381
FXUS65 KBOU 210532
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1132 PM MDT Mon Jul 20 2026

.KEY MESSAGES...

- Storm coverage and intensity increases through Wednesday and
  Thursday as monsoonal moisture moves into the region. Storms
  from Tuesday onward will be capable of producing locally heavy
  rainfall, and some threat of flash flooding.

- Storm coverage expected to decrease again by Friday into this
  upcoming weekend, although mountain areas should see at least
  scattered storm coverage each afternoon and evening.

&&

.DISCUSSION /Through Monday/...
Issued at 206 PM MDT Mon Jul 20 2026

Temperatures have soared early today with weak inversions and
further warming aloft. We`ll be pushing 100F or a bit more across
most of the plains and I-25 Corridor through late this afternoon,
before finally some slight relief arrives over the next few days.
The Heat Advisory for the I-25 Urban Corridor through Weld county
will be allowed to expire at 9 pm this evening.

Scattered storms were developing in the northern mountains as
expected this afternoon. While the majority of the storms stay in
the high country, isolated high based convection with gusty winds
and light rain may also spill onto the plains into this evening as
the hot temperatures help break the cap.

For Tuesday, the upper level high directly overhead will elongate
and shift to the southeast. This will allow for a slight bump in
monsoonal moisture as flow aloft turns light southerly. The
moisture bump would be much greater, however, if it wasn`t for the
current dry punch into southern Colorado seen in the waver vapor
imagery. A "cool" front will back into Colorado late tonight and
early Tuesday morning. This will support a slight increase of low
level moisture over the plains as well. Precipitable water (PW)
values still increase to 150-175% of normal over the higher
terrain, so locally heavy rainfall will be possible. The main
threat of any flooding would be over the most severe portions of
the 2020 burn scars and the 2024 Alexander burn scar, given an
inch of rain would be possible in 30 minutes from the strongest
storms. The best chance of storms on the plains would be late in
the day and especially in the evening as east/southeast low level
flow continues to support low level moisture advection into slow
moving storms. Given the very warm temperatures, these will likely
be outflow dominated so any heavier rain may be relatively short-
lived, but there is an opportunity for more balanced storms and
locally heavy rain especially if they persist later into the
evening.

The greatest risk for locally heavy rainfall and any threat of
flash flooding will enter the picture for Wednesday and Thursday.
PW values are still forecast to reach the daily maxes for this
time of year, with ensemble averages generally pointing to 1.3-1.4
inches along the I-25 Corridor, and 150-200% above normal across
the entire forecast area. That`s the common ingredient for heavy
rainfall potential for both days, but each day will also have
their own characteristics that may be more/less favorable for
heavy rain.

On Wednesday, we`ll have very slow storm motions, but rather
shallow warm cloud depth less than 3,000 feet. On Thursday, warm
cloud depths are forecast to increase to ~5,000 feet, but storm
motions will also be faster as mean wind/storm motions are
forecast to increase to 20-25 mph. At the same time, instability
grows slightly on Thursday so storms may be more organized and
intense (heavy rain but not much hail). We`ll also have to watch
the timing of a shortwave expected for Thursday. The GFS remains
slightly faster, while the EC is slightly slower and would be
favorably timed with peak daytime heating and convection. Whatever
the case, both days would at least have a couple ingredients
favorable for flash flooding, so we`ll monitor that closely over
the next couple days before issuing any Flood Watches for flash
flooding.

Looking ahead to Friday and the weekend, there is more agreement
now that we`ll be drying and stabilizing. This would focus mostly
on the plains with less storm coverage (mainly isolated near the
higher terrain), while mountain areas should be able to hold onto
more moisture and instability under the ridge. After a brief
respite from the heat and near normal temperatures Thursday, hot
weather will likely return with highs pushing back to the 95-100
degree range across the plains and I-25 Corridor, with potentially
another Heat Advisory by next Sunday.

&&

.AVIATION /06Z TAFS through 06Z Wednesday/...
Issued at 1123 PM MDT Mon Jul 20 2026

A weak cool front is making its way southward late this evening
and should arrive between 06z and 07z as winds shift to a more
E/ENE direction.  Sct -shra along with a few weak tstms have been
developing by this front so have kept in Prob30`s for -shra at
DEN and BJC from 07z-10z. Can`t rule out a weak tstm as well but
will leave out of current Tafs for now.  Meanwhile another surge
may move across by 10z with a wind shift to the north. This surge
could bring some smoke to the area by 10z.

On Tue, winds will be ENE by 15z and then shift to more easterly
by 18z. Smoke may linger thru 18z as well. By 21z winds will
become SE and continue thru Tue evening. At this time, it appears
best tstm chances Tue evening will stay north or the area.
However, if an outflow boundary from convection to the north were
to move into the area then tstm chances would increase. For now
have kept a Prob30 starting at 01z.

&&

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

&&

$$

DISCUSSION...Barjenbruch
AVIATION...RPK

NWS BOU Office Area Forecast Discussion