059
FXUS65 KBOU 201757
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1157 AM MDT Fri Sep 20 2024

.KEY MESSAGES...

- Dry today with above normal temperatures. Breezy and areas of
  elevated fire weather conditions - mainly over the Palmer Divide
  to Park County.

- Storm system will impact the area this weekend. 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.

- 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.

&&

.UPDATE...
Issued at 1037 AM MDT Fri Sep 20 2024

Still pretty quiet in the short term. There is an enhanced band of
upper level cloudiness along the southern fringe of the upper
level jet, and this feature moves very little today so expect a
continued but fairly narrow stream of high clouds. This could keep
temperatures down a couple degrees along/north of a Denver to
Sterling line. Will make a little adjustment in the sky and
forecast max temperatures today to account for that. Forecast is
on track with breezes, warmth, and dryness for elevated fire
weather conditions over the Palmer Divide to Park County.

&&

.SHORT TERM /Through Tonight/...
Issued at 341 AM MDT Fri Sep 20 2024

Southwest flow aloft continues over northeastern Colorado as our
upper level ridge pattern ends today. Cross sections indicate weak
mid-level moisture resulting in a few clouds for the eastern
plains. With limited moisture and a lack of instability,
precipitation is not expected today. 700mb temperatures between
12-14C should lead to above normal temperatures this afternoon.
Afternoon highs range between low to upper 80s for the plains and
low 60s to low 70s for the mountains and valleys. Near critical
fire weather conditions are possible mainly along the Palmer
Divide and adjacent plains due to low relative humidities in the
teens and breezy wind gusts up to 20 mph. Tonight, near normal
lows are expected and surface winds increase as the next system
approaches.

&&

.LONG TERM /Saturday through Thursday/...
Issued at 341 AM MDT Fri Sep 20 2024

The main focus of this period continues to be this weekend`s rain
and snow event. As we have messaged in the past few AFDs, there
remains considerable uncertainty in the forecast (mainly QPF),
despite growing agreement in the upper level features of this
system. In general, there has been a slightly southward/drier
shift across the 00z model suite, though there are still quite a
few very wet solutions offered by tonight`s guidance.

The synoptic pattern at this point is pretty well established. An
upper level low - currently centered near the southern California
coastline - is expected to drift into the desert southwest, then
lift northeastward from the Four Corners region into eastern
Colorado from Saturday into Sunday. Models are in good agreement
that this upper low will gradually weaken and transition to an
open wave as it begins to be picked up by the northern stream
flow. The mid-level low should also take a similar track, though
it`s also weak and as a result the upslope flow isn`t particularly
strong or long-lasting across our CWA.

At the surface, a strong cold front should be moving across the
forecast area early Saturday morning, with temperatures on
Saturday significantly cooler than what we`ve seen in the last
week or two. Forecast high temperatures will likely peak in the
upper 60s to 70s across the plains, which would be the coolest
temperatures so far this month. As a surface high strengthens
across the northern plains, it should promote continued cold air
advection/shallow upslope flow through much of the day.
Increasing synoptic scale lift will eventually lead to widespread
precipitation developing Saturday afternoon or evening, with the
bulk of precipitation expected to fall Saturday night into the
first half of Sunday. Sunday will be the coolest day in quite some
time, with highs likely struggling to get out of the 50s across
the plains. Showers should come to an end as the storm system
shifts off to our east and our source of lift diminishes.

Like the day shift, I am similarly perplexed by the lack of
consistency in the QPF forecast. There are no clear trends to work
with, and each of the main ensemble systems still have quite a
bit of spread. The most recent ECM ensemble for Boulder still has
a mean of just over 3/4" of liquid, though there is one member
with a trace, and a handful of members with >2" of rain over a 24
hour period. Our forecast lies a little on the wetter side of
guidance but is overall a touch drier than the previous forecast.
In terms of snowfall, the forecast hasn`t changed significantly.
We still expect several inches of snow above 10kft with snow
levels dropping to as low as 8500-9000ft. Again, we did consider a
Winter Weather Advisory for a couple mountain zones but held off
due to the unclear QPF forecast. We`ll punt that decision back to
the day shift and hope today`s guidance offers a little more
clarity.

We should see temperatures near normal (or perhaps a little
cooler) through the first half of next week, with northwesterly
flow aloft persisting. Models generally have one or two shortwaves
dropping down into the region on Monday/Tuesday, with hints at
another strong cold front for Tuesday. There would be a chance for
additional rain showers and mountain snow - albeit a lower chance
than this weekend - but at the very least a return of consistent
80/90 degree days looks unlikely.

Model solutions diverge as we get later into next week, but the
general pattern would favor a return to warmer and drier
conditions. Both the GFS/GEM build up a strong ridge by the latter
half of the week, while the ECM cuts off one of the previous
shortwaves, which then aimlessly drifts across the mountain west.
The broader ensemble still favors a return to above normal
temperatures and the NBM provides a reasonable starting point for
this 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/cylone 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.

&&

.FIRE WEATHER...
Issued at 341 AM MDT Fri Sep 20 2024

With relative humidities dropping to 11-15 percent this afternoon,
near critical fire weather conditions are possible mainly for
Lincoln county and the Palmer Divide. Southwest winds remain below
critical thresholds producing wind gusts up to 20 mph through
early evening.

&&

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

&&

$$

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