539
FXUS65 KBOU 270827
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
227 AM MDT Thu Jun 27 2024

.KEY MESSAGES...

- Numerous showers/storms for the mountains/mountain valleys.
  Heavy rainfall the greatest risk with a slight chance for a
  severe storm.

- Scattered showers/storms foothills, urban corridor, and nearby
  plains. Slight chance for a severe storm here.

- Scattered/numerous thunderstorms far northeast Colorado, some
  may be severe with large hail and damaging winds.

- Thunderstorms return Friday afternoon, strongest over our
  northern/northeast zones.

- Turning drier starting Saturday, with heat rebuilding Sunday
  onwards.

&&

.SHORT TERM /Through tonight/...
Issued at 227 AM MDT Thu Jun 27 2024

Upper level trough located over the Great Basin at this time is
producing night time showers and thunderstorms across Utah. This
system will progress east-northeastward today around the upper
level high centered over the Southern Rockies. Ahead of this
system, surface analysis shows a surface trough (dry line) forming
over eastern Colorado and northward into central Wyoming. The
forecast for the Front Range and eastern plains will be
challenging today because we will see lift from the trough this
afternoon and evening, but the surface trough over eastern
Colorado will help produce a drying westerly downslope flow.

For the mountains and mountain valleys today, lift from the
approaching trough is expected to produce numerous showers and
thunderstorms. Precipitable water values will be 150-200% of
normal. Heavy rain, small hail, and gusty winds will accompany the
thunderstorms. Can`t rule out a severe thunderstorm or two with
SBCAPE values of 500-1000J/kg.

Across the foothills, urban corridor, and nearby plains, westerly
downslope flow will produce subsidence and some drying. Still
enough moisture, instability, and lift from the approaching trough
that there will be a decent chance (30-50%) for showers and
storms this afternoon and evening. The strength of the westerly
winds will likely play a role in the amount and intensity of the
thunderstorms. Some of the hi-resolution models overdue the
westerly downslope winds (especially the HRRR). If this happens,
and we see weak westerly winds or just variable winds we could see
more showers and thunderstorms than the models are advertising.

The far eastern plains, east of the surface trough will see the
highest threat for severe weather. Here, southerly winds will keep
a moist (dew points 55-65) and unstable (SBCAPE 1000-2000 J/kg)
airmass in place. Storms that form, along and east of the boundary
may become severe with large hail and damaging winds. Main threat
area is expected to be over far northeast Colorado and northward.
Shear decreases to the south and thus lowers the severe threat.

&&

.LONG TERM /Friday through Wednesday/...
Issued at 227 AM MDT Thu Jun 27 2024

A 700mb shortwave beneath mostly zonal flow aloft will push across
Colorado Friday, with an associated surface front descending north
to south through our lower elevations from mid to late afternoon.
Although at first glance many parameters appear somewhat less
favorable for strong thunderstorms relative to previous days (more
marginal MLCAPE of a few hundred J/Kg, PW under 0.80-1.00", lower
dewpoints), the front combined with upper-level jet support should
provide for efficient lift. Deep-layer shear 30-45 kts, highest
across our northern/northeast counties, would also be sufficient to
support a few stronger to severe storms in the afternoon, especially
for areas from Weld County eastward. With steering level flow
between 25-40 kts, storms will move at a pretty good clip, helping
to taper the heavy rain/flood threat. The fairly dry downslope flow
will likely reduce coverage and intensity over the urban corridor,
with stronger storm clusters developing later in the afternoon
across the northeast plains. SPC`s Slight Risk area does a good job
of highlighting the corridor of higher threat, with large hail and
severe wind gusts the primary concern. Most of the lower elevations
will likely still break 90F, assuming the front indeed holds off
until mid afternoon or later for most.

A more stable airmass will be in place Saturday, with moderated high
temperatures in the mid 80`s east of the mountains. However,
isolated to scattered afternoon thunderstorms can still be expected,
being more favored for the southern Front Range mountains/foothills
under southwest flow aloft, as well as our southern plains later in
the afternoon, with returning light southeast winds at the surface
contributing to slightly higher dewpoints in a post-frontal regime.

Low-level warm air advection kicks up a notch on Sunday and that
will bump temperatures up around 10 degrees across the plains,
but with little if any change for the high country. Warmer
conditions aside, the overall setup will be similar to Saturday,
with mostly southerly flow favoring our southern mountains and
Palmer Divide for the highest chance of afternoon
showers/thunderstorms.

Ensemble guidance has pushed back slightly with the magnitude of the
heat next week, hinting at a more robust trough developing to our
north which should keep largely zonal flow in place over Colorado
early to mid-week. That`s not to say it won`t be hot - highs in 90`s
for plains/urban corridor appear a reasonably safe bet for now each
day. Still time for that to change, especially if we end up with a
stronger front on Monday than currently advertised. In any case,
precipitation chances will be lower for this period, with isolated
to locally scattered afternoon convection largely confined to the
higher elevations and perhaps eastern plains. Finally, worth keeping
an eye on humidity and wind trends for early in the week. Could see
pockets of elevated to near-critical fire weather conditions by
Monday as pre-frontal winds pick up and subsident flow sustains
lower humidity in the lee of the Continental Divide.

&&

.AVIATION /06Z TAFS through 06Z Friday/...
Issued at 1152 PM MDT Wed Jun 26 2024

Shower/thunderstorm threat has ended for the night. Southerly
winds to prevail most of the night. There could be a weak
northerly push 10-12Z, so added a variable wind group. General
wind direction for Thursday will be west-northwest, but at
generally weak speeds. With speeds being light, could see some
variability in direction through the afternoon. Scattered
thunderstorms are expected to form and track eastward across the
area after 21Z. Gusty outflow winds will likely produce wind
shifts. Thunderstorm coverage may be less than Wednesday due to
the downslope west-northwest wind component. Some models show
showers and weak storms lingering into the evening hours, so added
a VCTS to the forecast.

&&

.HYDROLOGY...
Issued at 227 AM MDT Thu Jun 27 2024

Over the mountains and mountain valleys, heavy rainfall will be
possible from thunderstorms with precipitable water values of 150-
200 percent of normal. Storm movement should be quick enough to
keep the flash flood threat low today. However, flood prone areas,
such as burn scars will see a greater risk for flash flooding
today.

&&

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

&&

$$

SHORT TERM...Meier
LONG TERM...Rodriguez
AVIATION...Meier
HYDROLOGY...Meier