Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 260107
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
707 PM MDT Tue Jul 25 2017

.UPDATE...
Issued at 707 PM MDT Tue Jul 25 2017

Increased pops earlier due to the better coverage of showers and
thunderstorms. Scattered storms are still possible over the far
eastern plains where the airmass is still unstable. Farther west
along the Front Range and over the higher terrain, the airmass is
more stable with isolated storms at best. The showers over the
area will continue push off to the east and slowly diminish late
this evening. Expect most if not all the showers to by gone by or
a little after midnight. Also adjusted pops to be a little higher
through the late evening hours.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 318 PM MDT Tue Jul 25 2017

Flow around the west side of a strong ridge across the southern
Plains has advected mid and upper level moisture across our area
for most of the day today. It has set up a battle between nearly
saturated mid and upper levels of the atmosphere and very dry air
below 500 mb. The low-level dry air seems to have *mostly* won
the battle this afternoon and evening as current radar and GOES-16
satellite trends show weak showers forming over the Front Range
mountains and mostly virga east of the mountains. Only a few
lightning strikes so far. The thick mid-level cloud deck and poor
mid level lapse rates associated with nearby ridge and the
monsoonal moisture plume limited instability across our area..
Not seeing any other significant forcing available this evening
into overnight to generate new convection or bolster the existing
convection. PoPs will be limited east of I-25 this evening through
midnight other than along the Palmer Divide. Will keep somewhat
higher PoPs across the far NE corner of the state where convective
temps may be reached by 00Z and much better moisture is available
in the low levels. Any storm that forms out there this evening
may be capable of 1" hail and wind gusts to 50 mph.

Tonight the ridge remains across west Texas and moisture from the
southwest continues to stream into Colorado, leading to mostly
cloudy skies and a warm night tonight, especially across the urban
areas along the I-25 corridor. Any lingering showers should end
by midnight. An exception could be Jackson/Larimer counties where
a weak short wave trough (barely visible in global models) skirts
the CO/WY border between midnight and 6 AM or so. Otherwise low
PoPs prevail.

Wednesday a significant change in the pattern occurs below 500 mb.
The passage of the weak short wave trough overnight looks to drive
a cool front south across the Plains of Colorado prior to sunrise.
As the surface high pressure area builds to the northeast across
the central Plains, the winds from the surface to 700 mb will turn
more east and northeast. Models differ on the amount of low-level
moisture behind the front on Wednesday but as it stands now the
60 degree isodrosotherm could make it west of I-25. Should be low
60s dewpoints across the eastern quarter of the Plains. Associated
with the cool front and cloud cover will be much cooler
temperatures throughout the day Wednesday, expecting highs only
around 80 across the Plains. With the much needed low- level
moisture injection behind the front, PWATs across the Plains may
exceed 1.4", which would be near the climatological max. Model
soundings and observational trends support thin CAPE profiles and
deep warm cloud depths with LCLs around 3-5 kft off the ground. Thus,
main concern Wednesday afternoon and evening would be the
potential for heavy rain. Upslope flow and some daytime heating
should initiate convection along and east of the Divide midday.
Not much synoptic support for rising motion and instability will
be limited. Storms would move towards the ESE at a decent clip,
reducing the chance of heavy rain sitting over one specific area
more than about 30 minutes. Any storms that form will be capable
of an inch in 30 minutes, but overall storm coverage and storm
motion means will hold off on a flash flood watch but will
carefully monitor convective evolution tomorrow.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 318 PM MDT Tue Jul 25 2017

Models keep the upper ridge axis over Colorado Wednesday night
through Friday night with weak westerly flow aloft. There is
benign synoptic scale energy over the CWA on the QG Omega fields
all five periods, Wednesday night through Friday night.
Southeasterly low level flow seems to dominate the CWA on the
models much of the time. Precipitable water values are in the 0.75
to 1.5 inch range Wednesday night through Friday night. Dew points
are progged in the 50 F west to lower 60s F east range all five
periods. There is some CAPE around through the periods, the
highest values to be over the western half of the plains late day
Thursday and again late day Friday. The QPF fields show decent
amounts of measurable rainfall through the periods. Will leave the
fairly high pops going in the mountains and western plains with
all the moisture available. For temperatures, Thursdy`s highs look
to be 1-2 C warmer than Wednesday`s. Friday`s highs will be will
be another 1-3 C warmer than Thursday`s. For the later days,
Saturday through Tuesday, the upper ridge pushes further west
Sunday and Monday. Medium range models show a weak trough over the
western U.S. plains on Tuesday. Temperatures look to be around
seasonal normals all four days.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 707 PM MDT Tue Jul 25 2017

There is just a slight chance at seeing an additional
thunderstorm in Denver area tonight. Scattered showers will
decrease as they push off the east this evening. There are a few
pockets of low clouds with ceilings of 1500 to 3000 feet in the
Denver. Expect these clouds to dissipate 03Z to 05Z as the showers
end. Winds turn northerly overnight ushering in cooler air and
will increase moisture. Expect low clouds to form 11Z to 13Z with
ceilings of 2000 to 4000 feet. There will be another good chance
for showers and thunderstorms Wednesday afternoon, with the better
chance over western and southern parts of the Denver area.


&&

.HYDROLOGY...
Issued at 318 PM MDT Tue Jul 25 2017

Very high precipitable water values and deep warm cloud layers
could result in very high precipitation rates for storms that can
form Wednesday afternoon and evening. Best chance for an inch in
30 minutes would be the southwest foothills east of the
Continental Divide of Jefferson/Clear Creek/Gilpin and Park
Counties. The Palmer Divide will also see a higher threat of heavy
rain after 2 PM Wednesday. Storms will mainly be moving out of
the west-northwest at 20-25 mph

&&

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

$$

UPDATE...Meier
SHORT TERM...Schlatter
LONG TERM...RJK
AVIATION...Meier
HYDROLOGY...Schlatter



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