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Area forecast discussion 
National Weather Service Riverton Wyoming
250 am MST Sat Jan 31 2015

Short through Monday night

Any left over snowfall in southern Sweetwater County this morning
will shut off shortly after sunrise as the low over Vegas propagates
south to old Mexico. The asct high and middle clouds that have been
over central and southern Wyoming Friday will clear out of Sweetwater and
Natrona counties to the southeast with mostly clear skies by 18z
today. Otherwise expect a mostly sunny day with light winds and
relatively mild temperatures today. The northern stream vorticity over
southern British Columbia will track across Wyoming from the northwest by 00z Sat. This
weather feature will deliver patchy light snow to areas north and
east of The Divide tonight as it drives a cold front south and 700 mb
temperatures drop to -16c in Johnson County by 12z Sunday. Post frontal north-northwest
winds will increase in Johnson County late today. Sunday will be
relatively chilly with warm air advecting in aloft from the
southwest so the cooler air will be short lived. Northern and
western Wyoming may not break freezing Sunday before warming back up
Monday. Still...seasonable temperatures slated for this time of year
Sunday. By Sunday night the steering flow will back from northwest to west
and increase in speed...more directly off the Pacific under a
flattening ridge. A batch of Pacific moisture combined with a weak
vorticity embedded in the backing flow...snow will increase in the west
Sunday night. Snow will lessen in intensity Monday in the far west.
Then the circulation now well off the Pacific coast at 45n/152w will
arrive as a vigorous shortwave bringing another slug of Pacific
moisture across Montana/Wyoming Monday night with snowfall increasing once
again in the far west. With favorable upslope conditions for the far
west Sunday night through Monday night with these two back to back
Pacific systems...there will be a potential of accumulating snowfall
for the far west and will have to be monitored. There will be some
isentropic lift in the far west Monday night as well with the second
Pacific system.

Long term...Tuesday through Saturday

The overall big picture is reasonably similar in the models tonight
but the timing/speed varies enough to make the forecast a challenge. still looks like a cold shortwave and associated cold
front will drop south out of Canada on Tuesday into Tuesday night
with potential for upslope east of The Divide...especially in the
north. In the west...a moist west-northwest flow will keep snow flying out
there with strong west-northwest wind ahead of the Canadian front east of The
Divide. GFS has more amplification of the upstream ridge Wednesday
with a colder...cyclonic northwest flow while the European model (ecmwf) pushes the ridge
axis more quickly off to the east with less amplification. The Gem
is more of a compromise. Will gradually push the probability of precipitation southward into
Wednesday to mainly cover the west and south half. As the ridge axis
starts to move in Wednesday night some isentropic upglide may be
enough for some light mountain snowfall out west. Driest day is shaping
up to be Thursday with ridge axis into the west. European model (ecmwf) keeps the Dry
Ridge axis over US through Friday while the GFS is flatter and more
moist with persistent at least light precipitation out west. Due to the
uncertainty this far out will keep just some probability of precipitation out west.
Potential for valley rain or snow and mountain snow increasing once
again by Saturday with potential shortwave or two spinning off the
eastern Pacific trough and moving east across the area. Overall...this
pattern could keep areas east of The Divide mixed off and on with
mild temperatures. Windy ahead of the Canadian front Tuesday. Brief
colder air Wednesday with decent surface high just to the NE...then
warm air aloft moving in again by Thursday through Saturday with
potential for enough flow aloft and pressure gradient to keep
quite a few areas east of The Divide mixed...especially Friday and
Saturday. Generally going with 40s and some 50s east of The Divide
for Friday and Saturday. Cooler in the west with inversions
holding in between systems and then of course cold rain or snow at
times with each system. So mainly 30s out west.


Aviation.../12z issuance/

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

Areas of fog will obscure terrain across the lower Bighorn Basin
through 14z...vicinity kgey-kwrl. A cold front will push south across
northern Wyoming late this morning...likely scouring out any
lingering fog. However...areas of lower MVFR/IFR ceilings and -sn will
follow the front into northern Wyoming after 20z...and southward
to near a vicinity kriw-kcpr line by 00z Sunday. Areas of lower
MVFR/IFR ceilings...local LIFR ceilings/fog will likely remain against the
east slopes of The Divide through 12z Sunday.

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

Areas of IFR/MVFR ceilings and -sn will gradually decrease across the
Interstate 80 corridor through 14z...but patchy fog may linger in
this area through the morning. Otherwise...VFR conditions will
prevail across the area 15z today-03z Sunday. A cold front will
plunge down the east slopes of The Divide late this morning
through this afternoon with some mountain obscurations possible along
The Divide. This front may push lower MVFR/IFR ceilings into vicinity
krks after 06z Sunday. Further out from the front....lower
MVFR/IFR ceilings from eastern Idaho may push into the far
west...vicinity kjac-kafo after 03z Sunday.


Fire weather...
fire danger is low for all areas...and will remain so through next
week. Measurable precipitation will not fall across the forecast
area until late today into Sunday morning east of The Divide.
From Sunday night through the next work week...a series of
disturbances will ride through the region...affecting mainly the
western and northern dispatch zones. Today will see improvement
in both mixing ability and dispersion...especially east of The
Divide and across the southern zones.


Riw watches/warnings/advisories...none.



Short term...lipson
fire weather...lipson

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