583
FXUS65 KBOU 160557
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1157 PM MDT Sun Sep 15 2024

.KEY MESSAGES...

- Scattered to isolated showers and thunderstorms this afternoon
  and evening with gusty outflow winds and little rainfall, with
  isolated severe storms possible over the northeast corner.

- Windy over parts of the plains Monday. The windy conditions
  become widespread on Tuesday.

- Scattered thunderstorms Tuesday afternoon. Some storms may be
  strong to severe over the plains.

- Cooler temperatures Friday and next weekend with good chances
  (30-60%) for rain and high mountain snow.

&&

.UPDATE...
Issued at 835 PM MDT Sun Sep 15 2024

After an afternoon and evening of gusty showers, radar is much
quieter across our forecast area. There is still a cluster of
showers producing some gusts up to 40-50 mph, mainly in Logan and
Washington counties, and this should generally lift north and east
over the next few hours. The main update to the grids was to add
some PoPs up there.

As these showers move out of the region, we`ll be left with quiet
conditions through most of the night. Overnight lows will end up
near or a little above normal, much like the last few nights.

&&

.SHORT TERM /Through Monday/...
Issued at 303 PM MDT Sun Sep 15 2024

The latest radar scans show showers have developed over the
mountains and are sliding northeast across the urban corridor. These
high-based virga showers have already produced some 25-30mph gusts,
with one RAWS site observing 45mph in Douglas County. We should
expect more observations like this over the next few hours as the
SPC mesoanalysis shows DCAPE values ranging from 1200-1500 J/kg
across the plains. Areas over the northeast corner of the CWA remain
capped at this time, but MLCAPE values have continued to climb
throughout the afternoon. If this energy can get tapped into, the
possibility of an isolated severe storm can`t be ruled out. As deep-
layer sheer is expected to remain modest at 20-25kts, the main
concern would be for severe wind gusts and hail. Above normal
overnight low temperatures are anticipated for the plains, with near
normal lows for the mountains.

For tomorrow, increasing mid-level moisture is expected to make it
to the southwestern flank of the CWA with the help of southwesterly
flow aloft tapping into moisture associated with the remnants of
what is now Tropical Depression Ileana. An upper-level trough is
expected to propagate eastward into the Great Basin by tomorrow
afternoon, placing Colorado downstream of its axis. Increasing PVA
and a 75kt jet will help initiate afternoon showers across the
mountains. Any chance for wetting rains should be confined to the
higher elevations of Park and Summit Counties and the Palmer Divide.
Across the plains, where drier conditions are expected, forecast
soundings indicate potential for another day of virga showers with
gusty outflow winds up to 35-40mph. The northeastern corner shows
enough instability and slightly more moisture in the lower-levels to
produce a few stronger storms. High temperatures are expected to be
above normal once again, though cloud cover is expected to keep them
a few degrees cooler than Sunday`s.

&&

.LONG TERM /Monday night through Sunday/...
Issued at 303 PM MDT Sun Sep 15 2024

A strong shortwave trough will become negatively tilted and will
move through our forecast area Tuesday afternoon and evening. Recent
model runs have been trending towards stronger forcing with this
shortwave and stronger frontogenesis in the low levels. There now
appears to be a healthy cold front that will be well-timed to
come across just after peak heating in the afternoon. High
resolution models show a line of storms that moves across the
majority of our forecast area. PoPs were raised to account for
this and may need to be increased further if models become
consistent with the increased forcing. Given the strong winds
throughout the low to mid levels, some of the stronger storms will
be able to bring that momentum to the surface and severe wind
gusts may be possible in a few of the storms. Despite the warm
southwesterly flow across the plains, there will be too much cloud
cover along with the early timing of the storms for high
temperatures to reach the 90s. Instead, highs will be in the mid
80s. Another aspect of the recent model trends has been towards
more moisture at the surface across the plains. As a result, no
critical fire weather conditions are expected.

There will be strong NVA on Wednesday behind the departing trough.
This will keep conditions dry. The cold front from the previous day
will help keep temperatures roughly 5 degrees cooler than Tuesday.

A shortwave trough will position itself over the Las Vegas area on
Thursday and will begin to move towards Colorado on Friday. The
trend in the ensembles has been towards a less progressive pattern
to end the week and the ECMWF ensembles have slightly less
precipitation. The main change to the forecast is lower PoPs on
Thursday which appears to be dry now. Friday into Saturday will
still have the best chance for widespread precipitation. There
remains considerable uncertainty but a healthy and widespread
precipitation event is still a possibility. The one detail about the
forecast that has higher confidence is cooler temperatures for the
weekend.

&&

.AVIATION /06Z TAFS through 06Z Tuesday/...
Issued at 1156 PM MDT Sun Sep 15 2024

Normal drainage wind patterns should continue through about 15Z
this morning. Fairly strong southwesterly winds aloft ahead of an
approaching upper trough to our west, should translate down into
the lower levels by afternoon. Will keep in the brisk
southwesterlies in the TAF this afternoon as well as put in a
TEMPO group for VRB20G40KT along with VCSH. Drainage winds are
expected again by 03Z this evening. There will be no ceiling
issues.

&&

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

&&

$$

UPDATE........Hiris
SHORT TERM....Bonner
LONG TERM.....Danielson
AVIATION......RJK