764
FXUS65 KBOU 211104
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
404 AM MST Sat Dec 21 2024

.KEY MESSAGES...

- Warm and dry today and Sunday.

- Periods of light snow in the mountains beginning Monday.

- Slight chance of rain or snow showers at lower elevations Monday
  night and again Christmas afternoon and evening.

&&

.SHORT TERM /Through Tonight/...
Issued at 404 AM MST Sat Dec 21 2024

Quiet and dry weather continues on this the shortest day of the
year. Though the sun angle at solar noon today is as low in the
sky as any day of the year (27 degrees above the southern
horizon), and thus incoming solar radiation is comparatively weak,
it will nevertheless be a very warm day across the area with a
500/700 mb ridge building in from the west. Temperatures shouldn`t
get as warm today as they were on Friday given the development of
a lee trough/Denver Cyclone, albeit a weak one. The
lee trough/Denver Cyclone will result in south to southeast winds
across the plains and north/northeast winds along and west of the
I-25 corridor, and thus no downslope-added warming. Expect highs
in the upper 50s to low 60s. A few spots could reach 65 near
Denver. The mountain valleys will warm nicely too other than
Middle Park, were temperatures will struggle to reach the mid 30s
with light winds and robust inversions to break. Elsewhere across
North and South Parks, and the upper Blue River Valley (Summit
County) temperatures should reach the mid to upper 40s.

Tonight the WNW flow aloft increases a bit resulting in strong
winds across the Continental Divide and extending down across the
East Slope foothills. Lapse rates are not favorable for mountain
wave enhancement, and QG forcing is neutral, meaning the wind
gusts 35-50 mph should remain confined to elevations above 7,000
ft late tonight into early Sunday. Wave clouds and mixing due to
the downslope winds at or just off the deck will keep
temperatures warm across the east slopes and I-25 corridor, with
lows in the 30s to low 40s. Across the plains and river valleys
lows should reach the 20s. Once again the coldest spots will be
across Middle Park with lows near 0 degF, and teens across the
rest of the mountain valleys.

&&

.LONG TERM /Sunday through Friday/...
Issued at 404 AM MST Sat Dec 21 2024

A series of weak weather systems will move through this week, but
we`ll still be in mild and mostly dry air most of the time.
There`s not much temperature gradient around, and therefore not
much wind either.

On Sunday, we`ll get clouds and a little surge of slightly cooler
air. Temperatures may be as warm as Saturday through midday, but
we do expect a little more cooling late in the day.

Shallow moisture will advect into the mountains Monday, possibly
starting some light orographic snow. Then a jet streak dropping
down the back side of the trough over the plains will bring a
ribbon of QG lift over the mountains Monday night and exiting
across the plains around sunrise Tuesday. The models have been
hesitant to produce much precipitation with this as the lower
level winds are weak, but there`s better forcing and steep lapse
rates in mid levels. We continue to think this feature is a little
underplayed in the guidance and bumped up PoPs a little bit for
Monday night. Whatever happens should be fairly light though, and
if there`s anything on the plains it will barely be cold enough
for snow and could be a mixture.

On Tuesday, there`s sharp warming/subsidence aloft and whatever
cooling there was from Monday night will be offset by the
sunshine and a little warm advection.

The strongest wave of the week will be on Christmas Day. Models
continue to have better agreement about this being south of us,
moving near the southern Colorado border during the day into
Wednesday night. They have also trended a little stronger/slower
in this set of runs. Of course, it`s a favorable track, but
there`s still not much moisture to work with and the winds with
this are pretty light. Our guidance gives a slight chance of rain
or snow on the plains south of Denver. There could be some upside
with this system if it`s a little further north, or stronger, or
slower, so we`ve bumped up the PoPs a little bit, but again
amounts will be light and it may be too warm for snow below 6000
feet. Light snow could have some impacts in the mountains late
Wednesday, but the chance of much impact at lower elevations looks
pretty low.

There`s quick ridging behind that system for Thursday, though this
could be a little slower. Another trough Friday but again it looks
like just a light mountain snow and there`s still not really any
cold air.

&&

.AVIATION /12Z TAFS through 12Z Sunday/...
Issued at 404 AM MST Sat Dec 21 2024

VFR through Sunday. Winds have gone SE at DEN as expected, and
they should continue SE to S into the afternoon. This afternoon when
a Denver Cyclone forms near DEN, the winds may go east for a few
hours, but should be under 10 kts. By 01 or 02Z winds at DEN
should be back to drainage out of the SSW around 10 kts, and
continue that way all morning and even into much of Sunday as the
pressure gradient favors S to SSW winds. APA will have a similar
evolution of the wind but will be less likely to change too much
in the afternoon hours, maybe going light and variable after 20Z
before coming back to S after 02Z. BJC should go south after 13Z
this morning, then east after 20Z due to the Denver Cyclone.
Overnight drainage winds out of the WSW are expected. All speeds
should remain under 10 kts at BJC, though there is a chance (20%)
of seeing west winds gust 25-30 kts between 06-14Z Sunday.

&&

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

&&

$$

SHORT TERM...Schlatter
LONG TERM...Gimmestad
AVIATION...Schlatter

NWS BOU Office Area Forecast Discussion