Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 111628

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1028 AM MDT Fri Aug 11 2017

Issued at 1028 AM MDT Fri Aug 11 2017

No changes to the forecast at this time, other than making
adjustments to the cloud cover during the morning hours. The
mountains started off mostly clear, while the plains had a fair
amount of stratus and areas of fog. Stratus on the plains is
lifting out, except in Lincoln and Elbert Counties where southerly
winds are bringing moist air up the southern side of the Palmer
Divide, producing more low clouds. Subtropical moisture is evident
over the San Juan Mountains this morning, with the northern edge
just to the south of the I-70 corridor. This will cause most of
the afternoon shower activity over the mountains to be across
Summit and Park Counties, but can not rule out a few showers also
moving across Grand and Jackson Counties, as an embedded small
disturbance is evident on satellite imagery over northwest
Colorado. Moderate westerly flow aloft will keep the showers
moving along. On the plains, clouds and the presence of low level
moisture have held temperatures in the upper 50s to mid 60s so
far. Afternoon heating and the embedded upper level disturbance
should kick off isolated showers through the afternoon. The latest
runs of the HRRR show heavier coverage of showers remaining over
southeast Colorado.


.SHORT TERM...(Today and tonight)
Issued at 357 AM MDT Fri Aug 11 2017

Areas of low clouds and fog this morning will slowly burn off mid
to late this morning. Visibility in a few locations will fall to
a quarter mile or less. Scattered/Isolated showers and
thunderstorms are expected to form over the foothills and
mountains early this afternoon. The convection is expected to
spread eastward through the afternoon and early evening. Models
are showing less convection than yesterday, likely due to less
moisture and a lack of a large scale short wave trough. There is a
weak wave over northern Utah. It will progress eastward and may
help trigger storms over northern Colorado. Will have slightly
higher pops over northern areas because of this. With decent shear
and CAPE up to 1000 J/kg, an isolated severe storm can`t be ruled
out with hail up to one inch and 60 mph wind gusts. Best chance
for this will be over the eastern plains. Temperatures will be a
little warmer with highs in the upper 70s to lower 80s across
northeast Colorado.

Storms will march eastward across the area this evening and exit
the state around midnight. It will be another cool night with lows
in the lower to mid 50s over northeast Colorado. Low clouds and
fog will be possible over the eastern plains late tonight. A
Denver Cyclone could pull this moisture into northern parts of the
metro area towards 12Z.

.LONG TERM...(Saturday through Thursday)
Issued at 357 AM MDT Fri Aug 11 2017

Subtropical moisture trapped beneath the upper ridge than has
persisted for the past several days over the Rocky Mtn region
will be periodically reinforced by intrusions of moist and
unstable air carried northward from the Desert Southwest. While
models show little west-east displacement in the mean position of
the ridge, they do indicate day-to-day fluctuations in its
amplitude, i.e., strength.

Over the weekend, models show the ridge flattening as a series of
weak pertabations race over the top of the ridge and down over
sern WY and nern CO. On the plains, a fairly moist low-level
southerly flow will keep areas of low clouds and patchy fog around
during the morning hours on Saturday. Strong daytime heating, and
the mid-level lift and instability provided by a shortwave
swinging down from WY should kick off scattered t-storms first in
the high country by early afternoon and out over the plains later
in the afternoon and evening. The swift flow aloft and a trend
towards lower precipitable water amounts should reduce the odds
for heavy rainfall, but t-storm chances should remain about the
same as that on Friday. A bit more sunshine on Saturday should
give us slightly warmer temperatures, with max temperatures
anywhere from 2 to 4 deg f warmer on the plains. Storms should
move out of the CWA by late evening or early morning with
subsidence and drying in the wake of the departing shortwave. On
Sunday, models show a weak surge of slightly cooler air slipping
down from WY in the morning which could generate pockets of low
clouds and patchy fog on the plains early. Later in the day the
passage of another weak disturbance in moderate northwest flow
aloft should kick off another round of t-storms by mid to late
afternoon, with the best chance for storms in the vicinity of the
Palmer Divide and along the eastern fringe of the CWA where
moisture and instability will be more favorable for storm
initiation. Expect highs on Sunday to be a couple degs cooler
with the slightly cooler nwly flow aloft.

Next week, models show the upper ridge building over the region and
a slow shift in the 500 mb axis east of the Continental Divide.
This returns a warmer and slightly more humid swly flow to the
region, which will favor t-storms production in the high country.
Storm chances east of the mountains will fluctuate daily with
drier downslope flow one day and moist upslope flow the next.
Right now, Tuesday looks to be the wettest day of the week with
low and mid-level moisture streaming up for the south and a
shortwave trough passing over the state in zonal flow aloft.
Storms should move quickly that day, so the threat of heavy rain
should be low. However, greater instability and shear on the
plains could support isolated severe t-storms. During the latter
half of the week...the airmass is forecast to dry out with a
warmer and drying zonal flow over the region. Could see daytime
temperatures flirting with the 90 deg mark on the plains again.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1028 AM MDT Fri Aug 11 2017

The HRRR shows southeasterly flow continuing over eastern Colorado
through this afternoon and evening with a weak Denver cyclone
circulation forming over the I-25 corridor. One or two showers or
thunderstorms may form over the Denver area through the afternoon
or early evening. Later tonight, the southeasterly flow on the
plains will help keep low level moisture in place, with more
stratus and fog developing overnight. The situation could be very
similar to what we saw this morning.




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