Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190842

Area Forecast Discussion
National Weather Service Riverton WY
142 AM MST Thu Jan 19 2017

.SHORT TERM...Today through Saturday night

Imagery shows split flow ending and being replaced weak upper level
southwest flow. The SFC, however, continues with high pressure over
the Great Basin and into WY west of the Divide with lower pressure
to the east. No precipitation anywhere around WY ATTM. Generally,
increasing moisture with modest upper level instability and forcing
will describe the state of the atmosphere through the forecast
period and somewhat limited storm potential.

The dry cold winter experienced lately will give way to snow
once again as the split today gives way to increasing southwest flow
aloft ahead of a digging and increasingly negatively tilted trof
across the western CONUS. Accumulating precip ahead of this
developing trof will begin to arrive over the far western FA later
this morning. Gusty southwest winds will continue across the wind
corridor from southeast Fremont county east across Natrona county
mainly due to a strong surface PGRAD and slowly increasing winds
aloft. Gusty near high wind over the Cody foothills will end
this morning. Still, no highlights planned, although there could be a
few near high wind gusts here and there. An SPS currently describes
the wind situation.

Then by this afternoon, as the digging trof heads into the
intermountain west with the main jet and upper level forcing digging
well to the south over the Desert Southwest, a weak embedded
shortwave will begin to lift across Wyoming this evening. This SW
trof will bring some chances for snow east of the Divide later
tonight with the best chance coming to the northern/northeastern FA
Friday, under the tail end of an exiting weak jet to the north.
Heavier accumulating snowfall out west will occur today and tonight
with what continues to look AOB Advisory level snowfall for the most
part (could have localized Advisory amounts in the Tetons/Gros
Ventre mountains).

East of the Divide, snowfall will arrive later tonight and through
the morning on Friday with the passage of the modest SW trof/vort max
and tail end/entrance region of modest jet, mentioned above, along
with a weak associated surface front, tapering off Friday
afternoon/evening. Generally, the lower elevations east of the
Divide can expect from a half inch to 2 inches across the southern
zones to 1 to 3 inches over the northern zones with perhaps as much
as 4 inches over portions of the northern Big Horn Basin by Friday
evening. Most mountain areas east of the Divide will receive from 2
to 6 inches with localized areas in the Bighorn mountains getting to
8 inches by Saturday morning. While some Advisory level snow may be
possible across portions of the Big Horn Basin and even over the
Bighorn mountains, will just SPS for the moment and let the day crew
take another look to see if continuity is present and a highlight is
warranted. Wind should not be an issue with this event.

Saturday morning through Saturday night, a broad, still digging,
negatively tilted trof will cover most of the western CONUS with the
jet still well to the south. Forcing and instability will be even
more limited with modest, but decreasing, moisture headed into the
western FA. This will keep the region west of the
Divide...particularly southwest WY...under chance of light precip.
Nothing too spectacular here.


Synopsis...A large trough will move onto the west coast on Sunday
and across the the Rockies Monday and Tuesday.  Snow showers will
increase across the west and south Sunday and Sunday night, and
spread into central Wyoming on Monday.  Snow will taper off from
west to east on Tuesday and Tuesday night.  High pressure will build
in Wednesday and Thursday with dry and cool conditions prevailing.

Discussion...Synoptic pattern on Sunday morning to feature upper low
upper ridge over the Rockies flanked by upper lows over the
southeast U.S. and along the northwest coast.  19/00z ECMWF and GFS
runs in good overall agreement with progression of longwave trough
across the western U.S. into the plains Monday and Tuesday, with
upper low lifting out of the central Rockies across Kansas/Nebraska
Monday night and Tuesday.  Within this longwave trough lie the
differences, with ECMWF having an overall further southward
placement of individual features and precipitation than GFS as upper
trough moves across the Continental Divide.  So far, the southward
solutions have been verifying better in this split flow regime.
Thus, expectations are that more significant snow amounts and
impacts will stay to our south Sunday and Monday.  Surface lee
cyclogenesis is forecast over eastern Colorado on Monday with low
level northerly flow developing east of the Divide through Tuesday,
favoring the Casper area, north facing slopes, with possible
moderate snow amounts.

GFS is more progressive than ECMWF folding upper ridge into the
northern Rockies Wednesday and Thursday. Forecast maintains some
low PoPs in the mountains due to the uncertainty, but upper ridge
will eventually prevail over the western U.S. by the latter part
of the week with dry weather expected into the last weekend in




Light snow and associated MVFR ceilings and visibilities are
expected to overspread the far west this morning, spreading into the
Upper Green River Basin and Sweetwater County during the afternoon
and early evening. The KJAC terminal should be most impacted by this
system with a prolonged period of MVFR/brief IFR conditions expected.
Farther southeast confidence decreases, but will still prevail MVFR
conditions at area terminals beginning in the late afternoon or
early evening. Mountains will already or become obscured this
morning, and remain obscured through the taf period. Winds are
expected to be mainly light in most areas.


Through the afternoon, the main concern will be strong/very strong
west/southwest wind in the wind corridor across SE Fremont and
Natrona County, on the lee side of mountain ranges, Cody Foothills
and Dubois area. Some westerly low-level wind shear will also be
present in the foothills. This will impact KCOD terminal. Winds will
be mainly light in the interior basins/valleys. Besides the wind,
VFR conditions with increasing mid-high level cloudiness are expected
today. Tonight, a weak system pushing across the region will result
in a chance of light snow and low ceilings breaking out especially
over northern Wyoming. The best chance appears be KCOD terminal
tonight, and have decided to prevail MVFR ceilings there with only
VCSH at the other terminals. Mountains are expected to become
obscured this evening.



Fire Danger low for all of WY through, at least, the weekend. Cold
seasonal temperatures and a good existing snow pack with light snow
beginning west of the Divide this morning will continue through
Friday morning. Light to moderate snow showers expected east of the
Divide later tonight and Friday. There will be a period of gusty
southwest wind ending across the Cody Foothills this morning and
then over the Wind Corridor by mid-day. Smoke dispersion will remain
poor east of the Divide through Friday morning before seeing
improvement...while west of the Divide will see better smoke
dispersal by this afternoon.




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