598
FXUS65 KBOU 200219
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
719 PM MST Sun Jan 19 2025

.KEY MESSAGES...

- Bitter cold through Tuesday morning.

- For the plains, wind chills of -15F to -25F late tonight
  through Tuesday morning.

- For the mountains and mountain valleys, the coldest conditions
  are expected Monday night and Tuesday morning with temperatures
  and wind chills falling into the -30F to -45F range.

- Another wave of light snow tonight and into Monday morning.

- Temperatures moderating somewhat through the rest of the week.

&&

.UPDATE...
Issued at 705 PM MST Sun Jan 19 2025

Snow continues to slowly develop and drift southward this evening,
with the leading edge fo the snow roughly from FNL-GXY-FMM. We
should see snow spread into the Denver metro over the next few
hours, with the peak snow rates likely coming close to midnight
tonight. The previous QPF/SnowAmt grids still look good, but we
did add a little refinement to the PoPs to capture the timing
offered by most hi-res guidance.

Temperatures remain a question mark tonight. Colder (yes, colder)
air should filter in through the overnight hours, but low
temperatures will be limited somewhat by widespread cloud cover.
Have a feeling this will keep temperatures from completely
bottoming out over the mountain valleys, but overnight lows should
still end up well below zero. Did make a few adjustments across
the area, but not anything significant.

&&

.SHORT TERM /Through Monday/...
Issued at 336 PM MST Sun Jan 19 2025

Snow has spread into the mountains over the past couple of
hours. Initially it`s light, driven by warm advection, but then
the instability showers will come behind it. We`ve sped up the
timing of the onset by an hour or two this evening, for Denver
probably between 7 and 10 PM. The best chance for a period of more
substantial snow should be in the late evening to early morning
hours as the more unstable air behind the shortwave arrives. By
morning, the QG lift will be departing, but there will still be
some instability driven by the very cold air aloft. Forcing,
moisture amount and depth are all fairly weak, but will occur in
favorable temperatures for dendritic growth this evening. Later as
we get colder, the instability will help with that but then it
will get cool and we`ll likely switch over to colder snowflake
types. With expected QPF around a tenth of an inch, this should
render a fairly widespread 1 to 2 inch snow with 3 inches in a few
places, most likely in the upslope areas west of Denver and
mountain ridges.

Monday will see decreasing showers in an unstable boundary layer
whose depth will be shrinking as warm advection aloft sets in by
afternoon. It may wind up much like Saturday, with fairly
widespread light showers but not much accumulation and gradually
improving roads.

Forecast temperatures for Monday are tricky with the timing of
clearing in doubt, but there`s fairly good consistency in the
models and it looks like we should be triggering that shallow
convection at around 5 degrees, so staying below 10 is looking
likely. We`ve left the forecast temperatures alone. There will
likely be a puff of wind (probably 10-20 mph) behind the front
overnight, then lighter winds on Monday, so the wind chills won`t
be a lot lower than the temperatures. The current advisory seems
to cover things well.

&&

.LONG TERM /Monday night through Sunday/...
Issued at 245 PM MST Sun Jan 19 2025

Strong northerly flow aloft is progged over the CWA Monday night
into Tuesday morning, with short wave upper ridging to be in place
much of Tuesday along with strong northwesterly flow aloft. By
late Tuesday night into Wednesday evening, upper troughing is
back over the CWA with a very strong north-northwesterly jet
maximum across central Colorado, north-to-south by 12Z Thursday
morning. The QG Omega fields have downward synoptic scale energy
for the forecast area Monday night and Tuesday. Weak upward motion
is progged Tuesday night through Wednesday night. For the lower
levels, models show lee troughing Monday night through Tuesday
night, then a weak pressure gradient is in place through
Wednesday night. The low level winds are dominated by downsloping
through the periods, with some weak drainage possible at night.
Cross sections do show a mountain wave set-up Tuesday, but winds
right along and just east of the divide should stay below high
wind criteria.

For moisture, there is some in the mid and upper levels from time to
time through the five periods.  There is some low level alpine
moisture progged Tuesday overnight into Wednesday. There should be
some orographic enhancement then too, so with go with 20-40% pops.
No pops on the plains.

For temperatures, thickness fields show decent warm air advection
kicking in Monday night into Tuesday.  Not too sure if it will be
enough to overcome the arctic air in place and most likely not in the
high mountains and high mountain valleys. Will hit the lows
pretty hard still for Monday overnight/Tuesday morning, with
the coldest spots in the Parks perhaps getting below -30 F. Lows
out on the plains could drop into the minus teens F in many
spots. Tuesday`s highs will be in the 30s and lower 40s over the
plains, significantly warmer than Monday`s readings. Wednesday`s
readings cool back down with more upper troughing.

For the later days, Thursday through Sunday, upper troughing and
strong north and northwesterly flow aloft will dominate through
Friday.  Zonal flow is progged Saturday, then more upper troughing on
Sunday. Moisture increases Saturday and Sunday, along with colder
air and upslope flow for the plains. Pops will be sparse Thursday
and Friday, then they will increase for the weekend. The only
"warm" day will be Friday, where readings could be near normal
seasonal values. It may actually be in the mid to upper 40s for
the plains that day.

&&

.AVIATION /00Z TAFS through 00Z Tuesday/...
Issued at 442 PM MST Sun Jan 19 2025

VFR expected in the near term, but that won`t last for more than
another couple of hours before we once again drop down to MVFR/IFR
conditions tonight through most of Monday. Upstream
radar/satellite show lower cigs/visby with some light snow closer
to the Wyoming border. We should see snow increase in coverage
after 03z with some potential for LIFR visby overnight at BJC.
Snow accumulation will generally be light with fairly consistent
north/northeast winds. Light snow is expected to stick around
through the first half of Monday, with continued MVFR cigs and
visby. A gradual clearing trend is expected into Monday evening
with VFR likely returning as we approach 00z.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Extreme Cold Watch from Monday evening through Tuesday morning
for COZ030>034-037.

Cold Weather Advisory until 9 AM MST Tuesday for COZ038>051.

&&

$$

UPDATE...Hiris
SHORT TERM...Gimmestad
LONG TERM....RJK
AVIATION...Hiris

NWS BOU Office Area Forecast Discussion