868
FXUS65 KBOU 112019
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
219 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 /18Z TAFS through 18Z Sunday/...
Issued at 1142 AM MDT Sat May 11 2024

Definitely a complicated aviation forecast from this afternoon all
the way through Sunday evening. Starting with today, winds should
increase out of the south at APA and DEN by mid afternoon. Showers
and thunderstorms are possible (>50%) after 21Z but especially
between 22 and 01Z this evening at all three terminals. They will
be moving southwest to northeast during the late afternoon and
evening hours. Lightning is a safe bet with the strongest showers,
and wind gusts to 25, maybe 30 kts could occur. We`ll need to
watch wind shifts with the showers and thunderstorms. Drainage
winds are expected after 05Z with SSW winds at APA and DEN, SW at
BJC.

MVFR CIGs are expected much of Sunday morning. We have low
confidence in the evolution of both the CIGs and
shower/thunderstorm potential on Sunday. Most models show a very
moist low to mid level of the atmosphere, which would support OVC
CIGs around 020-030 through at least midday. We have moderate
confidence in a band of showers forming late morning, probably
between 15-18Z, but the location of that band of showers is highly
uncertain. Have decided given the uncertainty, and the fact that
lightning is not expected with the band of morning showers, to
keep the MVFR CIGs in place and just handle the shower potential
with VCSH. Models are hinting at a second round of showers this
time with a low lightning potential later Sunday afternoon. This
seems like a low chance overall (30%), so have used a PROB30
group to handle that. Northeast winds 10-15 KT gusting to 25 kts
should be the norm in the absence of afternoon thunderstorms. VFR
could return by midday but without confidence either way, will
keep the lower CIGs in place in the TAF for the time being.

&&

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

&&

$$

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