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fxus65 kriw 081712 

Area forecast discussion
National Weather Service Riverton Wyoming
1012 am MST Thu Dec 8 2016

Short through Saturday night

A broken deck of stratocu over the central basins of Wyoming east of The
Divide is stuck within the inversion topping out at 750mb. This will
have an affect on the fate of our morning low temperatures this
morning. There does not appear to be an easy way for this trapped
moisture to escape beneath this inversion so low temperatures will not
bottom out quite as cold as they otherwise would tonight, namely 5
to 10 below zero. In contrast, areas to the east like Casper, that
have a healthy blanket of snow on the ground under clear skies and
light winds should have no problem bottoming out to 30 below zero,
considering is is already 24 below at kcpr. The infrared imagery is
showing -35f across Natrona County at this time. Where ever the infrared imagery
is not enhanced over the lower elevations, stratocu is interfering
with the radiational cooling. Out in western WY, still expecting to
see lows of 25 to 35 below this morning, comparable to what was
seen Monday morning.

Warm air will advect in aloft today from the west southwest allowing
basin and valley temperature inversions to persist through this
afternoon as 700 mb temperatures rise to -13c across central Wyoming today.
Highs around 8500 feet today could climb to near 20, with 10 to 15
degrees persisting in the valleys and basins.

Clouds will increase from the west today as the west northwest
steering flow tightens with isentropic lift developing in the far
west late this afternoon. As clouds thicken this afternoon over the
far west, steady snowfall will develop late this afternoon, first in
the far west mtns, with the snowfall working its way down into the
western valleys by sunset this evening as abundant Pacific moisture
advects in from the west. Steady snowfall will continue in the far
west tonight, piling up at a rate of around 1/2 to 3/4 of an inch an
hour, less so in the valleys. We should see a subtle respite from
the snowfall Friday, when we rely mainly on strengthening, slightly
difluent westerly upslope flow with the west facing slopes
maintaining a steady stream of moderate to heavy snowfall Friday.
Another bout of isentropic lift can be expected Friday night for
more enhanced steady snowfall everywhere in the hilited area. Then
on Saturday, after the steering flow backs to the west southwest,
the troughpa/Pacific cold front will occur later Saturday morning
for one last heavy snow squall before decreasing to less intensive
snow showers continuing in the far west under moist northwest flow favoring
northwest aspects through Saturday afternoon and night. Increasing west
southwest winds will result in the added element of considerable
blowing and drifting snow to this snow storm as 700 mb 50 knot winds
surface. South Pass should come close to a repeat of the white out
conditions that occurred there last Sunday evening. Will upgrade the
Winter Storm Watch to a warning and will introduce a Winter Weather
Advisory for the Upper Green river basin and the upper Wind River
basin for the same time frame, namely 3 PM today through 12 noon
Saturday. We will be dealing with optimal 700 mb temperatures for dendritic
growth. By the time all is said and done, we will likely be looking
at up to two and a half feet of snow in The Tetons and perhaps as
much as a foot to a foot and a half across northern Star and
northern Jackson valleys by middle day Saturday.

Meanwhile, along the southern wind corridor east of The Divide,
southwest winds will increase from the Red Desert to Casper with
blowing snow Friday morning before temperatures crack freezing Friday
afternoon as snowfall from last monday's snow event gets blown
around for the first time.

Some snow will spill over The Divide by middle day Saturday when the
Pacific frontal passage takes place.

Long term...Sunday through Thursday

Significant differences start showing up in the medium range
period as to how steep our upstream ridge will become early next
week. The GFS is quite strong with a long nly flow pushing the
coldest air of the season swd across US Monday into Tuesday. The
Euro is trending with a steeper upstream ridge but not enough to
bring the full brunt of the Arctic air swd. Current forecast is
much warmer than the GFS would indicate but we'll have to watch
this closely as this air originates in Siberia if it makes it
hear. The GFS for example has - 42c air making it into Fremont
County with -28c air at h7. We'll see how the big picture looks
tomorrow night. For now, we deal with a couple potentially
significant shortwaves in the wly flow Sunday and Monday with
periods of snow out west and possibly some snow east of The
Divide, especially nrn border and i80 corridor depending on their
strength. Developing north or northwest flow will either result in
a cold or very cold period by Tuesday into Wednesday. Potential
for at least mountain flurries/lgt snow and some light snow along
and behind Canadian/Arctic front later Monday through Tuesday. Low
confidence yet on details and timing. Potential warm advection
event out west by Wednesday night through Thursday followed by a
cold shortwave. So more accumulating snow west of The Divide off
and on through the medium range period and beyond with cold to
potentially bitter cold developing east.&&

Aviation.../12z issuance/

East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes

Mainly VFR conditions with skies mainly sky clear with light
winds this morning. Increasing mid-high level cloudiness is expected
to overspread the region between 21z and 03z as a storm system
begins to impact the western Wyoming. Winds will remain light in the
basins tonight, but winds will be picking up from the southwest over
the mountains and in the central wind corridor from southeast Fremont
County into the Casper area. Before the surface winds can respond
there could be a period of time where wind shear will be present at
kcpr this evening. A few showers may fell after 08z tonight in the
mountains outside of kcpr and klnd, otherwise dry conditions are
expected through 18z Friday.

West of The Divide...kbpi/kjac/kpna/krks routes

Widespread snowfall is expected to breakout across west and
southwest Wyoming between 23z and 05z. After 03z, persistent MVFR
conditions with occasional IFR conditions are expected through the
night at area terminals, except for krks where MVFR/IFR conditions
are expected to be more brief in nature. Mountains will become
obscured late afternoon into the early evening, and remain obscured
through Friday morning with IFR to LIFR conditions. The main
exception will be over South Pass where ceilings will be higher, and
confidence for continued snow is not as high. Periods of snow will
continue through 18z Friday, although it is difficult to pinppoint
times of heavier snowfall.


Fire weather...

A strong temperature inversion along with poor smoke dispersal will
be in place across the basins and valleys through much of the day
today. This morning will feature some of the coldest temperatures of
the season thus far, especially in zone 280 where lows this morning
should bottom out to 30 below in some areas. We should see lows of
25 to 35 below in the far west valleys and basins once again this
morning. A powerful winter storm will deliver 1 to 2 and a half feet
of snow to much of the far west this afternoon through Saturday.
West southwest winds will increase Friday and Saturday, especially
from the Red Desert to Casper and along The East Slope of the
Continental Divide. Temperatures will moderate later this week ahead
of a Pacific cold front that will track across Wyoming Saturday that
will spread some snowfall east of The Divide Saturday.


Riw watches/warnings/advisories...
Winter Storm Warning from 3 PM this afternoon to noon MST
Saturday for wyz001-002-012>015-023>025-027.

Winter Weather Advisory from 3 PM this afternoon to noon MST
Saturday for wyz016-026.



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