559
FXUS65 KBOU 202119
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
319 PM MDT Fri Sep 20 2024

.KEY MESSAGES...

- Dry and warm conditions replaced by cooler temperatures and late
  day showers Saturday.

- Storm system will impact the area Saturday evening through
  Sunday. 80-90% chance of precipitation, with rainfall amounts
  the last remaining uncertainty.

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

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

&&

.SHORT TERM /Through Saturday/...
Issued at 245 PM MDT Fri Sep 20 2024

Satellite and surface analysis shows a pretty dry airmass over the
region, with just a band of high level cirrus clouds. Those clouds
will dissipate this evening with mostly clear skies and relatively
cool temperatures overnight. A cold front arrives on the northern
plains late tonight into the early morning hours Saturday.

That cold front will be the start of a significant change in
weather for us. There will still be a fair amount of sunshine to
start the day Saturday, so despite the cold front and persistent
cold advection all day, high temperatures should still be able to
climb into the 65-70 degree range on the plains and I-25 Corridor.
A touch of fall will definitely be in the air, but nothing
compared to the cool expected for Sunday.

With regard to precipitation, there was a trend toward slightly
slower arrival of QG lift and precipitation, with most development
of showers holding off until late afternoon. Instability will be
limited, so only isolated thunderstorms possible. The showers will
be spreading in from the south and southwest, so places like the
northern tier of counties could still stay dry the entire day. The
bulk of precipitation and mountain snowfall is still on target
for Saturday night into Sunday morning. More on that in the Long
Term Discussion below.

&&

.LONG TERM /Saturday night through Friday/...
Issued at 245 PM MDT Fri Sep 20 2024

Winter enthusiasts rejoice! Northern Colorado will get its first
taste of the changing season as the first measurable snowfall is
expected this weekend for our highest elevations. Fun Fact: This
will also coincide with the autumnal equinox.

By Saturday evening, precipitation will shift east and north across
the forecast area. As temperatures decrease, snow-levels are
expected to lower to roughly 8700` to 9000`. Uncertainty in QPF
remains, however, the latest runs show yet another slight southward
shift with a drier trend from what the midnight shift discussed
in the previous AFD. Northeasterly upslope flow will help bring
the most snowfall accumulations to the Front Range Mountains of
Park and Clear Creek Counties where accumulations from 5-8" are
expected. With the anticipated southward trend, totals to the
north will be lighter, ranging from 2-5", with some greater
uncertainty with regards to locations in northern Larimer County.
The greatest expected impacts will likely be along I-70 from Vail
Pass to Georgetown where snow showers may cause some slick roads
and hazardous travel Saturday evening into Sunday morning above
10000`. Even though impacts are expected to be minimal, with this
being the first snow accumulation of the season, we have issued a
Winter Weather Advisory for the Front Range Mountains and Summit
County for elevations above 9000`. This will be in effect from 6PM
Saturday through noon Sunday. Flash Flood threats to the burn
scars should be minimal, with the Quarry Fire scar having a
limited threat.

Across the plains, ensembles still show some discrepancies with
amounts, but show agreement that the main stretch for the highest
accumulations of rainfall look to be from Boulder to Sterling,
along and south of I-76. The GFS ensemble shows a 40-50% chance of
greater than .5" falling in this location, where the ECMWF
ensemble shows 80-90% chance of this occurring.

Precipitation is expected to shift north and east throughout the
day, diminishing by the late afternoon. Temperatures will feel
quite different than what we have felt over the past few months,
with afternoon highs struggling to reach the 60s across the
plains. Overnight lows will drop into the 20s and 30s for the
mountains, and remain only slightly warmer across the plains, with
widespread mid to upper 30s expected.

Ridging is expected to build behind the exiting storm system to
kick off the work week. A shortwave is expected to pass through
bringing another chance for precipitation across the CWA on
Tuesday. By mid-week, temperatures are expected to climb back into
the 80s, but looks like the changing season is beginning to take
hold, with no 90s expected in the extended forecast period.


&&

.AVIATION /18Z TAFS through 18Z Saturday/...
Issued at 1157 AM MDT Fri Sep 20 2024

VFR conditions continue through the TAF period. Winds are expected
to hold from the ESE through the afternoon at KDEN and KBJC, with
a shear zone/cyclone development this afternoon to the south.
There is relatively high confidence this shear zone stays south
(70-80%) through the afternoon. However, KAPA could be just far
enough south for gusty south/southwest winds there (50-60%
confidence) toward peak heating/mixing around 21Z. Winds get a
little more tricky through this evening, with the shear
zone/cyclone potentially lending toward more north/northwest flow
01Z-05Z, and then disrupted or lighter than normal drainage winds
overnight. We`ve opted for a middle of the road approach with more
westerly component overnight. A cold front arrives 12Z-14Z
Saturday with switch to northerly winds. Low level moisture is
rather scant behind the front, so the threat of any stratus or
lower ceilings is still small (<20%). Showers will increase after
21Z Saturday with gradually lowering cloud decks then.

&&

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

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...Bonner
AVIATION...Barjenbruch