Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 161635
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
935 AM MST Fri Feb 16 2018

.UPDATE...
Issued at 935 AM MST Fri Feb 16 2018

Latest look at model cross sections gives us increasing
confidence of a high wind event for the Front Range Mountains and
Foothills starting late tonight. Mountain top stability is
forecast to increase considerably toward 12Z, and that should be
enough to generate a high amplitude mountain wave. Cross sections
show a cross mountain component of 45-50 knots impinging on the
Front Range, while upper air forecasts show the nose of the
subsident right exit region of the 120 knot upper jet arriving by
12Z. The combination of these factors should support the high
amplitude mountain wave and peak wind gusts reaching 75-85 mph in
wind prone areas. At this time, it looks like the strongest gusts
will occur north of the I-70 Corridor where winds aloft are
maximized. We have upgraded the High Wind Watch to a High Wind
Warning, starting at midnight. The Warning will continue into
Saturday night/early Sunday morning, but there should be a lull
during the late morning and afternoon hours Saturday before
another blast of brute force winds arrives Saturday night.

&&

.SHORT TERM...(Today and tonight)
Issued at 410 AM MST Fri Feb 16 2018

Northwest flow aloft will prevail today and increase tonight as a
short wave trough passes north of Colorado. Subsidence over the
area will bring mostly sunny skies today. Though lows clouds may
remain banked up against the foothills and Palmer Divide through
the early morning hours. Temperatures look to be near seasonal
normals today with readings in the 40s. Clouds begin to increase
over the northern mountains mid to late afternoon ahead of the
trough. Cross sections show the moisture with the system will be
shallow. Should be enough flow and moisture to produce light snow
over the higher mountains. It is not a favorable set up snow due
to poor lapse rates and shallow moisture. Snowfall is expected to
be light with up to 2 inches of snow overnight in the northern
mountains. Many locations will see little to no snow with this
system.

As far as strong winds tonight and the High Wind Watch goes,
forecast still unclear. Definitely looks windy tonight, with an
inversion at just above ridge top level. However, the 00Z run of
the NAM is weaker with winds after midnight than previous runs.
The set up looks promising for high winds even though the models
don`t show it with a stable layer at ridge top and subsidence
behind the exiting wave. Will keep the watch as is, due to the
uncertainty.

.LONG TERM...(Saturday through Thursday)
Issued at 410 AM MST Fri Feb 16 2018

Strong mid-level subsidence and warmer air moves over the area
Saturday morning in the wake of a weak mid-level shortwave
pertabation. Cross sections indicate 55-70kt mtn top cross barrier
flow during the early morning hours as a 95+kt jet speed maxima
passes overhead. Gusts of 50-75 mph a reasonable call for
ridgetops and higher east slopes of the Front Range mtns during
the morning hours. In the lower foothills, wind gusts no where as
strong, but still in the 25-40 mph range, particularly in areas
prone to high winds. Strengthening mtn top static stability should
also put an end to the spotty snow showers over windward slopes
in the nrn mtn ranges before noon. On the plains, resident airmass
will dry and warm on Saturday with the boundary layer flow
switching from northwest in the morning to south-southwest in the
afternoon. Should see at least an 8-10 deg F warmup from the day
before, with warmest readings in the Denver area and along the
Palmer Divide.

Saturday night, could see a repeat of very strong westerly winds on
the ridgetops and upper lee slope of the Front Range and Medicine
Bow Mtns with a steep cross state pressure gradient, and the
formation of a high amplitude mtn wave with strong warming aloft.
However, the cross barrier flow does not appear to be as strong
as the night before as the core of strongest winds looks higher
in altitude. That said, peak gusts in the 50-70 mph range still a
possibility. For that reason will hold onto the High Wind Watch
for the Front Range mtn and foothill zones 33..34..35 and 36.

Sunday, temperatures will continue to warm with downslope flow.
Expect to see highs on the plains in the upper 50s to mid 60s, and
in the 30s and 40s across the high country. All this warming
precedes a Pacific storm system which models show deepening/
organizing over the Great Basin. Models track this extensive area
of lift, moisture and cold air eastward over the northern and
central Rocky Mtn region Monday into Tuesday. The bulk of the
energy associated with this system appears to lift northeast out
of the Great Basin and over northwest Colorado and Wyoming, just
clipping the CWA. Models show a strong cold front backing into
nern Colorado on Monday. Some models indicate a morning arrival in
the Denver area, while others not until early afternoon. Air
behind this front appears quite cold and once it settles in, I
really do not expect it to flush out until Wednesday. So temps
Monday night thru Tuesday night will be quite cold, with lows in
the single digits above and below zero on the plains and highs on
Tuesday possibly not much above the lower 20s. ECMWF temp
guidance gives Denver a high of only 19 on Tuesday. An extended
period of light nely upslope flow should also keep low clouds
socked up against the Front Range through the period and from them
periods of light snowfall. At this time, snow amounts are
expected to be on the light side at lower elevations, while mtn
areas could pick up several inches of snow during the storm.

Models show this weather system moving out Wednesday and a return to
a warmer and drier zonal flow. This same flow could eventually
channel Pacific moisture into the state from southern California
by late in the week exposing mtn areas and maybe parts of the
plains to another chance of precip by late Thursday or Friday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 935 AM MST Fri Feb 16 2018

VFR conditions will prevail through tonight. South/southwesterly
winds around 10 knots may become light/variable under 8 knots
this afternoon through early evening as a weak Denver cyclone is
in place. That should eventually swing winds around to the
northwest at KDEN and KAPA 20Z-24Z, so will keep that trend in the
forecast for now. Gusty west winds will be possible after 06Z
tonight at KBJC due to mountain wave, with peak gusts of 35-45
knots possible there. KDEN and KAPA should see lighter southwest
winds around 10-15 knots prevail.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from midnight tonight to 8 AM MST Sunday for
COZ033>036.

&&

$$

UPDATE...Barjenbruch
SHORT TERM...Meier
LONG TERM...Baker
AVIATION...Barjenbruch



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