600
FXUS65 KBOU 172015
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
215 PM MDT Tue Sep 17 2024

.KEY MESSAGES...

- A line of thunderstorms with high and potentially damaging winds
  will shift east across the plains late this afternoon into
  early evening. Peak gusts from severe storms to around 60-65 mph
  in and near 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 accumulating snow in the Front Range mountains.

&&

.SHORT TERM /Through Wednesday/...
Issued at 215 PM MDT Tue Sep 17 2024

Well convection is ongoing, but this also reminds us how tricky
convective development and modeling is in reality. Earlier CAM
(convective allowing model) output showed mostly one organized
line of convection that should be ongoing at the present time. The
NAMNest had the most reality in the earlier 12Z runs suggesting a
split into a couple lines, while the HRRR is now finally getting
a handle on the real situation - with three lines clearly evident
on satellite and radar data. At this point, it appears the first,
or easternmost of these lines now organizing over the Front Range
as of early this afternoon will become the dominant line as it
pushes east across the plains through late afternoon and very
early evening. This first line is where the main threat of high
and potentially damaging winds can be expected, with the secondary
and tertiary lines expected to eventually weaken through late
afternoon. This means that roughly areas from east/southeast side
of Denver metro eastward across the plains will be the most
vulnerable for high and potentially damaging winds late this
afternoon. Ingredients are still 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. It`s just that organization may be a bit later than
originally anticipated.

Convection will pass to the east and into Nebraska and
Kansas by/before 7 pm. Clearing skies will then occur overnight
with a much drier airmass moving in. It will still be windy
overnight in the northern mountains and highest foothills with
stronger westerly flow aloft there.

Wednesday will feature dry conditions, mostly sunny skies, and
lighter winds. However, it will still be breezy in the northern
mountains and foothills. Temperatures will a few degrees cooler,
but still above normal for this time of year with highs on the
plains reaching the lower 80s.

&&

.LONG TERM /Wednesday night through Tuesday/...
Issued at 215 PM MDT Tue Sep 17 2024

Colorado will be in between storm systems Wednesday night through
Thursday with a moderate and dry southwesterly flow aloft. This
combined with some upper level ridging should result in dry weather
and slightly above normal temperatures across the forecast area. On
Friday, the southwesterly flow aloft increases as a closed upper low
drifts eastward from Southern California into West Central Arizona.
With a dry airmass still in place, precipitation chances across the
CWA will be limited. However, wouldn`t surprised to see a few light
showers across the high country by late in the afternoon due to
increasing QG ascent ahead of the upper low. In addition, gusty
south to southwesterly winds combined with low relative humidity and
very dry fuels may lead to elevated to near critical fire weather
conditons across the Palmer Divide and adjacent plains.

Much cooler unsettled weather is expected over the weekend as the
upper level storm system and associated cold front moves across the
region. The consensus of the models is that the upper low will track
across the Four Corners Region across Central or Southern Colorado.
At this time there is still quite a bit of uncertainty in QPF
amounts due to run to run inconsistencies and model discrepancies.
This can be seen by looking at the GEFS, EPS and the CMCE ensembles
which have a wide range of QPF amounts in Denver ranging between 0
and 3.5 inches. It all depends on the speed and exact track of
the storm and local mesoscale effects. The model averages are
suggesting precipitation amounts between 0.25" and 1.25" across
the CWA with 4 to 8 inches of snow across the higher Front Range
Mountains. However, there are no guarantees in this as minor
changes in the storm track could make a big difference in where
the heavier band of precipitation sets up. We will continue to
keep a close eye on this system, so stay tuned for the latest
forecast updates.

Dry and warmer weather is expected early next week as the upper
level storm system moves northeast into the Upper Midwest and upper
level high pressure builds over the Western U.S.

&&

.AVIATION /18Z TAFS through 18Z Wednesday/...
Issued at 1143 AM MDT Tue Sep 17 2024

Main concern is for very strong winds of 40-55 kts at the
airports with a line of thunderstorms this afternoon, as all
ingredients support these strong gusts. A 10-15% chance of a 60kt
gust. Most likely time for those will be between 21Z and 23Z. The
strongest winds should occur along and just ahead of the showers
and storms (and mainly from the SW/W). Winds will then turn more
easterly or light/variable behind the showers and storms,
eventually turning to fairly normal diurnal wind patterns from 06Z
through Wednesday.

&&

.FIRE WEATHER...
Issued at 215 PM MDT Tue Sep 17 2024

It will remain breezy to windy over the northern mountains and
foothills overnight, with gusts around 40 mph on some of the
higher slopes including locations in/near the existing Pearl Fire.
Drier air will move in overnight and Wednesday, with only
moderate humidity recovery tonight in/near the foothills, and then
low humidity readings most areas on Wednesday. Fortunately winds
will be lighter Wednesday, but still breezy across the northern
mountains and foothills. Humidity will remain above critical
thresholds there, but nonetheless given dry fuels and what we
think will be mostly limited rainfall this afternoon, pockets of
elevated fire weather conditions can still be expected in the
windier locations.

Near critical fire weather conditions are possible for the Palmer
Divide, Lincoln county, and the urban corridor during the late
afternoon and evening hours Thursday and Friday due to breezy
conditions and low humidity.

&&

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

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...Kalina
AVIATION...Barjenbruch
FIRE WEATHER...Barjenbruch