Lake John, Colorado Weather Forecast Discussion

889
FXUS65 KBOU 301114
AFDBOU
Area Forecast Discussion
National Weather Service Denver/Boulder CO
514 AM MDT Tue Sep 30 2025
.KEY MESSAGES...
- Isolated weak showers and thunderstorms this afternoon and
  evening.
- Temperatures will remain above normal through Friday.
- Precipitation potential will increase again come Saturday.
&&
.DISCUSSION /Through Monday/...
Issued at 114 PM MDT Mon Sep 29 2025
Showers and thunderstorms have thus far remained confined to the
mountains and Palmer Divide, mainly east of the Continental
Divide, with diurnal northeasterly flow aiding in initiation and
providing some surface convergence to keep things going. Largely
southerly flow aloft is also hampering their ability to extend
into the lower elevations, but with ACARS soundings showing the
weakly capped environment over the urban corridor inching closer
to eroding, the activity over the southern foothills and Palmer
Divide should begin to migrate north and drift into portions of
the Denver metro and plains with isolated to scattered coverage
later this afternoon. Instability and shear parameters are not
too robust however, and will certainly limit any organization.
Thus, main hazards with any convection will continue to be briefly
heavy rain, gusty outflows up to ~40 mph, and some small hail
with the strongest storms. Activity will dwindle quickly as we
enter the early evening hours, with a slightly cooler but still
milder than normal night in store for most.
The open wave evident at 500mb over Colorado today will depart to
the northeast tomorrow as it continues to fizzle. Southwest flow
will remain in place aloft stemming from a deepening low pressure
system offshore of the PacNW, but conditions will be inching drier
and less unstable. This should lead to much more isolated
coverage for afternoon showers or a weak thunderstorm, with the
best chances again in the high country.
For the Wednesday through Friday period we`ll see a reinforcement
of warmer than average conditions, with the increasingly negative
tilt of the western US trough keeping our area on the northwestern
periphery of the high pressure centered over the southern CONUS.
Highs will range from to upper 70`s to mid 80`s for a majority of
the plains and urban corridor. Precipitation chances will be near
zero for this period, although a few showers may creep into the
high country come Friday if the trough can speed up enough.
Ensemble guidance seems to be loosely coalescing around a
transition to a wetter pattern by Saturday as a closed low or
shortwave lifts into the Rocky Mountain region, with more
uncertainty as far as how this would progress Sunday onwards. Some
winter weather impacts can`t be ruled out for the mountains over
the weekend depending on system trajectory, but not much can be
said beyond that yet. At the very least, almost all ensemble
members suggest a return to at minimum near-seasonal
temperatures, if not below average, by Sunday.
&&
.AVIATION /12Z TAFS through 12Z Wednesday/...
Issued at 514 AM MDT Tue Sep 30 2025
VFR to prevail through the TAF period. Southerly winds will
continue into mid morning (15-16Z) and then become light and
variable as they transition to the northeast to east after 19Z.
Isolated high-based showers will be possible after 22Z Tuesday.
Soundings show a cap around 500mb, this will keep showers weak and
shallow and likely prevent thunderstorms from forming today. Will
cover the weak shower potential with a VCSH. Can`t rule out a wind
shift from the showers in the 22Z to 02Z time frame.
&&
.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION...Rodriguez
AVIATION...Meier

NWS BOU Office Area Forecast Discussion