Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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937
FXUS65 KBOU 281638
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1038 AM MDT THU JUL 28 2016

.UPDATE...
Issued at 1038 AM MDT Thu Jul 28 2016

Complicated weather pattern setting up for today. Northerly winds
have formed along the Front Range along with patchy low clouds.
Even though the low clouds are expected to burn off soon, the
northerly flow is expected to keep the area slightly cooler, thus
lowered highs a few degrees. Thunderstorms over western Nebraska
are expected to clip northeast Colorado in the next hour or two.
Outflow from these storms could help trigger convection later
today to the west...while the cooler air helps stabilize the
northeast corner.

Northeast winds moving into the Front Range later today and a
short wave trough passing to the north Colorado are expected to
produce at least scattered thunderstorms this afternoon and
evening. Airmass is set up for severe storms today with some super
cell thunderstorms likely. The 12z sounding at DNR showed very
steep lapse rate between 700mb and 500mb. Veering winds and CAPE
up to 3000 J/kg could lead to some very big hail.

Best chance for thunderstorms is expected to occur behind a
boundary that is expected to crash into the Front Range late this
afternoon. Northeast winds to 40 mph will be possible for a short
time behind this boundary. Thunderstorm activity is expected to
decrease around midnight as the airmass stabilizes. Low levels
will remain moist and low clouds and areas of fog will develop
late tonight and Thursday morning.

&&

.SHORT TERM...(Today and tonight)
Issued at 450 AM MDT Thu Jul 28 2016

Details of todays weather will be highly dependent on the
evolution of low level wind/temperature/moisture fields as a
result of several waves of convection dropping south across the
plains. Pinning down the details will be difficult, but the
environment looks favorable for severe thunderstorms at some point
on the plains. For areas near Denver, the prospects are much more
muddled.

Common themes in the models include:

A batch of convection moving near the northeast corner early this
morning (which looks like it should pass just east of our area),
followed by another round of storms dropping through the same area
around midday. These features are already present upstream with
the second one driven by a little shortwave that the models do
capture.

A tendency for easterly winds, reinforced at times by outflow from
convection, that maintains low level moisture across much of the
area.

A preference for the moisture to pool north of Denver, with the
outflow fighting a southerly wind component and some drying on the
Palmer Divide. Most models generate some convection off the
Laramie Range in Wyoming or in the Cheyenne area that tracks
southeast through this moisture in the late afternoon or evening.

The difficulty lies in the details of the wind field and the
pattern of available energy. A steady flow of east winds would
bring juicy air as far west as the Front Range by the end of the
day. However the threat of some drying over the higher elevations
south of Denver combined with southeast storm motions from the
more favored areas north of Denver appears to limit the threat
south of town, putting Denver on the edge of the severe weather
threat. Also worth noting that the runs with the strongest outflow
have quite cool temperatures by late afternoon and the incoming
moisture may not be enough to overcome the cooling, or at least
the cooling might limit the resulting evening convection near
Denver to one quick shot as the convergence is needed to overcome
capping.

Expected CAPEs vary wildly based on the above variables. There
should be an area of at least 1000 J/kg in place that gets
convection, and shear is adequate to generate tilted storms and
supercells. So there is a decent threat of severe weather. Main
threat should be large hail, with some wind potential as well.
Heavy rain also possible but storms should be moving enough to
keep the flash flood threat low. It does appear the Denver area is
on the edge of the threat, with the most likely timing here in the
early evening from outflow boundaries feeding new convection as
the moisture increases. Previous forecast and SPC outlooks were
pretty reasonable, I just made a few minor timing adjustments and
added the mention of severe storms across the plains.

IR satellite imagery has shown continued active burning on the
Beaver Creek wildfire near the border southwest of Laramie as it
has remained pretty warm, dry, and breezy overnight. There was an
impressive smoke plume at sunset, and there likely still is one
though I can`t find it in the pictures at this time. With similar
weather today the plume will likely persist moving toward the east
and southeast. For the most part the smoke will be elevated coming
across the plains, but it will be denser over parts of Jackson and
Larimer counties.

.LONG TERM...(Friday through Wednesday)
Issued at 450 AM MDT Thu Jul 28 2016

As upper ridging remains over the southern U.S. through the next
week, subtropical moisture is forecast to work its way under the
ridge and over Colorado. Temperatures will be remaining warm, and
each day will feature a chance of thunderstorms somewhere over
north central and northeast Colorado. Friday is expected to be the
coolest day of the lot, in the wake of anticipated outflows from
this evening`s thunderstorm activity over the plains. There will
also be a decent chance of showers Friday afternoon as high
pressure over the northern Great Plains will funnel easterly low
level flow into Colorado. Daytime warming and the presence of
low and mid-level moisture will combine to produce isolated to
scattered shower coverage, mainly in the evening.

Saturday and Sunday could see a slight drop in the coverage of
showers as temperatures aloft over the state warm up enough to cap
most shower activity. By Monday and Tuesday, models are showing
additional mid-level moisture moving into the state from the
south. That will set up a daily pattern of afternoon showers over
the mountains moving onto the plains in the late afternoons and
evenings. All in all, typical weather for the first part of
August. The threat of locally heavy rain will be on the increase
through the week, especially if the winds at mid and upper levels
diminish, leading to slow storm motions. These will be things to
assess later next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1038 AM MDT Thu Jul 28 2016

North winds this morning will start to shift northeast after 18z.
Northeast winds will increase around 22z with gusts to 30 knots as
a boundary pushes through. Scattered thunderstorms are expected to
form behind the boundary. The stronger storms will be capable of
producing large hail and very strong winds. Threat for storms will
decrease after 04z. Low clouds are expected to move into the area
after 06z with ceilings falling to 1000 feet or lower. Fog with
visibility less than a half mile will also be possible.

&&

.BOU Watches/Warnings/Advisories...
None.
&&

$$

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



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