National Weather Service Forecast Discussion

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FXUS65 KBOU 080324

Area Forecast Discussion
National Weather Service Denver/Boulder CO
824 PM MST Thu Dec 7 2023


- High winds in the Front Range, and elevated fire weather
  conditions on the plains come to end early this evening.

- Light snow showers tonight in the mountains, then more
  widespread snow, colder temperatures, and travel impacts Friday
  and especially Friday night.

- Considerable uncertainty to snow amounts. Higher totals
  favoring the mountains, southern foothills, and Palmer Divide.


Issued at 808 PM MST Thu Dec 7 2023

Let`s be frank...he or she who says they are confident with
regards to the snowfall forecast for the upcoming system is likely
not being entirely honest. Over the past few hours, model guidance
has spit out about as many different solutions as there are items
on a Cheesecake Factory menu, particularly in regards to our
second wave of snow Friday evening.

That being said, timing of potential impacts is the one aspect
where confidence has managed to increase. For Friday morning,
low-level flow will, for the most part, remain westerly. This
drying downslope component is simply not favorable for impactful
snow accumulations for the Denver metro, regardless of most other
factors. This appears well reflected in the latest CAM output,
suggesting minimal to nonexistent snow accumulations for the metro
in the morning. The Palmer Divide may still squeeze out some minor
accumulations, but really the focus for more widespread snow has
shifted to Friday evening and night.

Mid-level lapse rates do look robust, between 7-8.5 C/Km, and
generally collocated with the DGZ. Most guidance depicts heavier
snow bands developing after ~6-7PM Friday *somewhere* in the
plains or southern urban corridor, but the location varies wildly.
Our forecast continues to hedge towards our southern foothills and
Palmer Divide being favored for the higher accumulations (2-7"),
and with the possible contribution of shallow upslope flow in
those locations, we opted to hoist a Winter Weather Advisory for
these areas beginning Friday evening. That said, essentially
nowhere is out of the woods quite yet as far as receiving a
heavier band of snow.


.SHORT TERM...(This evening through Friday)
Issued at 300 PM MST Thu Dec 7 2023

Most of the Front Range and plains were seeing gusty west winds
this afternoon as the airmass became well mixed. There were still
a few pockets south and east of Denver that were protected. The
winds will be decreasing across the plains through early evening
as we decouple and the low level gradient decreases. The mountain
wave is still breaking in the foothills as evidenced by radar,
but the signature has weakened a little this afternoon as we start
to see some snow develop in the mountains. From our observations
over the years, mountain snowfall typically disrupts the mountain
wave, and thus we think the brunt of this event is over. We`ll
keep the High Wind Warning until 5 pm. So unless there`s
significant drying in the mountains behind the afternoon showers
we should be able to let the High Wind Warning expire on time.

On to snow...One shallow wave of moisture and light snow showers as
discussed above is moving through the mountains and will be
mostly over toward dark. Some impacts have been noted in the
Rabbit Ears Pass area this afternoon with snow covered roads/slick
travel, but that will likely improve again this evening as we dry
out again. Then, the next wave as seen in satellite imagery
moving into Nevada. This will arrive late tonight and Friday
morning. The lift will come in a couple notable features, the
right rear entrance region of an upper level jet and rather strong
frontogenesis as noted in the 700-500 mb layer. The latter can be
a "sneaky" hazard here in the lower elevations of northeast
Colorado, so will have to keep a close eye on that and a band of
heavier snowfall possibly developing for the morning commute. It
was interesting to see the mesoscale models/CAMs pointing that to
our south over the Palmer Divide, while the conceptual model and
synoptic scale models would show that farther north. Thus, we`ll
carry high PoPs up to about Denver and at least some light snow
accumulations in the morning. Regarding impacts, as long as snow
rates don`t increase too much we`ll likely be able to melt any
snowfall (outside of bridges and overpasses) due to sufficiently
warm road/pavement temps. The Palmer Divide, on the other hand
would be more prone to travel impacts should a heavier band move
across. We may need an Advisory to account for this (and the
Friday night snow - more on that below), but uncertainty is rather
high now so latter shifts will hopefully be able to narrow this
threat down.

Finally, given the mountains will likely see an uptick in snowfall
later tonight and Friday and impacts to the I-70 mountain
corridor, we`ve opted to expand the Winter Weather Advisory to
include the Front Range Mountains and Summit County as well.

.LONG TERM...(Friday night through Thursday)
Issued at 211 PM MST Thu Dec 7 2023

Snow is expected to increase across The Palmer Divide, Foothills and
adjacent plains Friday evening as the upper trough moves across the
region and another surge sweeps across the plains. At this time,
there is quite a bit of uncertainty with this system due to run to
run inconsistencies, model discrepancies and mesoscale influences.
The biggest uncertainty is how the winds set up behind the frontal
passage which will affect the amount of moisture and upslope flow
across the Front Range Urban Corridor and nearby foothills. Some of
the models show a downslope flow along the foothills into the Front
Range Urban Corridor which would result in very little snowfall.
This scenario would focus the heavier snowfall to the east and
southeast of Denver with little to no accumulation across the Front
Range Urban Corridor. On the other hand, some models such as the
ECMWF Hi-Res, RRFS A and UKMET models are showing deeper upslope
flow which would result in higher snowfall amounts further west
across the foothills and Denver metro. When looking at the SREF
Plumes, the general consensus of the models is that the snow across
the Front Range Urban Corridor and Palmer Divide will be somewhat
light ranging between 1 and 2 inches. However, if deeper upslope
flow develops, there is the potential for heavier snow amounts due
to the presence of quite a bit of instability and QG lift. SREF
Plumes are hinting at this with total amounts as high as 4.5 inches
at DIA and 9 inches in the southern/western suburbs. These amounts
seem somewhat overdone, however with this type of setup we can`t
completely rule it out. We`ve tweaked the forecasts to go middle of
the road for now, and also increased the 90% snow probs to account
for the potential for higher snowfall amounts. The snow is expected
to decrease after midnight with some additional light accumulation
possible over the Southern Foothills and Palmer Divide.

Across the mountains, snow and breezy conditions are expected to
continue into Saturday morning. Therefore, the Winter Weather
Advisory which was just issued by the short forecaster is expected
to continue until daybreak.

As the upper trough pushes east into the Central Plains States,
gusty northwesterly winds up to 45 mph are expected across the
plains making Saturday a cold and blustery day.

Dry and warmer weather is expected Sunday and Monday as upper level
high pressure briefly transitions across the Rocky Mountain Region.
On Sunday, windy conditons are expected across the higher mountains
and foothills due to a strong northwesterly flow aloft and mountain

For Tuesday through Thursday, models are showing another upper level
storm system taking shape across the the Great Basin and Southern
Rockies. At this time there great uncertainty on how this system
will develop as there is no agreement between the models. For now,
this system appears to be of little impact due to a lack of
moisture, however time will tell.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 434 PM MST Thu Dec 7 2023

Gusty west/west-northwest winds will relax starting 00-02Z,
before what we think is a turn to the 110-160 direction overnight.
There`s relatively good agreement here.

Near 13-14Z, a band of snow looks to develop in the vicinity of
the Denver metro and plains. Substantial uncertainty remains with
regards to the location of this band, which would produce a period
of snow through approximately 17-18Z. Based on current trends,
have leaned the forecast towards a more southern track, generally
south of KDEN, and will favor largely visuals prevailing during
the morning period around and north of Denver. That being said,
there`s still a distinct possibility for a brief period of IFR or
MVFR at KDEN during the above timeframe should the band develop
farther north.

Snow showers will taper off the in the afternoon and VFR
conditions are likely to prevail for all terminals. By Friday
evening, another bout of snow is likely shortly after the arrival
of northwesterly winds, with slightly higher potential for snow
across the Denver metro, mainly after 01-02Z Sat. This also is far
from a guarantee, but chances for reduced vis and CIGS below
050-060 look a little higher than with the initial band Friday


Winter Weather Advisory from midnight tonight to 5 AM MST
Saturday for COZ031-033-034.

Winter Weather Advisory from 8 PM Friday to 5 AM MST Saturday
for COZ036-041.



SHORT TERM...Barjenbruch

NWS BOU Office Area Forecast Discussion