Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 252136
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
336 PM MDT Mon Sep 25 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 301 PM MDT Mon Sep 25 2017

Upper low over Wyoming will push northeast into North Dakota
overnight, with the main upper trough hanging back over the Great
Basin and down towards Baja keeping southwesterly flow aloft
through Tuesday. Airmass over the plains should stay overall
stable this evening in a light upslope flow. Scattered showers
will continue moving northeast with mainly light rainfall. A few
stronger showers and even a thunderstorm possible may drift off
the higher terrain over the Palmer Divide area earlier this
evening. Snow levels are around 9-10 thousand feet, and should
drop down near 8000 feet overnight, though only a slight chance of
showers will remain south of I70 overnight. Skies should be
clearing with drier air pushing in, which should allow for quick
radiational cooling and a good chance for fog to form over the
plains and high mountain valleys. This should keep temperatures
moderated from hitting freezing over the plains - have opted not
to issue any sort of highlight.

On Tuesday, an extra push from the northeast from surface high
pressure will keep the plains stable, with any chance of showers
being relegated to the high country, mainly south of I70. The
airmass across the whole forecast area looks more stable, so have
not introduced thunder into the mountains either.  Should see
more sun as well as slight warm advection aloft, so temperatures
will be about 7-10 degrees F warmer.

.LONG TERM...(Tuesday night through Monday)
Issued at 301 PM MDT Mon Sep 25 2017

North central and northeast Colorado is still expected to enter into
a period of moist and unsettled weather beginning in the high
country on Wednesday. An upper low closing off over the Desert
Southwest on Tuesday is forecast by models to deepen before slowly
tracking north-northeast towards Utah and Colorado on Wednesday and
Thursday. Strengthening southerly flow out ahead of the upper low
will likely spread mid-level air laden with moisture (indicated by a
specific humidity around 6 g/kg at 700 mbs) over the forecast area
Wednesday night and Thursday. Most models indicate max 700-500 mb QG
ascent over eastern Colorado during this same period. Max bndry
layer QG ascent east of the mtns is predicted to occur during the 12
hour period from 06z-18z/Thursday with a moist east-southeast low-
level flow. Could see 12-hour rainfall amounts along the Front Range
ending at 18Z Thursday anywhere from a quarter inch near the WY
border to well over an inch in the foothills of Jefferson and
Douglas Counties and across Park County. As for the snow potential,
models, particularly the GFS and NAM are cranking out pretty high 24-
hour snow totals ending 00z/Friday, the bulk of which falling over
higher terrain south of I-70. Based on projected dry and wetbulb
temps and other factors, I do not see the snow level lowering much
below 9500 feet during the Wednesday night/Thursday morning time
frame, and the bulk of any snow accumulation will probably occur
above 10500 ft MSL in the Summit County and Park County area.
Overall believe snow totals will be less than 6 inches, but above
timberline could see 6-10 inch totals producing slushy and icy
pavement over the high mtn passes. With low to moderate confidence
in these snow amounts, will refrain from hoisting any winter wx
highlights at this time.

For Friday night and Saturday, the upper trough lifts out which
allows a drier and gradually warmer zonal flow to set up over the
state. Models still indicate areas of light QPF over higher
elevations, apparently the product of daytime heating and orographic
forcing. Areas east of the mtns should be dry for the most part on
Saturday with a drier southwesterly low-level flow. However, on
Saturday night, models show a weak upper air disturbance passing
over the area potentially producing isolated to scattered showers to
the CWA. Most of this shower activity will once again be found at
higher elevations.

For Sunday and Monday...fairly big differences in the models
prevents me from slaying far from the generally dry and warmer
forecast presently out there.


&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 301 PM MDT Mon Sep 25 2017

MVFR-VFR conditions will prevail into this evening as light
easterly upslope flow continues. Heavier showers over the
foothills will affect the area airports between 22-01z, which may
initially help to raise ceilings a bit, with brief decreases in
ceiling heights directly under the showers. Some clearing is
expected after 03z tonight, which should allow some patchy fog to
develop after 09z tonight and into early Tuesday
morning...bringing IFR conditions for Tuesday morning as well.
Slight westerly winds off the foothills may keep the fog out of
BJC however. MVFR to VFR conditions expected through Tuesday
afternoon, with less shower coverage and continued easterly flow.



&&

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

$$

SHORT TERM...Kriederman
LONG TERM...Baker
AVIATION...Kriederman



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