330
FXUS65 KBOU 161745
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1145 AM MDT Mon Sep 16 2024

.KEY MESSAGES...

- Scattered showers and isolated high based thunderstorms with
  strong, gusty outflow winds this afternoon into early evening.
  Highest coverage in/near the mountains.

- Windy conditions become widespread on Tuesday.

- Scattered thunderstorms Tuesday afternoon, some may be severe
  over the plains with high winds.

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

&&

.UPDATE...
Issued at 1022 AM MDT Mon Sep 16 2024

Mid and high level clouds were beginning to increase from the
southwest as a weak shortwave is moving up from the Four Corners
area. Instability will be limited as the bulk of this moisture,
barring the far northeast corner, is confined to the mid/upper
levels and the low levels are quite dry. Thus, with DCAPE near
1000-1200 J/kg, the main threat will be gusty outflow winds to
around 40-45 mph. We are seeing diminishing CIN over the plains
with some sunshine this morning, so we may have to spread the low
PoPs (mostly virga though!) a little farther east onto the plains
late this afternoon/early evening. The highest (scattered
coverage) is still anticipated to be over the mountains. We`ll
have to bump the forecast high temperatures a degree or so over
the northeast plains where there`ll be more sunshine and good
mixing.

&&

.SHORT TERM /Through Tonight/...
Issued at 350 AM MDT Mon Sep 16 2024

There a still a few showers over the eastern border at this
time.  They were moving east-northeastward out of the CWA.
Normal drainage wind patterns were in place for much of the CWA.

Models show a pretty strong upper low over northern California to
move into the western Great Basin by 00Z this afternoon, then to
be centered over northeastern Nevada by 12Z Tuesday morning. The
winds aloft for the CWA will be 45 to 80 knots from the southwest
both today and tonight. The synoptic scale energy for the CWA on
the QG Omega fields is somewhat noisy today and tonight and
pretty weak downward in nature anyway. The flow aloft seems to
translate somewhat down to the low levels this afternoon and
evening, with gusty southwesterlies surface winds progged fro a
bit this afternoon.

For moisture, the precipitable water fields show 0.50 to 1.10
inches over the forecast area today into this evening, with a tad
of drying over the plains tonight. Instability wise, the CAPE is
meager today into this evening, with the highest values over the
far northeast corner this afternoon only. There is nothing over
the CWA for the tonight period. The model QPF fields have limited
measurable rainfall over the mountains, foothills and Palmer Ridge
this afternoon and early evening only. Will keep the highest pops
going over the mountains and foothills this afternoon. The plains
can expect 0-10% pops for the first two periods. There may be
some outflows around, but not as strong as Monday`s were.

For temperatures, today`s highs are 2-5 C cooler than yesterday`s
highs.  The 90s F look to be over.

&&

.LONG TERM /Tuesday through Sunday/...
Issued at 300 AM MDT Mon Sep 16 2024

A strong negatively tilted upper level trough will lift northeast
across Colorado Tuesday. Center of the trough at 12Z Tuesday will
be over eastern Nevada with the axis extending into northwest
Arizona. By 18Z, the trough axis will be over southwest Colorado
and by 00Z it will be over northeast Colorado...very fast moving.
Ahead of the trough axis showers and thunderstorms will form
Tuesday morning over the Four Corners and northward into western
Colorado. Best lift over north central and northeast Colorado will
be Tuesday afternoon, lining up with peak heating. Pretty much
all the high resolution models show a line or broken line of
storms tracking across the area Tuesday afternoon. SBCAPE reaches
500-1000 J/kg, so the storms should have decent updrafts. Strong
lower and mid level winds, inverted-V sounding, and DCAPE of
1000-1500 J/kg all add up to strong to severe winds from the
thunderstorms. This looks to be a higher threat than most days
with the NAMNest and HRRR models showing some gusts to 70 mph. The
storms race east of the area by early to mid evening.

For Wednesday and Thursday, an upper level low drops south along
the west coast Wednesday and then slowly progresses eastward
Thursday. By late Thursday the upper level low will be centered
over California and Nevada. Ahead of this low, ridging in the
southwest flow aloft will prevail over Colorado. The airmass dries
out with mostly clear skies expected. Temperatures will be a
little cooler, but still above normal with highs in the lower to
mid 80s over northeast Colorado. Even though it will be dry (low
relative humidities), winds will generally be weak, easing fire
weather concerns.

On Friday, the upper level low slowly moves east across the Great
Basin and over the Central Rockies Saturday. Models still
somewhat struggling to find run to run consistency. The latest 00Z
runs of the GFS and ECMWF are slower than previous runs keeping
the area precipitation free until Saturday. Ensemble members are
not entirely on board with this solution. This low as it tracks
across the Central Rockies could stay north of Colorado resulting
in mainly dry conditions. If it takes a more southern and stronger
route, much of the area will see a good shot of rainfall. Because
of the uncertainty, will continue with the broad brush approach
with 20-50 PoPs for Friday and Saturday. Temperatures will run at
or below normal with highs expected to be in the 70s over
northeast Colorado. Though if we happen to see a cloudy/rainy day,
highs will likely be in the 60s. This storm system should be east
of the area Sunday. With the uncertainty in the weather pattern,
will keep low PoPs (10- 20%) in the forecast for Sunday.

&&

.AVIATION /18Z TAFS through 18Z Tuesday/...
Issued at 1145 AM MDT Mon Sep 16 2024

Main concern for relatively early in this TAF period is VRB and
gusty outflow winds from passing showers/isolated storm. Main
threat would be 21Z-24Z, but can`t rule out something as early as
20Z considering radar trends to our south. Also, the coverage and
propagation of showers to our south would suggest the main threat
of stronger outflows would be from the south/southwest, but can`t
rule out a lighter east/northeast flow behind any passing showers
given the dry sub-cloud layer. The chance of any TS at the
airports is quite low - about 10-20%. By 00Z-01Z, most of the
convection should have passed with a return to slightly enhanced
south/southwest winds expected overnight.

We have higher confidence of storms and stronger outflows
affecting the airports on Tuesday. The most likely timeframe for
that would be after 21Z. A linear feature of convection is
expected, which means more organization of strong, gusty outflow
winds to 40-50 kts with the storms that move across. Fortunately,
if the storms are linear, that should be mainly just a brief 30-60
minute blast of wind and high airport impacts before winds settle
down again. However, there`s a 20-30% chance a second batch would
be possible with gusty/variable winds - this time not as strong.
Before the storms arrive Tuesday afternoon, we`ll see gusty
southerly winds increase 16Z-21Z with daytime heating and
increased mixing/flow aloft driving the strengthening winds.

&&

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

&&

$$

UPDATE...Barjenbruch
SHORT TERM.....RJK
LONG TERM......Meier
AVIATION...Barjenbruch