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Area forecast discussion 
National Weather Service Riverton Wyoming
344 am MST Monday Nov 30 2015

Short term...Monday through Wednesday night

Looks like we have one more day of the white stuff for many areas
before we finally see some more settled weather move in across the
cowboy state. The persistent upper level low that has been over the
area will finally be kicked east Monday night. The models are fairly
consistent in keep the most snow showers across the west so the
higher probability of precipitation and quantitative precipitation forecast were put in that area. However...amounts should
remain fairly light. Most areas should have chance of snow showers
this morning before chances decrease in the afternoon from northeast
to southwest. All showers should be out of the area by later this
evening. High pressure should then build over the area and bring dry
weather. A weak shortwave will move through for Wednesday and
Wednesday night. The European has some light precipitation but the
other models are dry. So for now...since any precipitation would be
inconsequential flurries...we left the forecast dry from late Monday
night on.

The main dilemma is for temperatures and the effects of inversions.
We kept temperatures quite cold today with more cloud cover and some
flurries. There is more question for Tuesday since there will be
more sunshine. this point the weak December sun should
warm things somewhat but it will still be cold in the favored areas
like the Wind River basin...Green River basin and portions of the
Big Horn basin. Areas that mix better Buffalo...Rock
Springs and Casper should begin to see moderating temperatures. The
approaching shortwave for Wednesday will try to mix things out. But
for now we will keep things cold.

Long term...Thursday through Monday upper ridge of high pressure will bring very mild air
dry weather into the area on Thursday...but western valleys and snow-
covered central basins will remain trapped in shallow cold air under
steep inversions with areas of morning fog. A Pacific trough will
push east across the intermountain west and rockies Friday and
Saturday spreading some snow showers into the west on Friday and
into central areas Friday night through Saturday morning. An upper
ridge will then rebuild over the area Saturday night and Sunday with
clearing and cold conditions Saturday night...then warmer
temperatures for most areas Sunday with steep shallow inversions
setting up again over western valleys and lower central basins.

Discussion...30/00z GFS run has come in much better alignment with
more consistent European model (ecmwf) bringing split Pacific trough across the
intermountain west and rockies Friday and Saturday. This split
takes most of the energy south across The Four Corners area on
Saturday with weaker northern stream near the U.S. Canadian border.
A very mild airmass with 700 mb temperatures 1c to 3c will accompany
upper ridge translating the area on Thursday. This warmer air aloft
will steepen inversions over the lower central basins and western
valleys where high temperatures will likely remain below freezing
while mixed areas see highs 10-15f above normal. On the low end of
this spectrum will be the Wind River basin where highs will be in
the lower to middle 20s...and on the high end will be northeast Johnson
County where highs will be in the lower 50s.

Upper trough passage across the area Friday night will drag a
cold front south across the area that will spread some light snow
showers east of The Divide with this activity pushing south and
east of the area Saturday afternoon. Upper ridge will build into
the area Saturday night with advecting in warmer air at mountain
top level...while lower basins and valleys radiate out under clear
skies and light wind. Western valleys will likely have a hard time
recovering from single digit lows on Sunday. Another split Pacific
system is expected to push into the western U.S. On Monday...with
most of its energy once again headed south across the southern
Great Basin/Four Corners region.


Aviation.../12z issuance/

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

VFR conditions will prevail over the region through 00z. Areas of
MVFR conditions will occur due to lower ceilings...light snow or fog
until 19z. VFR conditions will occur after 00z through 12z Tuesday.
However...areas of valley fog may form after 08z Tuesday in the
basins...potentially impacting kriw...klnd...kwrl airports with
MVFR to IFR conditions.

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

VFR conditions will prevail through 00z. Areas of MVFR to IFR
conditions will occur due to light snow...lower ceilings or fog until
19z. After 00z VFR conditions will occur through 12z Tuesday. Areas of
valley fog will form and may potentially impact kjac...kbpi and kpna

Fire weather...

A very slow moving area of low pressure will keep snow showers
across portions of the area today...especially in the west. Expect
inversions in the basins and valleys with poor mixing and smoke
dispersal. Inversions are expected to hold in the basins through
Wednesday. Areas that mix from Rock Springs to Casper
may see better smoke dispersal. Mainly dry weather is expected from
late tonight through much of the week.

Riw watches/warnings/advisories...


Short term...hattings
long term...aem
fire weather...hattings

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