554
FXUS65 KBOU 250627
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1127 PM MST Sun Nov 24 2024

.KEY MESSAGES...

- Mountain snow continues through tonight, heaviest near Rabbit
  Ears Pass. The I-70 mountain corridor to see slippery roads this
  evening. Areas of snow light across the I-25 urban corridor.

- Winter Storm Watch for possible heavy snow Tuesday through
  Wednesday night in the mountains causing significant travel
  impacts.

- Potential for snowfall across the lower elevations Tuesday night
  into and Wednesday night.

- Mostly dry Thanksgiving Day into next weekend with below normal
  temperatures for all areas.

&&

.UPDATE...
Issued at 805 PM MST Sun Nov 24 2024

Combination of a weak upper level trough moving across, along
with the position of an upper level jet, is allowing for areas of
snow to continue in the mtns with a mix of light rain/snow at
lower elevations early this evening.

Meanwhile a secondary cold front is moving into nern CO which may
briefly enhance upslope component along the I-25 Corridor as it
continues southward.  Short range data continues to show some
precip enhancement in and around the Denver area between 9 pm and
11 pm with activity moving southward across the Palmer Divide by
2 am.  Expect this band of precip to be all snow as secondary surge
of colder air moves across. Some places may see an inch or two of
snow, on grassy areas, as this band moves across the I-25
Corridor mainly from Denver south to the Palmer Divide.

&&

.SHORT TERM /Through Monday/...
Issued at 156 PM MST Sun Nov 24 2024

Snow showers have moved into the northern mountains this
afternoon ahead of an upper level trough tracking across the
Northern Rockies. The snow will progress southward across the
mountains the rest of this afternoon and evening. Forecast appears
to be on track with 2 to 5 inches of snowfall for the mountains
with a little more (4-8 inches) over the mountains around Rabbit
Ears Pass. Webcams showing a Rabbit Ears pass is snow covered, but
most mountain roads are just wet at this time. This will change
late this afternoon after sunset when pavement temperatures cool
and snow increases. Expect slippery conditions to develop this
evening, including I-70.

Over northeast Colorado including the urban corridor, a cold
front is pushing southward and will be through the area by sunset.
Northeast upslope winds will prevail with convergence near the
base of the foothills. Dew points are in the upper teens to mid
20s, even behind the cold front. So it will take a little time to
moisten up the subcloud layer before we see rain/snow.
Temperatures get cold enough for snow 7-9pm and most of the
precipitation is expected to fall as snow. With the left exit
region of the jet over the area, snow banding will be possible and
could produce an inch or two on grassy surfaces.

For Monday, we could see a few clouds linger into the morning
south of Interstate 70, but sunny skies are expected for a good
part of the day as weak ridging in the westerly flow aloft moves
overhead. Today`s cold front will bring a cooler airmass for
Monday with highs in the 40s across northeast Colorado. High
clouds begin to enter the region mid to late afternoon in advance
of the next approaching system.

&&

.LONG TERM /Monday night through Sunday/...
Issued at 245 PM MST Sun Nov 24 2024

Models are showing strong westerly flow aloft over Colorado
Monday night through Wednesday, with an upper trough to push into
the CWA sometimes Wednesday/Wednesday evening depending on the
model.  For the most part, decent moisture advects into the western
CWA on the strong flow aloft by Tuesday morning, well ahead of the
upper trough.  Cross sections show deep moisture over the CWA
through much of Wednesday night.  The QG Omega fields do show upward
vertical velocity for the CWA through Thursday night around 06Z.
The snowfall should be enhanced as cross sections also show pretty
strong wind speeds and favorable wind directions for orographic
snow production in the highs mountains. As a result, we went with
a Winter Storm Watch for heavy snow in high mountains zones 31,
33, and 34 from 12Z Tuesday morning to 12Z Thursday morning for 12
to 24 inches of snow. Snow is also expected over the high Parks
and foothills zones commencing Tuesday morning. Advisory
Highlights may eventually be needed in these areas depending on
the how the storm pans out.

For the rest of the forecast area, the period of Monday night into
Tuesday morning looks to be dry with fairly normal diurnal wind
trends. Highs should be in the lower 50s on Tuesday.  A cold front
is expected to move in Tuesday night associated with the upper
trough. Weak low level upslope in progged for the plains. With the
available moisture, strong flow aloft, low level upslope and
limited synoptic scale lift, I went with 30-60% pops for the
western two-thirds of the plains Tuesday afternoon and night, with 40-
60% pops for all the plains Wednesday into Wednesday evening,
then decreasing pops Wednesday night. Snowfall amounts do not
look significant over the plains with this system at this time,
however model confidence is not high so higher snowfall amounts
are possible. Temperatures are expected to stay mainly in the 30s
over the plains for highs on Wednesday, about 10 degrees below
normals.

For the later days, Thursday through Sunday, models keep the CWA in
cool, dry northwesterly flow aloft.  There looks to be limited
moisture in the mountains next weekend for scattered alpine snow
showers. Temperatures will be below seasonal normals all four
days for entire forecast area.

&&

.AVIATION /06Z TAFS through 06Z Tuesday/...
Issued at 1051 PM MST Sun Nov 24 2024

Snow showers are in the Denver metro area although the majority of
the area is not seeing snow at the moment. At KDEN and KBJC, snow
showers are likely to redevelop shortly and take visibility as low
as 2 SM with ceilings of around 2,000 feet. That chance for snow
will mainly be between now and 09Z as drier air in the low levels
will move in at that time. At KAPA, light snow will continue
through about 09-10Z before tapering off. The low ceilings will
break at all airports between 12-14Z.

The rest of the day Monday and Monday night will have VFR
conditions. There will be light SE winds during the day that turn
to drainage during the evening and overnight.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM MST Monday for COZ031.

Winter Storm Watch from late Monday night through late Wednesday
night for COZ031-033-034.

&&

$$

UPDATE...RPK
SHORT TERM...Meier
LONG TERM....RJK
AVIATION...Danielson