781
FXUS65 KBOU 171117
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
517 AM MDT Tue Jun 17 2025

.KEY MESSAGES...

- Severe storm threat with high winds and large hail grow today,
  but still some uncertainties. Most areas will see some rain.
  Cooler.

- Strong signal for record breaking heat from Thursday into next
  weekend, along with potential for critical fire weather
  conditions.

&&

.UPDATE...
Issued at 248 AM MDT Tue Jun 17 2025

Rare nocturnal storms, several severe, have developed in the
previously unworked but increasingly moist and unstable airmass.
MLCAPE was still 1500-2000 J/kg in these wee morning hours, so a
few impressive storm updrafts were noted. Enough shear evidenced
on the straight-line hodographs for splitting storms, too. We
think storm intensity eventually settles down in the early morning
hours as the airmass eventually gets worked over, but a few
showers/isolated storms will remain with weak QG lift and a light
post-frontal upslope component.

The main question for today surrounds our afternoon severe storm
threat, which could start as early as noon but should be
essentially over by 6 pm as storms move rather quickly eastward
with the shortwave. Despite the rather good agreement of severe
storm development by early afternoon in the convective allowing
models (CAMs), there are some uncertainties. This largely
surrounds extent of stratus this morning, recovery in
temperatures, and then arrival time of shortwave. The stratus is
expected to be fairly widespread in the wake of these early
morning showers and storms, which will slow temperature recovery.
However, forecast soundings show that even temperatures around
70F with dewpoints in the mid 50s would yield MLCAPE of 1500 J/kg
but with a weak cap. Some models are warmer than that, which
would yield closer to 2000 J/kg. The shortwave noted in Utah
right now is on track to reach the Front Range by 18Z, a little
too early to coincide with peak heating. However, with cooling
aloft lapse rates steepen accordingly and the cap should be
broken, and be able to tap into the rich low level upslope and
higher MLCAPE values. Thus, we still expect a severe threat to
develop by noon-1 pm and then quickly progress east across the
plains as storms organize into more linear structures. Large hail
will be a threat, but a damaging high wind threat increases east
and southeast across the plains. Some model guidance suggests
gusts to 80 mph possible in enhanced bow echoes (something more
typically seen in the Central Plains). We collaborated with SPC
and pulled the Slight Risk back to the I-25 Corridor considering
the best MLCAPE likely building here. Meanwhile, there was an
upgrade to Enhanced Risk for east central Colorado where MCS
development and bowing will be more likely as storms organize
further.

&&

.DISCUSSION /Through Monday/...
Issued at 126 PM MDT Mon Jun 16 2025

Current satellite imagery shows cumulus clouds developing over the
Front Range mountains as light elevated convective showers are
just initiating, primarily in Larimer county. These showers will
continue for the next couple of hours, spilling across I-25
corridor and Denver metro. Severe threat is low at this time, as
SPC mesoanalysis shows decent CIN just east of the foothills due
to a weak cap still in place. However, daytime mixing will
eventually erode the cap. As these storms travel east, primarily
into eastern Weld, Logan Sedgwick and Phillips counties, they will
be entering into an environment that will support supercell
development. MLCAPE is expected to increase to 3,000+ J/Kg with
model Skew-T soundings showing 9 degrees C/km 500-700 mb lapse
rates and bulk shear >30 kts. Due to these expected conditions,
strong wind gusts up to 70 mph and large hail (golfball sized) are
possible, mainly between 6 PM and 10 PM. Tornado threat is very
low, but cannot rule out a brief spin up.

Overnight, a cold front will cross the region bringing gusty winds
and a few scattered light showers in the morning. Tuesday will be
a lot cooler than the past couple of days, with ensemble guidance
being in good agreement of the plains reaching up to the mid 70s.
An upper level shortwave is expected to track across the region
Tuesday afternoon, which could bring scattered thunderstorms. A
deep moisture field will be advected into the region from the
southeast, bringing dewpoint temperatures into the high 50s (and
low 60s far northeast), with MLCAPE values up to 2,000 J/Kg and
steep lapse rates. The only parameter that is lacking is bulk
shear, as models indicate less than 30 kts. If guidance starts
trending towards increasing shear, confidence will increase for
severe storms over the plains. If these storms do materialize,
wind and hail will be the primary threat.

The cooldown will be very brief as we will start warming up again
on Wednesday. Temperatures should reach up to the mid 80s across
the plains, with mostly dry conditions. However, the main concern
towards the end of the week will be the possibility of a heat
wave Thursday through the weekend. Ensemble guidance has continued
to trend towards multiple days of 97+ degree temperatures across
the lower elevations. Friday should have the best chance reaching
100 degrees, as each member of the ECMWF has the max temperature
being over 100 degrees. There is still a little uncertainty
however, as the GEFS has most of their members reaching up to 99
degrees. Regardless, it will be very hot until we get into next
week, as guidance has indicated a system moving through sometime
Monday or Tuesday.

Another concern with the scorching heat, is the fire weather
threat. As an upper level ridge travels east, southwesterly flow
aloft will bring dry conditions to the mountains and plains.
Relative humidity values could get as low as the single digits and
breezy wind gusts could bring about critical fire weather
conditions. We will continue to monitor.

&&

.AVIATION /12Z TAFS through 12Z Wednesday/...
Issued at 517 AM MDT Tue Jun 17 2025

Most of the thunderstorms were pushing to the south and east, and
the threat of any severe storms is going rapidly with it. However,
there may still be a few showers or even an isolated storm around
this morning. Areas of stratus with MVFR/IFR ceilings are expected
to gradually lift through 18Z. That`s when things for storms get
interesting again. A shortwave will move across the area, likely
sparking another round of storms. Those could start as early as
19Z and then organize into a linear storm complex which would
potentially produce high winds and less extensive large hail as
they move east across the I-25 Corridor. We have beefed up the
winds for now to ~G40kts for a most likely scenario in a TEMPO
group, but a 10-20% of reaching severe thresholds (50 kts) at
KDEN. We think the main threat of storms would be pushing east of
the TAF sites by 22-23Z, but still a chance of a lingering
shower/isolated storm til 02Z Wednesday. Then gradual clearing and
lighter drainage winds developing beyond 02Z.

&&

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

&&

$$

UPDATE...Barjenbruch
DISCUSSION...Ideker
AVIATION...Barjenbruch