133 
FXUS65 KBOU 182010
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
210 PM MDT Fri Sep 18 2026

.KEY MESSAGES...

- Increasing coverage and intensity of afternoon/evening showers
  and t-storms for the lower elevations, peaking Saturday when
  locally heavy rainfall can be expected across the plains. 

- Warm temperatures through Saturday to give way to more seasonal
  conditions Sunday onwards.

- Trending drier next week, although at least isolated to 
  scattered showers will remain each day.

&&

.DISCUSSION /Through Friday/...
Issued at 210 PM MDT Fri Sep 18 2026

The day started off mostly cloudy, which slightly slowed heating,
but skies cleared allowing temperatures to climb into the lower 
to mid 80s. ACARS soundings show 200-500 SBCAPE J/kg with little 
to no CIN. Precipitable water values are around an inch. We seem 
on track for scattered to numerous showers and thunderstorms later
this afternoon and evening. Main threat will be brief heavy rain,
small hail, and wind gusts up to 40 mph. Can't rule out a stray 
severe thunderstorm or two across the eastern plains where better 
instability resides. The showers and thunderstorms will progress 
eastward and end by mid-evening over eastern Colorado. 
 
The upper level low over the Great Basin slowly moves eastward 
tonight and Saturday, increasing QG ascent over the region. Cross 
sections show skies should be mostly clear to partly cloudy 
tomorrow morning, allowing temperatures to reach their convective 
temperature (mid to upper 70s). Scattered to numerous showers and 
thunderstorms are expected again, but they should begin a couple 
of hours earlier. Storms increase in coverage and intensity as 
they move onto the plains during the late afternoon and early 
evening. MLCAPE reaches 500-1500 J/kg and 0-6km bulk shear is 
30-40 knots. A few of the storms could become severe, mainly over 
the plains where the instability will be better. Models agree we 
will see a good amount of convection tomorrow, but they disagree 
by a few hours on the timing. Will it be an early to late 
afternoon event, or a mid afternoon to early evening one?

The upper level low will move east across Wyoming on Sunday. At 
the surface east winds will bring cooler temperatures and keep low
level moisture in place. We will likely see low clouds and if 
they are thick enough, there will be areas of drizzle Sunday 
morning. Highs are only expected to reach the mid 60s to lower 
70s. Even with this, the cold air aloft and moist low levels 
yield SBCAPE up to 1000J/kg. If temperatures can climb enough to 
break the cap, scattered thunderstorms are expected. Given the 
instability and good shear, a few severe storms will be possible. 
Historically, days like this favor the Palmer Divide where 
breaking the cap will be easier. Because of the cold air aloft, 
hail will be the main threat from the thunderstorms. 

On Monday, flow aloft returns to the southwest as the upper level
low shifts east of the region. A shortwave ridge embedded in the 
southwest flow is expected to bring a mild and mostly dry day. 
However the 12Z ECMWF slowed the eastward progression of the low 
and shows showers lingering into Monday morning over eastern 
Colorado.   For Tuesday through Friday, the models loosely agree, 
but details vary. It begins Tuesday when the closed low over 
southern California weakens and starts to lift northeast across 
the Great Basin. Models vary on the strengthen, timing, and track 
of this feature Tuesday and Wednesday, which sets up the pattern 
for the rest of the week.  Ridging will take over the 
intermountain west and plains mid week. Does this upper level 
trough get trapped underneath the ridge (GFS solution) or does it 
quickly lift northeast into the Northern Plains (ECMWF solution)? 
We'll see a chance for showers and thunderstorms Monday night and 
Tuesday as the upper-level feature embedded in the southwest flow 
lifts northeast across the region. If the slower solutions are 
correct, the chance for showers could linger into Wednesday. For 
Thursday and Friday, ridging is expected over the region. It could
end up being a "dirty ridge" with subtropical moisture trapped 
underneath, continuing the chance for showers and thunderstorms. 
Temperatures from Tuesday onward are expected to be near to 
slightly above normal.  

&&

.AVIATION /18Z TAFS through 18Z Saturday/...
Issued at 1140 AM MDT Fri Sep 18 2026

Afternoon convection still appears likely today. Recent clearing
evident on satellite has allowed temperatures to reach the upper
70s with dew points still hovering around 50F. ACARS profiles have
also shown a gradual increase in instability over the last couple
hours, with MLCAPE approaching 500 J/kg. Most guidance develops
convection over the southwest metro before 20z, which then lifts
northeastward through the terminals between roughly 21-23z. Some
SHRA may linger beyond 00z but most impacts from TS or gusty
outflow winds would be before that time.

Winds are a bit uncertain today. There should be light upslope as
a prevailing direction early this afternoon, before becoming more
variable in the vicinity of convection. A return to drainage is 
expected this evening. Some guidance hints at some stratus or fog 
potential overnight though this would be almost certainly be 
dependent on getting meaningful precipitation across a large area 
near the terminals today.

Tomorrow's forecast is largely similar to today, with convection
likely developing during the early afternoon hours, lingering into
the evening.

&&

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

&&

$$

DISCUSSION...BM
AVIATION...Hiris