712 
FXUS65 KBOU 292038
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
238 PM MDT Tue Sep 29 2026

.KEY MESSAGES...

- Widespread rainfall, heavy at times, remains likely across the
  far southern portions of our forecast area late tonight through
  early Thursday. A sharp gradient in QPF still likely across the
  Denver metro.

- Flood Watch in effect for southern Lincoln and far southeastern
  Elbert counties through Thursday morning. 

- Rain chances decrease late Thursday, leading to warmer and drier
  conditions for the weekend and early next week.

&&

.DISCUSSION /Through Tuesday/...
Issued at 158 PM MDT Tue Sep 29 2026

Quite a bit of uncertainty remains with this upcoming storm
system. A few clear trends appear to be emerging, mixed across the
noise of a full suite of 12z CAMs and global/ensemble solutions.

A well defined upper level low continues to spin near the CA/NV/AZ
border this afternoon. Meanwhile, Hurricane Polo made its second
landfall, this time over northwestern Mexico, earlier today and
continues to race northeastward thanks to the steering flow of the
aforementioned closed low. The upper low should continue drifting
eastward over the next 48 hours, with the associated vorticity
from the remnants of Polo slinging around the east/northeastern
periphery of the low on Wednesday. Deep moisture is expected to
lift northward tonight into early Wednesday, with a precipitation
shield developing over the southern half of the state. That
precipitation should crawl northward during the day, before
eventually pushing off to the east as the closed low slowly starts
to shear out by Wednesday night.

Many folks will not be surprised by today's forecast 
developments... with a pronounced drying trend noted across most 
deterministic and ensemble guidance since this time yesterday. As 
is often the case with closed lows south of the Four Corners 
region, it's rather difficult to get well aligned moisture/upslope
this far north. There remain a handful of holdouts across today's
guidance... most notably a few MPAS-core CAMS, a few REFS members,
and the(soon to be retired) NAM and 3km NAM, which continue to
suggest at the potential for significant rainfall totals as far
north as the Denver metro. Conversely, there are now more members
of the GEFS and ECMWF ensemble that have zero precipitation for
DEN (11, with several more with <0.05") than those with >1" of QPF
(8). 

While those CAMs have been bullish regarding the mid-level low 
and surface cyclogenesis in northern New Mexico/southeastern 
Colorado, I am rather skeptical of these solutions. These seem to
be dependent on a convectively driven feedback cycle that results
in a much stronger/westerly track of the surface low. While
possible, this feels implausible. Indeed, even comparing near 
term HRRR forecasts, it seems likely that there will be a 
continued correction to the southeast with it's QPF. Though we
can't completely ignore the wetter solutions, it's also difficult
to ignore a rather clear signal from the rest of 12z guidance 
that favors the weaker 700mb and surface low. Unsurprisingly, we 
did nudge lower PoPs and QPF down a bit, though this likely wasn't
nearly enough of a shift and I suspect the subsequent forecast 
will see more dramatic cuts there. We also trimmed the extent of 
our Flood Watch, which now only covers far southeastern Elbert and
southern Lincoln counties.

After this storm system departs, guidance is in good agreement
that a broad ridge will develop over the Rockies by the end of
this week. Outside of a weak shortwave or two working through the
northwest flow aloft, this should lead to an extended period of
near/above normal temperatures and little if any precipitation
through at least early next week, if not a bit longer than that.
MOS guidance suggests highs near 80F from Friday through most of
next week, with some deterministic guidance a bit warmer.

&&

.AVIATION /18Z TAFS through 18Z Wednesday/...
Issued at 1207 PM MDT Tue Sep 29 2026

CIGS will be tricky today as cloud base should be between 045 and
060, but for now FEW or SCT seem to be the most likely coverage at
that height. The chance for sprinkles this afternoon and evening
is under 15 percent so we left any mention out of the TAFs. North
winds 10-13kts will transition to east 7-11 kts this afternoon,
then southeast after 03Z. We are tracking moisture form Polo as it
merges with an approaching Pacific trough. Confidence in the
forecast continues to be low overall, including for aviation
impacts. For now, it looks like rain should hold off until 12Z or
later. BKN CIG 030-045 are expected after 09Z at all TAF sites. 
Best chance of rain is after 15Z at APA, 16Z at DEN and BJC, and 
rain chances are high throughout tomorrow afternoon and evening. 
It's rare, but we are keeping a prevailing SHRA all afternoon and 
evening. Winds should be north to maybe northeast 8-14kt with 
gusts to 20kt. We do not expect any TS. CIGs under the heavier 
showers may drop below 020, but shouldn't get any lower than that 
and VIS should be 4SM or better even in the heavier showers. Heavy
rain and CIGS dropping below 020 are not likely, but for now will
include with a PROB30 in the TAFs.

Looking ahead rain chances will remain in the forecast through 18Z
Thursday, along with continued MVFR CIGs. We should be able to 
transition from prevailing to TEMPO or prevailing after 06Z 
Thursday, stay tuned.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Flood Watch from midnight MDT tonight through Thursday morning 
for COZ047.

&&

$$

DISCUSSION...Hiris
AVIATION...Schlatter