368
FXUS65 KBOU 271713
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1113 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. Marginal (<20%) chance for a severe storm here.

- Scattered/numerous thunderstorms far northeast Colorado this
  evening, 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.

&&

.UPDATE...
Issued at 1113 AM MDT Thu Jun 27 2024

Minor updates have been made to this afternoon/evening`s forecast.
The updates were primarily based on observational trends and
surprisingly excellent agreement between consecutive runs of the
HRRR and the NAMnest, and consistencies between other members of
the HREF. The latest model data indicates that weak convection
should get going early afternoon across Larimer County and the
Palmer Divide. This convection is not likely to be severe given
limited instability, but could produce moderate rain and small
hail, along with lightning. It should move quick enough to
preclude most flooding concerns across Cameron Peak. Models want
to generate convection this evening across W/C Colorado associated
with the trough axis. This is a reasonable solution, but all CAMs
significantly weaken the convection as it crosses the Divide due
to rain shadow/downslope effects. We feel the drying/weakening is
a little overdone, but nevertheless impacts should be minor and
rainfall amounts look to be light after crossing the divide. Have
lowered PoPs accordingly but not too far because light rain and a
few rumbles of thunder could still occur, looking at 20-40% for
areas east of the Divide out to Fort Morgan. The line of
showers/storms could drop a quick 0.5" on the East Troublesome
burn scar, and areas of Jackson-Grand-Summit Counties, maybe as
far as South Park, but overall impacts look low from round two
this evening. Finally, there continues to be a slight risk of
severe thunderstorms east of Sterling this evening. Moisture is in
place but it looks less unstable across the northeast corner of
Colorado than it did 12 hours ago. Have thus lowered PoPs and will
message that the threat of severe weather is a little less, but
any storms that form could generate severe hail and gusts to 60
mph. Finally, we took a degree or two off the highs given current
temperature trends and expected partly to mostly cloud skies
across much of the area, but still expect lows in the lower 90s
across much of the plains and urban corridor.

&&

.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 /18Z TAFS through 18Z Friday/...
Issued at 1113 AM MDT Thu Jun 27 2024

VFR conditions are expected through Friday. WNW winds should be
the norm this afternoon, 10-15 kts with some gusts to 20 kts. The
convective potential looks a little less for this
afternoon/evening than it did 12 hours ago. For now, it looks like
the best bet for showers/thunderstorms impacting the terminals
should be after 00Z, maybe as late as 04Z. Chances are 20-40%
during this time window, which would include a wind shift or at
least a short period of strong/gusty winds as storms approach the
terminals. Will handle this potential with a TEMPO in the TAFs,
with lightning and gusty winds the main impacts. The best bet for
wind direction associated with the convection/convective outflows
would be WNW given the movement of the storms and the prevailing
flow, so about the same as before the storms, just more wind and
more gusts.

Storms should be east of the terminals after 06Z for sure, and
drainage winds (SSW at DEN and APA 8-12 kts, W at BJC 4-8
kts) will kick in all night long through 14Z Friday. On Friday,
there will again be chances (~30%) of afternoon thunderstorms, and
it`s safe to assume even without direct thunderstorm impacts, 1-2
wind shifts with strong/gusty winds are expected roughly between
2100-03Z. We will introduce a PROB30 group in the TAFs to handle
that.

&&

.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.

&&

$$

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