Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 231442
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
842 AM MDT Mon Oct 23 2017

.UPDATE...
Issued at 842 AM MDT Mon Oct 23 2017

Surface cold front has pushed across all of the plains this
morning. Temps dropped a quick 20 degrees along the Front Range
where downslope warming was replaced by cooler upslope and stratus
deck. Forecast adjusted for latest wind/cloud/temp trends.

&&

.SHORT TERM...(Today and tonight)
Issued at 400 AM MDT Mon Oct 23 2017

An upper level trough diving southeast across the Northern Plains
will produce a strong northwest flow aloft over northeast
Colorado today. The state will be on the subsident side of a 120+
knot jet. A surface cold front will push south through the area
this morning. This will be a glancing shot with shallow cold air.
Expected the mountains to remain on the mild side since the cold
air won`t make it over the mountains. There will be some moisture
embedded in the strong northwest flow. This will bring high clouds
to the area, mainly this morning. Orographic lift combined with
the moisture could produce light snow over the mountains this
morning. Little to no accumulation is expected. Not enough
moisture or lift for precipitation east of the mountains. However,
there may be some mid clouds this morning behind the front. As
drier air moves into the area. Clouds will decrease this afternoon
and by evening expect mostly clear skies over the state. Weaker
winds, mostly clear skies, and a cooler airmass will result in
chilly overnight lows with readings falling into the upper 20s to
mid 30s over northeast Colorado.

.LONG TERM...(Tuesday through Sunday)
Issued at 400 AM MDT Mon Oct 23 2017

Strong north-northwesterly flow aloft on the back side of a deep
high amplitude trough covering the eastern CONUS will keep dry and
gusty northerly sfc winds going on the plains of nern Colorado on
Tuesday. Could see gusts in the 30-40 mph range during the afternoon
in areas up along the Wyoming/Nebraska border. Elsewhere, should
see lighter winds due in part to the northerly component of the mean
layer flow. Temperatures on Tuesday will stay on the cool side,
generally 4-6 deg f below average. FYI, the average high for
Denver Tuesday is 62.

By Wednesday, the big upper trough to our east moves farther away
and the large upper ridge along the west coast begins to break
down, resulting in a weaker northwesterly flow aloft over northern
Colorado. Lee troughing resulting in 8-18 mph downslope flow will
help push daytime temperatures into the low-mid 70s on the
plains. High country temperatures will also rebound by as much as
6-10 deg F from the day before. The combination of a 10-18 mph
w-swly wind and min RH around 15 pct will produce high rangeland
fire danger up along the Wyoming/Nebraska border for at least a
few hours Wednesday afternoon. However, do not plan to issue any
fireweather highlights at this time.

Main focus for the past few days has been on the clipper weather
system which models have shown swinging down from the northern
Rockies Wednesday night and Thursday. With each model run, this
system appears weaker, drier and not as cold. However, models have
come into much better agreement as they all now show a relatively
strong sfc cold front driving south through nern Colorado during
the pre-dawn hours on Thursday, and the 700 mb front with the bulk
of the moisture, lift and cold air aloft passing over the CWA
during the late morning through mid-afternoon period. Latest
indications point to not as much post-frontal orographic forcing
but still plenty of cold air advection with gusty northerly winds
on the plains. QG omega fields now suggest that most of the
forcing for ascent will occur during the morning hours after
frontal passage when most of the moisture will still be up near
the Wyoming border. Therefore, QPF amounts over and along the
Front Range do not appear to be as great as they did a day or two
ago. Furthermore, the freeze/wetbulb-zero levels do not appear to
be as low either. Therefore, precip within the I-25 urban
corridor may remain as liquid during the entire 6-8 hour event,
except perhaps in higher areas of the Palmer Divide where a rain-
snow mix will be possible after say 3 PM. Meanwhile, may see only
a few inches of snow in the foothills, and perhaps an inch or two
more on higher east facing slopes of the Front Range by 00z/Friday.
By night fall, models show this quick hitting disturbance already
well south of the area, with rapid clearing in its wake. This
will allow strong radiational cooling to drop temps to the lower
20s on the plains.

On Friday, the models...particularly the GFS...show a much weaker
disturbance racing south out of Wyoming during the morning. Could
see a reinforcement of the cold air already on the plains and an
increase in cloud cover before noon, but really no precip with it.
Then quick clearing and slight warming in the afternoon with this
disturbance over the Oklahoma panhandle by then.

The weekend should be dry with temperatures gradually moderating as
a strong upper ridge builds to our west. Any gusty winds should be
confined to the nern corner of the CWA beneath a strong north-
northwesterly flow aloft. On Monday, could see a weak, dry cold
front back door into the area causing max temperatures to drop a few
degs on the plains.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 842 AM MDT Mon Oct 23 2017

Enough moisture and upslope component behind the front to produce
a stratus deck along the Front Range and across the Denver metro
area airports. There is some upslope component so hard to say
exactly how long it will take to burn off, but some hope deck
could break toward 18Z-19Z. Adjustments made for this through the
next few hours. Gusty winds behind the front have relaxed, but
still a few gusts to around 20 knots at KDEN through the day.

&&

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

$$

UPDATE...Barjenbruch
SHORT TERM...Meier
LONG TERM...Baker
AVIATION...Barjenbruch



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