791
FXUS65 KBOU 110739
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
139 AM MDT Sat May 11 2024

.KEY MESSAGES...

- Showers and isolated thunderstorms are possible Saturday
  afternoon across most of the area (20-50% plains; 50-80%
  mountains, mountain valleys, and foothills). Snow levels should
  remain around 10,000 ft into the evening hours, and no
  significant travel impacts are expected.

- Gradual warming this weekend and through Tuesday along with
  scattered showers and thunderstorms, most numerous this weekend
  and Tuesday onward.

&&

.UPDATE...
Issued at 744 PM MDT Fri May 10 2024

No significant updates this evening with mostly quiet conditions
across the region. A couple of lingering showers have been noted
across portions of the high country.

Main change to the grids this evening was to reduce... yes,
reduce... cloud cover across the lower elevations. Satellite
imagery this evening has shown the low/mid-level cloud cover has
decreased substantially in the past few hours, with most CAMs
showing mostly clear skies for the plains overnight.

&&

.SHORT TERM /Through Saturday/...
Issued at 119 PM MDT Fri May 10 2024

GOES-17 shows a well defined short wave trough over the CA/NV
border, with a trough axis extending northeast over Utah and into
NW Colorado. Weak instability near the trough axis and some QG
lift should lead to weak rain showers this evening, and some light
snow mainly above 9kft MSL. T-2 inches are possible through
midnight, but mostly along and south of I-70. Limited to no
travel impacts are expected this evening or overnight. CAMs
continue to show hardly any rainfall east of I-25, with limited
instability and less forcing away from the trough axis. Overnight
lows should remain up and well above freezing across all of the
plains below 6500 ft MSL, with lows in the mid 30s in the cool
spots to low 40s across the urban heat islands. The mountain
valleys should cool into the upper 20s despite partly cloudy
skies.

A note about the Aurora Borealis potential: Check the NWS Space
Weather Prediction center for best timing and updates, but for
now that looks like midnight to 3 AM for Colorado, and skies
should be partly to mostly clear during that time across the
northern 1/4, maybe northern 1/3 of Colorado. However, to have a
chance to see them, even under mostly clear skies, there needs to
be very little light pollution to your north, since that`s the
only direction you will be able to see them if visible.

On Saturday models show the trough moving into western Colorado,
which will increase instability over out area, as well as increase
QG lift. Low-level moisture will be limited with Td expected to
remain in the upper 30s to low 40s across the eastern plains.
There is a fair amount of mid and upper level moisture to work
with, resulting in a total of about 0.50" PW across our area, on
average. The trough is pretty cold for mid May, with 500 mb temps
around -20 degC. So despite meager low level moisture, the cold
air aloft should result in SBCAPE from 200-600 J/kg, supporting at
least isolated chances of thunder, and very small hail with the
strongest convection. Overall QPF amounts remain light, mostly
because of the dry low-levels. The best forcing will be across
central and southern Colorado, thus the northeast corner and
northern tier of counties across the plains may not see any
precip, while across Park County maybe as much as a half inch of
liquid could fall through the evening hours. A tenth of two are
possible for the Denver-Boulder metro area. Snow levels should
remain around 10kft MSL through Saturday evening. With ample cloud
cover much of the day, and even with some warming via southwest
flow aloft just ahead of the trough axis, highs will remain near
normals with highs in the upper 60s across the plains (70s
northeast corner where there will be more sunshine and little to
now precip during the day), and 50s in the mountain valleys.

&&

.LONG TERM /Saturday night through Friday/...
Issued at 119 PM MDT Fri May 10 2024

Sunday, the upper level low and positively tilted trough exits
across eastern Colorado. The leading edge of the upper ridge moves
in by the early evening, setting up thermal ridging and more
northerly flow over the region. This will bring warmer temperatures
with highs in the 60s for most of the lower elevations. Sufficient
moisture remains in the mid-levels on Sunday. Cooler air aloft and
daytime heating will provide instability (MLCAPE < 600J/kg) will
work to support increased showers and storms in the afternoon. There
is uncertainty in the rainfall amounts and location of highest
amounts across the lower elevations. Part of this is dependent on
the track of the low. Overall, both the GFS and EC ensembles show a
good signal for QPF > 0.10". Ensemble means also show a signal for
greater QPF (24hr 0.2-0.6") centered along I-70 through the Denver
metro and eastward onto the plains. A chunk of EC members show
amounts > 0.60" for the southern Denver metro. GFS is a bit drier
and this is likely due to some differences in low track. Overall
looking like a decent chance for a soggy Sunday.

There is general agreement among ensemble guidance that a weak upper
ridge moves in for Monday. Monday trends drier and more stable,
resulting in lower chances/coverage of showers/storms with the
higher chances confined to the higher terrain. The weak ridging will
support further warming temperatures with ensembles showing a high
chance at 70s across the lower elevations early next week with a
shot at upper 70s across the east plains. The upper pattern then
transitions to W to WNW flow aloft Tuesday and Wednesday.
Uncertainty increases in the details of a shortwave trough moving by
mid-week, particularly timing.

Tuesday and Wednesday display the higher chances for shower/storms
activity with the incoming trough providing synoptic ascent and
increased moisture. Hard to say where the axis of higher instability
will set up across the lower elevations and this will be dependent
on how the shortwave tracks as well as the timing of the wave.

&&

.AVIATION /06Z TAFS through 06Z Saturday/...
Issued at 127 AM MDT Sat May 11 2024

Models all show weak drainage winds overnight tonight, then
south-south-easterlies winds all day Saturday. I went with a TEMPO
group late in the day for -TSRA as there looks to be enough
instability for a few thunderstorms. From mid afternoon into the
evening hours DIA could see SCT-BKN030-050 and even BKN-OVC025-040
later tonight.


&&

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

&&

$$

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