Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 132000
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
200 PM MDT Fri Oct 13 2017

.SHORT TERM...(This evening through Monday)
Issued at 119 PM MDT Fri Oct 13 2017

Imagery continues to show a broad trof/ridge pattern (west to east)
across the CONUS with one small embedded SW exiting WY to the NE
and the nrn and srn jets begin to phase on either side of the main
upper trof over the nrn Great Basin. WY currently remains under
swrly flow aloft. The strength of the srn branch of the jet at 110
to 140 kts is leading, amplifying and pulling the main trof to
the east, eventually crossing WY sometime Saturday. The SFC has a
long cold front stretching across the middle portion of the CONUS
and then across the cntrl/srn Rockies. High pressure behind the
front is interrupted by growing embedded low pressure and
frontogenesis across the NW CONUS in response to the upper trof
moving over that area. Light isolated snow showers beginning to
fall across portions of NW WY.

Today/tonight the incoming nrn branch jet moves into/through the W
side of the main trof and begins to phase with the srn branch jet
over the nrn Great Basin. Light areas of precip, mostly as snow,
will begin to fall under divergent flow aloft across the nwrn
Mountains. Meanwhile, frontogenesis begins to take place from ern
ID to nrn California with swrly winds picking up and becoming
moderately gusty across the FA today. This will enhance fire
weather conditions for most of the lower elevations east of Divide
in addition to portions of srn WY. Localized near Red Flag will
also be possible across Natrona and Johnson Counties today.
Otherwise, lee side troffing will also occur this afternoon under
the flow regime described across ern WY and CO. Eventual lee side
cyclogenesis is then expected to take place over ern/sern CO by
Saturday morning. Generally, WY will be "cut-off" from an
tapped/extended moisture source with this incoming system as
return moisture out of the E or SE will not be possible behind
Thursday`s front. To the S and SW of the area, a very dry
atmosphere exists under the influences of a large EPAC high
pressure system. That leaves only limited incoming moisture from
the PAC NW that will accompany the main upper trof and which is
now, a little ironically, being cut-off from additional Pacific
moisture by the n/s oriented jet moving down through the west side
of the upper trof.

QPF values from this afternoon through Saturday morning west of the
Divide will generally range from a few hundredths to around a third
of an inch. This will equate. with H7 temps cold enough to snow down
to the valley floors from the beginning of this event, to 0.5 to 3
inches of new snow accumulation...with a few areas in the Teton and
Gros Ventre Mountains seeing (perhaps) up to 5 inches when all is
said and done. East of the Divide and across srn WY, the main cold
front kicks through the FA along/ahead of the upper trof axis,
bringing some chances for precipiation (liquid, frozen and mixed at
times) east of the Divide just before/after sunrise. Total snowfall
accumulation east of the Divide and across southern WY looks to
remain generally low at an inch or less for most locations...with
perhaps a couple of inches across portions of Natrona County in
addition to some of central WY`s foothills.

Sunday and Monday will see a general warming and drying trend begin
(again) under modest but increasing upper level ridging. Mostly
sunny/partly cloudy skies on Sunday will give way to largely
cloudless skies on Monday across most of the FA. Slightly elevated
fire weather conditions due to drying expected across the usual
spots east of the Divide, however winds look to behave for the most
part so no highlights expected at this time.

.LONG TERM...(Monday night through Friday)
Issued at 119 PM MDT Fri Oct 13 2017

Synopsis...Zonal flow/flat ridging pattern will be in place through
the middle part of next week. The flow aloft will begin to back
Thursday, with predominant SW flow Friday as a longwave trough digs
over the western CONUS. Dry and breezy winds will create elevated
fire weather conditions Tuesday through Thursday. A cold front
associated with the trough will enter western portions of the
forecast area Friday afternoon. A slight chance of PoPs remain
possible Friday morning and afternoon, ahead of the front. Any snow
will remain confined to the higher elevations early Friday morning,
with rain in the valleys.

Discussion...Temperatures continue to trend above normal Tuesday and
Thursday. Have bumped highs again on these days. Friday could be
well above normal as well ahead of the front, with increasing h7
winds and a well-mixed boundary layer.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 119 PM MDT Fri Oct 13 2017

VFR conditions will prevail across much of the area through 06Z
Saturday. Clouds and mountain obscurations will stay over the
western mountains, as light showers begin to make their way over
Yellowstone and vicinity KJAC. SKC will remain in place elsewhere,
before a cold front quickly makes its way through the area between
06z-18Z Saturday. Scattered to numerous snow showers will follow
this front over the northwest mountains, vcnty and north of a
KJAC-KDUB line, Friday evening into the overnight hours. Sct
MVFR/IFR valley and mountain snow showers are expected in this
area. A band of MVFR/IFR snow showers are expected to be centered
roughly along a vicinity KAFO-KRIW-KCPR line 09z-12z Sunday.
Precipitation over the area will quickly end from west to east
after 15Z Saturday, with some lingering showers possible near KCPR
Saturday afternoon. Surface SW winds 15-25 knots will begin to
subside after 00Z Saturday, before briefly weakening to around
10kt. Winds will increase again as the front approaches.

&&

.FIRE WEATHER...
Issued AT 119 PM MDT Fri Oct 13 2017

Fuels remain at critical levels across over Johnson and Natrona
Counties. (Please see the Fire Weather Forecast product for more
information).

...Elevated fire conditions this afternoon...

Today will remain near seasonal but quite dry with minimum RH values
ranging from the lower teens to around 25 percent across the lower
elevations but with increasing RH values over the mountains. No
precipitation expected today east of the Divide, however, chances
will increase for snow showers west of the Divide this afternoon
before ending through Saturday morning. Chances for light
precipitation (rain/snow and mixed) will then shift east of the
Divide along with a vigorous frontal passage late tonight through
Saturday morning. Liquid amounts of precipitation will generally
range to less than a third of an inch for most locations...with the
highest amounts over the western and central mountains. Winds will
be breezy and gusty at times out of the southwest especially over
southern/eastern Fremont and all of Natrona Counties. Today could
see a few areas across northeastern Sweetwater, southern Fremont and
all of Natrona Counties seeing Red Flag conditions for a short
period this afternoon...quieting down later this afternoon and into
the evening. Smoke dispersion will be good to excellent this
afternoon...good to very good Saturday afternoon.

&&

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

$$

SHORT TERM...Braun
LONG TERM...LaVoie
AVIATION...LaVoie
FIRE WEATHER...Braun



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