Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 261722

Area Forecast Discussion
National Weather Service Riverton WY
1122 AM MDT Wed Oct 26 2016

.SHORT TERM...Today through Friday night

Imagery shows large broad ridge across most of the CONUS with an
upstream trof well offshore the west coast and a downstream
shortwave trof over the central plains and upper midwest. Nose of a
broad jet currently moving across western and into central WY with a
few isolated but dissipating showers across northern portions of the
state. The SFC has mainly weak high pressure across the Great
Basin/Intermountain West and over most of WY with a weak trailing
nearly stationary front through the northeast corner of the state.

Today through Thursday: Early this morning, there will be a few
chances for patchy fog forming...primarily over the upper Green
River Basin and the Star/Jackson Valley locations. Otherwise, modest
ridging continues with increasing high pressure across the surface,
fewer clouds and no precipitaion. Patchy morning fog possible once
again across the Upper Green River Valley overnight tonight into Thu
early morning. Thursday will see even fewer clouds and more warmth
with weak lee side surface troffing occurring over the central basins
and east of the Bighorn Mountains increasing wind speeds a bit back
into the "gusty" category especially across the Wind Corridor and
over the mountains.

Thursday night through Friday night: Increasing in clouds and
possible chances for light precip begins across the northwest
portion of WY as a ridge riding shortwave/jet streak sweeps
across/near the area. Gusty southwest SFC winds will also continue
well into the evening/overnight period as SFC P GRAD increases from
southwest to northeast. A few possible near high wind gusts will be
possible over the northern portion of the Cody Foothills Thursday
evening. By mid-day Friday more significant changes begin to occur
in both the upper and lower levels as a vigorous fast moving
shortwave trof approaches western WY before overspreading the region
Friday night. This will quickly develop a low pressure cyclone
across eastern ID with a what appears to be a warm front moving up
and across southern and western WY. EPAC moisture...some graciously
lent from hurricane Seymour...will also accompany this system with
showers increasing quickly across the area west of the Divide.
Maximum convergence around the developing cyclone and into the
terrain features looks to occur over northwestern WY with the SW
trof axis moving through NW WY around midnight or
minus. As origin of this system is subtropical...H7 temps will only
fall to just below 0 degC late Friday night...with most significant
accumulating snowfall remaining above 8500 feet and over the Tetons
to southwest portion of YNP. Much lighter amounts below this level
and over the valleys. As it currently looks...especially considering
the speed of this system...all mountain locations across northwest
WY will get wet with 6 hr QPF values around a half inch Friday
evening(totaling near an inch over an 18 hr period), but snowfall
amounts should mostly remain at or below advisory levels under the
warmth of this nearly tropical-type system. However, the highest
reaches of the mountains...above 10k ft could see 6 to 10 inches.
Will continue monitoring for possible highlights. Additionally, as
sunrise nears Sat morning and the shortwave trof axis moves by,
there could be a period of good mountain wave activity across the
Absaroka and Wind River the point where a few near
high wind gust periods maybe felt. Too marginal for a watch at this
time...but perhaps coming up.

.LONG TERM...Saturday through Wednesday

There could be some lingering mountain snow/valley rain showers
over the northwest/north Saturday morning as a shortwave trough
quickly exits the area. Shortwave ridging is expected Saturday
afternoon and Saturday night with mainly dry and mild conditions.
Slight chances of precipitation confined to the extreme northwest.

Global models show an upper low/trough pushing into the West Coast
late Saturday night. This trough is expected to push into the
Great Basin/Intermountain West Sunday...and then across the
Northern Rockies Sunday night. Models are decent agreement with
precipitation quickly breaking out across west/southwest Wyoming
Sunday afternoon with a combination of general synoptic lift,
right entrance region dynamics, and a moist southwest flow
overspreading the region. On Sunday night, QG forcing and a
Pacific cold front pushing east should enhance precipiation out
west while spreading some precip east of the Divide. Through
Sunday evening, snow levels should remain above 8500-9000 feet,
but are expected to quickly lower to valley floors out west
shortly after frontal passage. This is a quick system with
precipitation ending across much of the area Monday morning,
except perhaps across the west/northwestern mountains where a
moist westerly flow remains. This system has the potential to
bring significant mid-high mountain snowfall, but the change over
to snow in the western valleys will be late enough to keep snow
accumulations on the light side.

Global models show the potential for another trough and associated
cold front late Tuesday into Wednesday time frame.

Temperatures are expected to be above average Saturday and Sunday,
with temperatures more seasonal for the rest of the period.




A few isolated showers will be possible over the higher elevations
of northern Wyoming through around 21z today. Additionally, some
low level clouds remain over the western basins, impacting KJAC.
Ceilings are expected to lift through the afternoon hours. VFR
conditions then expected through much of the TAF period, though
some lowered ceilings and patchy fog may return after 09z
Thursday. Lowered ceilings across the area will obscure mountains
at times.


High pressure will leave VFR conditions over the area through the
TAF period. Periods of gusty wind will be possible during the
afternoon hours both today and Thursday, impacting mainly KCPR.



Today through Friday, fire danger low most locations today and
Friday...moderate over portions of central WY Thursday. Minimum RH
values remain near or above 20 percent for most of the forecast area
through the forecast period with Natrona and Johnson counties seeing
the lowest RH values Thursday afternoon along with gusty southwest
wind of the 20 gust 30 mph variety. Dry forecast today through
Thursday. Then slight chances for precipitation return back into the
forecast across northwest WY later Thursday night as a small upper
level disturbance passes by. Significantly increasing precipitation
chances will then occur across much of the region west of the Divide
later Friday and Friday night as a fast moving and much stronger
developing weather system moves through the region.




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