280
FXUS65 KBOU 220243
AFDBOU

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

.KEY MESSAGES...

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

- The first real mountain snow is still 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 826 PM MDT Sat Sep 21 2024

Only minor changes to the forecast at this hour. A leading band of
showers has lifted north of the Denver area, with the more
substantial area of rain just approaching now. We still expect
this band to stall and pivot near or a little south of Denver
overnight. So far the heaviest precipitation has been in the
mountains of northern Park county which do really well in these
southeast flow/deformation band situations. SNOTEL data suggests
up to three-quarters of an inch of precipitation, with likely a
rain/snow mix at 11,000 feet for a couple inches of snow
accumulation. This focus will likely shift a bit further east, but
the idea of substantial accumulations above 10 or 11 thousand feet
in that stretch of mountains and perhaps as far north as Clear
Creek and Gilpin counties still looks good. Lighter accumulations
further north. We shifted the gradient in PoPs north and northeast
a bit given the position of that leading band, but the general
idea still looks good.

&&

.SHORT TERM /Through Sunday/...
Issued at 244 PM MDT Sat Sep 21 2024

Satellite shows the upper level low drifting northeast across
northeast Arizona, while convection has blossomed ahead of it
across central Colorado. From the looks of satellite imagery and
latent heat release, the upper level low will reorganize into
west central Colorado this evening before opening up and
elongating eastward into eastern Colorado by Sunday morning. At
the surface, upslope northeasterly winds dominated across the
plains, with the upslope component on the wind profilers seen
deepening over mountain top. Dewpoints were holding mostly in the
mid 30s across the plains, and that remains one of the most
concerning factors in this precipitation event.

One other thing of note is that the forcing has been delayed with
a slightly slower ejection of the upper level low, which means
we`ll also be slowing down the arrival of precipitation into our
forecast area this evening. As discussed earlier, the convection
now in Central Colorado will be the initiating factor for our
precipitation event, but given the slower motion of forcing we`ve
delayed most of the precipitation development along the Denver
metro area til this evening, and the rest of the plains through
the night. 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 and the persistent
upslope ahead of the ejecting upper level low.

The limiting factor with regard to precipitation amounts will be
the dry low levels. Latest model trends have caught onto this with
lower QPF amounts, although there is still a rather large
discrepancy in various model runs. Overall, we think most places
will see amounts of 0.25-0.50 inch, although a few spots (perhaps
one associated with mid level frontogenesis on the northern
fringe of the upper low) could see 0.75 to 1 inch yet as a band
rain could very well last through most of Sunday morning. There is
still reasonable agreement in all the newest data that the
northern border area will see little if any precipitation, as the
upper low and forcing have trended slightly southward.

With regard to the mountains, snow will be the main concern with
colder temperatures arriving this evening through a combination of
cold advection, latent heat of melting, and evaporative
cooling/wet bulbing. QPF will be the trickiest aspect, as again
the best precipitation will be generally south of Rocky Mountain
National Park. This favors the higher mountains and passes of
Park and Summit Counties, possibly into the Indian Peaks. We`ll
keep the Winter Weather Advisory in effect, with the main impacts
being slushy/snow covered roads over the higher passes (Vail Pass,
Berthoud Pass, approaches to the Eisenhower Tunnel) with the
heavier showers this evening into Sunday morning.

Temperatures will be much cooler Sunday with low clouds and
showers to start the day. But some sunshine is now expected to
return in the afternoon, enough to support highs into the upper
50s and lower 60s over most of the plains. It will still be the
coolest day in several months, quite appropriate for the first day
of Autumn!

&&

.LONG TERM /Sunday night through Saturday/...
Issued at 244 PM MDT Sat Sep 21 2024

Any lingering showers are expected to ease by Sunday night as the
upper-level low shifts east and out of Colorado. Overnight
temperatures will feel cold for this first day of the official
season change as widespread below normal temperatures are expected
across the CWA. This is expected to bring our first frost of the
season to portions of the plains overnight. Areas of fog may lower
visibility in the early morning, mainly for Weld and Morgan
Counties, but is expected to diminish as surface heating
intensifies.

Flow aloft will transition to a northwest regime as a building ridge
develops over the western CONUS to begin the week. Within the flow
aloft, a shortwave is expected to traverse the Rockies, bringing
increasing mid-level moisture throughout the day. Forecast soundings
indicate marginal instability, but there should be enough moisture
available for scattered afternoon high-based showers to develop
over the higher terrain, with little to no accumulations expected.
After Sunday`s cool afternoon temperatures of 50s and 60s,
Monday`s afternoon highs will rebound back up into the mid to
upper 70s across the plains, 50s/60s for the mountains. A cold
front looks likely to pass through northern Colorado Monday night
into Tuesday morning bringing cloudy conditions and below normal
temperatures for Tuesday.

Models are persistent in diverging from this point, with the
deterministic GFS closing off Monday`s passing shortwave once it
enters the Mississippi River Valley region, where it sits through
the remainder of the extended forecast period. The ECMWF pushes it
straight south into Texas where it closes off and sits until
ejecting to the northeast sometime over the weekend. Looking at the
their ensemble partners, PWAT anomalies indicate continued dry
conditions through the end of the forecast from Wednesday on with
values ranging from 80 to 105% of normal across the CWA. As
previous discussions have indicated, the NBM is a reasonable
middle ground for now until models show some stronger agreement.
Therefore, will let it ride for the latter half of the week.

&&

.AVIATION /00Z TAFS through 00Z Monday/...
Issued at 826 PM MDT Sat Sep 21 2024

Rain will increase around the Denver area through 06z, with MVFR
ceilings and visibilities developing and localized IFR possible.
These conditions will continue into Sunday morning, with rain
diminishing and conditions improving by 18z. Instrument approaches
to KDEN will likely (70% chance) still be needed into the
afternoon, with clearing between 21z-24z.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon MDT Sunday for COZ033-034.

&&

$$

UPDATE...Gimmestad
SHORT TERM...Barjenbruch
LONG TERM...Bonner
AVIATION...Gimmestad