Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 210300
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
900 PM MDT Thu Jul 20 2017

.UPDATE...
Issued at 900 PM MDT Thu Jul 20 2017

Updated the PoP and Wx grids for ongoing convection that should
keep going across Arapahoe and Lincoln/Elbert Counties this
evening. Overage coverage is low as previous convection from this
afternoon has stabilized the atmosphere across much of the Plains.
The thunderstorms should diminish by midnight tonight and skies
should gradually become partly cloudy then mostly clear by
sunrise.

Latest model trends are showing Friday with a similar set up to
today but with a bit more precipitable water and the addition of a
cool front that looks to be fairly well timed with early
afternoon heating the northern third of the Plains of Colorado.
The convective evolution Friday afternoon could be storms
initiating midday across the Front Range mountains and the Palmer
Divide, aided by upslope flow, which then move northeast across
the Plains propagating along the gust fronts generated by the
initial round of convection. Shear values still look too weak for
well organized convection but the threat of heavy rain will be
watched closely with any slow moving widely scattered
thunderstorms.

&&

.SHORT TERM...(This evening through Friday)
Issued at 332 PM MDT Thu Jul 20 2017

The slowly evolving upper air pattern over the southern United
States will continue funneling monsoonal moisture over Colorado
through tomorrow. This afternoon has seen another round of showers
develop over the mountains and move slowly north-northeastwards.
Moderate rainfall amounts and hail up to marble size has been
reported so far. The freezing level was 16,000 feet this morning,
showing how warm the airmass is. Precipitable water values along
the I-25 corridor now range from 0.85 to 1.10 inch, which is moist
enough for storms to produce periods of heavy rain. Satellite
imagery indicates that most of the plains should remain dry this
evening as a capping inversion is in place over the southern
plains into southeast Colorado. The shower activity over the
mountains this afternoon should begin taking a bit of a more
northeastward track this evening as the embedded short wave aloft
rotates around the strong upper ridge over the southern plains.
Shower activity will gradually diminish through the evening with
the loss of daytime heating.

For Friday, another round of convection is expected. The short and
medium range models indicate that moisture levels will be slightly
higher tomorrow, so the threat of heavy rain may be a little
higher. Temperatures are also forecast to be a couple degrees
cooler, owing to the slight increase in the amount of moisture
present. Like today, a sunny morning across the state will give
way to increasing clouds and showers through the afternoon. The
highest pops will be over the mountains and adjacent plains. Winds
aloft will remain weak, so that slow-moving storms will pose a
threat for locally heavy rain.

.LONG TERM...(Friday night through Thursday)
Issued at 332 PM MDT Thu Jul 20 2017

Upper level pattern has a persistent set of ridges with varying
shortwaves bringing an unsettled pattern for portions of the long
term. starting Friday night into Saturday the high pressure will
be over the Great Plains with a shortwave making its way over the
state as the ridge progresses eastward. Moisture PW values will
vary between 0.75 up to 1.5 further east over the plains. This
increase moist flow combined with NNE flow at the surface will
help to increase storm chances along the Foothills and Palmer
Divide Friday afternoon and evening. Storm motion will be slower
Friday evening so storms could drop a quick inch to 1.5 inches in
a short period of time increasing chances of flooding. Behind the
shortwave northerly flow will increase helping to usher in
slightly cooler temperatures for Sunday. This is due in part to
the high pressure over the plains transitioning eastward with a
ridge starting to build in over the Great Basin. Highs will drop
from upper 80s to lower 80s by Sunday. Subsidence will increase by
Sunday morning helping to dry out conditions through the
afternoon. There will only be a slight chance of storms for the
higher elevations

For next week the upper ridge over the Great Basin will make a
slow eastward progression moving over the state by Tuesday. Monday
and Tuesday will have storms over the higher country with
temperatures in upper 80s to lower 90s. Upper level flow will once
again be light, however PW moisture values will be slightly lower
then through the weekend with values from 0.75 to 1.0. by
Wednesday the upper ridge will shift east of the state helping to
bring another disturbance through CO. Increased storm potential is
possible by Wednesday afternoon with temperatures once again
getting into the mid 90s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 809 PM MDT Thu Jul 20 2017

VFR conditions through at least Friday. After -TSRA early this
evening which generated a few gust fronts with winds to 20-25
kts, things have quieted down across the metro area. An isolated
thunderstorm 20 miles south of Denver should be the last storm of
the evening. It is moving away from the airfield. Radar and
GOES-16 satellite trends do not show any further gust fronts
heading for the TAF sites, and we do not expect any further
thunderstorms after 03Z. We are watching the current convection
over Arapahoe/Elbert/Lincoln Counties this evening which in
theory could generate a gust front moving out of the southeast
towards DEN. Chances of a gust front forming and making it to DEN
are very low, thus expect S and SSW winds at APA and DEN tonight
and Friday morning. Light WNW winds at BJC. Tomorrow a cool front
will reach the terminals around 22Z, with north flow 10-12 kts and
slight chances of thunderstorms along and behind the front.
Handling Friday afternoon TAFs with a prevailing VCTS for now
after 22Z...and as the convective evolution becomes clearer on
Friday more details will be put into the TAF.

&&

.HYDROLOGY...
Issued at 332 PM MDT Thu Jul 20 2017

Slow moving storms this evening and again Friday afternoon will be
capable of producing locally heavy rainfall. Areas that have had
rainfall over the past couple days will be at the highest risk of
flooding, if more slow moving storms pass over.

&&

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

$$

UPDATE...Schlatter
SHORT TERM...Dankers
LONG TERM...Bowen
AVIATION...Schlatter
HYDROLOGY...Dankers


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