Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 151110

Area Forecast Discussion
National Weather Service Denver/Boulder CO
410 AM MST Wed Nov 15 2017

.SHORT TERM...(Today and tonight)
Issued at 400 AM MST Wed Nov 15 2017

An upper level ridge and a surface high pressure system will
bring quiet weather conditions today. There will be enough
moisture aloft to produce high clouds, meanwhile lower levels of
the airmass will remain dry. Last night`s cold front ushered in
cooler air. Highs look to be in the upper 40s to mid 50s today
across northeast Colorado.

Southwest flow aloft behind the exiting ridge will start to
transport warmer air into the area overnight. A lee side surface
trough will deep over eastern Colorado. This will cause winds to
increase over the mountains and foothills. Overnight lows will be
mild in the foothills and nearby plains where winds keep the airmass
mixed. Farther east across the northeast plains, lows will fall into
the mid 20s to lower 30s.

.LONG TERM...(Thursday through Tuesday)
Issued at 400 AM MST Wed Nov 15 2017

Abnormally warm and dry conditions beneath a passing upper ridge
Thursday will likely give way to blast of snow and strong winds
across the high country Thursday night into Friday. An abnormally
strong nearly zonal flow aloft, at its core a 150-160kt Pacific
jet stream, will slide south from the Pacific Northwest/northern
Rockies and eventually over Colorado on Friday. Strong warming on
Thursday in advance of this jet, aided by a southwest/downslope
bndry layer flow should have little problem sending daytime
temperatures into the low/mid-70s on the plains. Along the base
of the foothills and across the Palmer Divide, could see gusts to
around 25 mph by late afternoon. Up around timberline, should see
much stronger gusts to around 55 mph by afternoon with even
higher gusts over night as the jet nears the state. Furthermore,
models also indicate a fairly rapid moistening and destabilization
of the mid-layer over the nrn mtns Thursday afternoon leading to
isolated to scattered showers at higher elevations. Freeze level
will be quite high at that time, somewhere up around 10000 ft ASL
during the afternoon. So any snowfall will likely be confined to
areas up around timberline.

Thursday night into Friday morning, the powerful jet at the
bottom of the upper trough carving out over the Great Basin is
progged to shift south over srn WY/nwrn CO overnight and be
aligned west-east across Colorado during the day Friday. Even
with the jet sagging southward over the area, the snow level in
the high country will be slow to lower Friday morning as low/mid-
level winds will be generally from southwesterly direction. It`s
during the afternoon when temperatures are likely to take a nose
dive with passage of the 700mb trough axis and accompanying sfc
cold front. The saturated layer also maxes out along and west of
the Continental Divide around this time. West-northwest winds of
20-35 mph and gusts to around 60 mph coupled with a period of
moderate to heavy snowfall could produce very difficult travel
conditions in the high country through Friday evening. Uncertainty
in the timing and intensity of this snowfall prevents me from
hoisting any winter weather highlights at this time. However, one
may be issued later today or tonight. As for the lower elevations,
prefrontal warming and drying Thursday night into Friday morning
should give way to gusty northerly winds and elevated RHs with
passage of the sfc cold front. When this occurs is still in
question. The GFS and ECMWF show it sweeping across the nern CO
plains during the late afternoon hours, while the NAM and Canadian
models hold off on frontal passage until early to mid- evening on
Friday. Temperatures look too warm for any snow, at least not
until after frontal passage. And still, the snow level overnight
doesn`t appear to fall much below 4800 feet. So, rain first, then
a rain/snow mix, then possibly a short period of light snow with
little to no accumulation. The banded precip generated by the
passing jet and post-frontal upslope flow appears to shift south
of the fcst area before morning, with fairly rapid clearing after
sunrise with strong drying and subsidence aloft.

Saturday should turn out to be the coldest day of the week with
highs on the plains struggling to climb out of the 40s. It will
continue to be gusty as well in the mtns and across the northeast
corner of the CWA for a good part of the day. Should we see any
fog in low lying areas, it should burn off quickly by mid-morning.

For the period Sunday through Tuesday, expect dry and gradually
warmer conditions with a flat upper ridge slipping in from the
Great Basin. Temperatures should run slightly above average for
this time of month.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 400 AM MST Wed Nov 15 2017

VFR conditions will prevail through the tonight with sct-bkn high
clouds. Southeast winds will increase after 18Z with gusts up to
20 knots. A Denver Cyclone is expected to form after 21Z and cause
a wind shift to the north/northwest around 23Z.




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