Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 130249
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
849 PM MDT Tue Mar 12 2024

.KEY MESSAGES...

- A winter storm will bring significant snowfall to most of the
  Front Range Mountains, Foothills and Palmer Divide. Travel may
  be impossible for these areas Wednesday night through Thursday.

- There is increasing confidence in snowfall across lower
  elevations. The Denver metro and western suburbs will see
  hazardous travel impacts with 10 to 20 inches of snow possible.
  There remains more uncertainty with regards to snow amounts in
  Fort Collins, Greeley, and I-70 east of Denver including Limon.

- Snow will tapper off late Thursday into Friday. A warming and
  drying trend will begin Friday afternoon.

&&

.UPDATE...
Issued at 842 PM MDT Tue Mar 12 2024

Mid and upper level moisture will continue over the area
overnight, however lower levels will remain rather dry,
espcially across the plains.  Meanwhile, QG ascent will
develop overnight as a few disturbances embedded in the WSW
flow aloft move across. Overall, we see periods of snow
showers in the mtns with mainly virga over the plains. Late
tonight, lower levels may saturate enough for some light
showers over the plains.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 400 PM MDT Tue Mar 12 2024

Only minor changes to the forecast through Wednesday, mainly to
add timing details and slow things down a little. The next
shortwave nicely centered over Utah at this hour will produce a
batch of showers over the mountains tonight. Models have been
trending more aggressive about keeping the showers together as
they move east of the mountains around sunrise, so we`ve trended
the forecast in that direction.

There should be a pretty good break behind that shortwave later
Wednesday morning. There are hints in a few models of some low
clouds and rain cooled air from Denver northward in the first
part of the morning, but this looks overdone as it seems like that
would take quite a bit more rain. Showers will develop over the
mountains by early afternoon, while north to northeast winds
develop on the plains. Upslope convergence should be producing
showers at first and then more sustained heavier snow on east
facing slopes by late afternoon. Given the timing of all of this,
we`ve decided to push back the start of the Winter Storm Warning
in the mountains until noon.

At lower elevations we should have showers developing in the
afternoon, but the cooling will be gradual at first. We expect the
snow level to stay above 6000 feet through at least mid afternoon,
though it could be starting to lower by rush hour. Air and roads
should be warm enough for minimal impact during rush hour except
for areas above about 6500 feet. If the precipitation gets heavy
quickly things would be changing in the early evening.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 400 PM MDT Tue Mar 12 2024

Overall, the significant winter storm remains on track with
greatly increasing confidence in heavy snow amounts and severe
impacts for some areas.

A strong 500 mb trough will move from Idaho on Wednesday to the
Arizona/Nevada border by midday Thursday. This will be the main
feature that not only brings us a major winter storm with week, but
also persists in the Desert Southwest area all the way into next
week. As this trough digs southward on Thursday, there will be a
weak shortwave trough that moves from eastern Colorado out towards
Iowa. There has been a small but important trend in model data
with this initial shortwave. Models now show this shortwave to be
very weak and the surface low pressure center that this trough
creates is shown as roughly 3-5 mb weaker (higher values) than
previous predictions. This allows the cut off trough over the
western US to have much more of an influence for our CWA during
the entire event. As a result, low to mid level winds will have
more of an easterly component for a longer period of time, a sign
that points to higher QPF and snowfall amounts. What will
determine a large portion of the snow amounts will be the strength
of the low to mid level wind speeds. This is because the 500 mb
trough is too far away from our CWA to provide a great source of
QG ascent. Some models, like the NAM, show 700 mb northeasterly
flow above 50 knots. Other models, like the ECMWF, show peak
values around 35 knots. It is not surprising that the NAM shows a
significant amount more QPF and snow for this event. Given the
uncertainty in the northeasterly flow, the forecast was trended
towards a combination of the EPS and GEFS means. The GEFS
increased QPF in the latest run while the EPS kept it largely the
same with a slight increase for the foothills. Given the spread in
the model guidance is decreasing, there is greater confidence
that there will be significant impacts in our forecast area. This
increased confidence lead to the upgrade to warnings for the
mountains, foothills, and much of the I-25 corridor. In the
sections below, snow amounts, timing, and impacts are discussed
in three separate geographical areas.

Mountains and Foothills...

The deep, moist, upslope flow will result in remarkable snow
amounts in the Front Range foothills and mountains to the east of
the Continental Divide. Given such strong upslope flow even above
500mb, there appears to be little chance that a barrier jet
pushes the heaviest snow to the east of the foothills. However, if
the low end scenario were to play out in the mountains and
foothills, it would likely be due to a barrier jet interrupting
the best forcing. Given that ensemble guidance from all global
models show more than 1.5 inches of QPF, a good base case for
snow amounts is around 18 inches which is indicated in the Winter
Storm Warning. 3 inches of QPF is a reasonable 75th percentile for
QPF amounts which would equate to around 36 inches of snow due to
compaction and slightly below normal snow ratios. The reasonable
"worst case" scenario is snow amounts around 4 feet. If areas were
to get 4 feet, it would lead to impossible travel conditions that
could last multiple days. Our office has increased the wording in
our messaging to alert people that these extreme snowfall amounts
and travel impacts are a legitimate possibility. If you are
living in the mountains and foothills or planning to travel there,
travel is not advised beginning late Wednesday evening through
Thursday. That means bad news for skiers who may be eagerly
looking forward to big time snow amounts in the mountains. Please
do not put yourself into excessive danger and try to travel to the
mountains or foothills if you are coming from the I-25 corridor
or plains. The snow will wind down Friday morning and travel will
slowly improve throughout the day, especially if the sun comes
out.

I-25 corridor and Palmer Divide...

Wednesday evening, surface convergence along with ample moisture
will create thunderstorms over much of the I-25 corridor. These
will initially begin as rain and then will transition to snow due
to heavy precipitation rates and dynamic cooling. The
precipitation over the I-25 corridor will shift from being more
convective in nature Wednesday night to more stratiform on
Thursday as a deformation zone forms over northern Colorado.
Widespread moderate to heavy snow is expected throughout the day
Thursday and snowfall rates will begin to tapper off Thursday
night. Snowfall amounts will be between 14-24 inches with
localized amounts up to 32 inches across the Palmer Divide. This
will be where the worst travel conditions exist. Golden and
Boulder, along with other areas to the west of I-25 in th Denver
metro, will see amounts between 10-20 inches with localized
amounts that could exceed 2 feet. To the east of I-25, there will
be more hours of rainfall which will result in snowfall amounts
between 8-16 inches. The watches were upgraded to warnings to give
the best lead time for this storm since there is high confidence
in significant impacts. Since there will be strong north-
northeasterly winds coming off the Cheyenne Ridge, it will produce
downslope flow in Fort Collins and Greeley. This will not only
reduce the amount of QPF but will also result in more rainfall
during the event. Having said that, snow amounts will be in the
4-10 inch range and if the higher end verifies, it will bring
hazardous travel conditions. Therefore, a Winter Storm Watch was
issued for the zones that include Fort Collins and Greeley.

Eastern plains...

Rain will fall Wednesday night across much of the plains east of
DIA. It is not until the daytime on Thursday that cold enough air
aloft comes in to change the majority of the rain over to snow.
Then, a challenge to receive accumulating snow will be that it has
to be snowing hard enough to overcome the solar radiation of a mid
March sun angle. This uncertainty is the reason the Winter Storm
Watch was left for eastern Arapahoe and Adams Counties as well as
northern Lincoln County. In the worst case, I-70 could be shut
down due to snow and blowing snow. In the best case, little
snowfall accumulates on the roads and they are able to remains
open on the eastern plains.

Rest of the long term forecast...

Our forecast area will be under weak ridging in-between the same
cut off trough over the western US and another trough digging down
through the Midwest. This should lead to a period of warming
temperatures and drying conditions. However, either one of these
troughs could end up being much closer to, or over, our forecast
area. If this were to be the case, mountain snow showers would be
possible and the I-25 corridor could see cold temperatures and
precipitation. Light PoPs are in the forecast to account for this
chance.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 540 PM MDT Tue Mar 12 2024

Some virga will be possible this evening with a few light
showers possible late tonight between 10z-14z. Ceilings
will gradually lower to 6000 ft by 06z with brief
ceilings down to MVFR between 10Z and 14z.  Winds will be ENE
this evening and then may become more north by 06z.

On Wed, winds will be light N or NE in the morning and then
increase around 21z from the NE.  There will be a chc of
showers with MVFR ceilings after 21z.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT Thursday for COZ031.

Winter Storm Warning from noon Wednesday to 6 AM MDT Friday for
COZ033>037.

Winter Storm Watch from Wednesday evening through late Thursday
night for COZ038-043-045-046.

Winter Storm Warning from 9 PM Wednesday to 6 AM MDT Friday for
COZ039>041.

&&

$$

UPDATE...RPK
SHORT TERM...Gimmestad
LONG TERM...Danielson
AVIATION...RPK


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