Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 230144
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
744 PM MDT Thu Jun 22 2017

.UPDATE...
Issued at 712 PM MDT Thu Jun 22 2017

Convection across the area has dramatically underperformed this
afternoon into the evening. The primary reasons for weak and poor
coverage of the thunderstorms this afternoon and this evening came
about just before noon when the convective temperature was
reached across the Foothills and adjacent Plains...thunderstorms
quickly formed and developed a thick cirrus shield that overspread
much of our area. As the storms moved east of I-25 they
encountered a robust cap, and storms weakened then dissipated as
they moved further east. The cool outflows from the initial storms
and thick cirrus limited instability this afternoon and evening.
Where the models were predicting 1500+ J/kg MUCAPE, probably more
like 500 J/kg were available. Given the convective stability east
of the Continental Divide this afternoon that will continue into
the overnight hours, updates were made to the forecast...namely
eliminating any threat of severe weather and reducing PoPs overall
from chance to slight chance or less.

After 1 AM tonight the HRRR and ESRL HRRR, and the NAM all show a
cold front entering the state from Wyoming and Nebraska. They
have been consistent for the past 4-5 hourly runs. Current surface
obs across the central high Plains are showing fairly strong N/NE
flow, and there is a strong surface high over Montana moving
south. GOES-16 WV currently shows a weak short wave trough moving
south into Wyoming. Feel these are the features the high res
models are latching onto. Fully buying into the northerly frontal
surge solution will take a leap of faith given how poorly the high
res models handled today`s convective evolution. Additionally, the
global models are also overdoing precipitation tonight...so the
updates made to PoPs were to lessen them and synch the PoPs with
the timing of the front. Given all this, and assuming the frontal
passage occurs as shown in the high res models, there should be
an hour or two of light rain showers occurring behind the front
along with low clouds and much cooler temperatures. The cool
temperatures and cloud cover will continue through Friday, and
given how hot it was this week, it will feel quite cool to most of
us throughout the day.


&&

.SHORT TERM...(This evening through Friday)
Issued at 255 PM MDT Thu Jun 22 2017

Storms continues to spread west to east across the forecast area.
With the front now all the way through the area, upslope flow
continues to increase. However, the better moisture has been slow
to work its way in, so strength of storms are weaker at this point
than initially thought. More threat right now is strong outflow
winds, with hail threat increasing as the afternoon goes on.

A shortwave currently in southern Idaho into central Wyoming is
progged to push south tonight, to push additional moisture and the
cooler air in with an increase in upslope flow. Will see a
continuation of showers and storms likely through midnight, with a
possibility of seeing convection for a few hours after midnight
with the shortwave. Forecast soundings show lower levels
moistening up with stratus expected to push in, however upper
levels still remaining unstable. For now will remove mention of
thunder after midnight for the high terrain and after 3am over
the plains, with stratus spreading across the plains and into the
foothills up to about 10kft. This will create areas of fog along
the foothills and adjacent plains.

High surface pressure will remain over the Great Plains to keep
upslope into NE Colorado Friday. Clouds will likely remain over
the most of the plains to keep temperatures readings in the 60s,
a nice respite from the heat of late. Further west of the divide,
temperatures will still be cooler than today, but some instability
may allow for isolated weak afternoon thunderstorms.

.LONG TERM...(Friday night through Thursday)
Issued at 255 PM MDT Thu Jun 22 2017

Cooling trend continues through the weekend with a moist and cool
airmass in place on the plains. A cold front will move into NE
Colorado Saturday ushering in cooler air. Easterly winds will
help to keep moisture pooling into the foothills with clouds
persisting through the evening into Saturday afternoon. Model air
soundings show increased mid-level subsidence through the weekend.
Some light precipitation will be possible but expecting more
showers and closer to the foothills vs convection. The convection
potential will increase over the higher terrain for Saturday and
Sunday as conditions will be warmer and more unstable then on the
plains. Brief heavy rain, deadly lightning and small hail with
gusty winds will be possible with any convection. Temperatures
will continue to be below seasonal normals with highs in the 70s
through the weekend.

For Monday into mid week the upper level ridge will start to build
back into the Great Basin ushering in a warmer airmass for Monday
and slowly increasing temperatures through the week. With the
warming airmass...at least for Monday into Tuesday do not expect
much in the way of convection due to a stable mid level. A lee
side trough will help to increase downsloping and warming
temperatures for Monday evening into Tuesday with lows in the 60s
and highs on Tuesday reaching back above normals in the low 90s.
Chances of convection increase on Wednesday with the possibility
of breaking the cap and having some isolated storms in the
afternoon on the plains. By Thursday GFS and EC showing a front
moving in late Wednesday that could help to destabilize the
atmosphere and increase storm chances for Thursday. Will continue
to monitor to see how models time the incoming front.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 712 PM MDT Thu Jun 22 2017

Convective potential has waned this evening and thunderstorms are
not expected through the TAF period. Light winds from an easterly
direction should occur this evening and into the overnight hours.
Between 10 and 11Z, expect a strong northerly push to occur
across the three TAF sites, with north winds 15-20kt with higher
gusts at DEN, and around 10-15 kt at APA and BJC. Rain showers are
possible (20-30% chance) for an hour or so behind the front and
have handled this with TEMPO groups. Low stratus development is
also a strong possibility ahead of the front in the upslope flow,
and especially after frontal passage when CIGs may drop to 010. In
precipitation CIGS may briefly drop to 006 or 008. At this time
VIS should stay up around 5 SM, but in precipitation it could
briefly drop to 3 SM. It looks like CIGS will lift with drier air
and the late June sun Friday morning, such that by 16/17Z and
beyond VFR conditions should prevail.

&&

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

$$

UPDATE...Schlatter
SHORT TERM...Kriederman
LONG TERM...Bowen
AVIATION...Schlatter



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