Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 232133
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
333 PM MDT Sat Mar 23 2024

.KEY MESSAGES...

- A weather system will affect the area Sunday through early
  Monday morning. This system may bring heavier snowfall to
  portions of the mountains, southern foothills and Palmer Divide
  Sunday night. However, confidence in exact details still remain
  low at this time.

- Blowing snow impacts possible across the Palmer Divide Sunday
  night through early Monday.

- Snow shower activity will continue in the mountains Tuesday and
  Wednesday with mainly dry conditions over the plains.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 327 PM MDT Sat Mar 23 2024

Water vapor imagery reveals there is a shortwave trough moving
onshore over California. There is weak QG ascent across Colorado
under diffluent flow aloft ahead of the shortwave. At the surface,
there is weak lee cyclogenesis across eastern Colorado. A boundary
has formed along I-70 east of Denver and this boundary appears it
will be the genesis point for showers and storms to form later this
afternoon. These showers and storms will move northeastward along
the I-76 corridor through the evening. After midnight, the plains
will remain mostly dry as a pocket of drier air aloft moves in.

In the mountains, radar is beginning to pick up on light showers
this afternoon. These showers will stay light throughout the
afternoon and evening before more low level moisture arrives after
midnight. In contrast to the plains, snow showers will increase in
the overnight hours with a couple inches of snow along the western
slopes possible.

The aforementioned shortwave trough will move northeastward tomorrow
eventually ending up in eastern Colorado. Strong lee cyclogenesis
will rapidly deepen a surface low pressure system that will position
itself in the Arkansas River Valley. The combination of these
features will create strong QG ascent and precipitation will form
across the majority of our forecast area in the afternoon after many
dry hours in the morning. There will be a few focus areas for
heavier precipitation. The first will be the higher Front Range
mountains and foothills as strong lapse rates, healthy moisture, and
QG ascent will lead to moderate to briefly heavy snowfall. Another
focus area for precipitation will be the base of the foothills onto
the adjacent plains and I-25 corridor. There will be widespread
thunderstorms in this area with briefly heavy rainfall. Some of the
CAMs have rainfall up to an inch with these storms but most of the
area will pick up about a quarter of an inch of rain. The final
focus area for precipitation will be along I-70 in Elbert and
Lincoln Counties. A boundary will setup there in the afternoon with
most unstable CAPE above 500 j/kg to the north of it. Thunderstorms
will form along this boundary with brief heavy rain and hail likely.
Soundings indicate steep lapse rates around 9 C/km with strong
forcing in the hail growth zone. While it appears instability will
be a bit too weak for severe hail, hailstones up to three-quarters
of an inch will be possible.

By the late afternoon, a cold front/modified outflow boundary will
form over the northern plains and will begin to rapidly move
southward. This front will decrease temperatures to the mid 30s by
6pm and some snow will begin mixing in with the rain across the
plains. No snow accumulation is expected across the plains before
6pm.

.LONG TERM...(Sunday night through Saturday)
Issued at 327 PM MDT Sat Mar 23 2024

...Late weekend system to bring snow to the mountains, foothills,
and portions of the plains...

Sun night into Mon, an open wave trough moves across the region.
It will contain a more pronounced southerly branch with an upper
jet that will provide sufficient synoptic ascent. There is a
general consensus among model guidance that this trough moves east
across Colorado into Sun PM with a closed low developing along
the CO-KS border before it lifts northeast across KS. Above normal
moisture will accompany this system, further supporting
precipitation. There is still notable uncertainty in the placement
of this low which will have impacts on amounts. This will be
discussed in more detail later on in this discussion.

By Sun evening, a cold front will have settled across the region
with enhanced northerly upslope flow filling in behind it across
the lower elevations. Lapse rates will still hold rather unstable
across the mountains, although not as unstable as the afternoon.
This will keep snow going in the mountains. However, the real
change will be the changeover to snow across the Palmer Divide
then lower elevations as well as an uptick in snow for the
southern foothills. This is reinforced by the arrival of colder
air and enhanced upslope flow favoring the south foothills. Right
now, seeing a changeover in the early evening timeframe (23-01Z)
across the Palmer Divide, then across the adjacent plains as it
continues to cool. Most model guidance is highlighting the
potential for a favorable window for accumulating snowfall Sunday
evening, mainly across the southern foothills and into the Palmer
Divide areas. Model soundings reflect this quite well with deeply
saturated profiles with notable lift/instability in the dendritic
growth zone.

After midnight, lapse rates weaken and moisture decreases as the
larger scale forcing moves out. Snow gradually decreases in coverage
and intensity toward sunrise. Monday, temperatures will be below
normal in the 30s across the lower elevations and 20s/30s for the
high country. Residual moisture and rebounding lapse rates in the
toward the afternoon support some mountain snow showers.

As tracking of the system remains lower confidence, the snow totals
will be as well. Some models have been more consistent than others.
The GFS (deterministic & ens) has been generally wetter than the
ECMWF suite. GFS ens mean QPF puts around .7-.95" for the south
foothills, front range and .5-.7 for the adjacent plains. The 12z
ECMWF, although trended wetter than its previous run, but still
remains the drier of the models. Higher-res models have also been on
the wetter side, similar to the GFS. The GFS and higher-res guidance
have unfortunately been less consistent from run to run, so although
a good chunk of guidance is trending upward in amounts, there is
still not a whole lot of consistency on where the higher amounts may
be and exactly how much.

For now, leaning on the more conservative end of guidance with
snow totals. Front Range and central mountains in that 8-12 inch
range with localized higher amounts. More uncertainty in store for
the southern Foothills, Palmer Divide, and plains. Palmer Divide
around 2-8" for now. Southern foothills 4-10". Despite this being
below Winter Storm Warning criteria, there is the lower
probability scenario it being closer to a foot. Given the
potential, but low confidence at this time, went with Winter Storm
Watches for these areas Sun eve through Mon AM. Denver metro
currently has no highlights, but will be evaluated for advisories
for the evening package since it may impact the Mon AM commute.

And now the last concern we need to discuss...wind. This system will
be accompanied by a well developed low with a sufficiently tined
pressure gradient wrapped around it. This will provide plenty of
wind across the plains as the low exits Colorado Sunday night into
Monday. Gusty northerly winds 40-50 mph will become consistent
across the east plains and Palmer Divide Sunday night. There are
also areas that will be favored for higher gusts such as the east
plains along WY border, Washington County, and Lincoln County.
These areas have potential for gusts to peak 55-65 mph at times.
As a result, a High Wind Watch has been issued. Wind and snow are
a hazardous combination (blowing snow) and have potential to
impact the Palmer Divide since they have a much higher likelihood
for blowable snow than east. A Winter Storm Watch for blizzard
conditions was issued for zone 41 (Palmer Divide) Sun eve through
Mon morning.

Early next week, the mountains will see periodic chance for
orographic snow while the plains remain mostly dry. A ridge builds
in midweek decreasing moisture in the mountains and warming
temperatures.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1230 PM MDT Sat Mar 23 2024

The main concern this afternoon and early evening will be gusty
outflow winds from showers that develop and move off the terrain.
Gusts could reach 30 knots at KDEN and KAPA. The most likely time
for these winds would be between 21-00Z. Otherwise, winds will
generally shift to the southeast and then trend towards drainage
overnight. There are no concerns tomorrow morning but tomorrow
afternoon will have very impactful weather at all terminals.
Thunderstorms with low ceilings and rain will develop. Strong
northerly winds will also arrive behind a cold front. Shortly
after the cold front`s arrival, the rain will transition to snow
most likely around 00Z. IFR ceilings and visibility is likely
tomorrow evening with wind gusts up to 40 knots out of the north.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from midnight tonight to 6 AM MDT Monday
for COZ031-033-034.

Winter Storm Watch from Sunday evening through Monday morning
for COZ036-041.

High Wind Watch from Sunday evening through Monday morning for
COZ038-042-048.

High Wind Watch from Sunday evening through Monday morning for
COZ046-047-049.

&&

$$

SHORT TERM...Danielson
LONG TERM...Mensch
AVIATION...Danielson


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