Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 231652
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1052 AM MDT Sat Sep 23 2017

.UPDATE...
Issued at 1045 AM MDT Sat Sep 23 2017

Made a number of changes to today`s forecast starting with
temperatures. New guidance would support lowering max temps on
the plains by 2-3 deg f and that would appear reasonable with the
extensive low cloud coverage and the ongoing weak east- northeast
upslope flow in place. Next, beefed up pops significantly along
the eastern fringe of the CWA to account for the clutter of
showers/t-storms tracking northeast at a fairly fast pace. May
actually see this convection exiting the CWA in the next hour or
so. Even so, will hold onto the high PoPs out there as this area
will remain under the forcing along a jet axis straddling the sern
corner of the state for the remainder of the day. Elsewhere, have
either removed or lower PoPs particularly along the Front Range
where the showery precip has yet to form. Same goes for much of
the high country as this area under the influence of a dry
subsident layer of air out ahead of the nearly stationary deep
upper level trough over the Great Basin. Updated forecast with
these changes will be out momentarily.

&&

.SHORT TERM...(Today and tonight)
Issued at 507 AM MDT Sat Sep 23 2017

Moisture will continue to stream northward across Colorado
through tonight, with both the upper level features and the
surface cold front moving ever so slowly eastward. Jet streak
passes by northwest of our area tonight, bringing enhance lift
tonight and forcing better low level convergence in the frontal
zone.

Lots of little things happening this morning, but not well
connected. There are more breaks in the clouds than previously
expected, but low clouds are starting to fill in from the north at
this time. Meanwhile, showers are occasionally forming on the cold
side of the cold front across the northwest corner of Kansas. All
of this should come together better by midday with a bit more
cooling/moistening in the cold air mass, widespread lift moving
in, and lift over the frontal zone also increasing. As a result,
more widespread rain should develop this afternoon and continue
much of the night. Indications are two areas of focus. Best lift
will be toward the northwest closer to the jet streak, helped by a
modest orographic lift into the mountains. The other area will be
on the cold side of the frontal zone, which should run from
southeastern Colorado across northwest Kansas into central
Nebraska. The best of this should be east of our area, but
significant rain could come back as far as Limon and Akron. Areas
east of there could get an inch or two of rain out of this
event. More intense convection that could present a flooding
threat should be further east.

Snow level is a bit lower than expected at the moment, between 9
and 10 thousand feet, probably due to the western part of the area
being a little drier at low levels. This should come up about a
thousand feet during the day, then drop back tonight with a little
cooling. There could be significant accumulations above 10-11
thousand feet, but much less at 9-10 thousand feet accompanied by
quite a bit of melting as air and ground temperatures will be
above freezing most of the time.

Models are a bit colder than our forecast. I compromised on
following this trend due to some sunshine in some areas this
morning. If the low clouds thicken up quickly, we may still be too
warm for today.

.LONG TERM...(Sunday through Friday)
Issued at 507 AM MDT Sat Sep 23 2017

Sunday and Sunday night, the closed low over east central UT
Sunday morning is expected to lift into southwest WY by 00z
Monday, with a moderate to strong southwesterly flow aloft. A
stationary front will be settled along the Front Range and Palmer
Divide with a shallow and weak north to northeast upslope
component in place. There will be a period of light to moderate
mid level qg ascent that passes across the region during the day,
strongest over northwest CO. This will result in a good chance of
showers through the day Sunday then decreasing overnight across
the north and west as the strongest qg ascent shifts to the
north. As far as the eastern plains are concerned, a strong upper
level jet will still be in place across southern and eastern CO
which should allow for showers to persist over the eastern zones
overnight. Monday and Monday night, a trough axis will stretch
from western WY into the Four Corners with a persistent
southwesterly flow in place through the afternoon. The best chc of
showers will be in the vicinity of the upper jet over the eastern
plains along with a slight chance of thunderstorms, with lighter
showers across the western part of the cwa. The models show
generally light qpf across the cwa Monday night, just to the east
of the trough axis from roughly the Front Range Foothills
eastward. The snow level will be dropping as well, 7500-8000 feet
by 12z Tuesday.

The trough splits on Tuesday with the northern branch moving into
the northern Great Plains and another piece of energy over the
desert southwest. This will result in the drier west/northwest
flow in the mid levels Tuesday into Wednesday. The GFS shows a
mid level ridge stretching southeast to northwest across CO
Wednesday into Thursday with a drier airmass and lesser pops
across the cwa. A closed low over the Desert Southwest eventually
lifts north into UT late Thursday into Friday. This feature churns
another batch of subtropical moisture into western CO which
should allow for a better chance of showers in the high country
Friday afternoon. Overall, temperatures will be below normal
through the period, coldest on Sunday and Monday then closer to
normal by the middle of the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1045 AM MDT Sat Sep 23 2017

ILS CIGs likely in the Denver metro area for the remainder of
the day. The low ceilings are not necessarily expected to
produce any appreciable precip, at least not until this
afternoon. Light east-northeast winds will prevail through 06z
tonight which will be another reason for the ILS CIGs. Rain
showers are still expected to become widespread after 23z today.
Could see patchy fog with this precip overnight, but VSBYs
probably not falling below 3 miles.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

UPDATE...Baker
SHORT TERM...Gimmestad
LONG TERM...Cooper
AVIATION...Baker



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