Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 141032

Area Forecast Discussion
National Weather Service Denver/Boulder CO
332 AM MST Wed Feb 14 2018

.SHORT TERM...(Today and tonight)
Issued at 330 AM MST Wed Feb 14 2018

Changing weather in the short term period with 500 mb shortwave
trough digging south along Canadian coast into Pacific NW showing
up nicely on water vapor imagery. The trough drops south along the
Sierra Nevada today and tonight, kicking out a closed low off the
southern California coast in the process. Meanwhile a
strengthening 100-120 knot 300 mb jet in southwest flow points
into Colorado from southern California ahead of the southern low,
and by 06z Thursday the wind max increases to over 140kts in CO.
Two things result...a windy day on tap today in a mountain wave
cloud, and mountain snows developing later this evening and
continuing through the remainder of the short term period.

For wind, vertical cross section showing 50-70 knots above the
mountaintops, and indications of mountain wave developing along
immediate foothills. As with previous forecaster do not expect
much progression out onto the plains, however foothill regions can
expect to see gusts of 40-50 knots. A fly in the ointment is more
of a southwest component of the jet, which is not the more common
scenario. Expect stronger winds in mountain wave to favor
northern foothills, primarily northern Larimer county, as well in
the mountains. Foothills further south will not be entirely out of
the woods either however. Expect boundary layer to decouple later
this evening and allow wind in the plains to subside, however
higher elevation wind to persist.

Mountain snows enter the forecast later today into tonight in
upward vertical motion associated with left entrance region of jet
streak, enhanced by orographic lift. Snow mainly relegated to the
higher elevations at the start this evening, becoming more
widespread in the mountains toward 12z Thursday. Snowfall combined
with wind will create blowing and drifting snow, as well as poor
visibility and difficult travel. As a result have issued a Winter
Storm Warning for higher elevation zones beginning 00z Thursday
and continuing into extended period.

Temperatures...westerly downslope flow will warm max temperatures
into the upper 50s to near 60s out on the plains, with 30s and 40s
in the mountains. Warmer southerly component to surface winds and
some cloud cover will prevent temperatures from cooling
excessively, with lows in the 20s and 30s tonight.

.LONG TERM...(Thursday through Tuesday)
Issued at 259 AM MST Wed Feb 14 2018

Snow is expected to be falling in the mountains come Thursday
morning. Lapse rates of 6-8 C/km will prevail through the moist
layer up to 500mb of Thursday. This combined with westerly ridge
top winds of 25 to 40 knots will result in good orographic lift
over the mountains. In addition, a strong jet should produce bands
of heavy late Wednesday night and Thursday Morning. Over the
Front Range and eastern plains, a cold front will push through
early in the day. Highs will be cooler with 40s expected. Light
snow will be possible behind the front Thursday evening when
upslope flow is the strongest. Best moisture and chance for snow
with this will be along and south of I-70.

Surface high pressure drops south across the area late Thursday
night bringing dry conditions. Northwest flow aloft will result in
cool and dry conditions for Friday. Though light snow will be
possible over the northern mountains ahead of a weak system
embedded in the northwest flow aloft. Winds will increase with
this system Friday night with strong winds possible in the
mountains and foothills. These strong winds could spread to the
base of the foothills and areas nearby Friday night when a
mountain wave may form. Upcoming forecast shifts will need to
monitor this system for strong winds.

Northwest flow aloft Saturday will turn westerly on Sunday as an
upper level trough moves over the Pacific Northwest. Not much
moisture will be embedded in the flow aloft, so chances for
precipitation will be low this weekend. Temperatures will be mild
with highs climbing into the mid to upper 50s on Sunday.

Models are coming into better agreement with an upper level
trough digging south across western CONUS early next week. This
will allow an arctic cold front to push south through Colorado.
Lowered temperatures for this system Monday and Tuesday. Moisture
is expected to increase with snow beginning late Sunday/early
Monday in the mountains. If models hold true, snow would develop
sometime Monday and Tuesday across the lower elevations behind the
arctic front.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 330 AM MST Wed Feb 14 2018

Wind is the main impact this forecast cycle, otherwise VFR
conditions. Mountain wave setup for today into tonight, with
westerly jet of 50-60 knots at summit level breaking along eastern
slopes. Plenty of uncertainty with the wind forecast, but still
forecasting strongest winds to impact KBJC with 40-45 kt gusts,
and not make it all the way east to KDEN. Nevertheless forecasting
westerly winds gusting to 25 kts at KDEN, and gust to 30 kts not
out of the question. KAPA similar to KDEN forecast with gusts
remaining near 25 kts. VFR cigs to develop after 00z, with no
obstructions to visibility.




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