143 
FXUS65 KBOU 242009
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
209 PM MDT Thu Sep 24 2026

.KEY MESSAGES...

- Some showers this evening and overnight, followed by some patchy
  fog.

- One last round of showers/storms likely Friday afternoon.
 
- Drier and warmer this weekend before an upper level trough
  brings cooler temperatures and potentially our next chances for
  showers/storms early next week. 

- Medium chance (30% to 50%) for mountain snow above 10,000ft 
  late Monday into Tuesday. 

&&

.DISCUSSION /Through Thursday/...
Issued at 124 PM MDT Thu Sep 24 2026

It's been a relatively cool and cloudy day across most of our
forecast area, though there's been little rain to speak of so far.
There have been a few areas of slightly deeper cumulus across the
northern mountains and along a weak surface boundary in Weld
county which have produce some light showers. A few additional
showers could develop along any convergent boundary this afternoon
and evening, but otherwise the rest of the afternoon should remain
fairly quiet. 

As we get into the evening hours, a very weak mid-level low is
forecast to slide into northwestern Colorado, leading to a brief
period of east-southeast upslope developing across the forecast
area. Combined with some modest surface convergence and increasing
MUCAPE, this should allow for a cluster of showers and 
thunderstorms to develop over the northern foothills this evening
into the early overnight hours. If the HREF/REFS members are to 
be believed, some locally heavy rainfall would be possible before
upslope quickly diminishes after midnight. Moist easterly surface
flow overnight should allow for widespread fog and stratus to
develop across the plains, with that wrapping into the
northern/northwestern half of the Denver metro by early Friday.

The overall forecast changes very little tomorrow. The
broad/disorganized upper trough will kick out one last weak 
shortwave on Friday. We should see isolated to scattered
showers/thunderstorms develop by midday over the higher
elevations, with this activity pushing eastward through the
afternoon and evening. Deterministic guidance remains fairly
bullish with metro precip chances, though I'd want to see how the
overnight fog/stratus plays out before committing to higher PoPs.

The exiting upper trough will quickly be replaced by weak ridging
across the region by Saturday, leading to a warm and dry weekend.
Most statistical guidance warms temperatures into the mid 80s on
both Saturday and Sunday, with only enough moisture for some
scattered cloud cover across the higher elevations on Sunday.

Attention then turns to the potential for a stronger storm system
to impact the region by sometime early/mid-week. Guidance
generally agrees that a sharp upper trough axis will reach the
northern California coastline by Sunday evening, which then drifts
into the desert southwest towards the Four Corners region by
Tuesday. Forecast confidence quickly decreases during this period,
with solutions ranging from a strong cutoff low over Baja
California to a positively tilted trough that quickly swings into
western Texas, with a few more favorable solutions scattered in
between.

There are a few interesting tidbits across the past couple model
cycles that are worth noting. First, while ensemble mean solutions
look favorable for precipitation here, there is a distinct bi-
modal distribution in forecast QPF within each respective ensemble
system. This would imply that this storm is "boom or bust", with a
fairly even split between boom/bust at this point in time. There
are also additional complications from the interaction with what
would be the future remnants of Hurricane Polo. While the
deterministic forecast (NBM) is a fairly reasonable middle ground
(i.e., cooler and increasing chances of rain/mountain snow), 
predictability and forecast confidence will be low for the next 
several days.

&&

.AVIATION /18Z TAFS through 18Z Friday/...
Issued at 1154 AM MDT Thu Sep 24 2026
 
Winds will turn north/northeast as the afternoon continues. 
Rain showers are possible this afternoon, so PROB030s will start
around 22z this afternoon. Isolated showers could linger past evening
hours (~4z), so some PROB30s may need extensions depending on 
convective development. Winds should turn to drainage tonight for
KAPA and KBJC, but some uncertainty remains for KDEN tonight as
most models favor a southeasterly direction (~150) with some
indicating more of a northeast flow. NAM and GFS MOS guidance
favors the southerly drainage flow (~170-210). Guidance also 
shows a cyclone developing tonight, so wind directions could be 
changed for tonight. Also in play is a TEMPO for fog/low stratus 
at KDEN and KBJC from ~11-15z. Dewpoint depressions are forecasted
to be ~1 or < 1 for KDEN and KBJC, but KAPA is not expected to 
get this potential. Visibility could drop to as low as 1/2 SM for 
KDEN and KBJC if the fog comes to fruition. 

Winds in the morning look to turn variable for KBJC and KAPA while
we wait for the lower ceilings to rise (~15z). Northeast flow will
settle in the afternoon for KBJC and southeast flow for KAPA. KDEN
may remain in a southeast flow. A PROB30 for -TSRA is in place 
Friday afternoon for Denver, (22z-03z), and will likely be 
introduced for the other sites in the next TAF amend. A TEMPO was
almost used considering a majority of the models are in good 
agreement for convective development tomorrow, but for now we 
stuck with the PROB30. Brief decrease in visibilities and gusty 
winds up to 35mph are possible. After 0z chances drop quickly as
the convection shift east, but isolated may chances may linger for
a few more hours. 

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

&&

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

&&

$$

DISCUSSION...Hiris
AVIATION...Schnell