Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 260217

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

Issued at 758 PM MDT Mon Sep 25 2017

Latest GOES-16 WV imagery shows the strong jet streak well east of
our area into northwest Kansas and translating further east with
time. Have thus reduced PoPs for the eastern Plains north of I-70.
Weak synoptic lift associated with the strong trough to our
northwest has generated very light showers across the central
mountains and Palmer Divide, including south metro Denver. These
showers will not amount to much rain accumulation and should
weaken/diminish after midnight. Clouds should hang on near the
foothills and they have been hard to clear anywhere east of the
Continental Divide with shallow cool air in place. With good cloud
cover tonight and high dewpoints, will not need a frost advisory.
The other potential impact is fog formation. With light winds and
low T/Td spreads radiation fog is a strong possibility in any
areas that clear out the stratus deck. Clearing skies before
sunrise not likely in in most spots, thus have used "Patchy" fog.
Across Lincoln County especially along the east side of the
Palmer, and the South Platte Valley east of Greeley seem to have
the best changes for significant fog development tonight and will
keep "Areas" of fog in the forecast. Any fog that forms should
burn off by 9/10 AM.


.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 758 PM MDT Mon Sep 25 2017

MVFR ceilings should continue tonight with light east and then
south winds. Best bet is around the 020 level with SCT to BKN
decks at the TAF sites. Fog is possible near DEN but most
confident in the scenario that keeps the fog well northeast of the
airfield. Winds should be light or south/southeast, keeping any
fog that forms across the South Platte River valley at bay. There
is a remote chance if skies clear earlier than normal, radiation
fog could form at any of the metro area terminals. Chances of that
scenario playing out are too low to include in the TAF ATTM. There
is a bit of uncertainty in when skies will clear/scatter out to
VFR tomorrow. Late morning to midday looks like the most likely
evolution. Otherwise no impacts via aviation weather through
Tuesday night with light winds (less than 10 kts) expected area-




SHORT TERM...Kriederman
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