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Area forecast discussion
National Weather Service Denver/Boulder Colorado
839 PM MST Fri Dec 13 2019

Update...
issued at 839 PM MST Fri Dec 13 2019

Latest satellite and radar shows clouds and precipitation filling
in across much of western Colorado and northern Utah as the next
plume of Pacific moisture arrives. A look at the 00z sounding
from Grand Junction showed some stabilization and warm advection
occurring, so despite the increase in moisture, snowfall rates
will be capped off a bit by the 4-5c/km lapse rates above 700 mb.
However, those lapse rates will be improving by Saturday morning
again as slight cooling in the 650-500 mb layer arrives. In fact,
we already see signs of this destabilization over northern Nevada
at this time as radar returns show precipitation there is
becoming more convective. All in all, the forecast is on track
which indicated the lull in mountain snowfall through this evening
and then intensity increasing tonight through Saturday morning.
Perhaps another little lull in the I-70 mountain corridor around
mid day Saturday, before cold advection arrives with heavier
snowfall again by late Saturday afternoon and evening. The
forecast with additional snowfall through Saturday evening of 8-16
inches looks good. Basically, we're just a little over halfway
through this event.

On the plains, there's a chance a few showers could push off the
mountains and foothills late tonight or tomorrow morning. This
would be due to improving lapse rates and the proximity of the
upper level jet (overhead). The best chance would likely be over
the northern border area where low level convergence and less
pronounced downslope would help the cause.

&&

Short term...(this evening through saturday)
issued at 245 PM MST Fri Dec 13 2019

A strong winter storm continues to impact the mountains as a west-northwest-
oriented jet brings waves of moisture into the high country.
While snowfall rates have decreased today in a lull of moisture, and
will continue to do so through the evening, strong winds are
creating blowing and drifting snow making for hazardous travel
conditions. The 160+kt jet is currently right over the forecast
area, helping push strong winds over the plains as well where
gusts have reached 40 to 55 mph. As the sun sets and the jet aloft
weakens, the surface winds will be weakening as well. The next
wave of moisture is pushing through Utah and should spread into
the northern and central mountains by midnight. Snowfall rates
will rapidly increase, with some areas seeing 1 - 1.5 inches per
hour. Forecast soundings show moist adiabatic conditions through
most of the morning and afternoon for continued snow over the
mountains. Winds will still be slightly weaker tomorrow in the
mountains, but gusts of 30 to 50 mph will still create areas of
blowing and drifting snow.

Over the plains, winds will decrease this evening, then have a
northeasterly push due to high pressure sinking south over the
Great Plains. This push likely won't make it all the way through
the Metro areas, and where it stops, some light snow will be
possible towards sunrise over the eastern plains in the area of
convergence. Forecast soundings show the saturated airmass
reaching down to around 4kft above the ground tomorrow morning
around the Metro area, as the westerly winds try to keep it dry in
the lower levels east of the mountains. Could see a few light
flurries however in the early morning with little accumulation.
The flow aloft bends as slight ridging moves in the fast flow,
which will create some relatively warm advection pushing up from
the south. This will combine with moisture moving down from
Wyoming to bring some areas of light snow, less than an inch.
Overall, temperatures will be about 10 degrees colder tomorrow.

Long term...(saturday night through friday)
issued at 245 PM MST Fri Dec 13 2019

On Saturday night, an upper level trough digs southeast and will
be near southern Utah 12z Sunday. Ridge top winds will decrease
through Saturday night as the trough approaches Colorado. This
will cause snow to decrease and in many cases end in the mountains
Saturday night. Running the Winter Storm Warning through Saturday
evening still looks like a good plan. Main forcing transitions
from strong orographic lift across the mountains, to ascent ahead
of the trough and lift from weak northeast upslope flow along the
Front Range. The heaviest snow is expected to be over central and
southern Colorado Sunday and Sunday night. The European model (ecmwf) continues to
show the snow being a little farther north and lasting longer than
other models show. Will keep a compromise of the models for the
forecast with light snow (1 to 4 inches) across the Denver area
Sunday. If the European model (ecmwf) pans out a Winter Weather Advisory may be
needed with up to 3-6 inches of snow for the Denver area.

Upper level trough axis will be east of Colorado Monday leaving a
chilly northwest flow aloft over Colorado. For the mountains and
foothills highs are expected to be in the single digits to teens.
Across the lower elevations, highs should be in the 20s. There may
be enough moisture embedded in the northwest flow aloft to
produce flurries or light snow in the mountains. Any snow
accumulation is expected to be light. The European model (ecmwf) keeps light snow
over the area early Monday. Will have pops around 10 percent for
this as the European model (ecmwf) is the only model showing snow lingering all the
way to Monday.

An upper level ridge will amplify over the western states Monday
night and Tuesday. This will cause flow aloft over Colorado to
turn northerly. Cool temperatures will continue for Tuesday.
Subsidence around the ridge will result in dry conditions for the
state.

The ridge shifts eastward across the intermountain west
Wednesday. This will usher in warmer temperatures for Colorado and
keep dry conditions in place. A progressive weather pattern will
prevail over the country for last part of the weekend. Models are
still trying to resolve the details. There could be a weak system
on Thursday that would bring snow to the mountains, but most
models do not show this. Will have very low pops in the mountains
for this system and not mention a chance for snow at this time.
Otherwise, dry conditions are expected through Friday.
Temperatures look to be near normal for Thursday and Friday.

&&

Aviation...(for the 06z tafs through 06z Saturday night)
issued at 839 PM MST Fri Dec 13 2019

Winds have become light southeasterly at kden and kapa and should
stay generally south/southeast through the rest of the night.
Overall, VFR conditions will persist, but a period of ils
approaches may exist from 10z-17z as a mid level moisture plume
and better mid level instability arrive. That could also spawn a
brief light snow shower, but no accumulation is expected. Winds
may be light and variable for a while on Saturday, trending to
east/northeast after 19z.

&&

Bou watches/warnings/advisories...
Winter Storm Warning until 11 PM MST Saturday for coz031-033-034.

&&

$$

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