551
FXUS65 KBOU 180201
AFDBOU

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

.KEY MESSAGES...

- Dry Wednesday and Thursday with increasing fire conditions by
  Thursday.

- Cooler temperatures this weekend with good chances (30-60%) for
  rain, and accumulating snow in the Front Range mountains.

&&

.UPDATE...
Issued at 758 PM MDT Tue Sep 17 2024

Convection has exited the far nern plains early this evening with
no additonal additional activity expected over the CWA overnight.
It will be windy at times over the higher mtns with a few gusts
up to 60 mph overnight.

&&

.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 some 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 /00Z TAFS through 00Z Thursday/...
Issued at 526 PM MDT Tue Sep 17 2024

There is a boundary to the NW of DIA which could briefly switch
winds to a more NW direction between 00z and 0030z if it holds
together. If it washes out then winds will stay south or go
southeast by 02z. Overnight winds should be drainage.  On
Wed, winds will be light and variable by late morning and then
transition to light easterly by 21z. VFR conditions will be in
place overnight through Wed.

&&

.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.

&&

$$

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