098
FXUS65 KBOU 151830
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1230 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.

- 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
  (30-50%) for rain and high mountain snow.

&&

.UPDATE...
Issued at 927 AM MDT Sun Sep 15 2024

Satellite imagery shows patchy fog over the northeastern plains
is burning off as solar heating intensifies, leaving behind a
patch of stratus stretching into the Nebraska panhandle.
Temperatures here are slightly cooler, but should warm up quickly
after the rest of the stratus burn off. With regards to convection
this afternoon, with marginal amounts of instability expected, and
dry lower-levels, the likely outcome will be scattered virga
showers that could produce some hefty outflow gusts, and
potential for some microbursts developing as showers move off the
higher terrain. The stronger gusts could near 50 mph. Beware if
you`re out on the water, as these gusts can seem to appear without
any warning, and in the absence of rainfall! There is a chance
for an isolated severe or near-severe storm to form over the
northeastern portions of the plains later this afternoon where
dewpoints are expected to remain in the mid-50s throughout the
day. The main threat would be strong winds and hail. The current
forecast is in good standing with only a few adjustments made to
include the addition of this morning`s clouds.

&&

.SHORT TERM /Through Tonight/...
Issued at 349 AM MDT Sun Sep 15 2024

Satellite pictures and observation show much of the CWA in clear
skies early this morning. There is a bit mid level cloudiness over
the far northeast corner. Temperatures are mostly in the mid and
upper 50s over the plains at this time. The high mountains,
valleys and parks are in the 30s to 40s right now.

Models have weak southwesterly flow aloft over the CWA today and
tonight.  They also have some weak upward synoptic scale energy
progged for the CWA today and tonight. Precipitable water proggs
increase today into the 0.50 to 1.00 inch range by late this
afternoon and this evening. There is higher CAPE around today
compared to yesterday with the highest values over the mountains.
The high resolution models mainly have 10-40% convective coverage
from around 20Z this afternoon over the western CWA to around 05Z
over the far eastern CWA this evening. The best coverage is over
the mountains. Soundings show the storms will produce decent
outflow winds and not too much moisture. Left the mention of fog
in over the northeast corner early this morning, as it is
possible.

Thickness fields continue to show above normal temperatures for the
CWA this afternoon for high temperatures.  Highs across the plains
look very close to yesterday`s highs.

&&

.LONG TERM /Monday through Saturday/...
Issued at 326 AM MDT Sun Sep 15 2024

Weak ridging in the southwest flow aloft moves over the area
Sunday night and Monday morning. A trough on the heels of the
ridging brings an area of rain to southwest Colorado Monday
morning. The rain tracks northeast into central Colorado by
midday. For the Front Range and eastern plains, clouds increase
mid to late morning likely keeping temperatures from topping 90F.
The rain has a tough time surviving crossing the southwest and
central mountains. Any rainfall over the Front Range and eastern
plains is expected to be light. A strong storm or two will be
possible over the northeast plains where better low level moisture
will reside. Lee side surface trough over eastern Montana and
Wyoming will produce gusty south winds over the eastern plains.

For Tuesday, the closed low over the Great Basin on Monday
weakens and lifts into the Northern Rockies. The trough axis
passes through northeast Colorado Tuesday afternoon. Surface low
pressure deepens to the north resulting in gusty south to
southwest winds throughout the area. Appears there is enough
moisture in the southwest flow to keep dew points in the 40s. This
yields SBCAPE of 500-1000 J/kg. Lift from the trough is expected
to trigger scattered showers and thunderstorms. Strong to severe
outflow winds will be possible from the showers and storms due to
the strong flow aloft and quick moving nature of the storms,
mainly over the plans. The showers and storms are expected to move
east of the area by mid evening, leaving dry conditions for the
overnight hours Tuesday.

For Wednesday and Thursday, an upper level trough digs southward
along the west coast and closes off over California Wednesday. The
low then slowly spins eastward into the Great Basin Thursday.
Over Colorado, southwest flow aloft will prevail. Temperatures
take a small step backwards behind Tuesday`s system with highs
cooling into the lower to mid 80s. Dry conditions are expected for
Wednesday and most of Thursday. We may start to see showers and
storms over northwest and north central Colorado late Thursday
ahead of the upper level low over the Great Basin.

For Friday and next weekend, the 00Z models completely flip
flopped and now again show the upper level low being stronger and
taking a more southern track. Still seeing a wide range of
solutions from the ensembles, including more showing a second
trough digging south across the Rockies late Saturday and into
Sunday/Monday. Temperatures will be cooler with troughiness over
the region. Forecast will have highs in the 70s over northeast
Colorado. However if the wetter solutions end up correct, we`ll
likely see highs only in the 60s for a day or two. As far as PoPs
go, will continue with mainly 30-40 PoPs for Friday and Saturday.
The NBM decreases to 20s for Sunday (likely too low), but it
hasn`t ingested the 00Z runs which show this second trough being
stronger and digging farther south.

&&

.AVIATION /18Z TAFS through 18Z Monday/...
Issued at 1152 AM MDT Sun Sep 15 2024

Light and VRB winds are present across all TAF sites as we
approach the noon hour. High-based showers are beginning to form
over the mountains and plains that are expected to produce gusty
outflows in the coming hours. The current SPC mesoanalysis shows
values of DCAPE ranging from 1200 to 1400 J/kg across all TAF
sites. This may translate to the production of microbursts that
could gust up to 35-45kts associated with passing virga showers.
Aside from the gusty and erratic winds for a few hours this
afternoon, there may be some lingering outflow boundaries that
tonight that could shift winds across the terminals for a brief
time. However, once the environment stabilizes, winds should
transition to a typical drainage for the overnight hours around
5/6Z.

For tomorrow, winds will likely be light and VRB in the morning
before settling into a southerly pattern as possible
showers/storms develop over Park County and the Palmer Divide. There
is less confidence in this afternoon convection with gusty
outflows than today. Forecast soundings continue to show potential
for VCSH with gusty outflows possible around 20Z, but instability
is lacking as expected increasing cloud cover through the morning
may slow surface heating. There was enough confidence to go with
a PROB30 at this time, but will monitor to see if this needs to be
adjusted.

&&

.FIRE WEATHER...
Issued at 326 AM MDT Sun Sep 15 2024

Minimum relative humidity levels will drop into the teens and
lower 20s over all but the higher mountains and northeastern
corner this afternoon for a few hours so elevated fire weather
conditions are expected.  Wind speeds will not be significant in
most of these areas unless there is are short lived convective
outflow wind gusts around.

Strong southwest flow aloft will bring spotty gusty winds on
Monday to the plains. As the main storm system moves across the
region on Tuesday, windy conditions are expected to spread across
the area. This system will be somewhat moist with scattered
showers and thunderstorms. Strong outflow winds will be possible
with the showers and storms. This will keep minimum relative
humidities in the 20s with some upper teens possible Monday. It
dries out Wednesday behind this system with relative humidities
falling into the 10-15% range. Winds look to be weaker Wednesday
with gusts less than 20 mph.

&&

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

&&

$$

UPDATE...Bonner
SHORT TERM.....RJK
LONG TERM......Meier
AVIATION...Bonner
FIRE WEATHER...RJK/Meier