486
FXUS65 KBOU 191636
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1036 AM MDT Sun May 19 2024

.KEY MESSAGES...

- Risk (50-60% chance) of a severe storm or two across the far
  northeast plains this afternoon. Isolated to scattered
  showers/storms with gusty outflow winds elsewhere.

- Another round of strong to severe storms possible Monday
  afternoon into Monday evening across most of the eastern plains.

- Cooler, wetter pattern late Monday into Tuesday as a couple weak
  systems track across the region

- Warming/drying trend for the second half of next week

&&

.UPDATE...
Issued at 1036 AM MDT Sun May 19 2024

Satellite shows a broad trough to our northwest, resulting in
moderate west/southwest flow aloft. At the surface, moisture
advection was occurring across the northeast plains with dewpoints
increasing into the lower to mid 50s roughly along and east of a
Fort Morgan to Limon line. Satellite also shows some cumulus
already developing across the mountains, with sufficient mid level
moisture still in place per water vapor imagery and 12Z Grand
Junction sounding. ACARS soundings from Denver show a rather
shallow inversion, resulting in a fast warmup and probably an
earlier than normal day for convection.

Overall the forecast is on track for isolated to scattered showers
and relatively high based storms with gusty winds this afternoon,
and a threat of severe storms over the northeast corner.  A
dryline will develop with further mixing (once we hit convective
temps along the Front Range), and then shift eastward across the
plains during the afternoon. The highest MLCAPE is expected to set
up roughly along and east of a Sterling to Akron line, where
values ~1500 J/kg expected with low to mid 50s dewpoints. Thus,
we`ll have a threat of severe storms for those locations mid to
late afternoon with sufficient instability and bulk shear.
Exact location of the severe storms is still a little uncertain
when taking a look at a couple cirrus cloud bands and CAM output,
but the ingredients including sufficient shear are there.
Straightline hodographs would favor large hail and damaging winds
as the primary threats.

West of the dryline, gusty outflow winds will be the primary
threat with higher based showers and isolated storms, along with
DCAPE near 1300 J/kg. Thus, would not be surprised to see some
gusts to 50 mph with that activity. Things should settle down a
little earlier than normal with weak subsidence spreading across
the forecast area this afternoon and evening.

&&

.SHORT TERM /Through tonight/...
Issued at 415 AM MDT Sun May 19 2024

Water vapor imagery shows a broad trough has moved over the
western US with southwesterly flow over Colorado. There is decent
mid level moisture and weak large scale ascent which has led to a
few showers forming over the far northeast plains this morning.
Drier air moving in along with downslope surface winds will help
high temperatures reach well above normal with highs in the 80s
across the urban corridor and pains this afternoon.

During the afternoon, there will be slight instability that forms
across much of the CWA which will allow for weak showers and
perhaps a thunderstorm or two to form. The dry low level airmass
with steep lapse rates means that any showers or storms that form
could create strong wind gusts potentially up to 45 mph. Across
the far northeast plains, it will be a different story. There will
be much higher low level moisture to the east of a dryline.
Instability will be much greater with most unstable CAPE values
exceeding 1,500 j/kg. With sufficient shear and forcing, strong to
severe storms may form with large hail and damaging wind gusts
possible. The most likely area for the severe storms would be east
of a line from Sterling to Akron.

Showers and storms will dissipate tonight. There will be a couple
weak cold fronts that will allow for temperatures to cool into the
40s across the plains.

&&

.LONG TERM /Monday through Saturday/...
Issued at 315 AM MDT Sun May 19 2024

More organized convection is expected across portions of the
forecast area on Monday. Guidance is largely in good agreement
with the synoptic scale pattern here, but important mesoscale
details remain uncertain across our forecast. Broad southwesterly
flow aloft is anticipated ahead of an approaching positively
tilted trough axis. A weak southern stream shortwave is expected
to race across the region from Monday afternoon into Monday night,
with lee cyclogenesis expected somewhere in eastern Colorado. To
the north of this low, moist, east/northeasterly flow is expected
to develop, with surface dew points gradually increasing into the
upper 40s to mid 50s in northeastern Colorado. Wherever this
warm/moist sector develops, guidance depicts a favorable
environment for strong to severe storms for a brief period late
Monday afternoon into Monday evening, with MLCAPE values near
500-1000 J/kg in along with good 0-6km speed/directional wind
shear. A few hi-res models try to develop storms as far west as
the I-25 corridor, though HREF/NCAR FV3 ensembles generally keep
the higher probability corridor near the eastern edge of our CWA.
These marginal events are always difficult to forecast, and
therefore it wouldn`t be surprising to see shifts in the current
SPC outlook as we get closer to the event... but at the very
least, a conditional severe storm threat is becoming more likely
for Monday.

By Tuesday, the first shortwave should be exiting the region,
with some lingering showers/storms possible across the northeast
plains. With the passage of a strong cold front, highs should be
much cooler across the region... only peaking in the upper 50s to
low 60s. Still, modest boundary-layer moisture combined with
larger scale ascent from a trailing shortwave should be enough for
additional showers and a few storms by the afternoon, especially
across the Front Range/urban corridor where shallow upslope flow
should develop. Model soundings show a humidity rich 700-400mb
layer with only modest instability, which in all likelihood would
limit the severe threat. Still couldn`t rule out briefly heavy
rain along with some hail, but the potential certainly looks lower
than Monday`s threat.

Wednesday will be much quieter across the forecast area with
temperatures warming back up into the 60s/70s. A drier airmass
should advect into the region with typical inverted-V type
soundings with minimal CAPE. Can`t rule out a few high-based,
mostly virga, showers but these should stay isolated in coverage.

A continued warming/drying trend is expected into Thursday as
southwesterly flow aloft strengthens. A weak shortwave is forecast
to track to our north with minimal impacts from this feature.
Again a few showers and storms will be possible across the higher
elevations and along the northern border of our CWA.

As we enter the latter half of the week into next weekend,
forecast uncertainty increases quickly. There is generally good
agreement that a broad upper trough/closed upper low will meander
across the Pacific Northwest, with one or more shortwaves pivoting
around the main trough axis. However, there is little agreement
in the timing and placement of any of these finer-scale details
and thus little reason to deviate from the multi-model mean. The
general pattern would favor near normal temperatures with daily
chances (20-40%) of showers and thunderstorms across the region.

&&

.AVIATION /18Z TAFS through 18Z Monday/...
Issued at 1036 AM MDT Sun May 19 2024

Main concern is gusty outflow winds from high based showers or
virga this afternoon.

ACARS soundings show the inversion weakening and we`re starting to
mix a little better now. While we could go variable winds for an
hour or two initially, overall a west to northwest wind should
prevail through 00Z. We are already starting to see cumulus
develop in/near the mountains, and thermal/moisture profiles would
suggest strong and gusty outflow wind potential to ~40 knots
possible with any high based convective activity. Coverage is
expected to be great enough per upstream satellite imagery to
warrant VRB gusty winds with a VCSH from around 19Z to ~00Z. The
threat of any thunder is less than 10%.

Winds should settle down toward 00Z but could still be variable
between northerly and east or southeast. A cold front then arrives
toward 09Z-11Z with a northeasterly push of winds. That will also
usher in higher low level moisture with potential for stratus. At
this point, the upslope appears pretty shallow and weak, so we`ll
just go with SCT clouds around 3000 ft AGL for the Denver area TAF
sites 12Z-16Z. There`s about a 30% chance it goes BKN for MVFR
ceilings.

&&

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

&&

$$

UPDATE...Barjenbruch
SHORT TERM...Danielson
LONG TERM...Hiris
AVIATION...Barjenbruch