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Area forecast discussion
National Weather Service Denver/Boulder Colorado
954 am MST Wed Nov 20 2019

Update...
issued at 954 am MST Wed Nov 20 2019

Overall, forecast is in good shape and didn't make any big
changes. Satellite and radar are showing increasing rain and snow
across western and southern Colorado. This area of precipitation
will spread northeast through the afternoon and evening. Mountain
snow with rain across the Front Range is still expected for this
afternoon. With this looking more certain raised pops even more.
Still some uncertainty on when the rain changes to snow this
afternoon/evening. The hrrr/rap are the warmest models which keeps
the airmass warm enough for rain through mid evening. If this
were to occur, little to no snow would accumulate across the lower
elevations. Believe there should be a change over to snow a
little earlier and closer to 00z like the 12z runs of the NAM and
GFS. If this pans out, would expect up to 2 inches of snow. With
temperatures staying near freezing, don't expect much for impacts
across the lower elevations. Meanwhile in the mountains, roads
should become snow covered and slippery by early evening, thus the
Winter Weather Advisory looks on track.

&&

Short term...(today and tonight)
issued at 322 am MST Wed Nov 20 2019

We've been talking about today's weather system for several days
now, and here it comes. Moisture will be increasing from the south
through the day as an upper level disturbance from the south
tracks over Colorado. Upper air soundings from Arizona last
evening showed precipitable water values near records for this
date, and nearly two times the average precipitable water value
for mid to late November. Not all of this deep moisture will be
making it to Colorado, but enough will arrive to produce mountain
snowfalls this afternoon and evening. Expected snow accumulations
in the mountain zones are 3 to 8 inches, so will issue a Winter
Weather Advisory for today and tonight. The upper disturbance will
be leaving the state by midnight, so most snow accumulations are
expected before that. Subsidence is expected after the upper
trough passes which should diminish precipitation across our
forecast area.

On the plains, temperatures have remained mild this morning in the
south to southwesterly flow ahead of the approaching weather
system. At the same time, a cold front is moving down out of
Wyoming which will sweep across the plains by late this morning.
The cold front moved through Casper and Douglas, Wyoming in the
past hour with good surface pressure surges developing over
northern Wyoming. The front should accelerate over the next few
hours with the wind shift crossing the northern Colorado boundary
by 6 or 7 am and then arriving in Denver by 10 am. Northerly winds
through the afternoon should hold temperatures in the 40s through
the afternoon, even though some places are already starting out in
the lower 40s. Light rain showers are expected to move out over
the plains during the afternoon as upslope flow continues behind
the cold front passage. Diabatic cooling effects and cold
advection behind the front should cause precipitation on the
plains to change over to snow by about the time the sun is going
down, then light snow will continue overnight. Forecast quantitative precipitation forecast values
are light on the plains, so only 1 or 2 inches of snow
accumulation is expected. Snow is also expected to be diminishing
after midnight as the upper system will be moving northeast of the
state.

Long term...(thursday through tuesday)
issued at 322 am MST Wed Nov 20 2019

An upper level low will move into the Great Basin on Thu with south
to southwest flow aloft. Moisture will increase over the higher
terrain by aftn along with some weak mid lvl ascent. Thus will
see an increasing chc of snow showers in the mtns by aftn. Over
lower elevations shallow upslope will be in place, and as weak
mid lvl ascent moves in, may see a chc of light snow showers
along the Front Range and portions of the plains during the aftn.
As for highs, readings over nern Colorado will be in the lower to mid
30s.

For Thu night, a piece of energy from the upper level low over
the Great Basin will move into ern Colorado. As a result, the main
upper level low may reform over ern Colorado and linger into Fri
morning. As this system moves across there will be some mid level
ascent. Cross-sections show increasing moisture with favorable
orographic flow in the mtns Thu night into Fri morning. Thus may
see some potential for heavier snow overnight into Fri morning.

At lower elevations, the fcst is a bit tricky due to how the
upper level reforms over ern Colorado. In addition, the low level flow
will become more southeast as well. There is some agreement among
the models that there could be a period of steadier snow Thu
night, in and near the foothills and along portions of the I-25
corridor.

On Fri, the upper level low will begin to move eastward during the
day. There still could be some lingering light snow in the
foothills and along the I-25 corridor in the morning. Across the
mtns, favorable orographics will still be in place thru the day,
but moisture should begin to decrease in the aftn with snow ending.
As for highs on Fri, will keep readings in the upper 30s to lower
40s across nern Colorado.

For the weekend, the flow aloft will be northwesterly. Cross-
sections show some mid and high level moisture embedded in the flow,
however, both Sat and sun look dry at this point. Meanwhile, with
downslope flow developing, will see warmer temps across nern Colorado
with highs in the 50s.

By early next week, there are major differences between the European model (ecmwf)
and GFS. The European model (ecmwf) has a cold front affecting nern Colorado on Mon as a
disturbance in northwest flow tracks across. This would bring a chc of
snow to the area on Mon. Meanwhile, the GFS has a much stronger
system affecting the area Mon night into early Tue, which brings
substantially more snow to the area. For now will trend towards
colder temps and keep in a good chc of snow.

&&

Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 954 am MST Wed Nov 20 2019

North to northeast winds usher in cooler air through the rest of
the day. Clouds will lower through the afternoon with rain showers
forming around 21z. Rain is expected to change to snow around
00z. The snow is expected to end in the Denver area 03z to 05z.
Total snowfall of up to 2 inches will be possible, most of which
is expected to be on grassy and elevated surfaces. Low clouds are
expected to persist through tonight and into Thursday with
ceilings of 500 to 1500 feet. Some light snow or freezing drizzle
will be possible through late tonight and Thursday morning.

&&

Bou watches/warnings/advisories...
Winter Weather Advisory until midnight MST tonight for coz031-
033-034.

&&

$$

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