Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 071716
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1016 AM MST Thu Dec 7 2017

.SHORT TERM...Today through Saturday night

Mid to high level clouds continue to stream over the area this
morning, riding along the exit region of the jet. The main axis of
the jet will shift eastward over the Plains through the day today.
The ridge axis will also shift eastward slightly, allowing for H7
temperatures to warm over the state. These temperatures will warm
10 degrees C over the next 24 hours, with -1 to -3C readings by
12Z Friday. These warmer temperatures will be in place through the
rest of the weekend.

The main forecast challenge today will be winds impacting northern
Johnson County again. Boundary layer and H7 winds of 50kts will
occur over NE WY after 18Z Thursday and 00Z Saturday. Although
gusty winds will occur again near KBYG and the higher elevations
of the Bighorns, am not confident of another high wind event
repeating itself this afternoon. The main differences are a lack
of upper level support and a strong surface pressure gradient. As
previously mentioned, the jet is moving eastward today and
tonight. A strong jet positioned parallel with the Bighorns would
help translate those winds to the surface and that pattern will
not repeat itself today. Also, the surface pressure gradient does
not look as strong today as it did yesterday.

With clouds continue over the area this morning, fog should not be
an issue. These clouds will remain over the area, before finally
clearing out of the area by early Friday afternoon. Mostly clear
skies will be into the weekend.

.LONG TERM...Sunday through Thursday night


Overview...A dry and overall mild pattern will continue through
the extended period. Cooler temperatures will be found under
inversions over the western valleys and Wind River Basin, along
with patchy morning fog.

Discussion...Medium-range models remain in good overall agreement
with persistent western ridge, eastern trough over the U.S. through
the extended period and beyond, with some retrogression of western
ridge mid to late next week.  A series of clippers will continue to
plunge down the front side of the ridge into the northern and
central plains.  The next in this series of clippers will dive down
into the northern High Plains Sunday afternoon/evening with both GFS
and ECMWF now keeping QPF east of the area, so main effect of this
clipper will be to kick up the N-NW winds across Johnson County
again Sunday night into Monday morning.  Afterwards, there is poor
agreement on timing and track of subsequent clippers moving
over/through the ridge mid to late week, so just kept the model
blended low PoPs in the Bighorn Mtns Wed-Thur.  Main adjustments
were made to temperatures, with no model guidance fully accounting
for inversions over western valleys and snow-covered central basins.
Lowered Max/Min temperatures several degrees below guidance for
these areas and added patchy morning fog.  Bias-corrected guidance
should begin to pick up on these inversions as this pattern persists
in the short-term.

&&

.AVIATION...For the 18Z TAFS through 18Z Friday

West of the Continental Divide...KJAC, KPNA, KBPI, and KRKS

VFR conditions will prevail through the period. A gusty wind will
occur at most sites today with the wind decreasing after 01Z.
VFR conditions are expected Friday.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

VFR conditions will prevail through the period. A gusty wind will
occur in northern Johnson county this afternoon and tonight. A gusty
wind will develop this afternoon at KRIW airport with decreasing
wind after 02Z. VFR conditions are expected Friday.

&&

.FIRE WEATHER...

Temperature inversions will remain in place through the rest of
the week, mainly over the central basins and western valleys.
Expect poor mixing in these areas. Considerable warming will occur
in the higher elevations through the weekend. Light snow and high
winds will continue over the Bighorns today. Accumulations will
remain light, but gusty winds will impact northern Johnson County
again, as well as portions of Natrona County.

&&

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

$$

SHORT TERM...LaVoie
LONG TERM...Meunier
AVIATION...Ross/Lispon
FIRE WEATHER...LaVoie



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