508
FXUS65 KBOU 131135
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
535 AM MDT Sat Sep 13 2025

.KEY MESSAGES...

- Scattered showers and thunderstorms expected into Sunday, with
  an isolated threat of severe storms possible Saturday afternoon
  and evening across the lower elevations.

- Cooling temperatures through the weekend.

- Unsettled, uncertain pattern for most of next week.

&&

.DISCUSSION /Through Friday/...
Issued at 204 PM MDT Fri Sep 12 2025

A slow moving trough over the west is still expected to dominate
the upper-level pattern through the weekend, keeping cooler
temperatures in place, and widespread precipitation chances into
Sunday, including some light snow for the highest elevations.
Another upper-level low is on track to enter the Pacific
Northwest early next week, but guidance still struggles to agree
on specifics.

This morning`s upper-air analysis shows that Colorado remains
downstream of an approaching upper-level low centered over the Great
Basin, keeping southwesterly flow aloft over the region. A near
100kt jet stretches downstream of the trough axis, from southern
California into Western Colorado and Wyoming. With an upper level
high centered over the Central CONUS, and the trough to the west,
mid-level moisture from both the Gulf and the Pacific are being
advected into the Intermountain West, which has already aided in the
development of severe thunderstorms across eastern Utah and Western
Colorado this morning. As the day progresses, the low is expected to
slowly slide eastward, increasing vertical ascent across our
forecast area. Though the strongest forcings are expected to remain
west of our forecast area today, expect showers and a few
thunderstorms to develop over the higher terrain through the
afternoon, and push eastward through the evening. With ensembles
showing elevated PWAT values approaching 180% to 200% of normal by
this afternoon along and adjacent to the Continental Divide, there
is potential for a few pockets of heavier rainfall, with the HREF
LPMM 6-hr precipitation zeroing in on areas near the Indian Peaks
and the higher elevations in Larimer County to see the highest
accumulations between 1 to 2". A little more uncertainty remains
for the lower elevations as patchy cloud cover this morning has
kept instability limited. Despite temperatures climbing to the
mid to upper 80s across a large majority of the plains, SPC
Mesoanalysis still shows MLCAPE values remain at 250 J/kg or
less across the lower elevations. The latest ACARS sounding
resembles an inverted-v profile with DCAPE values approaching 800
J/kg, so expect to see some gusty outflows between 45 to 55 mph
with any showers or weak storms that manage to make it off the
terrain. Some of the hi-res models keep showers in place through
late tonight, moving west to east across the lower elevations.

Saturday is still on track to see the best chances for more
widespread precipitation as mid-level moisture continues to advect
in from both the Gulf and the Pacific, and QG forcings reach their
height as the trough draws near. With increasing southwesterly flow
aloft, 0-6km bulk shear values look to increase to between 35 to
45 kts. Mixed with patchy enhanced instability in areas where
less cloud cover will allow for the best surface heating, will be
suffice for an isolated threat of marginal to severe storms across
the lower elevations in the afternoon, with the main hazards
being gusty outflows to 50 mph and hail up to 1.5" in diameter.

Showers are expected to persist into Sunday afternoon before weak
ridging moves in behind the exiting trough by the evening hours.
Cooler temperatures are expected to remain through the weekend
before a slight warmup returns with the new work week on Monday.
With ensembles still showing some discrepancies with the next
expected shortwave progged to enter the Pacific Northwest early
next week, have let the NBM ride from Monday onward. NBM seems
reasonable with a few days of warmer and drier conditions expected Monday
and Tuesday, before another potential cold front associated with
the Pacific Northwest low cools things off again by midweek.

&&

.AVIATION /12Z TAFS through 12Z Sunday/...
Issued at 529 AM MDT Sat Sep 13 2025

VFR conditions will prevail for all terminals through the TAF
period. Surface winds have taken on a more westerly component this
morning and, after a short period of weak and VRB winds after
sunrise, should favor a transition to NE flow by ~16-17Z, with the
timing of this directional shift having trended slightly earlier
over recent model runs.

Wind direction uncertainty increases substantially starting mid-
afternoon when convection is expected to push east of the higher
terrain. TAFs continue to favor predominantly NW flow owing to
storm motions, but this is low confidence, and winds will likely
be outflow-dominant much of the afternoon. Similar to previous
days, gusts 30-35 kts will be possible in the vicinity of stronger
convective cells.

As for timing of -TSRA potential, most favorable window will be
21-01Z, but -SHRA will likely persist in the general region
through much of the evening. Expect a temporary return to SW
drainage flow tonight.

&&

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

&&

$$

DISCUSSION...Bonner
AVIATION...Rodriguez