Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 10/22/19

Area Forecast Discussion

National Weather Service Denver/Boulder CO

 923 PM MDT Mon Oct 21 2019

.UPDATE...
Issued at 912 PM MDT Mon Oct 21 2019

Latest CDOT cams this evening have been showing a general trend
of snow tapering off across the higher ranges. Latest runs of the
short range models are in line with this trend along with mosaic
radar indicating the back edge of the precip near the Steamboat
Springs vicinity. Will let the current Winter Weather Advisory
for the northern and central mountains expire at its intended
time of 03z. Will expect periods of light snow to continue over
the higher northern mountains tonight with additional
accumulations of 1 to 2 inches mainly above 10500ft. Across the
lower elevations, a dry and cool night is expected with light
winds gradually following the typical drainage flow patterns.

&&

.SHORT TERM...(This evening through Tuesday)
Issued at 308 PM MDT Mon Oct 21 2019

As a deep upper trough continues to move east over the central
plain states, a 125-kt jet is blowing over the forecast area.
Even though we`re on the backside of a trough, the favorable
northwesterly flow continues to bring orographic snow to our
mountains. Strong wind gusts between 25 and 45 mph are creating
blowing and drifting snow along with reduced visibility,
especially over the passes. Another 1 to 4 inches is expected the
rest of the afternoon into this evening, when the Winter Weather
Advisory will expire at 9pm. After this time, moisture will be
tapering off, however light occasional snow will still fall due to
the continued favorable northwest flow aloft. Only another 1 to 2
inches of snow would be expected through morning.

Over the plains, ACARS data show dry adiabatic sounding in the
lower levels, verified by the cumulus traveling southeast with
very light radar echoes. A dry airmass is keeping precipitation
limited to virga, confirmed by area webcams. Could see some
sprinkles over the plains, but not measurable. Will keep a dry
forecast.

500 mb heights will be slowly rising through tomorrow with warmer
air expected to advect in from the west. 700mb temperatures will
increase 5 to 7 degrees C tomorrow over today, through some high
cloud cover is expected to sneak in from the north. Therefore
forecasting temperatures around 10 degrees warmer. Scattered
orographic snow showers will continue over the day with flow still
from the northwest. Winds aloft will be lighter as upper ridging
moves in, however a surface trough will be forming along the
urban corridor and eastern plains. This will increase winds along
and east of the divide, bringing strong speeds into the foothills.
The winds over the plains will be lighter compared to the last
couple days.

.LONG TERM...(Tuesday night through Monday)
Issued at 308 PM MDT Mon Oct 21 2019

On Wednesday a storm will approach Colorado in the northwest
flow. Before getting to the details on precipitation, we need to
highlight the potential for a downslope wind event along the
eastern foothills and adjacent plains. Cross-barrier flow of 50
kts, maybe a bit stronger, will combine with weak lapse rates (5
deg/km) above the ridge lines. This could lead to the formation
of mountain wave(s) in the lee of the rockies, especially across
Boulder and Jefferson Counties were the inversion is strongest. At
this point it does not look as windy as early Sunday morning`s
bora event, but we have increased winds through mid-morning
Wednesday with gusts to 30 mph in the forecast grids for the lower
eastern foothills and nearby plains, though they could be
stronger than that.

The details of the short wave trough/storm system are somewhat
complicated regarding how it will evolve from later Wednesday
into Thursday morning. On the synoptic scale, there will be a long
wave trough across the northern Plains on which moves northeast
into Canada on Wednesday afternoon. On the backside of the long
wave trough, a fairly strong short wave trough digs quickly almost
due south Wednesday evening into Thursday. The bulk of the
forcing aloft arrives Wednesday evening with strong QG lift from
differential vorticity advection. Moisture should be transported
along a 120 kt jet out of the north, which pushes into western
Colorado overnight into early Thursday. There could be potential
for banded snow early Thursday morning across the northern
mountains and foothills with the arrival of the left exit region
of the jet. As the trough digs nearly straight south early
Thursday morning, deep cold air advection occurs at 500 and 700
mb, with -26 to -28 degC at 500 mb and -10 to -11 degC at 700 mb
by 12Z Thursday, so it will be plenty cold for snow across our
entire area. Near the surface the northerly flow aloft will drive
a cold front across northeast Colorado, with falling temps likely
during the afternoon hours. There is a 1034 mb surface high across
Idaho by 00Z Thursday, and clockwise flow around the high will
lead to good upslope flow from Wednesday evening through about
sunrise Thursday morning. Finally, frontogenesis near the surface
up to 600 mb may play a role in developing precipitation though
it is strongest before the column saturates, so despite how strong
it is projected to be, it should be too far south of us before
the moisture arrives. Moisture arrives in the mid levels Wednesday
afternoon with light snow possible across the northern mountains.
Low-level moisture takes a while to arrive behind the cold front
across the Plains, probably not until late afternoon/early
evening. We are also concerned about convective snowfall potential
given the strong cold advection aloft, with model soundings and
cross sections showing some potential instability, including CSI,
starting Wednesday afternoon and into the evening hours.

There are many sources of lift to contend with, and not all line
up with the moisture or with each other. Here is the current line
of thinking regarding timing and expected impacts. Based on when
the atmospheric column saturates, currently expected by late
afternoon/early evening, snow should begin across the I-25 urban
corridor by Wednesday evening. Roads will start off warm but by
sunset the combination of cold advection and snow rates could
mean snow will start sticking Wednesday evening. Exact timing and
total amounts are uncertain but the evening commute could be
impacted along the I-25 urban corridor. By midnight the aforementioned
sources of lift plus ample low-level moisture should lead to
snowfall across much of the area, especially near the eastern
foothills and Palmer Divide where upslope will be strongest. The
QG lift and strength/depth of the upslope flow are still uncertain
three days out, but depending on which way this evolves there
could be a decent snowfall for the I-25 corridor and Palmer
Divide. This is especially going to be the case if the ECMWF is
correct which digs the trough further west and has fairly deep,
moderate intensity east upslope flow around a closed 700 mb
circulation in southwest Colorado. EC is also the wetter of the
two when comparing ensemble output as well. By midday thursday
much drier air works in from the north and the flow pattern turns
downslope east of the Continental Divide, and with temps across
the Plains warming above freezing (perhaps low 40s), road
conditions should improve by Thursday afternoon. In the mountains,
snow production should be ongoing throughout Wednesday and pick
up Wednesday afternoon with the best QG forcing aloft and some
cold advection at 700 mb, but taper off by midnight Thursday as
the west upslope component is lost and the best QG forcing aloft
moves south. Advisory level amounts are possible if not likely
through early Thursday in the mountains.

Despite model differences on the intensity and track of the
winter storm, by Thursday afternoon they all clear out our area
with strong and dry northwest flow on the backside of the trough.
The bulk of the impacts should be improving by Thursday evening`s
commute across the plains and the mountain areas.

Friday and Saturday will be dry across our area as the trough
pulls further east and a ridge builds in from the west. Northwest
flow aloft around the ridge will continue into Sunday as a trough
approaches from the northwest sometime during the day on Sunday.
Temperatures Friday and Saturday will see a significant warming
trend from the 60s on Friday to perhaps as warm as the low 70s on
Saturday with west to even WSW flow at 700 mb bringing much warmer
air into Colorado and with downslope east of the Divide. The GFS
is more bullish on the warming with > 10 degC at 700 mb. NAEFS and
ECMWF standardized anomalies show 1-2 standard deviations for 700
mb temperature, lending support for a really warm day Saturday.
Even the mountains will be in the 50s up to around 9 kft. Model
agreement in the deterministic models and their ensembles are low
regarding the timing and evolution for Sunday`s trough, but it
looks to be a lot colder and chances of snow increase across our
area, including the I-25 urban corridor. Highs will probably only
be in the 40s for the plains with the passage of a strong cold
front late Saturday/early Sunday. Ensemble output does not point
to higher than normal moisture associated with the trough at this
time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 912 PM MDT Mon Oct 21 2019

Under VFR conditions, light winds will follow the typical
overnight drainage flow pattern in the vicinity of the Denver
metro terminals. By Tuesday afternoon, a weak surface lee trough
will develop across southeast and eastern Colorado. This, along
with strong northwesterly flow aloft, will allow for a west to
northwesterly component to the surface winds for Tuesday afternoon
in a dry VFR environment.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

UPDATE...Fredin
SHORT TERM...Kriederman
LONG TERM...Schlatter
AVIATION...Fredin