943
FXUS65 KBOU 300029
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
629 PM MDT Tue Oct 29 2024

.KEY MESSAGES...

- Critical fire weather conditions today across the Palmer Divide
  and east-central plains due to warm, dry, and very windy
  conditions.

- Snow likely across the Front Range including the Denver area
  Wednesday morning with a light snow accumulation on grassy
  areas.

- Hard freeze Wednesday night/Thursday morning.

- Dry with near normal temperatures Thursday through Sunday
  including for Halloween.

- Low confidence (10 percent) for next weather system from late
  Sunday through Monday.

&&

.UPDATE...
Issued at 629 PM MDT Tue Oct 29 2024

The Red Flag Warning was allowed to expire due to decreasing
northerly winds, and higher humidities as the next batch of snow
showers arrive Wednesday morning. Additionally, this update
includes patchy fog along the I-25 and I-76 corridor for Wednesday
morning. There are some hints of freezing drizzle across a few
model soundings over the urban corridor during the morning
commute Wednesday but will leave this outcome out of the forecast
package for now until 00Z data arrives.

&&

.SHORT TERM /Through Wednesday/...
Issued at 215 PM MDT Tue Oct 29 2024

Water vapor shows a strong trough over the area where the CO/WY/UT
borders meet. Southwesterly flow is out ahead of this trough with
a dry intrusion beginning to make its way across the northern
Front Range mountains. This has lead to a substantial decrease in
precipitation in the mountains and a good chunk of the area has
seen the sun come out. The forcing ahead of the dry intrusion has
helped to create rain showers and graupel in Boulder and Fort
Collins this afternoon but the southwesterly flow aloft has worked
to dissipate the showers as they have moved northeastward. There
will continue to be showers from around Copper Mountain to
Berthoud Pass and northward along the Continental Divide this
afternoon. Elsewhere will generally be dry as the southwesterly
flow aloft is rather dry.

Across the plains, a cold front has slowly moved southward
throughout the day and is currently over Monument Hill to Limon.
To the south of this boundary, Red Flag conditions exist with
Limon gusting to 55 mph. A Red Flag Warning will continue through
6pm. On the north side of this boundary, there are northerly winds
and cooler temperatures. This has decreased the fire weather
concerns across the northern portion of the Red Flag Warning but
has not eliminated concerns.

Tonight, our forecast area will generally be under the dry slot
of this storm. While there could be a few convective snow showers
over the higher terrain, snow amounts will be minor and impacts
will be minimal. The main forecast concern tonight will be the
chance for fog or light freezing drizzle across the Urban Corridor
and plains. As colder air arrives from Wyoming, the air will be
close to saturation towards sunrise. Some models, like the HRRR,
have fog developing with temperatures at or slightly below
freezing. That could lead to slick travel conditions for the
morning commute so this will need to be watched closely. With that
being said, the more likely scenario is that persist northerly
winds will keep temperatures a couple of degrees above freezing
and will keep most of the area free from fog. The forecast
currently does not indicate fog but the next shift may need to
include it.

The center of a very cold trough will move directly over Denver
around midday tomorrow. This trough has 500 mb temperatures as
cold as -26 C and there will be steep mid level lapse rates as a
result. The QG ascent associated with this trough along with the
instability will create scattered convective showers over our CWA
throughout the day tomorrow. The majority of these showers will
fall as snow even across the plains. 1-4 inches of snow is
expected across the mountains and foothills with some of the
mountain passes seeing slick travel conditions. What is very
uncertain is the chance for accumulating snow across the Urban
Corridor and plains. Due to the convective nature of the showers,
there may be snowfall rates strong enough to produce an inch of
snow on grassy surfaces. However, due to the convective nature,
there is very low confidence of where that might occur and models
don`t seem to be much help. A general trace to 1 inch of snow is
forecast across the Urban Corridor and plains. Highs will be much
cooler tomorrow and will struggle to reach 40 in Denver.

&&

.LONG TERM /Wednesday night through Tuesday/...
Issued at 215 PM MDT Tue Oct 29 2024

The upper level trough will exit the area Wed evening with some
lingering light snow in the early evening hours.  Meanwhile, late
Wed night into Thu morning will see a Hard Freeze across all of the
plains.

On Thu, zonal flow aloft will develop over the area as a weak sfc
lee trough develops along the front range.  This will allow for
moderating temps and dry conditions over the area.  Highs will be in
the mid to upper 50s across the plains.

For Fri, dry WSW flow aloft with downslope low level flow. This will
allow for warmer temperatures as highs reach the lower to mid 60`s
across the plains.

On Sat, an upper level trough will move into the wrn US with SW flow
aloft.  This will lead to another dry day with highs remaining in
the 60`s over nern CO.

For Sun into early next week, there are major differences between
the operational models on the handling of the wrn US trough.  The
GFS moves the system eastward and develops a strong storm system
over the Great Basin late Sun into Mon.  In addition, it has a cold
front moving into northeast Colorado as well with upslope flow
developing.  As a result, this leads to widespread pcpn and
colder weather across most of the area by Mon.

On the other hand, the ECMWF digs a substantial amount of energy
into srn California/Baja area late Sun with a closed low moving into
srn Arizona/nrn Mexico by Mon.  Furthermore, this is a secondary
piece of energy which moves across the nrn Rockies.  This solution
would lead a mainly dry pattern across the area late Sun through Mon.

Naturally, confidence in either of these solutions is low at best
this far out.

For Tue and Wed, another system may affect the area by the middle
part of next week, however, just like the previous system mentioned
above, there will likely be differing solutions going forward as
well.

&&

.AVIATION /00Z TAFS through 00Z Thursday/...
Issued at 629 PM MDT Tue Oct 29 2024

VFR conditions are possible until 09Z for all sites. As the colder
system pushes into the region, increasing moisture arrives. This
may lead to fog as early as 10Z. It is possible IMC could begin
slightly early near 09Z. Overall, IFR conditions are possible at
all sites 11Z-15Z due to potential fog and incoming snow showers
where cigs sit near 500-800 ft. By 21-22Z Wednesday afternoon
ceilings should improve to VFR for all sites.

&&

.FIRE WEATHER...
Issued at 215 PM MDT Tue Oct 29 2024

Critical fire weather conditions are occurring in Elbert and
Lincoln Counties currently. These conditions will continue through
6pm with the area of critical fire weather conditions decreasing
as a cold front moves southward.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Freeze Warning from 10 PM Wednesday to 9 AM MDT Thursday for
COZ038>041-043-045.

&&

$$

UPDATE...AD
SHORT TERM...Danielson
LONG TERM...RPK
AVIATION...AD
FIRE WEATHER...Danielson

NWS BOU Office Area Forecast Discussion