373
FXUS65 KBOU 240917
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
317 AM MDT Wed Jun 24 2026

.KEY MESSAGES...

- Numerous strong to severe thunderstorms expected today for the
  lower elevations, particularly late afternoon through tonight.
  Strongest storms may produce very large hail exceeding 2-3" in
  diameter.

- Additional round of strong to severe thunderstorms expected
  Thursday. Localized flash flood concerns will exist both
  Wednesday and Thursday.

- Critical fire weather conditions this afternoon in the mountains
  given warm, dry and breezy weather and the potential for
  isolated dry thunderstorms. Notably higher fire danger will
  develop this weekend as hotter and windier weather returns.

&&

.UPDATE...
Issued at 239 AM MDT Wed Jun 24 2026

Given ongoing convection, this update will focus on relevant
changes, trends and nuances with regard to the short-term (~36 hr)
forecast period. See below for yesterday`s discussion covering more
of the long term forecast.

Our eastern plains are sternly holding on to a moist, unstable
airmass with a continued pocket of ample shear especially from
Washington County northeastward. These conditions will sustain
potential for scattered strong thunderstorms through about
sunrise. Localized flooding remains possible in areas of training
convection given observed rainfall over the past ~12 hours.

Today`s setup is again a complex one, but key ingredients for
severe weather remain in play. Yesterday, a moderately capped
environment limited significant convection over the urban
corridor, perhaps aided by both cooler post-frontal temperatures
and also smoke aloft. The former doesn`t appear to be as likely of
a limiting factor today per forecast soundings, with most showing
more pronounced mixing with a few degrees of warming at low to
mid-levels. We`ll still be contending with some smoke, but that
alone shouldn`t be sufficient to substantially buffer convective
initiation. We do have a deck of low stratus across the urban
corridor early this morning, but guidance suggests this should
scour out reasonably quickly after sunrise.

Instability, shear profiles, steep lapse rates and
climatologically anomalous moisture will collectively fuel
scattered to widespread thunderstorms by late afternoon/early
evening, supporting intense supercells once again capable of
producing hail locally exceeding baseball size. The more
pronounced upper-level shortwave will arrive in the evening and
overnight, resulting in another atypically late bout of severe
weather which looks to continue through the majority of the
night. Considering antecedent precipitation and the propensity for
multiple rounds of convection today and tonight, expect a
steadily increasing flash flood threat into the overnight period.

Lastly, Red Flag Warnings were expanded to encompass all of out
mountains zones for this afternoon. Although sustained winds may
be on the marginal side, high-based convection may spur periods of
much gustier conditions, particularly north of I-70, as well as a
potential for isolated dry thunderstorms - a dynamic enough
combination of factors to warrant the upgrade.

&&

.DISCUSSION /Through Tuesday/...
Issued at 1255 PM MDT Tue Jun 23 2026

The busy week of severe weather rolls on for our area with today
likely bringing the highest risk of the week for the urban corridor.
This is a complex forecast given the risk each day will depend on
earlier convection. Thus, the risk for this evening will depend on
how the afternoon evolves, but multiple rounds of strong to severe
thunderstorms are possible today. Following this morning`s cold
front, winds are northeast and eventually are expected to turn to
the east behind an outflow boundary from storms in Kansas and
Nebraska by later this afternoon. This will further advect moisture
into our area. A subtle shortwave trough should move across northern
Colorado this evening providing better lift to the area.

The ingredients look to be in place for a potentially lengthy severe
weather threat today, but a strong cap is currently in place that
will need to be overcome this afternoon/evening. Unusually high
dewpoints for our area (> 55 degrees) are possible for much of the
day into the evening. MLCAPE values could exceed 1500-2000 J/kg for
much of the afternoon and evening across the entire plains. 0 to 6
km bulk shear will be around 50 to 70 kts, possibly even reaching 90
kts. 0 to 3 km storm relative helicity values could be around or
even slightly exceed 100 to 200 m2/s2. CAMs are showing a wide
variety of solutions, especially once we reach this evening, likely
due to the aforementioned potential for a cap to be in place as well
as the strength/timing of the shortwave. With the incoming
shortwave, we believe the cap should be able to be overcome, leading
to widespread showers and thunderstorms tonight and overnight.

The Storm Prediction Center has included nearly the entire I-25
corridor and plains in an enhanced (3/5) risk for today. The
greatest threats today will be large hail (2 inches or greater in
diameter followed by damaging wind gusts (> 70 mph). There is also
the threat for a tornado or two and/or landspouts.

Isolated to scattered storms are expected to initiate along the
Cheyenne Ridge and Palmer Divide around 1pm to 4pm this afternoon.
By this evening, as the better lift arrives, convection should
increase in coverage along the I-25 corridor and move through our
area from NW to SE. The highest threat window for the Denver metro
would be between about 9pm and 3am tonight with the threat
continuing possibly through as late as sunrise for our far
southeastern plains. We want to emphasize that this is a somewhat
unusual nocturnal threat for our area.  Large hail looks to be the
main impact from evening and overnight convection.

The flooding threat will increase as the week goes on given the
amount of rainfall that has fallen, especially on the eastern
plains, where 0.25-0.50" has already fallen in the last few days.
Even tonight, some areas on the eastern plains could see a quick
inch of rainfall. Between Wednesday and Thursday, some areas could
pick up an additional 1 to 1.5 inches of rainfall.

A Red Flag Warning remains in effect for today for the mountains due
to the threat of locally critical fire weather conditions. This
threat will continue tomorrow. See the Fire Weather Discussion for
more details. Wildfires in Utah will continue to produce smoke and
the flow aloft will carry it into Colorado for at least the next few
days. While the near surface smoke density looks quite minimal, the
smoke will be thicker at mid and high levels and will keep some hazy
conditions around the area particularly each night and early morning.

Wednesday and Thursday will continue to bring a risk for severe
weather as shortwaves continue to rotate around the high pressure
centered in Texas. Thus, much of the plains is already highlighted
in a slight (2/5) risk from the Storm Prediction Center for
Wednesday and Thursday. The threat each day will evolve based on how
previous convection plays out. We currently expect dewpoints in the
50s and low 60s to continue as well as MLCAPE values between 1000
and 2000 J/kg particularly on Wednesday with only slightly lower
dewpoints and MLCAPE on Thursday expected as of now.

Friday through the weekend should bring less showers and
thunderstorms, but there will still be a chance especially for the
eastern plains, given only zonal flow aloft. The heat will return
for the weekend into next week with temperatures well into the 90s
forecast as the ridge builds to our east. Fire weather concerns will
also increase for the mountains and mountain valleys on Saturday and
Sunday, looking increasingly concerning due to some stronger wind
gusts.

&&

.AVIATION /06Z TAFS through 06Z Thursday/...
Issued at 1122 PM MDT Tue Jun 23 2026

Still can`t rule out a tstm (20 percent chance) moving across DIA
and possibly APA by 07z as there are a few storms to the north
moving southeast. Otherwise may see ceilings between 6000 and 7000
ft thru 08z. After 08z stratus down to 2500 ft may affect all
three airports thru 15z on Wed. Winds should be E or ENE overnight
and then may become light and variable by 12z.

On Wed, should see stratus burn off by 15z or 16z with widely sct
tstms developing by 20z or 21z. For now have left prob30 in the
tafs for tstms between 20z and 01z with brief gusts up to 35 kts.
Depending on how things setup there could be some threat for a
strong/severe storm as well. Winds will become ESE/SE by 17z and
continue thru the aftn.

After a lull in tstm activity by 00Z or 01z there is some threat
of additional activity by 04z so have included a prob30 for this
potential.

&&

.FIRE WEATHER...
Issued at 1255 PM MDT Tue Jun 23 2026

Widespread elevated to critical fire weather conditions are
possible each of the next two days in the mountains and mountain
valleys. Humidity will drop to around 10% in the valleys today,
with only a slight improvement Wednesday. Wind gusts up to 35 mph
are possible today with only slightly lower wind speeds on
Wednesday.

Fire weather concerns will be very low on Thursday and Friday as
moisture increases and thunderstorms return. These will be most
widespread Thursday with localized wetting rains.

Warmer, drier, and windier weather remain increasingly likely for
Saturday, Sunday, and early next week. Wind gusts in the
mountains, including both valleys and higher elevations, are
likely to exceed 40-50 mph (80% chance), sustaining highly
critical fire weather conditions both days.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 11 AM this morning to 6 PM MDT this
evening for COZ211>214-217-218.

&&

$$

UPDATE...BRQ
DISCUSSION...MV
AVIATION...RPK
FIRE WEATHER...MV

NWS BOU Office Area Forecast Discussion