Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 201654

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1054 AM MDT Thu Jul 20 2017

Issued at 1054 AM MDT Thu Jul 20 2017

Will scale back the chance of showers this afternoon slightly.
Still expecting scattered thunderstorms to develop over the
mountains this afternoon, but the strongest flow of monsoonal
moisture will be moving across Utah and western Colorado according
to the latest water vapor satellite imagery. There is a weak
circulation left over from last evening`s convection that is
moving over southwest Colorado at the present time. This feature
will become a focusing mechanism for afternoon showers over the
San Juan Mountains over the next few hours. The presence of this
feature will tend to bring the heaviest shower activity today to
that region. Showers over the north central Colorado mountains
today are expected to be slow-moving again this afternoon, and
more capable of producing locally heavy rain than large hail.
Showers spreading onto the plains laster this evening still seems
possible as outflow from mountain showers produces boundaries for
additional shower development.


.SHORT TERM...(Today and tonight)
Issued at 400 AM MDT Thu Jul 20 2017

GOES-16 satellite imagery showing the main monsoon moisture plume
over Arizona, Utah, and western Colorado. Even though models are
all over on the specific amount of moisture, they all agree it
will increase today. Based on the models and satellite imagery,
the best moisture should be north of I-70 today and will have the
highest pops over northern areas. HiRes models show convection
beginning over the higher terrain early afternoon. This convection
throws outflow boundaries eastward with additional storms firing
off them as they progress eastward. Main threats with the storms
will be heavy rainfall and outflow wind gusts. CAPE increases to
around 1000 J/kg, so a few severe storms will be possible with
wind gusts to 60 mph the main threat. However, can`t rule out hail
up to one inch in diameter.

Temperatures will not be as hot as yesterday due to an increase in
mid and high clouds and easterly surface winds. Along the Front
Range, expect highs in the lower to mid 90s where more cloud cover
is expected. Out on the plain, it should be warmer with less cloud
cover with highs climbing into the mid to upper 90s.

.LONG TERM...(Friday through Wednesday)
Issued at 400 AM MDT Thu Jul 20 2017

Picking out the subtle ripples slowly revolving around the upper
high over the central Great Plains will continue to add a degree of
uncertainty to the upcoming forecast, at least through Saturday.
Models show the monsoon moisture plume draped across all of Colorado
through Friday night. The plume is then progged to shift southward
more over southern Colorado on Saturday with the Great Plains ridge
breaking down and a new ridge building over the Great Basin. From
specific humidity and theta-e plots, it appears the greatest
moisture will be passing over northeast Colorado Friday afternoon
and evening. Models indicate mean layer PW values on the plains
anywhere from 1.2 to 1.8 inches around 00z/Sat. In addition to the
available moisture, sfc-700mb are fcstd to turn n-nely Friday
morning behind what appears to be a weak cold front. By evening
model soundings indicate e-nely upslope winds to around 600 mb which
will potentially provide the necessary lift to generate scattered
showers/t-storms on the plains and scattered to numerous shwrs/t-
storms over the Front Range mtns. Furthermore, weak w-swly transport
flow will elevate the chance for locally heavy rainfall, more so
over and along the Palmer Divide where effects of the upslope slope
will be most pronounced. Rainfall rates of 1.5 to 2.0 inches per
hour are possible with the stronger slow moving storms. That evening
storms are expected to drift out across the plains in advance of
with one of those subtle ripples in the mid-level flow. Models show
this wave taking its sweet time passing over the fcst area, and
possibly not exiting the area until Saturday afternoon. This could
cause t-storms on the plains and in the high country to linger all
night and well through Saturday morning. With PWs on the decline,
rainfall rates and amounts should not be as great, though would not
rule out a local 0.5 to 0.75 rainfall somewhere acrs southern
portions of the CWA.

By Saturday night with the moisture plume continuing to sag south
away for the CWA, rain chances should drop off. This southward shift
in this moisture is due in large part to a transition to nwly flow
aloft with Great Plains ridge collapsing and the Great Basin ridge
building. Precip chances decrease even further on Sunday with
continued drying. Late day storms should largely be confined to
higher elevations. The shift to nwly flow should result in cooler
temperatures across the CWA starting Saturday. Highs during the
weekend are forecast to be as much as 5-10 deg f below average.

Next week, medium range models show the Great Basin 500mb high
making a slow eastward migration, reaching western Colorado by
Tuesday. Monday and Tuesday look dry for the most part east of the
mtns, with isolated to scattered convection in the high country.
By Wednesday, the upper high is predicted to shift east of
Colorado, which returns the state to light south-southwesterly
moist flow which should increase our chance for showers and
t-storms esply in the high country. Lastly, should see
temperatures gradually moderate during this three day period.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1054 AM MDT Thu Jul 20 2017

Some thunderstorm activity is expected over the Denver area this
afternoon and evening. Locally heavy rain will be possible along
with gusty outflow winds. Showers will dissipate by late evening
with skies clearing out overnight. No other aviation impacts are


Issued at 400 AM MDT Thu Jul 20 2017

Models showing precipitable water values climbing above an inch
today. Heavy rain is expected with the stronger storms that form.
Winds aloft are expected to push storms to the northeast at 15 to
25 mph, so the flash flooding threat will be low today. If
hydrology problems occur today it will be due to storms
regenerating over the same.

The chance for flash flooding should remain elevated on Friday and
Friday night with mean layer precipitable water values on the
plains running anywhere from 1.1 to 1.8 inches. Storms with the
best chance for producing heavy rainfall are more likely to be
found over and adjacent to the Palmer Divide with ongoing post-
frontal northeasterly/upslope flow. Mountain areas may also
see pockets of intense rainfall produced by slow moving storms.
Heavy rainfall chances should steadily diminish through the day
Saturday which a shift to a drier northwest flow.




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