968
FXUS65 KBOU 180014
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
514 PM MST Fri Jan 17 2025

.KEY MESSAGES...

- Flash freeze potential and threat of icy/hazardous travel
  conditions is increasing for the pm commute. Snow will arrive
  for most of the I-25 Corridor between 4 and 6pm, and then
  continue into Saturday morning.

- Snowfall amounts for the I-25 corridor will range from 3 to 7
  inches, with up to 10 inches on the east slopes of the Front
  Range. The eastern plains will have an inch or two of snow, but
  that could still produce some travel impacts.

- Much colder tonight and Saturday.

- Bitter cold late Saturday through Tuesday morning. Lower
  elevations will have an extended period of temperatures in the
  single digits above or below zero. Wind chills on the plains may
  be in the minus 20s. The coldest period will be from Monday
  morning through Tuesday morning.

- Another wave of light snow is possible Sunday night into
  Monday.

&&

.SHORT TERM /Through Saturday/...
Issued at 338 PM MST Fri Jan 17 2025

Surface analysis shows the first cold front that blasted through
northeast Colorado late this morning through the noon hour has
pushed into southeast Colorado. Snow had developed behind the
front, but through early afternoon it had mostly stayed along and
north of the Wyoming border. However, that will be changing very
soon as we now have deepening upslope and increasing forcing in
the form of mid level frontogenesis and QG ascent. Thus, clouds
are quickly developing along the Front Range, and snow will be
able to spread south and fill in rather efficiently across the
foothills, I-25 Corridor, and nearby adjacent plains through 5-6
pm.

At the same time, road temperatures were quite warm (still near
50F along the I-25 Corridor as of early this afternoon). Those
will be dropping as air temperatures cool and cloud thicken, but
likely be above freezing with onset of snow. The initial snow
band also appears rather impressive near the Wyoming border, so
with the favorable synoptic and mesoscale forcing, there is
growing concern initial snowfall will be heavy enough to support a
flash freeze A flash freeze is where road temps are warm enough
to initially melt snow on contact with roads, and then freeze
after colder air arrives. Note: it`s already down to 22F in
Cheyenne, so with the onset of snow, both air and road
temperatures are expected to drop rather quickly. This "flash
freeze" could make road conditions very icy or even treacherous in
some spots for the latter portions of the PM commute, lasting
into this evening.

With regard to snow amounts, not much has changed. QPF forecasts
have remained quite consistent with near 0.25" of liquid
precipitation near downtown Denver, but upwards of 0.50-0.60" in
some of the foothills, with Boulder County under the gun for the
heaviest. On top of that, snow ratios will be quite high, with
deep dendritic growth profiles in a shallow upslope/isothermal
regime. It appears we`ll have DGZ depth near 5,000 feet through
about 03Z, then gradually decreasing to 3,000 feet late this
evening. Still a decent number, and with temperatures dropping
into the teens we think the dendrites will still be flying in/near
the foothills due to shallow forcing and the DGZ in that exact
shallow upslope layer. Thus, snow ratios or SLR will likely end up
close to 20:1 or more. Intensity is expected to gradually
diminish late tonight/Saturday morning as the upslope component
weakens, but still some light snow/flurries lingering into
Saturday afternoon - especially in/near the Front Range. Most of
the accumulation will occur tonight into early Saturday morning,
with totals still expected to be 3-6" from Fort Collins south to
Denver, and locally 10-12" in/near the Boulder County Foothills.

Finally, temperatures will be much colder. Upstream observations
show teens advecting through eastern Montana today, and that`s the
airmass that will infiltrate our forecast area. Therefore, with
fresh snow cover we think overnight lows will likely drop to
single digits, and then only "warm" into the mid teens for Saturday.

&&

.LONG TERM /Saturday night through Friday/...
Issued at 338 PM MST Fri Jan 17 2025

Not a lot of change in the longer term forecast today. The GFS and
its ensemble members remain warmer and still look too warm. We
lowered temperatures a few degrees below the NBM guidance
throughout because of this, and then for snow cover and stagnant
air under inversions once we "warm up" from Tuesday onward.

Some trends...

It is looking like there`s a good break between systems Saturday
night into Sunday. There may be some lingering flurries Saturday
evening due to the radiational cooling offsetting any drying and a
little lingering upslope, but we think this will all end with
at least partial clearing. That could allow temperatures to drop
more than we have forecast for Sunday morning if the clearing is
more/earlier than we`ve been thinking. Since we`re already in the
lower half of guidance, we didn`t trim this further yet. On the
flip side, there should be some sun over the plains on Sunday for
temperatures recovering into the teens. Mid and high level clouds
will move over the mountains by afternoon limiting the recovery
there. Lower level moisture and light snow associated with the
next shortwave looks to come over the mountains mainly during the
night and the plains late Sunday night into Monday morning. It`s
not a lot, but it shouldn`t take much to generate a little
precipitation and could be enough to freshen up the snow/ice on
the roads in places. There will be another gentle push of colder
air into the plains, along with more cooling aloft, so Monday will
likely be the coldest day. There could be some clearing that could
provide some sun and get us up around 10 degrees in the
afternoon, if not we might be closer to zero during the day.

Monday night is interesting as we`ll have competing interests. It
will start with the coldest low level air, skies should be
clearing, and light winds. But as the night goes on there`s
warming off the ground and some southwest/drainage winds should
begin in most areas. There`s a good chance of having evening or
midnight lows in the foothills and most of the plains. If the
little wind starts early it won`t be that cold, but if it waits
until the middle of the night we could see a quick drop in the
evening (as low as -15 at DEN for example) before the temperature
recovers. All the low lying normally coldest places (South Platte
valley, mountain parks, etc) should be quite cold through sunrise
Tuesday. Areas that do get some mixing Tuesday will likely reach
near freezing (lower elevations) and upper teens/20s (mountains),
but the cold pools may lag behind more than we`re showing.

For the rest of the week, there will be a bit more wind/mixing but
we`ll still be fighting the cold pools. There`s another shortwave
Wednesday night or Thursday and the models have varying strength
to it but agree there will be a quick shot of colder air again
behind it. This should bring some light snow to the mountains and
a chance to the plains. Highs Thursday will depend on the timing
of that cold air and any snow. Noteworthy that the operational EC
is on the cold end of its ensemble range (around 30 while some are
in the 40s) and the GFS is not much warmer. Another question is
how much wind is actually able to surface with this trough, it
could be warmer in most areas Wednesday ahead of it. Similarly,
Friday`s highs will depend on how good the mixing is behind the
trough.

&&

.AVIATION /00Z TAFS through 00Z Sunday/...
Issued at 451 PM MST Fri Jan 17 2025

We are beginning to see some light snow develop this evening as
expected. Ceilings quickly lowered to MVFR conditions and
visibility is expected to follow shortly at all Denver TAF sites.
KBJC is now reporting visibility of 2.5 miles, which will be
progressing eastward towards KAPA and KDEN in the next hour or so.
We are still expecting the worst of the snow to be between 01-07Z
with lowest visibility dropping to 1/2 to 1SM during a portion of
this time with IFR conditions expected. Some uncertainties remain
with regards to the ceiling heights tomorrow morning. With this
cold air, ceilings typically stay a little higher than what
guidance suggests, therefore we were a little more optimistic with
keeping them higher than what guidance suggested, but still
expect MVFR to return by early tomorrow morning. Light snow and
flurries are expected to remain through tomorrow morning.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST Saturday for COZ031-
033>036-038>041-043.

Cold Weather Advisory from midnight Saturday night to 9 AM MST
Tuesday for COZ038>051.

Winter Weather Advisory until noon MST Saturday for COZ045>047.

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...Gimmestad
AVIATION...Bonner