Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 201700

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1100 AM MDT Thu Sep 20 2018

Issued at 1100 AM MDT Thu Sep 20 2018

Shower activity from last night`s upper trough passage is clearing
out of the eastern parts of the state at the present time. Showers
will linger over southern Lincoln, Washington, Sedgwick and
Phillips Counties will continue for another hour or so, and then
move east. Satellite imagery shows some shower development over
the mountains with a few weak echoes also showing up on radar.
Subsidence should be increasing across the state through the
afternoon, so showers should remain weak and then diminish through
the afternoon hours. Temperatures should remain cooler through the
afternoon, spite of increasing sunshine as the upper trough
progresses out of the state. The lack of warm temperatures will
help ease the fire weather concerns in the mountains, but gusty
west winds will also provide fresh air to the wildfires being
managed in Jackson and Grand Counties.


.SHORT TERM...(Today and tonight)
Issued at 232 AM MDT Thu Sep 20 2018

The main forcing provided by the upper level trough is currently
moving through central Colorado helping to sustain light showers
over the foothills and plains. Lingering showers are expected to
continue through the morning hours for the plains with lowered
overcast skies further east ending between 10 and 11 am. The
backend of the trough is currently over the western portions of
the state with an increasingly subsident airmass behind it. A
surface low will deepen and move slightly south helping to
increase WNW winds to scatter out clouds in the lower levels by
late morning. Temperatures will be slightly cooler with highs in
the lower 80s on the plains and 60s in the high country. Clearing
skies and light winds will move in the the evening and overnight
hours. Fire weather concerns continue to be heightened over the
northern mountains south of the Wyoming border. Increased pressure
gradients provided by the deepening sfc low during the afternoon
will help to increase winds over the higher terrain with gusts
into the 25-30 range for a varied period of time. Relative
humidity values will drop during the say with the dry and
subsident airmass with lower teens expected.

.LONG TERM...(Friday through Wednesday)
Issued at 232 AM MDT Thu Sep 20 2018

The flow aloft will become more westerly on Friday and then
southwesterly for the weekend.  Overall, cross-sections show a
relatively dry airmass across the area Friday thru the weekend.
As for temps, highs will be near seasonal levels on Friday but rise
back into the 80`s for the weekend over nern CO.

On Monday a weak upper level trough is forecast to move quickly
across the area with some mid level qg ascent.  The ECMWF has better
moisture with this feature versus the GFS which is drier.  As a
result, the ECWMF has a better chance of showers over the higher
terrain.  Across nern CO a cold front will move across during the
day which will drop temps back into the 70`s.  As the weak upper
level trough moves across, in conjunction with the front, this
could lead to at least a slight chance of showers across the

For Monday night into Tuesday both the ECMWF and GFS have northwest
flow aloft over the area.  The ECMWF has a disturbance embedded in
the flow which affects nrn CO late Monday night thru Tuesday morning
as a secondary cold front moves across nern CO. As a result, this
leads to a chc of pcpn mainly across the plains. Meanwhile the
GFS has no such disturbance and maintains dry northwest flow aloft
over the area. For now have included some low pops for late
Monday night into Tuesday morning. As for highs, it looks like
readings will only be in the 60s for Tuesday over nern CO.

By Wednesday dry northwest flow aloft will will be over the area
with temps slightly below seasonal normals.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1100 AM MDT Thu Sep 20 2018

Skies should clear out through the afternoon, with westerly winds
but minimal aviation impact. Later tonight, the westerly flow
aloft is forecast to induce an anti-cyclonic circulation along the
front range with winds turning easterly at KDEN after about 11 PM.
This may produce some low clouds and maybe some fog to the Denver
airports through the early Friday morning hours. Will mention some
MVFR ceilings in the next set of TAFs through about 10 AM.


Issued at 232 AM MDT Thu Sep 20 2018

While temperatures behind the trough and front will be cooler, the
dry airmass behind it will still help to create elevated fire
weather conditions across the northern mountains of Grand and
Larimer counties. The pressure gradient will increase due to a
deepening surface cyclone that will raise wind speeds by the
afternoon. Sustained winds of 10 to 15 with gusting from 15 to 20 mph
will be possible for a few hours with RH values dropping into the
lower teens.

Elevated fire danger will be in place across the higher terrain
Friday through the weekend.  Winds will be fairly light in most
areas Friday and Saturday but will increase on Sunday.




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