Area Forecast Discussion
Issued by NWS Riverton, WY

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380
FXUS65 KRIW 190759
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
159 AM MDT Sat Aug 19 2017

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

Imagery shows continues to show trof/ridge/trof pattern across the
CONUS with the upstream trof axis down the west coast and ridge axis
from nern MT down through srn ID. The SFC has broad low pressure
across the swrn CONUS, a cold front moving through the PAC NW, and a
weak trof over ern WY with a meso-high to the west of the trof. No
precipitation currently falling anywhere in WY.

Today, the ridge axis translates ewd across WY and is quickly
replaced by zonal flow aloft while the SFC becomes represented by
low pressure with a weak embedded SFC trof from cntrl WY through the
sern corner and down the Front Range of CO. Even as an embedded
shortwave trof aloft zips through the flow later today/tonight,
the boundary layer will be so dry that precipitation will be hard
to come by anywhere. Outside chance would arrive this evening over
the far northern reaches of the Absarokas as a weak front pushes
into the area. Otherwise, elevated fire weather will be present
across WY today mainly due to low RH values this afternoon. The
highest fire danger will be located across the southwest quadrant
of WY where west to northwest wind could briefly gust 20+ mph at
times...flirting with marginal Red Flag conditions. SPSs have be
posted for the entire FA. Sunday will a marginal increase in mid
level moisture into southwestern WY as it is funnels into the
region around an upper high centered well to the southeast and a
developing nearly cut-off upper low near srn California. However,
this moisture will be advecting into very dry air and will only
increase cloud and low precip chances marginally across the swrn
corning of WY later in the day/evening as weak embedded wave/vort
max ejects/rotates over the area from the same two upper high/low
previously mentioned.

Late Sunday night through Monday night: A slightly better moisture
feed into WY from the southwest - between the same two upper
high/low mentioned before - along with the approach and passing of
the main upstream trof (now over the PAC), along with some modest
attending PAC moisture, will increase upper/mid level clouds
across western WY early in the morning...with decreasing cloud
through the rest of the morning hours and into the afternoon.
Northern and portions of west central WY will see an upper level
induced front push into/through the area in the morning with
perhaps a very few mid level clouds early...but with a modest
increase in upper/mid level clouds from late morning and into the
early afternoon as the upper trof moves through. The eastern FA
(Johnson/Natrona) will likely be the last to be affected from this
weak system. From an eclipse point of view, the
western/southwestern zones will generally see improving conditions
after a partly cloudy start to the morning. The northern/central
zones will generally see few(er) clouds early but increase clouds
as the morning wears on. While the eastern forecast zones may have
a very few clouds around early...with the modest increase in
clouds holding off until after noon. Small chances exist for
precip will exist late in the afternoon and into the evening as
the upper trof and SFC front continue to move through and out of
the the FA.

.LONG TERM...(Tuesday through Saturday)

Overview...Upper ridge will bring mainly dry and warmer weather
Tuesday and Wednesday.  There will be slight chances of
thunderstorms along and south of an Afton to Casper line on
Wednesday. A Pacific cold front will push across the northern
Rockies Thursday night and Friday, with breezy and warm conditions
on Thursday bringing chances of showers and thunderstorms across
the north and west Thursday and Friday, with a cooling trend
Friday and Saturday.

Discussion...Upper air pattern on Tuesday features ridge centered
near the Continental Divide with a weak low off the southern
California coast, and a stronger low over the Gulf of Alaska.
Broad surface high pressure will be over the northern and central
Plains with low pressure over the Intermountain West. Wyoming will
be on the northern fringe of moisture and instability across the
Great Basin and Central Rockies with a few thunderstorms possible
near the Uinta Mountains Tuesday afternoon/evening. Other areas
will see mainly clear skies (what else would one expect on the day
AFTER the eclipse). Spread increases between GFS and ECMWF in
handling the two upper lows along the western N. American coast.
GFS continues to show stronger closed low from the Gulf of Alaska
dropping down to near Vancouver B.C. Thursday before lifting NE
into downstream ridge. ECMWF shows a bit weaker system lifting out
faster and further north, These differences, in turn, likely
affecting the ejection of weaker upper low from Southern CA across
the Four Corners region Wednesday and Thursday. GFS kicks system
out faster on a further northward track, ECMWF, maintaining more
separation between northern and southern streams, drifts a weaker
system on a slower, more southward track. More weight is given to
the ECMWF scenario, with GFS likely crashing northern stream low
too far into downstream ridge before lifting it out. Wyoming will
lie between these two systems as they lift through the Rockies on
Friday, with low-end chances of showers and thunderstorms. Upper
ridge is then expected to build back up across the Intermountain
West next weekend.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday)

VFR to rule at all terminals through 12Z/Sun. Gusty west to
northwest wind the main weather concern for today, particularly at
KBPI, KPNA, and KRKS. Winds will increase between 18Z-20Z/Sat with
speeds generally 12G22kts. KCPR will see 12-15kt winds most of the
day on Saturday. These winds will be diurnally driven, with a
weakening to 10kts or less between 01Z-02Z/Sun as the atmosphere
decouples. Limited instability and moisture associated with a weak
shortwave along and near the Montana border late today. Convection
will be confined to the northern Absaroka Range with little more
than some increased mid-high cloud at KCOD.

&&

.FIRE WEATHER...

...Elevated Fire Weather Concerns Today Due To Very Dry
Conditions...

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). Elevated fire weather concerns for the entire
forecast area today due mainly to very low RH values expected this
afternoon. Very dry conditions with minimum RH values dropping into
the low to mid teens across the lower elevations, upper teens in the
foothills, and 20 to 30 percent in the mountains. Winds, however,
will remain on the light side and less gusty overall than on Friday
with southwestern Wyoming perhaps seeing a few gusts of 20 to 25
mph. Sunday will be similar in many respects to the conditions pre-
sent today except for slightly higher RH values and perhaps even a
little less wind, especially across the southwestern zones.

&&

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

$$

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



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