305
FXUS65 KBOU 181639
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1039 AM MDT Sun Aug 18 2024

.KEY MESSAGES...

- Precipitation chances increase today, with scattered storms
  expected through next week.

- There will be a limited threat of flash flooding over the burn
  areas today but a higher threat on Monday.

- Above normal temperatures will continue through the week. A few
  days may see mid-90s highs across the metro and plains.

&&

.UPDATE...
Issued at 1039 AM MDT Sun Aug 18 2024

Shallow inversions have allowed a very fast warmup here in the
Denver metro area. Farther east, over northeast Colorado, we have
a cool surge enhanced by convection in Nebraska. That should act
to cool temperatures a few more degrees than forecast over the
northeast corner, while the I-25 Corridor will still be hot with
highs in the lower to mid 90s.

With regard to convection, it is interesting to see the delay in
CAMs producing showers and storms today, as inversions are fairly
weak. However, there is also another stable layer noted in the
ACARS soundings near/just above 700 mb that is the likely culprit.
The current forecast seems reasonable with convection developing
over the mountains early to mid afternoon, and then spreading
northeast across the plains late this afternoon and evening. Gusty
winds and lightning will be the main threats as low level moisture
is limited in most locations, although the plains could see a
strong/severe storm along the axis of higher bouyancy/weaker
shear.

&&

.SHORT TERM /Through Tonight/...
Issued at 418 AM MDT Sun Aug 18 2024

Current satellite pictures are showing plenty of mid and upper level
cloudiness moving and up and over the ridge aloft. The upper ridge
axis is over far eastern Colorado. The area radars indicate that
convection has ended across the CWA early this morning. Current
temperatures are still pretty warm for this time of night with mid
60s to lower 70s F still over most of the plains. 50s F are
common over the mountains and foothills, with colder readings
right along the divide.

Models keep the upper ridge axis over the CWA today and tonight with
20 to 40 knot jet level westerly flow in place.  The synoptic scale
energy is mostly benign for the first two periods.  Precipitable
water values are progged to increase later day and tonight with
values getting into the 0.80 to 1.30 inch range by tonight. The
low level dew points increase this later this afternoon and
evening with 50s to mid 60s F over the plains. The 60s F dew
points are mainly progged over the eastern half of the plains.

The best CAPE is progged over the eastern half of the plains later
this afternoon and evening, with limited amounts over the rest of
the CWA.  Not sure how well the CIN will be able to erode away
across the CWA this afternoon and evening.  Cross sections are
pointing to some downdraft producing thunderstorms for later today
and this evening.  The QPF fields from the various models are not
very impressive with rainfall amounts for the first two periods,
with the best rainfall to be over the southwestern quarter of the
forecast area.  For pops, will keep the 30-60%s going in the
mountains with the highest over Park County. For the plains, 20-
30%s should suffice at this time. Still some brief heavy rain is
possible, especially over the High County south of I-70.

&&

.LONG TERM /Monday through Saturday/...
Issued at 207 AM MDT Sun Aug 18 2024

The long term period will be dominated by the presence of a large
upper ridge centered near the TX/OK panhandle.

Monday should be one of the more active days of the period, with
well above normal moisture in our region. Deterministic guidance
suggests ~1.25" PWATs across the metro by the afternoon hours,
which is roughly 150-175% of normal. The main question is how much
instability will be available to work with, with substantial
spread among the HREF and global ensemble members. While
mean/median surface CAPE is less than 500 J/kg for KDEN Monday,
there is a notable right skew to the data, with a handful of LREF
members showing >2500 CAPE. While the answer is likely somewhere
in between, there`s no obvious solution at this point. Weaker
(stronger) instability would likely lead to less (more)
organized/weaker storms and a lower (higher) threat of heavy
rainfall.

The ridge is expected to strengthen a drift a little closer to
the forecast area on Tuesday, which should shove the plume of
better moisture to the north and west of the forecast area. Enough
moisture does linger across the high country for at least a few
showers/storms, but it should be much drier across the plains.

As we get towards the latter half of the week, the ridge is
expected to gradually weaken and shift eastward, leading to a
return of modest southwesterly flow aloft. This should allow
better moisture to advect back into the region, with precipitation
chances increasing... especially towards Thursday/Friday. We`ll
have to wait and see how active the pattern looks as we get into
next week. Eventually drier air should work back into the region
as we get into next weekend.

Meanwhile... as mentioned in the past several AFDs... the pattern
broadly supports above normal temperatures across the region for
the foreseeable future. Our forecast grids have 90F at KDEN every
day in the long term period, with a couple of mid 90s highs
forecast on Tuesday and Wednesday. It`s possible a day or two ends
up a bit cooler from convection/outflows, but otherwise our
streak of 90F days looks like it will continue on for the next
several days.

&&

.AVIATION /12Z TAFS through 12Z Monday/...
Issued at 538 AM MDT Sun Aug 18 2024

Normal diurnal wind patterns are expected today at DIA with drainage
winds kicking back in around 02Z this evening. Will again leave the
TEMPO group in for -TSRA later this afternoon as models show some
instability and limited measurable rainfall. Wind gusts to 35 knots
are possible in and near the storms.  There will be no ceiling
issues.

&&

.HYDROLOGY...
Issued at 425 AM MDT Sun Aug 18 2024

Moisture availability is expected to increase later today and
tonight. The highest pops are expected over the southwestern
quarter of the CWA late this afternoon and this evening. The
threat of flash flooding will be limited mainly to the newer burn
areas and Park County.

Better moisture is expected again on Monday, with PWAT
standardized anomalies of 2-3 sigma noted on the GFS/ECMWF by
Monday afternoon. There is still some question as to how much
instability will develop across the Front Range and urban corridor
despite the above average moisture content, as surface dew points
only rise into the low 50s. Still, the combination of modest
instability, abundant moisture, and relatively slow storm motions
leads to a limited threat of burn area flash flooding, with an
emphasis on the recent burn areas from this summer where rainfall
thresholds are lower.

The overall rainfall threat diminishes on Tuesday as the upper
ridge expands and the better moisture is temporarily pushed out of
the region. The pattern gradually turns wetter again for the
latter half of the week, with slow moving thunderstorms becoming
more widespread. At least a limited threat of burn area flash
flooding is forecast through the rest of the forecast period, with
the potential for a locally higher threat on a day or two as we
approach next weekend.

&&

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

&&

$$

UPDATE...Barjenbruch
SHORT TERM....RJK
LONG TERM.....Hiris
AVIATION...RJK
HYDROLOGY.....RJK/Hiris