Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 221534
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
934 AM MDT Sun Jul 22 2018

.UPDATE...
Issued at 934 AM MDT Sun Jul 22 2018

Going forecast through this evening looks on track. Temperatures
will be a little cooler today due to an increase in cloud cover
and a northerly push. Models showing a similar set up to yesterday
with most of the convection initiating over the foothills and
mountains and then spreading east along outflow boundaries. Severe
threat will be confined to the eastern plains where ML CAPE
climbs to 1000-2000 J/kg.

Forecast for tonight and Monday is quite uncertain. Airmass
becomes very moist with precipitable water values reaching 1.25 to
1.5 inches along the Front Range and over northeast Colorado.
Models greatly vary on the amount and placement of precipitation.
The ECMWF shows a few areas receiving 3 inches of rainfall. The
06Z GFS, seems to be a dry outlier with precipitable water a half
inch lower in places and a lot less precipitation. The RAP, NAM,
GEM, are somewhere between the GFS and ECMWF. Will increase pops
and QPF for tonight and Monday. May need to consider Flash Flood
Watch for parts of the area. Will watch what the 12Z models show
before making any decisions on a watch.

&&

.SHORT TERM...(Today and tonight)
Issued at 453 AM MDT Sun Jul 22 2018

The airmass continues to gradually moisten, but like yesterday the
increase is deep moisture without a corresponding increase in the
surface moisture. As a result, we will not really be more unstable
than yesterday, and the increased cloud cover should result in a
drop in high temperatures of a few degrees, which may actually
reduce the instability. Winds aloft will become more northwesterly
today, with low level winds becoming northerly this afternoon as
what is currently a very poorly defined cold front develops. It`s
not really colder though, but perhaps a little drier air coming in
from southeast Wyoming toward the Front Range this afternoon. As
this flow becomes more northerly tonight, some of the better low
level moisture now from Nebraska into northeast Wyoming will wrap
back into northeastern Colorado.

Meanwhile, what appears to be an area of weak lift, or at least
better moisture, will slowly move across the area. This will come
faster than is optimal for the Front Range. Some light showers are
already spreading into the mountains, and this should result in
it clouding up early in Denver today. Normally, this would hold
our temperatures back more, but the air aloft is so warm it will
still likely get into the 90s even with the reduced sunshine.
Better timing for the eastern border areas though the cloud cover
may still be a factor in convective development.

We should wind up with CAPEs of 500-1000 J/kg over the mountains
and Front Range, with up to 1500 J/kg on the plains. Shear is
fairly weak, and the wind profile will favor forward propagating
storms, so there is a limited threat of marginal hail and wind
where there is more energy. High precipitable water (from 1 inch
over the Front Range up to 1.5 inches on the plains )and deeper
warm cloud depth by evening suggest a heavy rain threat, but the
wind profile does not favor slow moving storms. Motions from 10-20
mph with this much moisture would suggest potential for a quick
1-2 inches, but durations of an hour or less. We`ll be alert for
anything that could organize storms differently, like an outflow
boundary stalling parallel to the flow, or sustained foothills
convergence this evening when the deeper flow is more northerly,
but at this time the threat of flash flooding looks low so we will
just mention heavy rain with some of the storms.

Showers may continue overnight as the moisture will still be here
and possibly even increasing as the plains moisture wraps around
this developing system. It could become a steadier rain in an area
of convergence on the plains for a little while. In this time,
storm motion could be slower, but hopefully the intensity will
also be less. Models are suggesting some overnight cooling, but
with as much clouds and moisture as we`ll have that may be
doubtful. For now we`re a little above guidance.

.LONG TERM...(Monday through Saturday)
Issued at 400 AM MDT Sun Jul 22 2018

The upper ridge center is over New Mexico on Monday then moves
slowly westward. By Wednesday morning at 12Z it is located over
south central Arizona. The jet level flow is 30 to 60 knots from
the west and southwest Monday into Monday night. On Tuesday, it is
west and west-northwesterly. There is nothing indicated on the QG
Omega fields in the way of synoptic scale energy all four periods.
Low level winds look to be east and southeasterly on Monday and
Monday night. They are south and southeasterly Tuesday and Tuesday
night. Precipitable water progs show values in the 0.75 to 1.50
inch range Monday in Monday evening, then they decrease a bit into
the 0.50 to 1.25 inch range late Monday night through Tuesday
night. Looking at CAPE, the highest values are over the mountains
and foothills late day Monday and evening. On Tuesday, the highest
CAPE is over the eastern and southern CWA. For pops, will go with
40-60%s in the mountains Monday afternoon and evening. On the
plains, 40%s west to 10%s east look good. Will go with lower pops
on Tuesday, with the highest over the mountains and Park County.
Little to nothing on the plains. For temperatures, Monday`s highs
looks to be 3-7 C cooler than today`s highs. Tuesday`s highs come
back up 4-8 C from Monday`s. For the later days, Wednesday
through Saturday, models keep the upper ridge centered over the
Desert Southwestern United States. The flow aloft is 40 to 70
knots and west-northwesterly in direction much of the time.
Fairly decent moisture remains in place through the later days,
with 0.50 to 1.50 inch of precipitable water all week into next
weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 934 AM MDT Sun Jul 22 2018

VFR conditions will prevail today. Scattered thunderstorms will
develop after 20z, and could produce localized MVFR visibilities and
wind shifts with gusts to 30 knots this afternoon. For tonight
moisture will continue to increase with scattered showers and
thunderstorms. A prolonged period of low clouds and rain is possible
for late tonight and Monday morning.

&&

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

$$

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


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