879 
FXUS65 KBOU 300030
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
630 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 /00Z TAFS through 00Z Thursday/...
Issued at 629 PM MDT Tue Sep 29 2026

Winds are currently still very light (<7KT) across the region 
although a shift to a southeasterly component can be seen starting 
to take shape on radar. We expect shower activity this evening to be 
limited to the mountains and foothills. Winds will turn light and
variable late tonight before becoming northerly in the early 
morning hours.

Scattered light showers will begin to arrive at 12Z-15Z. Ceilings 
may drop down to 2,500-4,000 ft AGL at times, particularly during 
periods of persistent rainfall. While uncertainty remains regarding 
the intensity of the rainfall, the timing of the most persistent 
appears to be in the afternoon, between 19Z and 24Z. Winds will 
remain N-NE through most of the day, with some fluctuations and 
brief gusts up to 25KT near the heaviest downpours. 

&&

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

&&

$$

DISCUSSION...Hiris
AVIATION...AA