753
FXUS65 KBOU 121019
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
419 AM MDT Mon Aug 12 2024

.KEY MESSAGES...

- Another round of thunderstorms this afternoon/evening with a
  greater threat of heavy rain and possible flooding. Marginal
  risk for severe storms as well.

- Chance of severe storms continues on the plains for Tuesday.

- Gradual drying trend expected from Wednesday through the end of
  the work week with near to slightly above normal temperatures.

&&

.SHORT TERM /Through Tonight/...
Issued at 419 AM MDT Mon Aug 12 2024

The main features of today`s weather are increased moisture at all
levels and lighter winds aloft. That adds up to an increased
threat of heavy rains and flooding. As usual when the moisture
increases, cooler temperatures and increased cloud cover will
likely be limiting factors. We will likely have to wait for
diurnal convection and then lift from what looks like the short
wave/jet streak of the day which is now entering eastern Utah and
is probably well time for late afternoon mountain and evening
plains convection.

Once we do get storms going though, they should produce outflow
boundaries that create slow moving convergence lines in a moist
unstable environment. Late afternoon/evening soundings show
1000-2000 J/kg CAPE and precipitable waters up to about 1.3
inches. Storm motion will ultimately depend on the
strength/organization of outflows, but sounding winds suggest
something around 10 mph, perhaps even slower early in the
afternoon. By evening, there`s potential for backbuilding/training
as southerly low level winds will create inflow on the back side
of cells. It`s probably not enough for long-term stationary
storms, but it will slow the movement of storm clusters. We should
also have a pretty productive environment, with little shear or
drying aloft to produce entrainment and warm cloud depths ranging
from about 5000 feet near the mountains to as much as 9000 feet on
the plains. We should be able to have some storm clusters that
produce up to 3 inches of rain in a couple of hours. We considered
a flash flood watch, but plains FFG is relatively high and we`ll
likely have enough organization for some storm motion with outflow
winds coming off the mountains, generated by the rain, or from the
shortwave. For the burn areas, the early storms are the most
likely problem as they won`t have the organization to create storm
motion at the beginning. So the mid afternoon hours are the most
likely problem time there, shifting to the plains in the evening.

Forecast temperatures looked alright. We upped the cloud cover and
adjusted PoP timing a bit later due to the temperatures and
shortwave timing.

&&

.LONG TERM /Tuesday through Sunday/...
Issued at 419 AM MDT Mon Aug 12 2024

Ridging will amplify and move eastward on Tuesday placing Colorado
on its backside. This will allow for southwesterly flow aloft to
transport moisture into the forecast area from the southwest.
Southeasterly surface flow is expected to advance low-level
moisture northward, keeping dewpoints in the 50s and 60s across
the plains. An upper-level shortwave disturbance mixed with
steepening mid-level lapse rates will help promote scattered
showers and storms over the high country in the afternoon,
shifting eastward across the plains into the evening. PWATs are
expected to be ~150-160% of normal for the high country and
eastern plains. With this heightened moisture content, some storms
may be possible of producing heavy rainfall at times. Storm
motion is expected to be slightly faster than Monday`s, but there
will likely still be an elevated threat for localized flash
flooding across sensitive areas, especially our newest burn scars.
As the storms move eastward into the 50 and 60 degree dewpoints
across the plains, forecast soundings show MLCAPE values between
1800-2000 J/kg, and 43kt sfc to 8 km shear that would help promote
more organized thunderstorm development. There will be an
isolated threat for hail and severe winds in the stronger storms
that form, mainly east of the I-25 corridor. Afternoon high
temperatures are forecast to be near normal with 80s expected for
the plains, except for areas in the far northeastern corner where
low level clouds are likely to keep solar heating minimized. The
mountains are forecast to be in the upper 60s- mid 70s, and the
foothills will be slightly warmer in the 70s to low 80s. Overnight
temperatures will be in the upper 50s for the plains, low 50s for
the foothills, and 40s for the mountains.

Models look to come into more agreement that a shortwave trough is
expected to move across the PNW Wednesday that will help flatten out
the ridge bringing a more zonal flow aloft over the forecast area.
This will bring drier conditions as PWATs look to decrease to 110-
120% of normal, however, there will still be enough moisture and
enough synoptic support from the shortwave to the north to allow for
afternoon scattered storms and showers to develop over the mountains
and move east across the plains. High temperatures are expected to
be within a few degrees of normal across the forecast area. A cool
front will move across the CWA overnight Wednesday that will drop
overnight low temperatures 3-5 degrees from Tuesday`s.

A drying trend looks likely for Thursday through Saturday as
Colorado looks to be on the front side of a 500 mb ridge bringing NW
flow to the CWA, cutting off our monsoonal moisture supply. PWATs
look to drop to 80-90% of normal, and temperatures will rebound
to the 90s for Friday and Saturday. Some scattered afternoon
showers and storms look possible for the high country on Saturday
afternoon, but Sunday looks to see a more significant moisture
return with more widespread chances for precipitation with
southwesterly flow returning on the backside of an amplifying
ridge.

&&

.AVIATION /12Z TAFS through 12Z Tuesday/...
Issued at 419 AM MDT Mon Aug 12 2024

Low clouds with IFR ceilings have moved across KDEN. There is less
certainty about whether they will get to KBJC (70% chance) and
KAPA (50% chance). LIFR ceilings are possible for a few hours,
until about 15z, but dense fog is not expected.

After that, VFR conditions will prevail. Thunderstorms are
expected to be in the area between 21z-03z. The main impact will
be a few wind shifts with gusts up to 35 knots, but localized IFR
visibilities and gusts up to 50 knots are possible with the
strongest storms. The threat of fog is lower for Wednesday
morning.

&&

.HYDROLOGY...
Issued at 419 AM MDT Mon Aug 12 2024

Meteorological reasoning is above...but there is an elevated risk
for flash flooding in the more sensitive burn areas mainly this
afternoon. Elsewhere, some of the storms will produce heavy
rainfall of 1 to 3 inches, with the heavier rains presenting a
risk of at least minor flooding mainly on the plains this
evening. If one of the heavier storms moves over an urban area in
the late afternoon/early evening that could also be a flood risk.

Tuesday will bring another day of heightened threat for heavy
rains across the forecast area. Above normal moisture will still
remain in place that could lead to localized heavy rainfall in
stronger storms that develop. We have left the Alexander Mountain
Burn scar in an elevated threat with a limited threat for older
scars. There will be another day of afternoon showers and storms
across the high country that could create a limited threat for
our burn scars as moisture content begins to decrease. By Thursday
and into the weekend, drier conditions are expected.

&&

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

&&

$$

SHORT TERM...Gimmestad
LONG TERM...Bonner
AVIATION...RPK
HYDROLOGY...Bonner/Gimmestad