Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 300839
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
239 AM MDT SAT JUL 30 2016

.SHORT TERM...Saturday through Monday night

Much like yesterday, the radar is starting quiet and should remain
quiet through the morning hours. However, with a plume of mid level
moisture moving in from the southwest mainly isolated thunderstorms
should develop west of the divide this afternoon and a couple could
spread east of the divide this evening. With the large temperature
/dew point spreads and inverted V soundings, gusty downburst
winds will once again be the main threat. With the flow going
southwesterly, it will be a warm to hot day today, especially east
of the divide. Highs in the 90s should be fairly widespread in the
lower elevations. The second concern is for possible critical fire
weather. We were on the fence this morning with upgrading. Relative
humidity should be drop into the lower teens for many areas. Winds,
however, look marginal at best. We decided to upgrade mainly based
on the chance of lighting strikes and high Haines indices.
Thunderstorms should wane tonight but a few could linger all night
and Sunday morning.

And speaking of Sunday morning, this looks to be the best chance of
thunderstorms across the area with the mid level moisture in place
and a shortwave moving through the flow. There will also be more
clouds around, which should keep temperatures a bit cooler. Fire
concerns may lower a bit with the cooler temperatures and somewhat
higher relative humidity. However, there will still be the threat of
some high based thunderstorms for everyone. The threat out west
looks to be early though with the models showing some very dry air
with precipitable water values as low as 0.25 inches moving in
Sunday night. Drier air will work its way across the area during the
evening with any convection ending after midnight. At this point, we
made few changes to Monday. With heights over 5900 and little
moisture in the atmosphere, it looks like a dry and warm to hot day
for almost everyone. Winds look to remain on the light side however.
The one exception may be in southern Sweetwater where just enough
moisture may hang on but even here storms would be few and far
between.

.LONG TERM...Tuesday through Saturday

On Tuesday, models are showing some monsoonal moisture being
pulled northward ahead of a compact upper low pushing east across
Washington. This with peak heating and weak instability should
result in isolated convection forming over the mountains and
trying to spread into the lower elevations. A few areas could see
critical fire weather, but winds look to be marginal at best. The
exception will be on the lee side of Continental Divide /zone 286
and 288/ where winds have a better shot, but then RH will be
marginal in the mountains as some monsoonal moisture pulls
northward.

A weak cold front is expected to slide south, east of the Divide
Tuesday night, but wash out Wednesday before a secondary cold
front Wednesday night. Yesterday`s 00z model solutions were
stronger with the front Tuesday night with the front not washing
out during the day. Thus have warmed temperatures up a bit for
Wednesday. Also, instead of moist low levels in wake of the front
with an easterly flow east of the Divide, models show this air
mass being displaced by a dry westerly flow Wednesday afternoon
with critical fire weather conditions now possible. Models
continue to agree the monsoonal moisture will be push southward
with the only chance of convection Wednesday over the far south.

On Thursday, lighter winds are expected with monsoonal moisture
confined to the far south. This could change Friday and Saturday
as both the 00Z GFS and ECMWF are showing monsoonal moisture
lifting northward into the area. I have added some mountain
convection Friday and Saturday, but left most of the lower
elevations dry as models yesterday were in agreement in keeping
the monsoonal moisture south of the forecast area.

&&

.AVIATION.../12Z ISSUANCE/

West of the Continental Divide...KJAC/KBPI/KPNA/KRKS

Smoke from the Cliff Creek Fire near Bondurant and the Lava Mountain
Fire west of Dubois may bring some terrain obscurations over
Togwotee Pass and the northern Wind River Range through the period
Otherwise, VFR conditions will prevail through 12z Sunday.  Some
high level monsoonal moisture will rotate into southwest Wyoming
Saturday afternoon while a weak upper level disturbance pushes into
northwest Wyoming.  These elements will combine to produce isolated
showers and thunderstorms primarily between 21z Saturday - 03z
Sunday.  A dry sub-cloud layer will result in the potential for
erratic wind gusts 30-40 knots near any showers or thunderstorms.
Surface W-NW winds of 10-20 knots will otherwise prevail Saturday
afternoon and early evening.

East of the Continental Divide...KCOD/KCPR/KLND/KRIW/KWRL

VFR conditions will prevail through 12z Sunday, with smoky areas
continuing in the primarily in the vicinity of the northern Wind
River Range and Togwotee Pass.  Some monsoonal moisture will rotate
across southern Wyoming Saturday afternoon where some high-based
showers and thunderstorms will develop primarily vcnty and south of
a KLND-KCPR line.  A dry sub-cloud layer will result in the
potential for erratic wind gusts of 30-40 knots in the vicinity of
any showers or thunderstorms.  A weak upper level disturbance may
bring some isolated showers with a few thunderstorms across northern
Wyoming Saturday evening into early Sunday morning.
&&

.FIRE WEATHER...

...RED FLAG WARNING FOR PORTIONS OF WESTERN AND CENTRAL WYOMING...

A combination of low relative humidity, Haines indices of 6 and a
gusty breeze will bring critical fire conditions this afternoon and
evening to portions of western and central Wyoming. Isolated
thunderstorms will fire West of the Divide this afternoon with
erratic wind. Smoke dispersal and mixing should be generally good to
excellent. Fire concerns should ease a bit on Sunday with slightly
cooler temperatures, more cloud cover and somewhat higher relative
humidity.
&&

.RIW Watches/Warnings/Advisories...
Red Flag Warning from 1 PM this afternoon to 9 PM MDT this
evening for WYZ283-288-289-414>416.

&&

$$

SHORT TERM...Hattings
LONG TERM...WM
AVIATION...AEM
FIRE WEATHER...Hattings


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