Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus65 kriw 180625 
afdriw

Area forecast discussion
National Weather Service Riverton Wyoming
1125 PM MST sun Dec 17 2017

Short term...(this evening through wednesday)
issued at 117 PM MST sun Dec 17 2017

A weak high pressure ridge over Idaho and Montana will move east of
the western and central Wyoming with west northwest flow increasing
behind the ridge. Strong winds will increase over and near the east
slopes of the mountains with a high wind threat in the Cody foothills
tonight and Monday. Other areas will have strong winds and local
high winds may develop through the wind corridor between Rock Springs
and Casper. Some wind gusts to 65 mph have developed in the Green
Mountains near Muddy Gap already. Non windy areas with fresh snow
cover will be cold with potential fog over night into Monday morning
along the rivers and unfrozen lakes. Pacific moisture will gradually
increase in the flow through mid week keeping clouds and areas of
mountain snow with rather light accumulations each day. Wind will
gradually shift to the southwest Monday night and Tuesday with
warming temperatures and increasingly gusty winds through Wednesday.

Long term...(wednesday night through sunday)
issued at 117 PM MST sun Dec 17 2017

It makes sense that we will finally descend into the depths of winter
as we head into the first day of winter, which arrives at 9:28 am
Thursday. The coldest air mass of the season will move toward the
area as a powerful cold front sweeps into the area from the north.
As expected, the models have slowed the progress of the front down
somewhat. As a result, much of Wednesday evening may end up
remaining precipitation free. Snow will spread across the area
Wednesday night however. There is decent frontogenic forcing along
with some jet support. However, the limiting factor will be, like
with many of the other Canadian and Arctic fronts, will be moisture.
So at this time, amounts look to fall in the advisory category
rather than warning. There are some other hazards however. First
will be the possible high wind across the northern Big Horn basin
and Johnson County associated with the strong cold advection behind
the front later Wednesday night and Thursday morning. In addition,
the snow will be of the light and powdery variety, Aka easy to blow
around and reduce visibility. This snow should end by Thursday at
sunset, the shortest day of the year. In addition, much colder
temperatures will move in with highs possibly occurring at midnight
for many areas and the temperatures falling through the day.

The next question is how cold will it get Thursday night. We will
have the Arctic air mass, the long nights and fresh snow cover. The
one thing that is questionable though is a clear sky. Below zero low
temperatures are certainly possible though for many areas.

Another cold front with a reinforcing shot of cold air then moves in
for Friday. The GFS tries to bring the system into the northwest
later Thursday night but given the bias for rushing systems in, we
held off precipitation until Friday. This looks to be another shot
of snow, although this system has even less moisture to work with,
so it would be a mainly light snow, although maybe more than one
would expect with very high snow ratios given the very cold air in
place. Light snow will likely continue through Friday night and
possibly into Saturday as well. Saturday looks to be even colder
with highs possible remaining below zero in many areas. And again,
lows Saturday and Sunday night will depend on how much clearing
there is. However, brutally cold temperatures at night look likely
with some lows below minus 20 looking likely for some areas at this
point. Stay tuned.

&&

Aviation...(for the 06z tafs through 06z Monday night)
issued at 1122 PM MST sun Dec 17 2017

West of the Continental Divide...kjac, kpna, kbpi, and krks

Stratus decks mainly around fl100 vicinity and north of a kafo-
kpna line will persist through the morning, with some lowering
of these ceilings into MVFR category with flurries or light snow
developing vicinity kjac by 12z. Ridge top west winds will
increase late Monday morning with some mixing dissipation of lower
stratus; areas of low level wind shear developing vicinity kjac through the
afternoon. Low level wind shear will decrease Monday evening, with some lowering
of stratus decks into low VFR/MVFR categories, areas of -sn
returning Monday night.

East of the Continental Divide...kcpr, kcod, kriw, klnd, and kwrl

VFR conditions will prevail across the area through 12z Tuesday
with mainly some broken high clouds. The main aviation hazard will
be mountain wave activity vicinity kcod where some gusts of
50-60kts are possible through 15z Monday. The potential for
mountain wave activity will decrease after 15z, but low level wind shear will
persist vicinity kcod through the period. Low level wind shear will also develop
further south along The Divide to vicinity klnd by 16z Monday.
Surface southwest winds 20-30g40kts will occur vicinity 50sm NE
krks-kcpr through around sunset Monday.

&&

Fire weather...
issued at 117 PM MST sun Dec 17 2017

A weak high pressure ridge over Idaho and Montana will move east of
the western and central Wyoming with west northwest flow increasing
behind the ridge. Strong winds will increase over and near the east
slopes of the mountains with a high wind threat in the Cody foothills
tonight and Monday. Other areas will have strong winds and local
high winds may develop through the wind corridor between Rock Springs
and Casper. Some wind gusts to 65 mph have developed in the Green
Mountains near Muddy Gap already. Non windy areas with fresh snow
cover will be cold with potential fog over night into Monday morning
along the rivers and unfrozen lakes. Pacific moisture will gradually
increase in the flow through mid week keeping clouds and areas of
mountain snow with rather light accumulations each day. Wind will
gradually shift to the southwest Monday night and Tuesday with
warming temperatures and increasingly gusty winds through Wednesday.

&&

Riw watches/warnings/advisories...
High Wind Warning until 8 am MST Monday for wyz003.

&&

$$

National Weather Service Glossary of Abbreviations