Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Riverton Wyoming
257 am MST Thursday Jan 29 2015

Short through Saturday night

Imagery showing ridge over low across the western Continental U.S. With Wyoming under northwest flow
once again. Extreme west side of the SW trough that passed through the forecast area
today now moving out of the eastern portion of the state this
morning...taking light precipitation with it and leaving developing areas
of patchy fog around the County Warning Area. Yellowstone and the northern Bighorn Basin
currently have the most fog in the valleys and basin bottoms.

Beginning of forecast...fog is expected to have to have developed and
persist into the morning hours over most basins...many western
valleys...and the lower elevations of Johnson and Natrona counties.
Although currently fcsting the fog to lift by late
could persist over some areas into the afternoon...with another round
of fog scheduled across many of the same areas overnight tonight and
into Friday morning.

We will be largely devoid of precipitation until late Saturday...except for
an ever so slight possibility of a sprinkle or two across the southern
border area on Friday due to the development of a large closed low
system across the desert SW out of what is currently the weak trough
under the ridge as mentioned above. Jet phasing of the southern branch
and cyclogenesis should occur by Friday night near the northern Baja California region.
However the strength and position of the southern branch of the jet will
keep the track for this wet storm system well south of the forecast area this
time around...only scraping the southern border of Wyoming with modest
moisture through Sat morning.

Otherwise...well upstream and currently over the Gulf of Alaska...a
portion of the northern branch of the jet will move around the ridge over
the epac/western Continental U.S....eventually making its way into the northern County Warning Area by
late Sat...pushing a developing low and front through Montana then Wyoming by
late afternoon/eve. Post frontal pop chances will increase mainly
across the northern and eastern zones...east of The Divide...behind this clipper
like front. The previous days relatively dry atmosphere along with
the projected trajectory will keep moisture at a only
expecting relatively light precipitation east of The Divide overnight
Sat...but all as opposed to the most recent weather
system. This will usher in the beginnings of a week's Worth of
several upper level disturbances that will move quickly through the
persistent upper level northwest flow...bringing increased chances for snowfall
across the west and north.

Long term...Sunday through Thursday

In wake of the system Saturday/Saturday night...a mainly dry
northwest flow is expected Sunday. Would still expect the
possibility for some mountain snow showers over the northwest and
north. Medium range models continue to show a Pacific shortwave
trough briefly flattening out the West Coast ridge as it pushes into
the Pacific northwest Sunday night. This shortwave trough is then
projected to push across the northern rockies in the Monday/Monday
night time frame. This should result in a good chance of snowfall
over the far west/northwest and Bighorn mts...with breezy to windy
conditions in the wind corridor. In wake of this system...the
western upper ridge builds back...with models showing a relatively
moist northwest flow over the forecast area...with perhaps a
disturbance or two Tuesday and Wednesday. This will keep chances of
snowfall in the forecast...especially over the northwest/northern
mountains...possibly spilling into the lower elevations east of The
Divide at times. Models have continued to struggle with the
placement/timing of an Arctic cold front in the Tuesday/Wednesday
time frame. The GFS keeps the Arctic air mass north and east of the
area...while the European model (ecmwf) and Gem are showing this cold air mass
impacting the areas east of The Divide Tuesday and Wednesday. Models
are hinting at the western ridge shifting a bit over The Rockies
Thursday with milder and mainly dry conditions.


Aviation.../12z issuance/

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

Drier air pushing in early this morning will result in clearing middle
level cloud decks through 12z with areas of fog or IFR/LIFR ceilings
likely forming and persisting through 19z. The Wind River basin and
Bighorn Basin...vicinity kriw-kwrl-kgey...stand the best chance of the
most widespread...persistent fog into the early afternoon...but fog
may also become fairly widespread further east...vicinity kriw-kcpr
through the morning. Most fog should dissipate by early Thursday
afternoon. VFR conditions will prevail afterwards...through
Thursday night with fog more patchy and mainly over the lower
basins...vicinity kriw and kwrl Thursday night.

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

Some clearing of middle-level cloud decks will occur early this morning
resulting in formation of lower IFR/LIFR ceilings or fog over the
western valleys and southwest through Thursday morning. Most of the
fog or lower ceilings will erode by early Thursday afternoon. An upper
level disturbance will spread in isolated to widely scattered -shrasn into the
southwest after 06z Friday with most of this activity expected to
stay vicinity Interstate 80 and south.


Fire weather...

Fire danger is low for all areas...and will remain so through next
week as. Measurable precipitation will not fall again across the
forecast area until late Saturday 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. Smoke dispersion will be poor
across most of the region today and again Friday. Saturday 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...Braun
long term...wm
fire weather...Braun

National Weather Service Glossary of Abbreviations