Winter Weather Advisory 21, NWS Denver/Boulder, CO

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1020 AM MDT Wed Apr 15 2020

.UPDATE...
Issued at 1012 AM MDT Wed Apr 15 2020

Steady as she goes, kind of. Colorado remains under a moderate
northwest flow aloft while a 90kt jet streak has nosed into
northwest Colorado this morning. This has created some dynamical
lift with snow increasing over far northern mountains and up along
the Continental Divide. Looking at web cameras the only real
impact appears to be on Rabbit Ears Pass with icy/snowpacked
roads. QG lift will continue to increase this afternoon and
evening ahead of the upper trof. This coupled with steeping lapse
rates will lead to favorable snowfall production. Will push up the
start of the winter storm warning in zone 31 and 33 to mid
afternoon more in line with above factors of lift/instability.

Main cold front has pushed into northern Wyoming with increasing
northerly winds and snow behind the front. Front should push
southward over the plains late afternoon/early evening with
increasing northeast winds behind it. Both HRRR/RAP develop quite a
bit of convective showers this evening behind the front which may be
a bit overdone. Some of this may start as rain showers before
secondary surge turns everything to snow later tonight.  For now
would change the current watch to advisories across the plains
tonight and advisories for Boulder/Larimer counties for now. Main
impact for roads would be tonight while daytime Thursday impacts
would be less. May be able to drop plains advisories Thursday
morning.  Still quite a few unknowns and details with qpf as
models continue to differ and will try and iron those out for the
afternoon package.

&&

.SHORT TERM...(Today and tonight)
Issued at 434 AM MDT Wed Apr 15 2020

Quite a bit of uncertainty over the details of the upcoming
weather system. The models are still struggling to resolve the
structure of the trough which results in differing placement of
the heavy snow band associated with the lead speed max in the jet
stream as well as the intensity and the amount of lift outside of
the main snow band. The models had been trending toward more
digging of the back of the trough, but now seem to be converging
on an unusual compromise with the strongest lift associated with
the lead speed max tonight and less separation between that
feature and a second jet streak on the back side of the trough.
This results in a heavy snow band tonight, then the trough just
stalls in place rather than either dropping south or having a
solid second round of lift along the same axis. There's not
convincing evidence in observational data yet that the first part
is so strong, though the models have fair agreement on this.

The main question for the first part, tonight into early Thursday,
is the position of the jet-induced snow band and its intensity.
The NAM is the extreme solution, focusing this over southeast
Wyoming into Nebraska and never sagging it south, resulting in
almost no snow around Denver until later Thursday. The current GFS
and EC have similar positions focused more on the Colorado-Wyoming
border area with some north to south motion as this feature
passes. We're accepting this as a reasonable solution, with some
hesitation about the exact details. The GFS has more precipitation
than the EC, and may be right as the environment should provide a
good response to even modest forcing with a deep neutral lapse
rate. Another question is how much upslope component there will be
into the foothills. This relates to the back part of the trough as
the strung out, stalling east-west trough axis either won't have
much upslope or will keep the deep upslope north of Colorado.
Either way, the upslope will be secondary to the jet streak
through tonight with limited shallow upslope possible later
tonight. Meanwhile, the high mountains should see a decent
response to the orographic forcing from west winds at mountaintop
level, plus the jet related lift wherever that is. Most likely
heavy snow will be north of Berthoud Pass, though this may sag to
the I-70 corridor by morning.

The current highlights will stand for now, as there's uncertainty
over both the snow band location and the changeover to snow this
evening. Northern border areas with the watch do look most likely
to get heavy snow, though it can't be ruled out in and near the
southern foothills eventually. Northern areas may start before
midnight too if it cools quickly enough, while southern areas
should be later. Snow amounts are just nudged a bit upward in the
most likely areas. WPC amounts are similar to an EC/GFS combo and
are accepted.

.LONG TERM...(Thursday through Tuesday)
Issued at 434 AM MDT Wed Apr 15 2020

Uncertainty regarding this system will continue into Thursday. The
models generally keep the strongest QG ascent over Wyoming where
the mid level trough stalls out. The overall QG ascent appears to
weaken through the day, but low level upslope during the day in
addition to a 90+ kt jet max stretched across northern CO may help
enhance the snowfall. Model spatial cross-sections show limited
Equivalent Potential Vorticity (EPV) which would support the
enhanced bands of snowfall related to the upper level jet. The
bottom line is that the models are having a hard time phasing
everything together with this system. The GFS20 shows a more
favorable northeast upslope component in the 850-700mb layer vs
the NAM12 and ECMWF solutions. Consequently, the GFS20 shows the
highest storm totals near the foothills vs the other models.
Highest confidence at this time is in the mountains and foothills
north of I-70. Across the I-25 corridor and northeast plains,
lower confidence as the GFS and NAM12 are complete opposites.
Storm Total grid a blend of the GFS20/ECMWF solutions and this
still look like the most reasonable option at this time. No
additional winter highlights planned with some much uncertainty,
but will still need to keep a close eye on zones 36, 39 and 41
and see how things shape up. The focus will shift more to the
south and east by late Thursday night into Friday as the trough
exits the region and the snowfall tapers off. Over the weekend it
will be warmer but still unsettled. The models still show a weak
disturbance moving out the Great Basin and across CO. Not much
impact with this system, with sct showers in the mountains and
more isolated coverage across the eastern plains. For early next
week, with warming trend will continue with an upper level ridge
over western CO on Monday. There could be another system possibly
impacting CO on Tuesday but not much agreement in the models with
this feature at this time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1012 AM MDT Wed Apr 15 2020

Surface winds will be shifting west and northwest and becoming
gusty this afternoon. Initial front will be into DEN around 00z
with showers developing behind it, and could initially be rain
showers. Stronger surge develops later tonight with more
widespread snow developing with IFR conditions.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 10 PM this evening to 6 PM MDT
Thursday for COZ038-039-042-043-048-050.

Winter Storm Warning from 9 PM this evening to 6 PM MDT Thursday
for COZ035.

Winter Weather Advisory from 6 AM Thursday to midnight MDT
Thursday night for COZ034.

Winter Storm Warning from 3 PM this afternoon to midnight MDT
Thursday night for COZ031-033.

&&

UPDATE...Entrekin
SHORT TERM...Gimmestad
LONG TERM...Cooper
AVIATION...Entrekin

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