National Weather Service Forecast Discussion

000
FXUS65 KBOU 192107
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
307 PM MDT Thu May 19 2022

.SHORT TERM...(This evening through Friday)
Issued at 241 PM MDT Thu May 19 2022

Long wave trough over the Pacific Northwest and Northern Rockies
will dig south and be over the Great Basin and Central Rockies by
Friday morning. A short wave trough embedded in the long wave trough
near the Washington coast digs southeast into Utah by Friday. In
addition to lift from the trough, a strong jet and frontogenesis
will provide large scale ascent. At the surface a strong cold front
pushes south across northeast Colorado this evening. Expect gusts to
45 mph to accompany the front. Then winds decrease, but remain a
northeast upslope direction for tonight and Friday.

The 12Z soundings across the Pacific Northwest and Northern Rockies
showed the cold air in place and 700mb temperatures a little (~1C)
colder then the models show. This cold air will track south with the
trough move into the area late tonight and Friday morning. By 18Z,
Friday 700mb temperatures fall to -5C to -7C. Cold enough for snow
with moderate to heavy precipitation down to 5000 to 6000 feet MSL.
All snow is expected for the foothills and mountains Friday due to
this cold air. As the airmass continues to slowly cool, mostly snow
is expected by late Friday afternoon across the Urban Corridor.

For the Denver area, main point is at least a half inch of
precipitation will fall as snow and more likely around an inch. The
snow will accumulate on tree branches, causing broken branches and
power outages if they happen to fall on/break power lines. Don`t
park under trees and avoid walking under trees. Travel is expected
to become difficult in the mountains and foothills Friday
afternoon. Winter Storm Watches will be upgraded to Winter Storm
Warnings. Even if amounts don`t reach criteria, the impacts of the
heavy snow warrant the Warning. To the east of the Warning, plan
to have a Winter Weather Advisory.


.LONG TERM...(Friday night through Thursday)
Issued at 241 PM MDT Thu May 19 2022

A high impact winter storm will be ongoing at the start of the
long term forecast period, and this will be the timeframe in which
the lower elevations sees the majority of its snow. While there
is high confidence in the forecast for the Foothills and the rest
of the high country, considerable uncertainty remains for the
Denver metro. Let`s dive in.

As discussed in the short term discussion, snow will be ongoing
across much of the high country as a potent storm system slowly
moves across the forecast area. Aloft, northeast Colorado will be
in the right entrance region of a 130-140kt jet streak, with a
broad 500mb trough over the western United States. Embedded in
this larger-scale trough, a robust shortwave is progged to swing
east from southern Utah into Colorado overnight. Closer to the
surface, upslope flow will continue to establish during the
evening hours, with cold mid-level air slowly filtering in... with
700mb temperatures near -9 to -10C by midnight. The combination of
the aforementioned synoptic scale ascent and deepening upslope
flow/mid-level frontogenesis will set the stage for heavy snow
for the high country and I-25 corridor Friday night.

For the Foothills/High country...
Heavy snow will continue in the evening and overnight hours, with
banded snowfall likely. The heaviest snow will slowly shift south
with time as the mid-level front sags south towards the Palmer
Divide. Within the heavier bands, snowfall rates of 1-2 inches per
hour looks likely. BUFKIT soundings show the dendritic growth
zone deepening to about 10kft, which will be collocated with the
maximum lift, resulting in efficient snow growth. Expect travel
conditions to quickly detieorate as the sun sets, with
considerable impacts to fully-leafed trees likely. HREF
probability matched mean QPF from 6pm to midnight shows as much as
an inch of liquid over the southern Foothills/Palmer Divide...
which loosely translates to "a lot" of snow falling in a short
window. PMM Snowfall also indicates the potential for 12 to 27
inches across the Foothills and 10 to 20 inches for the higher
portions of the Palmer Divide. Slightly lower totals are expected
on the west side of the divide, but low- end warning criteria
should be met.

For the Urban corridor and portions of the plains...
The looming question in this forecast package is the transition
time from rain to snow across I-25 and the Denver metro, with 12z
guidance offering a variety of options, but little agreement. A
few CAMS (namely the WRF-ARW and HRRR) are much warmer with
boundary layer temperatures through roughly 03z, while others
(NAM) are much cooler. We favor the colder side of guidance and
argue that mostly snow will be falling at 00z/6pm with 700mb
temperatures already cooler than -7C by this time. The other key
question going into tomorrow is how quickly/efficiently snow
manages to accumulate in the lower elevations. Guidance in general
has slowly trended towards cooler overnight temperatures and more
QPF in the overnight hours. Despite marginal surface
temperatures, I`m slowly gaining confidence in a snowier solution
for Denver. Even if most of this snow melts on hard surfaces, the
4-8 inches of wet snow on our foliage would cause tree damage and
some power outages. Locations closer to the Foothills/Palmer
divide will see the highest totals, with a sharp gradient in snow
totals likely across the metro.

By Saturday morning, our moisture will slowly be shifting out of
the forecast area into southeastern Colorado, with snow ending
from north to south through the late morning/early afternoon
hours. Guidance has generally favored a slightly slower end time
to the snow, and light accumulations are likely through Saturday.
Temperatures will also struggle to make it out of the upper 30s or
low 40s, with colder temperatures in the higher elevations.
Overnight lows will be near to well below freezing across the
area.

The pattern stays relatively cool and unsettled into early next
week, with another (weaker) shortwave bringing additional moisture
by late Sunday. This moisture looks to stick around through at
least Tuesday, though forecast guidance varies on the speed of
this system. After this trough departs, warmer and drier weather
will return as ridge begins to build over the west.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1133 AM MDT Thu May 19 2022

Gusty west winds still on track to develop between 19-20Z with
gusts to 35 knots possible. These winds continue until sunset and
may briefly decrease before a strong cold front pushes through.
This front is expected around 0230-0300Z with northeast winds up
to 40 knots. Winds quickly decrease behind the front, but remain
breezy with gusts to 25 knots possible overnight. As the storm
system nears, ceilings fall below 6000 feet 0800-1000Z. Rain
showers form mid to late morning. Snow will be possible 18z Friday
and likely after 21Z. Moderate to heavy snow is then expected for
DEN Friday night and early Saturday with 4-8 inches of heavy
slushy snow expected.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 8 PM MDT this evening for COZ214-216-
238>251.

Winter Storm Warning from midnight tonight to noon MDT Saturday
for COZ030-032-033-035.

Winter Storm Warning from noon Friday to noon MDT Saturday for
COZ034-036>041.

Winter Weather Advisory from midnight tonight to noon MDT
Saturday for COZ031.

Winter Weather Advisory from 6 PM Friday to noon MDT Sunday for
COZ045-046.

&&

$$

SHORT TERM...Meier
LONG TERM...Hiris
AVIATION...Meier

NWS BOU Office Area Forecast Discussion