496
FXUS65 KBOU 151131
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
531 AM MDT Tue Sep 15 2026

.KEY MESSAGES...

- Cooler weather arrives today. Limited shower and storm chances,
  mostly over the higher terrain.

- Unsettled pattern rest of this week, but still some uncertainty
  with regard to daily precipitation chances. Thursday looks like
  the highest opportunity of measurable rain (50-70%) and a
  locally heavy rainfall threat.

- Temperatures bounce back Wednesday, but then average near or
  even slightly below seasonal normals through next weekend.

&&

.DISCUSSION /Through Monday/...
Issued at 228 AM MDT Tue Sep 15 2026

A cold front moved across the plains early this morning, which
will bring a marked change to the weather. Cooler temperatures
can be expected in all areas east of the Front Range mountains,
with highs mostly in the upper 60s to lower 70s. There is still
some uncertainty with the amount of cooling, as models vary
considerably on the low level moisture and stratus potential. At
this point, we do think we`ll see at least areas of stratus this
morning, with the highest concentration generally Denver and
Boulder to points south and southeast. Given the somewhat limited
extent, we do expect that to break up by late morning although
mid level clouds could slow that dissipation and warming. As a
result, we`ll lean toward the cooler side of any short term
guidance. With regard to shower and storm chances, a few showers
and even a couple rumbles of thunder continues over the mountains
early this morning. Those should gradually dissipate as we lose
any upper level support (right entrance region of the upper jet).
However, sufficient daytime heating over the mountains will allow
scattered showers and a few storms to redevelop this afternoon.
The plains will be much more stable, making it difficult for any
showers drifting off the mountains to survive very far east onto
the I-25 Corridor and adjacent plains.

For Wednesday, we`ll see flat upper level ridging which will allow
temperatures to warm to above normal readings again (mid 80s for
most of the plains). Instability will increase slightly, with a
few hundred J/kg of MLCAPE. Most convective initiation will once
again be over the mountains, but given improving thermodynamic
profiles we should see eventual development and movement of
scattered showers and storms into the I-25 Corridor and adjacent
plains through the late afternoon and evening hours. Some of these
showers and storms will likely continue overnight given further
moisture and mid level theta-e advection.

Thursday is still shaping up to be a more active weather day. Two
reasons for this; Precipitable water values climbing to 200% of
normal and upper level support from a disturbance originating from
the northeastern tropical Pacific (currently near the southern
tip of the Baja California peninsula, or vicinity of Cabo San
Lucas). However, timing of this feature may be key in potential
convective coverage and intensity, as too early of an arrival
(like the GFS) could mean we`ll just see scattered and lighter
showers and a few storms. A later arrival during peak heating
could mean more numerous showers and storms with locally heavy
rainfall. While the EC and GEM are a little slower than the GFS,
they could still be a bit on the early side. In any case,
something to monitor over the next couple days as warm cloud
depths are forecast to increase to 3,500-6,000 feet, fairly
impressive for this time of year.

The weather still looks fairly active into Friday. While there is
less moisture, there is still some upper level support due to a
trough drifting slowly east across the Northern Rockies and
northern Great Basin. Timing is uncertain here, but appears
Saturday could still be quite active before drier weather more
likely arrives late in the weekend. Temperatures will be holding
near or even slightly below normal for this period.

&&

.AVIATION /12Z TAFS through 12Z Wednesday/...
Issued at 531 AM MDT Tue Sep 15 2026

Stratus is developing a little ahead of schedule, and also with
more extent. That`s despite the rather meager low level moisture
along the I-25 Corridor and Denver area TAF sites (dewpoints only
in the lower 30s). We`re leaning toward the NAM on this forecast
given the recent developments, and with the winds turning more
easterly in the low levels, slight low level moisture advection,
and some mid level clouds aloft any breakup will likely be
delayed til after 18Z. In fact, depending on clouds and thickness
of the stratus deck some of our stratus breakout runs with more
clouds and deeper moisture kept it in most of the day. We don`t
think the stratus would be that deep (less than 1,000 feet) so
we`ll shoot for something closer to 19-20Z, but a 30% chance it
could last til at least 21Z.

Regarding winds, a light N wind will start, transitioning more
E-ESE 17Z-20Z. Winds are expected to increase after 21Z but
potentially not til 00Z as Denver cyclone develops. That cyclone
means a chance (30%) that winds could turn NW at KBJC as early as
00Z and KAPA and KDEN as early as 02Z, so something to watch.
Given limited low level moisture, the threat of any fog even with
the developing cyclone is less than 10%.  Otherwise enhanced SE
winds expected this evening with a few gusts around 20 kts. Just a
slight chance of showers after 21-22Z, but most of these will
dissipate as they move off the higher terrain. Also, low levels
would be too stable to support VRB outflow winds.

&&

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

&&

$$

DISCUSSION...20
AVIATION...20

NWS BOU Office Area Forecast Discussion