982
FXUS65 KBOU 171647
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1047 AM MDT Tue Sep 17 2024

.KEY MESSAGES...

- A line of thunderstorms with high and potentially damaging
  winds are likely this afternoon. Peak gusts from severe storms
  up to 60 mph likely in the mountains, 60-70 mph along the I-25
  Corridor, and a few gusts of 70-80 mph possible over the eastern
  plains.

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

&&

.UPDATE...
Issued at 1046 AM MDT Tue Sep 17 2024

Forecast for a high impact weather day is coming together pretty
much as anticipated. Satellite shows a fair amount of sunshine
with warming temperatures across the Front Range and plains,
while scattered thunderstorms had already developed on the West
Slope of Colorado. There is excellent agreement in the short range
convective allowing models (CAMs) showing storms organizing
linearly over the mountains and then pushing east onto the I-25
Corridor 2-4 pm, and then moving quickly east across the plains
4-7 pm, and should be pretty much over by 8 pm. The one thing
there`s a little uncertainty with is the storms potentially
having too much outflow along the Front Range in a low MLCAPE/high
DCAPE environment. That could lead to new storm initiation east
of original convection, and then organizing again more
effectively east of the I-25 Corridor where damaging winds would
be even more likely.

Ingredients are all there for high winds with the storms,
including DCAPE >1200 J/kg, strong mid level environmental flow
(35-40 kts at 700 mb), and linear (QLCS-like) organization
although MLCAPE a bit limited as discussed above. Thus, we think
peak gusts from severe storms would be near 60 mph in the
mountains, 60-70 mph along the I-25 Corridor, and potentially
70-80 mph over the plains east of I-25. Those would be the
strongest, but almost all areas should see gusts of at least 40-55
mph. Winds will reach a given location well ahead of the showers
and storms, so we`re messaging safety for water recreationists.
Also, scattered power outages can be expected in areas that see
the strongest winds.

Fire danger is also a threat with strong southwesterly winds and
warm temperatures on the plains this afternoon, and the gust
fronts along and ahead of storms. We are expected wetting rains
with most storms in the mountains - hopeful we see that over the
new Pearl fire or significant fire behavior possible there with
the gusty winds associated with storms and/or post-frontal
downslope winds.

&&

.SHORT TERM /Through Tonight/...
Issued at 352 AM MDT Tue Sep 17 2024

Water vapor satellite imagery showing a broad upper level low
spinning over Nevada. Ahead of the low, lift is producing isolated
showers over western Colorado already. The center of the low
lifts into Wyoming by this evening, but as the trough axis races
northeast across Colorado it will provide plenty of lift. By mid
morning expect scattered thunderstorms across western Colorado.
Daytime heating will destabilize the airmass ahead of the trough
over central and eastern Colorado. Still good model agreement that
a line of storms form by late morning over western/central
Colorado. Mid level flow increases in the afternoon with 700 mb
winds increasing to 35-40 kts by 21Z-00Z. Storm movement looks to
be 35-40 knots as well and as mentioned in the last discussion a
20 knot forward outflow out of a storm moving 35 knots yields 55
knot winds. So it won`t take much to produce strong winds today.
Greatest instability will be over the eastern plains with SBCAPE
of 500-1000 J/kg. The greater instability should produce more
organized storm segments, leading to winds of 70 mph or locally
stronger. The enhanced severe risk over the northeast plains looks
on track. No big changes to the forecast for today. Will continue
to highlight the strong wind threat from the thunderstorms this
afternoon.

For this evening and tonight, storms continue to race
northeastward and exit the state by early to mid evening.
Subsidence quickly moves in and brings mostly clear skies
overnight. This may keep windy conditions going across the higher
foothills and Front Range Mountains this evening. Expect a chilly
night compared to recent nights due to the clear skies and dry
airmass. Lows are expected to fall into the 40s overnight across
northeast Colorado. The mountain valleys, likely fall below
freezing with lows in the mid 20s to lower 30s. A little moisture
hangs on overnight over the northeast plains where a little fog
will be possible towards sunrise Wednesday morning.

&&

.LONG TERM /Wednesday through Monday/...
Issued at 352 AM MDT Tue Sep 17 2024

A few small changes were made this update to long-term forecast
package. Strong subsidence values are displayed by QG fields
along northeastern Colorado Wednesday. Strong southwesterly flow
continues under high pressure creating mostly sunny skies and dry
conditions. 700mb temperatures between 11-13C will lead to above
normal temperatures Wednesday and Thursday. Additionally, low
relative humidities are possible. Although wind gusts remain below
25 mph for the lower elevations, there is an increasing chance
southwesterly winds could increase mid- afternoon given the upper
level stationed in southeastern Wyoming especially for the
foothills areas. This may lead to elevated fire weather conditions
for the foothills briefly but mainly for Jackson and Larimer
counties Wednesday afternoon. Additionally, Thursday similar
conditions could occur with slightly stronger winds for the lower
elevations leading to near critical conditions for the plains,
Palmer Divide, and Lincoln county.

There is good agreement for a weak shortwave increasing mid-level
moisture starting Friday late afternoon. While MLCAPE values are
low near 100 J/kg, isolated high based showers and storms with
little rainfall are possible through late evening for both the
mountains and plains. DCAPE values approach 1000-1300 J/kg
therefore expect gusty winds associated with storms and showers.
This forecast package included a decrease to PoPs Friday.

Our next upper level trough pack a punch which could arrive this
weekend. Majority of ensemble members are favoring precipitation
arriving late Saturday morning first for the higher elevations. QPF
fields range between 0.5-2 inches for the region. There is some
uncertainty if this system hovers in the northeastern corner
through Sunday afternoon. This resulted in NBM isolated PoPs for
northeast Colorado through Sunday evening. ECMWF also keeps
showers and storms lingering through Sunday evening. Either way,
there is high confidence in cooler afternoon temperatures this
weekend through early next week especially Saturday and Sunday.
Saturday compared to Friday, temperatures are almost 10-15 degrees
cooler for both the mountains and plains. Maybe even a chance for
sweater weather?

&&

.AVIATION /12Z TAFS through 12Z Wednesday/...
Issued at 533 AM MDT Tue Sep 17 2024

Strong southwest flow aloft will bring gusty south to southwest
winds through this afternoon. South winds increase to 30 knots
around 18Z. Forecast guidance is in good agreement with a line of
thunderstorms moving across the mountains and into the Denver area
between 20-23Z. This is expected to bring a brief period of
strong outflow winds (40-50kts) as the storms move through. Winds
decrease after 00Z and may turn northerly or become variable as
weak cold front moves through in the 02-04Z window. Drier air
moves in after 00Z Wednesday with mostly clear skies tonight and
Wednesday.

&&

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

&&

$$

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