356
FXUS65 KBOU 112357
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
557 PM MDT Sat May 11 2024

.KEY MESSAGES...

- Showers/thunderstorms are possible (>50%) for much of the day
  Sunday, with locally moderate rainfall. Thunderstorms could
  begin as early as 9 AM, but become more favorable during the
  afternoon hours. Several inches of snow are possible above 9,500
  ft elevation.

- Drier and milder Monday outside of a few mountain
  showers/storms.

- Turning more active again Tuesday and Wednesday with increased
  chances for showers and thunderstorms across high country and
  lower elevations.

&&

.SHORT TERM /Through Sunday/...
Issued at 131 PM MDT Sat May 11 2024

GOES-17 water vapor imagery shows a slowly moving vorticity max
moving slowly to the east across the AZ/UT border. By sunrise
Sunday the trough should be directly overhead the central part of
Colorado, then over western Kansas by Sunday evening. Several
sources of lift should be in place as the trough moves overhead.
There is a fair amount of QG lift as well as a deformation zone
that wraps around the 500 mb low, especially Sunday morning.
Additionally, the core of the trough is -18 degC at 500 mb, and
not to be outdone, there is strong northeast flow across the
plains starting mid-morning Sunday. Overall it will be hard to pin
down best periods and locations of rain because of a few
competing factors, including the potential for a stratus layer
Sunday morning. Let`s start with overnight first.

Scattered showers and isolated thunderstorms should dissipate
and/or move east of our area before midnight tonight. We`ve
lowered PoPs a bit overnight between midnight and sunrise as most
CAMs do not break out much precip, which makes sense with the loss
of sun/instability and before the arrival of the QG lift and
deformation zone. Those features look to arrive between 8-10 AM
more or less, but models have been backing off QPF associated with
this banded feature. Instability is lacking, so much that if the
band forms, it`ll probably just be light rain without any thunder.
CAMs have likewise been wishy-washy on the location of this
banded rainfall, but latest trends have the band across I-70 or
points south. The uncertainty has led to PoPs going from 20
percent across the northern tier Colorado Counties to 60 percent
across I-70 and the Palmer Divide during the morning hours. Most
CAMs, other than the HRRR, have low clouds in place which would
limit instability as mentioned earlier, and even without the
clouds the HRRR has been backing off on QPF from this first round
of showers. Assuming low clouds either don`t form or evaporate by
midday, there should be enough instability to form showers and
thunderstorms in the afternoon across all but the areas near the
Wyoming border. These will be widely scattered and thus the
highest PoPs are still only about 60%, but if you happen to be
under one of these showers you might experience moderate rain,
gusty winds 25-30 mph, and very small hail are possible. In terms
of temperatures, it`s interesting that after the trough axis
passes and northerly flow kicks in aloft by late morning, there
will actually be warm advection in the 700-500 mb layer, and areas
with more sun like along and north of US-34, high temps will top
out near 70 deg. In the thicker cloud cover, including
Denver/Boulder metro areas and the plains to the east and south,
highs will likely only be in the low to mid 60s. Middle/North
Parks should also be in the 50s to near 60, but thick clouds and
also probably the best chance of precip across Park County and
the foothills from Boulder County to the Palmer Divide should
result in temps barely reaching 50. Snow levels throughout Sunday
should remain above 9500 ft MSL. 36 hour snowfall totals above
10,000 ft could be over 6", but a good chunk of that amount
depends on the banded precipitation Sunday morning through midday,
as well as the convection Sunday afternoon, so totals are likely
to be widely varying or short distances.

&&

.LONG TERM /Sunday night through Saturday/...
Issued at 131 PM MDT Sat May 11 2024

Sunday evening, northeast Colorado will be under the backside of the
exiting upper low/trough system. Weak synoptic ascent builds in with
showers and storms tapering off generally from north to south.

Monday, an upper level ridge moves in with northwesterly flow aloft.
Subsident flow under the ridge will keep it much drier than Sunday.
Model cross sections show residual moisture in the flow coupled with
marginal instability (MLCAPE < 400 J/kg), mainly across the high
country. This will support scattered showers/storms, primarily
confined to the higher elevations. A shower or two may stray onto
the plains although, with dry lower levels it is more likely to be
virga. With the ridge overhead, 700mb temperatures warm, supporting
highs in the low-mid 70s across the lower elevations and 40s/50s for
most of the high country.

There is a general consensus among ensemble guidance that flow aloft
transitions to westerly Tues/Weds in response to an approach
shortwave trough. While there is good agreement on a trough moving
through mid-week, there is still uncertainty in the details.
Moisture returns to Colorado ahead of the trough with synoptic
ascent and cooler air aloft bringing instability. Expect increased
chances for showers/thunderstorm those days. Instability still stays
on the marginal side with MLCAPE < 500 J/kg. Tuesday afternoon the
"higher" instability will likely be positioned across the far east
plains. A few stronger storms can`t be ruled out on the east plains
as a result. Despite small differences in track, timing with the
trough there will be another shot at afternoon showers/storms
Wednesday. Expect mild temperatures Tuesday with mid to upper 70s
across the lower elevations. Cooler temperatures Wednesday with the
trough/front passage. Zonal flow over the region for Thursday with
drier conditions expected and temperatures closer to normal. Warm-up
Friday into the weekend.

&&

.AVIATION /00Z TAFS through 00Z Sunday/...
Issued at 540 PM MDT Sat May 11 2024

Complicated TAF forecast through the majority of the period.
Ongoing SHRA/TSRA has been disorganized and has thus far stayed
reasonably far away from the terminals. A more organized batch of
precip is attempting to drift towards APA this evening, along with
a few showers in the periphery of a weak Denver Cyclone near DEN.

The first area of uncertainty is how this remaining activity
evolves over the next few hours. With decreasing instability, the
main concern this evening would be briefly lowered cigs, but
guidance offers little insight here. Can`t rule out some brief
MVFR/ILS cigs and briefly reduced visibility. In general, expect
a gradual lowering cigs overnight but confidence in this timing is
low.

Next question is if a band of rain makes it into the Denver metro
on Sunday AM. Recent trends have generally bumped this north to
the point where even BJC/DEN could see a few hours of rain near
daybreak Sunday, with greater confidence towards APA. Again main
impacts here would be reduced visibility and MVFR cigs/visby with
a limited chance of any TS.

Finally, there is the potential for additional scattered
showers/isolated thunder in the afternoon hours. This will likely
be dependent on how the AM rain evolves and little change was made
beyond 18z for the DEN TAF.

&&

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

&&

$$

SHORT TERM...Schlatter
LONG TERM...Mensch
AVIATION...Hiris