevations east of the Divide
and across south and southwest WY. The area will be post-frontal
with weak high pressure building across the surface and modest
ridging taking place aloft. No precipitation is expected across the
FA and SFC winds should largely remain below 15 kts for all
locations. Late Sunday afternoon and evening...another dry clipper
type front will move through northeastern/eastern WY...but with
little impact of any kind save a wind shift.

.LONG TERM...Monday through Friday

Synopsis...Upper ridge will rebuild over the four corners region
through this period, bringing above normal temperatures (most basin
and valley highs in the 80s to lower 90S).  A few thunderstorms will
develop along and east of a frontal boundary along the east slopes
of the divide Tuesday and Wednesday with a few stronger storms
possible over Johnson county.  On Thursday and Friday, chances for
thunderstorms will shift to the south on the edge of monsoonal
moisture plume.


Discussion...ECMWF and GFS in overall agreement with synoptic
features over the CONUS through Thursday with upper ridge
strengthening over the Rockies, flanked by troughs over the eastern
U.S. and NE Pacific/Gulf of Alaska.  The latter trough will eject a
few shortwaves over/through the ridge across the northern Rockies
Monday through Wednesday.  The airmass over Wyoming will remain dry
with only slight instability over/near the northern mountains where
a few thunderstorms cannot be ruled out.   Both ECMWF and GFS are
weaker and further north with the strongest shortwave in this
series, topping the ridge across Montana into the northern High
Plains Tuesday night.  This shortwave will encounter the better low
level moisture and instability mostly east of the area, but a few
stronger storms will be possible along and east of I25 Tuesday
afternoon and evening.  Incoming Pacific shortwaves become less of a
factor late in the week as upper ridge amplifies across central
Canada.  Monsoonal moisture is expected to begin a northward surge
across the Great Basin late in the week along with a few `ridge
riders` rotating north across Utah, turning east across Wyoming.
Forecast brings isolated thunderstorms across the south on Thursday
with a northward trend in coverage on Friday.


&&

.AVIATION.../12Z ISSUANCE/

The main weather concern during the forecast period will be
gusty surface wind ahead of and immediately behind the incoming
Pacific Northwest trough. Pressure gradient tightens Friday
afternoon as surface low develops over the eastern Wyoming
plains by midday. Winds will increase between 18Z-20Z/Friday
with most terminals seeing southwest surface wind 20-35kts
ahead of a cold front. This front should be through the
western terminals between 22Z-23Z/Friday and to a KWRL-KRKS
line by 00Z/Saturday before pushing farther east prior to
03Z/Saturday. Northwest surface winds will prevail in the
wake of this front, which will favor continued gusty wind
of 20-35kts Friday evening at KBPI, KPNA, KRKS, and KRIW.
Weak convection possible this morning from KRKS to KCPR in
association with passing shortwave. Afternoon convection will
be hampered as a dry slot pushes across the forecast area
ahead of the trough. Winds are expected to decrease around
06Z/Saturday at all terminals.

&&

.FIRE WEATHER...

...Red Flag Warning for the lower elevations of Natrona county
through Saturday evening...

...Red Flag Warning for Johnson county...the southern Bighorn
Mountains...and Casper Mountain beginning noon today and
continuing through Saturday evening...

...Red Flag Warning for the Salt and Wyoming Ranges in addition to
the Western Wind River Mountains beginning noon today and continuing
through Saturday evening...

Fire danger elevated to extreme for most of the forecast area
today and Saturday. Warm/hot and very dry conditions continue
today with winds increasing even further, ahead of and with a
strong frontal passage. Southwest fire zones 414 and 416 have just
gone critical in fuel dryness and have been added to the Red Flag
Warnings beginning at noon today and continuing through this
evening...mainly for a strong frontal passage and associated very
windy conditions with moderately low RH values. Smoke dispersal
during the daytime hours will be fair to excellent east and west
of the Divide during the daytime periods through the weekend.

&&

.RIW Watches/Warnings/Advisories...
Red Flag Warning from noon today to 9 PM MDT Saturday for WYZ281-
285-300.

Red Flag Warning from noon today to 9 PM MDT this evening for
WYZ414-416.

Red Flag Warning until 9 PM MDT Saturday for WYZ280.

&&

$$

SHORT TERM...Braun
LONG TERM...AEM
AVIATION...CNJ
FIRE WEATHER...Braun



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