Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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000
FXUS65 KBOU 290200
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
800 PM MDT Wed Sep 28 2016

.UPDATE...
Issued at 800 PM MDT Wed Sep 28 2016

Another quiet night under a weak southerly flow aloft. Convective
cells formed over the mountains earlier this afternoon and
evening. Temperature-dew point spreads were 30 to 40F, so little
if any precipitation reached the ground. The cloud cover may help
keep overnight lows slight warmer tonight over the mountain
valleys. Other than small changes to line up with current
trends...will leave forecast as is for tonight.

&&

.SHORT TERM...(This evening through Thursday)
Issued at 347 PM MDT Wed Sep 28 2016

The upslope flow gradient has developed on the plains but the
magnitude of the winds generated by this gradient have not been as
strong as anticipated earlier today. Winds have already begun to
transition to a sely component with the sfc high slipping
southeast acrs nwrn Nebraska. Meanwhile...low top convection is
well underway in the high country as evident on satellite imagery
in response to the influx of mid-level moisture circulating around
the closed upper low over Arizona and the relatively strong solar
heating. Regional radar network has yet to detect any precip
inside the CWA but its not too far away in Eagle County where a
few light rain showers struggle to hold together.

Tonight...the upper ridge over the state at the present time
will slowly slip east causing winds aloft to shift from a nwly to
swly direction by morning. The warmest air aloft will pass over the
area with the ridge which should result in another night of very
mild temperatures for the end of September. High country convection
will have little reason to sustain itself much beyond sunset...although
should see a gradual increase in cirrus/cirrostratus overnight
with high level moisture gradually spreading over the four corners
region with strengthening s-sely flow aloft.

Thursday...should see cloud cover steadily increasing from south-
to-north mainly over the high country during the day as PW values
continue to creep up. Could see a few showers forming over the
higher elevations by late morning and through the day would expect
to see coverage increase over the mtns and high valleys even
though forcing aloft will be weak. East of the mtns...a general
swly downslope component should keep the boundary layer quite
dry...although can not rule out a stray shower/t-storm drifting
northeast off the high terrain and over the nearby plains late in
the afternoon with little more than gusty winds. Highs on
Thursday expected to be about the same as those today even with
the increase in cloud cover.

.LONG TERM...(Thursday night through Wednesday)
Issued at 347 PM MDT Wed Sep 28 2016

Subtropical jet streak will move over Colorado Thursday night into
Friday bringing weak lift in an increasingly moist airmass. There
is still a fair amount of stability so the showers that do develop
will likely be weak. Best chance for the mountains is probably
Thursday night with the main period of lift. For the plains there
is probably a better chance of showers/storms on Friday afternoon
due to better low level moisture. Still just low PoPs on the
plains though. For the weekend there will still be some moisture
around but the lift should be gone. There may be enough warming to
produce some diurnal showers/storms but this is also questionable.

Models are coming into better agreement on the trough moving
across the west early next week. Solutions still diverge during
this time, but there is better agreement on the speed as the GFS
has been slowing down and the ECMWF has sped up a bit. Current
solutions have the stronger southwest flow developing ahead of the
trough on Monday with the cooling coming sometime on Tuesday.
There is also pretty good agreement that the center of the trough
will come over or north of us, which means meager PoPs for the
plains mainly with the trough axis, and a likely shower band for
the mountains at the same time, then a chance of some trailing
orographic mountain showers behind the trough. If the trough is
far enough south, there could be several inches of accumulation in
the middle to higher mountain elevations, but most solutions are
too far north for a significant snow. For the plains the prospects
of significant precipitation are not looking good, but it will
turn colder and there will likely be a breezy day or two. Some
solutions have a secondary trough moving through on Wednesday
which could prolong any mountain precipitation, but this is also
lacking in other model runs. Model consensus looks alright for
most of this, though I did nudge temperatures down a little for
Tuesday and Wednesday as the model trends are looking good for
this. Significant snow (if there is any) should stay above 7-8
thousand feet, but an outside chance of snow showers down to
around 6000 feet with the coldest and furthest south solutions.
But that is looking like a pretty low probability.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 800 PM MDT Wed Sep 28 2016

Mostly clear skies will continue tonight and Thursday. Easterly
winds will slowly turn southeast and end up southerly around 06z.
Wind gusts to 25 knots will be possible at KDEN through 06z. Winds
are expected to weaken some after 06z. For Thursday...a Denver
Cyclone is forecasted to develop over the Denver area, so
forecasting wind direction will be challenging Thursday after 18z.

&&

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

$$

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



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