Severe Thunderstorm Warning 309, NWS Denver/Boulder, CO

Area Forecast Discussion
National Weather Service Denver/Boulder CO
627 PM MDT Thu Aug 13 2026

.KEY MESSAGES...

- Localized flash flooding possible due to heavy rain producing
  thunderstorms. Highest risk is over the eastern Colorado
  plains, where a Flood Watch is in effect from 3 pm to 10 pm.

- Another round of storms Friday afternoon/evening with a
  continued threat of locally heavy rain.

- Monsoon moisture lasts through the weekend, bringing scattered
  showers and thunderstorms most days.

- Warming and drying trend still set to begin Monday.

&&

.DISCUSSION /Through Thursday/...
Issued at 120 PM MDT Thu Aug 13 2026

Main threat today will be flash flooding from heavy rain producing
thunderstorms. Satellite showed the rather thick mid level cloud
deck over the mountains and I-25 Corridor is thinning, allowing
for stronger solar insolation and warming. Observed ACARS
soundings show that we've nearly broken the cap as of 1 pm. In
fact, we're now seeing towering cumulus over the mountains and
even moderate cumulus over portions of the plains. In addition,
one could argue a weak shortwave (in the form of a departing right
entrance region of an upper level jet branch) was riding up
through central/north central Colorado. Finally, weak pressure
rises are occurring over the northeast plains which means a more
easterly upslope low level component is expected just prior to
storm initiation. Therefore, all ingredients are in place for
scattered to numerous storms this afternoon, pushing eastward
across the plains through this evening. There could even be a
round or two of weaker (but still briefly heavy) storms behind the
initial stronger storms. For more hydro details about the threat
of flash flooding, see the HYDROLOGY section of this Area
Forecast Discussion below.

We can't rule out an isolated severe storm as MLCAPE grows to
800-1400 J/kg over the plains, with a wet microburst being the
main threat and just an isolated marginal hail threat in this
monsoonal, heavy rain environment.

Friday's weather is shaping up to be quite similar to today's.
We'll still be under the deeper monsoonal moisture plume although
there are signals for slight drying and warming. However, that
could very well be offset by the increase in boundary layer
moisture and 60F plus dewpoints and patchy fog/stratus tonight.
While there is no discernible shortwave at this point, we still
expect a minimally capped environment. That will allow scattered
to numerous storms to fire over the mountains by early afternoon
and spread east across the plains late in the afternoon and
evening. See Hydrology discussion again for more details for
Friday's heavy rain/flash flood threat.

Saturday and Sunday are also shaping up to be active weather days.
While there is slight drying noted as flow aloft turns slightly
more W-SW, we'll likely see daily shortwaves as we'll be in the
favorable right entrance region of an upper level jet. Bulk shear
will also be increasing again which means a greater risk of a
couple severe storms. Temperatures will remain below seasonal
normals by several degrees with potential for backdoor cool
fronts. Highs on the plains are expected to average in the lower
80s.

For early next week, there is still good agreement for a drier
westerly component as the ridge begins to amplify over the
Central/Southern Rockies. That will support less storm coverage
starting Monday, and temperatures warming to or just above
seasonal norms. There's more uncertainty for later next week
whether we return to monsoonal southwest flow or stay drier and
more westerly.

&&

.AVIATION /00Z TAFS through 00Z Saturday/...
Issued at 555 PM MDT Thu Aug 13 2026

Showers and thunderstorms have impacted all of the TAF sites this
afternoon with KAPA receiving the strongest storm thus far.
Scattered showers and thunderstorms will continue for the next few
hours, but after 02z things become more uncertain. For now,
introduced a prob30 from 2-6z to account for the chance of a
shower or thunderstorm to make it to the TAF sites. Gusty outflow
winds up to 30kts will be possible along with a brief drop in
visibility.

After convection ends tonight, all sites should be light and
variable for a few hours before more of a drainage pattern settles
in around ~10-12z for KDEN and KAPA. Overnight, ceilings around
1000 - 1500 feet are possible for KDEN and KBJC (with the
potential of as low as 500 ft for KBJC). Tomorrow will be a
pretty similar day with scattered showers and thunderstorms
expected in the afternoon and into the evening, so introduced a
tempo from 21z till 00z, and then a prob30 for the rest of the
evening to account for any lingering showers or storms. Gusts up
to 30 kts are possible with the strongest storms and visiblities
could be reduced at times.

&&

.HYDROLOGY...
Issued at 120 PM MDT Thu Aug 13 2026

There are several ingredients in place for heavy rain this
afternoon into the evening;

1) Warm cloud depth is 5,000-6,500 feet, deepest on the plains.
2) Precipitable Water (PW) 150-180% of normal.
3) Average storm motion near 15 kts, and some training of cells
also possible given potential for shallow/non-existant cold pools
in storms.
4) Moderate instability, leading to stronger storms.

The above mentioned ingredients are expected to produce heavy
precipitation rates, on the order of 1.5" or greater in 30-45
minutes. Therefore, it's not impossible for localized flash
flooding in urban areas should stronger storms develop. The
highest risk would be just east of Denver, unless we can surge in
the stronger easterly low level winds with the weak surge on the
eastern plains (greater uncertainty here). Short range models and
HREF still point to the highest probability just east of Denver,
per latest PMM and LPMM suggesting local 2-4" rain totals in the
stronger storms. And it should be noted that could fall in just an
hour or slightly more given the favorable heavy rain parameters
in place. Therefore, we'll maintain the Flood Watch for flash
flooding over the eastern plains, while expanding it slightly into
southern Lincoln County where a greater risk is foreseen into
this evening.

For Friday, the setup and ingredients look quite similar again, with
the highest risk just east of the I-25 Corridor. However, forward
storm motion is expected to increase slightly to around 20 kts.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 10 PM MDT this evening for COZ042-044>049.

&&

DISCUSSION...20
AVIATION...MAI/Schnell
HYDROLOGY...20

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