104
FXUS65 KBOU 182100
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
300 PM MDT Wed Sep 18 2024

.KEY MESSAGES...

- Dry and warm through Friday with elevated fire weather
  conditions.

- Storm system will impact the area this weekend. 70-80% chance
  of precipitation, but considerable uncertainty regarding how
  much. There is a chance of significant rainfall, and several
  inches of mountain snow depending on track/evolution.

- Turning much cooler by late Saturday and Sunday. Below normal
  temperatures early next week, then likely (60% chance) back to
  drier and warmer weather toward the middle of next week.

&&

.SHORT TERM /Through Thursday/...
Issued at 245 PM MDT Wed Sep 18 2024

Water vapor imagery shows very dry air within southwesterly flow
aloft. Subsident flow over our CWA has lead to a quiet weather day
with mostly sunny skies. There are gusty winds especially across
the high country but winds have been rather weak across the
plains.

Tonight and into tomorrow, a ridge aloft over Texas will
strengthen and there will be ridging developing over Colorado
within the southwesterly flow aloft. This ridging will increase
subsidence and dry weather is expected to continue. There will be
weak lee cyclogenesis in northeast Colorado tomorrow which will
lead to light southeast winds. Without any downslope winds, highs
will be similar to today. Highs will top out in the low to mid 80s
across the plains. With the weak winds, critical fire weather
conditions are not expected.

&&

.LONG TERM /Thursday night through Wednesday/...
Issued at 245 PM MDT Wed Sep 18 2024

We`ll start this forecast period with upper level ridging across
Colorado, resulting in dry and unseasonably warm conditions
through Friday. For now, it appears enough mid level ridging will
occur for lighter flow aloft and weaker pressure gradients, thus
marginal breezes for fire weather concerns. High temperatures
Friday will reach the mid to upper 80s across the plains with the
warm mid level temperatures and bountiful sunshine.

The main concern for this forecast period will be this weekend`s
potential storm system and resultant weather. While we`d like to
say confidence is growing as we approach this weekend, there is
still a considerable amount of uncertainty with regard to the
track and speed. This uncertainty is rather typical for a storm
system in this synoptic setup (strong upper level ridging over
Texas and then a rather compact low getting ejected from the
Desert Southwest). Thus, we expect to gain only slow confidence
one way or another on the deterministic QPF forecast. One can see
the significant differences between each and every operational run
(e.g. 00Z and 06Z GFS having a whopping ~3" precip for Denver
metro, but then the 12Z only showing ~0.1"!). Therefore, we`ll
need to keep an eye on the big picture for now and generally align
our forecasts with ensemble averages.

Speaking of ensembles, it was interesting to note the 00Z GEFS
output favored areas to the north of Denver, while EC ENS showed
generally the heaviest QPF swath from the Front Range Foothills
due east across the plains, while the CMCE favored south of
Denver. Now the latest 12Z GEFS had also shifted south toward the
CMCE, so we`re even seeing some gyrations in the ensemble
averages. Individual ensemble ranges were impressive to say the
least, showing anywhere between nothing and 4 inches of liquid
precipitation. There`s even a large spread in the 25-75th
percentiles - which suggest roughly 0.25 - 1.5 inches of
precipitation. One thing we are more certain about is temperatures
turning cooler, with highs likely struggling to the lower 60s
across the plains by Sunday.

The main message is this...Considerable uncertainty exists, but
there is a very good chance (70-80% chance) of precipitation this
weekend. We have a good chance (50-60%) of seeing a moderate
amounts of precipitation, and low (~20% chance) of seeing light
precipitation, and a similar ~20% chance of seeing a high end
precipitation event. If the moderate to high end event unfolds,
then we`ll be looking at several inches of high mountain snowfall
(above 10,000 feet), but some accumulation down to ~8,500-9,000
feet if the heaviest/ coldest solution verifies.

By early next week, there is decent agreement of northwesterly
flow aloft developing as a ridge amplifies over the West Coast.
This would allow additional shortwaves toward Monday and Tuesday
with at least a small chance of showers, and temperatures
remaining below normal on average.

A warmup is expected toward the middle of next week with a return
of upper level ridging.

&&

.AVIATION /18Z TAFS through 18Z Thursday/...
Issued at 1248 PM MDT Wed Sep 18 2024

VFR through the TAF period. Light southwest winds will eventually
become light northeasterly. Tonight, light drainage winds are
expected. Southeasterly winds will increase tomorrow afternoon
with gusts up to 20 knots possible.

&&

.HYDROLOGY...
Issued at 245 PM MDT Wed Sep 18 2024

We discussed the probabilities of precipitation amounts in the
Long Term section above. Saturday afternoon and evening will have
some threat of burn scar flooding if we see more convection
develop ahead of the storm system. Following that, more stratiform
type rainfall would be expected late Saturday night into Sunday,
which typically doesn`t have a significant impact on burn scars.
Nonetheless, if heavier scenarios unfold then some issues could
still develop.

&&

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

&&

$$

SHORT TERM...Danielson
LONG TERM...Barjenbruch
AVIATION...Danielson
HYDROLOGY...Barjenbruch