179
FXUS65 KBOU 211809
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1209 PM MDT Sat Sep 21 2024

.KEY MESSAGES...

- Cooler today, with below normal temperatures.

- Storm system will impact the area this evening through Sunday,
  with numerous showers and a couple storms. But expected rainfall
  amounts trending toward the lower side of earlier guidance.

- The first real mountain snow is likely, with >70% probabilities
  of accumulating snow above 9,000ft. Minor travel impacts over
  high mountain passes this evening - Sunday morning.

- Well below normal temperatures for Sunday, but likely a short-
  lived cooldown before a return to near or above normal
  temperatures next week.

&&

.UPDATE...
Issued at 940 AM MDT Sat Sep 21 2024

Cold front moved across the plains and back through the foothills
early this morning. Temperatures cooled with gusty north/northeast
winds across the plains. Those winds will slowly diminish through
the day and turn more northeasterly. Unfortunately the low levels
are quite dry behind the front, with dewpoints holding mostly in
the mid 30s. That`s one strike in what could otherwise be a good
QPF event for us.

Forcing has also been delayed, which means we`ll also be slowing
down the arrival of precipitation into our forecast area. There
was one surge and precipitation shield in northeast New Mexico,
but for the most part that will slide east of our forecast area,
only clipping Lincoln County. In between that there is a large dry
slot noted on water vapor and satellite imagery, before the main
synoptically forced precipitation area in northeast AZ and
southeast UT. We should see that gap fill in later this afternoon
and evening as enough moisture and instability is present in the
low/mid levels for convective development in southern/central CO.
This should end up being the start of our precipitation event, but
given the slower motion of forcing we`ve delayed most of the
precipitation development along the Denver metro area and adjacent
plains til this evening. Even then, there`s more uncertainty as to
how this unfolds with the dry low levels, but would think
eventually tonight we`ll see more widespread showers through a
combination of upper level forcing as seen in the QG fields as
well as persistent upslope ahead of the ejecting upper level low.

However, due to the dry low levels the effective intensity of
precipitation will be limited, so we`re trending toward less QPF
in this forecast. In fact, the latest model averages are really
dropping QPF numbers - mostly under 0.5 inch for Denver, and much
less than that farther north toward Longmont and Fort Collins.
The more southern trend continues as well, and this is noted in
the latest QG forcing and upper low track being farther south, and
a little weaker too. This puts into question the Winter Weather
Advisory for the mountains north of I-70 with less QPF to work
with. We`ll continue to watch that trend as well.

&&

.SHORT TERM /Through Tonight/...
Issued at 344 AM MDT Sat Sep 21 2024

A few changes to precipitation chances and temperatures the short-
term forecast. Overall, model guidance has a good handle on the
location of the upper level low compared to current GOES-18 water
vapor satellite imagery. As this system continues to slowly push
into western Colorado, there will be a delay in precipitation as
majority of forcing will arrive late this afternoon.

Before the arrival of our storm system, a cold front should drop
temperatures along the forecast area. Afternoon temperatures will
peak between the upper 60s to low 70s for the plains.
Additionally, the Front Range mountains and valleys should drop
between the mid 40s to low 60s. As the trough approaches, mid
level to surface winds increase shifting to northeast this
afternoon. Lower elevations could experience wind gusts up to 40
mph before any precipitation develops. Any heating and instability
remains limited, thus the flash flood threat over the burn areas
has significantly decreased given the isolated thunderstorm
threat. There is a low chance of early showers developing this
afternoon but will likely fight drier air near surface. Stratiform
rainfall should begin this evening across the lower elevations.
Although majority of ensemble members for both the GFS and ECMWF
have decreased, QPF fields between 0.30-0.90 inches seem
reasonable for areas below I-76 corridor. There is some
uncertainty for areas such as Fort Collins and northern Weld
county due to this upper level low becoming weaker. The only
advantage these areas have are periods of upslope flow possibly
tonight. It is evident majority of the forecast areas will receive
much need precipitation especially for areas in severe drought.
Snowfall amounts remain unchanged in the forecast with areas
above 9,000 feet receiving 3-8 inches therefore the Winter Weather
Advisory continues starting this evening. Tonight, low
temperatures drop below normal for the entire forecast areas and
rainfall continues overnight.

&&

.LONG TERM /Sunday through Friday/...
Issued at 344 AM MDT Sat Sep 21 2024

The 500mb vort max should be pretty close to the Denver metro by
Sunday morning, and there should be a narrow band of showers
arcing around this vort max at the start of the forecast period.
Guidance still isn`t in great agreement on where this sets up, but
the multi-model mean would place this somewhere near Denver and
off to the east-northeast. A couple slower solutions keep some
rain around through around noon, but most of the area should by
dry as we get into the afternoon hours. Gradually clearing skies
are expected through the day, but this will do little to warm up
temperatures during the day. 700mb temperatures do manage to climb
back to around +2-4C by the afternoon, and high temperatures
should reach the upper 50s to perhaps the low 60s across the
plains.

Northwesterly flow aloft is expected on Monday, with temperatures
warming back up into the 70s. Moisture aloft gradually increases
by the late afternoon hours ahead of an approaching shortwave and
it may be enough for a few rain or snow showers across the higher
mountains by the evening hours.

Guidance begins to quickly diverge by Monday night/Tuesday. A
shortwave tracking out of the northern Rockies should dive
southeastward towards the Central Great Plains, with an
accompanying cold front pushing towards the region late Monday or
early Tuesday. The GFS/UKMET keep this feature much closer to the
parent trough over the upper Midwest, while most other models have
the s/w detached from the longwave trough (and eventually cuts it
off into a closed low), which ushers in cooler weather into our
area again. There isn`t any particularly clear reason to go with
one solution over the other, and for now the NBM is a reasonable
middle ground.

Forecast uncertainty grows as we get into the latter half of the
week, with the GFS developing a strong ridge over the region,
while other models still have lingering effects from the weakening
upper low over the Central/Southern Great Plains. At this point
the GFS and its ensemble are outliers compared to the rest of the
long range guidance, and our forecast is closer to the non
GFS/GEFS mean. Despite the uncertainty in temperatures, there is
rather high confidence that this period will be dry with no
meaningful PoPs.

&&

.AVIATION /18Z TAFS through 18Z Sunday/...
Issued at 1209 PM MDT Sat Sep 21 2024

VFR conditions will give way to showers and lower ceilings through
the course of the evening and overnight. It was difficult to
determine the exact start time of showers, since the low levels
are quite dry. We do think there will be some light showers in the
vicinity or virga toward 00Z, with a higher probability of showers
toward 02Z, and especially after 06Z as low levels slowly moisten.
The lack of initial moisture also means there`s considerable
uncertainty in low cloud extent and height, but eventually we
should be in enough rain or showers in the vicinity to saturate
the low levels with stratus and at least MVFR conditions
developing. About a 30% chance we drop to IFR and some fog at
times 08Z-16Z. The ceilings will likely break after rain showers
end toward 18Z Sunday.

North/northeast winds will prevail, trending more easterly tonight
and then southeasterly on Sunday.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM this evening to noon MDT
Sunday for COZ033-034.

&&

$$

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