574
FXUS65 KBOU 300212
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
712 PM MST Wed Jan 29 2025

.KEY MESSAGES...

- Snow develops across the plains tonight, spreads into I-70
  corridor east of Denver and over the Palmer Divide by early
  morning.

- Snow will lead to a slick morning commute for areas south and
  east of Denver.

- Warmer but breezy Friday through the weekend. Lots of
  uncertainty with the magnitude of a backdoor cold front Sunday
  night into early next week.

&&

.UPDATE...
Issued at 712 PM MST Wed Jan 29 2025

The 18Z model runs as well as the 00Z NAM and HRRR seem to have
finally closed in on a solution, keeping the heavier band of snow
associated with the TROWAL east of the Denver area. General
consensus is this band ends up forming over Elbert, eastern
Adam/Arapahoe, and Washington Counties. Lighter snowfall is still
expected east of this band. To the west, most of the models show
light snowfall (less than 2 inches) and some models now showing no
snowfall for Denver. The best chance for snow in the Denver area
will be late tonight into Thursday morning (3AM to 10AM) before
the band of heavy snow and gusty north winds form. Dry is over the
area and to the north over Wyoming. Once the northerly winds
develop, snow is expected to end along and west of the I-25
corridor as dry air is transported into the area.

As far as changes to the forecast goes, increased PoPs and snowfall
amounts to the east of the Denver area, across Elbert, eastern
Adam/Arapahoe, and Washington Counties. If heading east on I-70 out
of Denver, expect to encounter snow covered roads and hazardous
travel through early to mid afternoon. We also lowered PoPs and
snowfall amounts west of I-25, all the way west into the mountains
and foothills. Conditions for freezing rain/drizzle still exist over
the northeast plains, but the 18Z models trended away from freezing
rain/drizzle. Though wouldn`t be surprised to still see a little, so
left the mention of freezing rain in the forecast, but decreased the
chances and ice amounts in the forecast.

&&

.SHORT TERM /Through Thursday/...
Issued at 231 PM MST Wed Jan 29 2025

There are still a few questions left unanswered as they relate to
our upcoming storm system, but there`s one aspect where confidence
is high: Whether in the snow-lover camp or not, many folks will be
sorely disappointed, and many will be quite pleased once the snow
wraps up. Such is the beauty of a banded snow event!

Let`s first take a few steps back though. Sun abounds this afternoon
as we await the approaching system, with mild southerly flow in
place along the leading edge of the closed low, which is currently
sitting over north-central Arizona. In our southeast plains, clear
skies are rapidly giving way to an expanding stratus deck as this
southerly flow advects a tongue of healthy Gulf moisture into the
region. By this evening, cross-sections indicate deepening mid-level
moisture that would support gradual development of showers in the
eastern plains, especially Lincoln/Washington Counties. Marginal
surface temperatures could support a brief rain shower or wintry mix
during the onset of precipitation, but a changeover to snow
shouldn`t be far behind for *most* areas (caveat to come shortly).

As the front occludes overnight, a pronounced NE-SW TROWAL will
develop through the early morning hours from the northern plains
into the Palmer Divide. One effect of the warmer air aloft will be
to sustain a warm nose near 800mb across the northeast plains, which
opens the door to a period of freezing rain or a wintry mix for
those areas as surface temperatures drop below freezing. Have thus
introduced this into the forecast.

The TROWAL will also enhance low-level frontogenesis, promoting a
transition to more of a banded snow event by Thursday morning, which
will carry potential for a period of moderate to heavy snow with
rates near 1"/hr for several hours to the southeast of Denver.
Models have a loose idea - rather than a firm hold - on the
placement of this band of heavier snow, but the area of greatest
confidence in heavier snow Thursday morning extends from the Palmer
Divide northeastward (where northerly upslope flow will be
maximized), and across the I-70 corridor between Bennett and Limon.
With high confidence in impacts for the morning commute, a Winter
Weather Advisory was issued for these areas. Adjacent plains zones
(e.g. Morgan, Washington, southern Lincoln Counties) were included
to capture spatial uncertainty in the banding. Additionally, hi-
resolution guidance remains rather steadfast in amplifying QPF
across our southern foothills. However, still believe this is
overdone in some of the recent runs, considering less-than-favorable
north/northwest winds. This will also keep areas to the north of
Denver largely dry.

So with all of that said, there`s reasonable confidence in little to
no impacts for the mountains, northern foothills, and northern I-25
corridor extending from the NW Denver suburbs to the WY border. The
axis of greatest uncertainty will be focused across the SW Denver
suburbs towards DIA and the northeast plains, where a steep snowfall
gradient can be expected. The most favored locations under the
heavier snow band may well receive 5-10" when all is said and done
in/near the Palmer Divide, but such amounts won`t be overly
widespread. Snow will ramp down quickly by midday as breezy
northerly winds develop across the lower elevations, with
temperatures likely climbing into the 40`s for snow-free areas of
the plains and urban corridor.

&&

.LONG TERM /Thursday night through Wednesday/...
Issued at 231 PM MST Wed Jan 29 2025

Dry conditions are expected across the forecast area Thursday night
as the 500MB upper low moves be over the Central plains states and a
dry northerly flow sets up over Colorado. On Friday, dry and warmer
conditions are expected over the CWA as an upper level ridge of high
pressure builds over the state. Colorado will be under the influence
of a zonal flow aloft this weekend with some downsloping flow east
of the mountains. This pattern will result in mild and dry
conditions across the plains with high temperatures ranging from the
mid 50s to lower 60s. There may be enough mid level moisture caught
up in the flow to produce a 20-50% chance of light snow across the
high country over the weekend. In addition, the pattern will produce
breezy to windy conditions across the high country Friday night
through the weekend with wind gusts to 65 mph possible across the
Front Range Mountains and foothills.

For the Monday through Wednesday period, the zonal flow aloft
continues with some weak upper level ridging over the region. The
high country should see a continued 20-50% chance for light snow due
to some mid level moisture combined with orographic lift. There is
quite a bit of forecast uncertainty east of the mountains due to
model differences. The GFS Deterministic and GEFS Ensembles are
suggesting a cold front moving across the eastern plains Monday
morning with a shallow cold air mass settling in behind it. This
scenario would result in much colder temperatures on the plains with
daytime readings only climbing into the 30s along with the
possibility for some very light snowfall. On the other hand, most of
the other models keep the front north of our CWA with warm and dry
weather continuing. An example of this would be the ECMWF and GEM
deterministic solutions which are showing Denver`s max temperatures
climbing into the lower 70s both Monday and Tuesday afternoon.
However, it would most likely be cooler than this since these
solutions are outliers. For now have decided to go with the
forecast model blend which has high temperatures in the mid 40s to
mid 50s on the plains.

&&

.AVIATION /00Z TAFS through 00Z Friday/...
Issued at 438 PM MST Wed Jan 29 2025

Clouds will continue to creep north bringing ceilings of 6000 to
8000 feet after 00Z. Ceilings are expected to fall below 6000 feet
03Z to 06Z and continue to lower, falling below 3000 feet towards
12Z Thursday. Growing confidence the heavier snow stays just east
and southeast of DEN. Though light snow is still expected, mainly
in the 10-17Z window. Winds to stay light the rest of the
evening, and then pick up out of the north around 12Z with gusts
to 25 knots possible much of the day Thursday. The snow and
clouds shift east of the area after 18Z with VFR conditions
expected by 21Z.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 AM to 2 PM MST Thursday for
COZ041-044-045-049.

Winter Weather Advisory from 11 PM this evening to noon MST
Thursday for COZ046-047.

&&

$$

UPDATE...Meier
SHORT TERM...Rodriguez
LONG TERM...JK
AVIATION...Meier

NWS BOU Office Area Forecast Discussion