613
FXUS65 KBOU 170212
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
812 PM MDT Mon Sep 16 2024

.KEY MESSAGES...

- Severe storms with high and potentially damaging winds will be
  likely Tuesday afternoon.

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

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

&&

.UPDATE...
Issued at 759 PM MDT Mon Sep 16 2024

It`s relatively quiet across the forecast area this evening, with
just a few dissipating showers over the higher elevations. Outside
of some mid/high cloud cover, not a whole lot is expected
overnight tonight. Didn`t need to make any significant changes to
the grids, but did update the PoPs through the night.

&&

.SHORT TERM /Through Tuesday/...
Issued at 300 PM MDT Mon Sep 16 2024

Isolated to scattered weak convection is attempting to develop
across the forecast area this afternoon. MLCAPE is negligible in
all but the far northeast corner, where the airmass is more
strongly capped. Thus, we think the main threat over the next few
hours will be gusty outflow winds to around 40 mph, with only
light rain. Most of this activity is expected to decrease rather
quickly this evening with stabilization and passage of the weak
shortwave moving northeast across the area right now. Clearing
skies are expected overnight, with low temperatures near normal
and lighter winds.

Tuesday is still shaping up to be an atypical active September day
with severe/damaging winds possible from fast moving showers and
storms in the afternoon. Many ingredients are coming together for
this threat. A fast moving shortwave kicks out of the Great Basin
and lifts northeast across Colorado in the afternoon - coinciding
with peak heating and destabilization. Mid level flow increases
in the afternoon with 700 mb winds increasing to 35-40 kts by
21Z-00Z. DCAPE increases to 1000-1200 J/kg (not great for high
winds, but will definitely lead to acceleration of the
environmental flow). Thus, just 20 kts of acceleration added onto
35 kt mean flow in a well mixed environment would suggest shower
and storm outflows easily reaching 55 kts (~65 mph). In addition,
these storms may align linearly, suggesting more widespread high
winds possible than our typical thunderstorm days and similar to a
QLCS type structure. That said, our instability is lacking which
will be the one limiting factor in our severe weather/high wind
threat for tomorrow. Still, it was interesting to see some high
resolution model output showing a couple pockets of 70-80+ mph
storm outflows tomorrow afternoon. Whatever the case, tomorrow is
shaping up to be a rare day with a legitimate severe storm/high
wind threat.

Boaters, paddleboarders, and water enthusiasts beware of the
weather tomorrow! It is best to be off the water before noon. Sudden
onset of gusty winds are likely on open waters, which could leave
you in peril with little advanced warning.

&&

.LONG TERM /Tuesday night through Monday/...
Issued at 300 PM MDT Mon Sep 16 2024

A potent upper level shortwave will be move across the Central and
Northern Rockies Tuesday night with a moderate southwesterly flow
aloft over Colorado. There may still be a line of convection across
the far eastern zones during the early evening which should quickly
move out of our forecast area. Some of these storms could be severe
with strong wind gusts being the main threat. In addition, the
models are hinting at a mountain wave setting up Wednesday evening
with wind gust to 65 mph possible across the higher foothills and
Front Range Mountains.

For Wednesday through Friday, Colorado will be in between storm
systems under the influence of a moderate southwesterly flow aloft
and weak upper level ridging. With limited moisture, dry weather is
expected across all of the CWA with breezy conditions across the
high country. The combination of low relative humidity and gusty
southwesterly winds may lead to elevated to near critical fire
weather conditions from South Park eastward across the Southern
Foothills, Palmer Divide and east across Lincoln County. There
should be a uptick in shower and thunderstorm activity on Friday as
mid level moisture and QG ascent increases ahead of a large closed
upper low centered over the Mohave Basin. Despite the increase in
moisture, the airmass is expected to remain quite dry. Consequently,
the showers and storms are expected to be high based with gusty
winds and mainly light rain.

Cooler unsettled weather is expected Friday night into the weekend
as the next storm system and associated cold front moves across the
Central and Southern Rockies. At this time, there is quite a bit of
uncertainty on how the pattern will unfold due to run to run
inconsistencies and model discrepancies. For example, the latest GFS
run has trended further south and has a weaker upper low tracking
across the Colorado/New Mexico border. This solution only produces
light QPF across the CWA. On the other hand, the ECMWF, suggest a
heavy swath of precipitation stretching from the Central and
Northern Mountains across the plains north of I-76. This is in
response to a stronger upper low tracking from the Four Corners
Region into Northeastern Colorado. If the ECMWF solution verifies,
we could even see some severe weather across the far Northeastern
Plains as well as several inches of snow across the high mountains.
Due to all the uncertainty, have decided to stay with the pops given
by the model blend.

Mostly dry and warmer weather is expected early next week as the
storm system moves east of the region and a dry west to
northwesterly flow aloft sets up over Colorado.

&&

.AVIATION /00Z TAFS through 00Z Wednesday/...
Issued at 532 PM MDT Mon Sep 16 2024

VFR conditions will continue through tomorrow morning. Gusty
southerly winds should gradually diminish over the next few hours
leaving typical drainage flow speeds (10-15kt) in place through
the overnight hours. At BJC winds should become light and variable
tonight.

Tomorrow will see a return of enhanced southerly flow by the late
morning or early afternoon. Forecast guidance is in good agreement
with a line of convection moving across the mountains and into the
metro. As mentioned in the previous AFD, this would likely be a
brief period of outflow winds as the storms move through, with the
potential for 40-50kt gusts for a brief period. Best timing of
this generally looks to be 20-23z. Additional convection beyond
that seems less likely and winds should diminish beyond 00z as
daytime heating ends.

&&

.FIRE WEATHER...
Issued at 300 PM MDT Mon Sep 16 2024

Good humidity recovery can be expected in most areas tonight, with
winds also decreasing after showers/isolated storms end this
evening.

Tuesday is shaping up to have elevated fire weather conditions
ahead of the approaching cold front, with southwest winds
increasing in most areas in a still warm/dry environment. Showers
and storms will provide for scattered wetting rains, but also
produce very strong outflow winds.

&&

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

&&

$$

UPDATE...Hiris
SHORT TERM...Barjenbruch
LONG TERM...JK
AVIATION...Hiris
FIRE WEATHER...Barjenbruch