208
FXUS65 KBOU 201143
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
543 AM MDT Mon May 20 2024

.KEY MESSAGES...

- Strong to severe storms are expected Monday afternoon and
  evening across most of the eastern plains, with all hazards
  possible.

- Another round of widespread showers and storms is expected
  Tuesday afternoon and evening. Small hail and gusty winds are
  possible from these storms but they will likely stay below
  severe limits.

- Light snow accumulation will be possible across the higher
  mountain ridges. Some travel impacts could occur on mountain
  passes tonight and Tuesday night.

- Warming/drying trend for the second half of next week

&&

.SHORT TERM /Through tonight/...
Issued at 311 AM MDT Mon May 20 2024

A complicated convective forecast is in store for today, with the
potential for scattered to numerous strong to severe storms
across most of the plains from this afternoon into the overnight
hours.

Water vapor satellite and 500mb RAP analysis data show a
positively tilted trough axis extending from southwestern Montana
into Nevada this morning, with a southern stream shortwave just
off the coast of southern California. Broad west-southwesterly
flow is in place in the region ahead of these features, and the
upper level flow is expected to strengthen later today as the
trough approaches. Meanwhile, the southern stream shortwave is
expected to race across the Four Corners region and eject into
eastern Colorado by late tonight, with accompanying lee
cyclogenesis also expected later today. As this surface low
develops, east/southeasterly surface flow should gradually
increase with moisture advecting into the eastern plains through
the afternoon hours.

With these larger scale features in reasonably good agreement
across guidance at this point, our attention turns more to the
finer-scale details. The first and most pressing question is how
quickly the airmass recovers behind this morning`s cool front.
Surface dew points this AM are generally in the upper 30s to upper
40s across the plains, though across the border into KS/NE Td`s
are closer to 55-60F. High resolution guidance has been
relatively inconsistent with the westward advection of the higher
theta-e airmass into our CWA. The NAM/NAMnest are notably cooler
across the plains today with pronounced capping... while most
other guidance warms the plains into the low/mid 70s with a
quickly eroding cap noted by 00z.

This leads us into the second important question... which is the
timing of convective initiation across the plains. This generally
looks to occur during the late afternoon or early evening hours as
the shortwave approaches and height falls become more pronounced,
along with increasing surface convergence along a stalled frontal
boundary. If CI occurs closer to 00z, there would be a narrow
window for surface based convection before transitioning to a more
elevated supercell environment later into the evening. Along the
aforementioned boundary, forecast soundings are quite impressive,
with strong deep-layer shear with elongated/curved hodographs as
the low-level jet strengthens. Assuming guidance is close on the
timing, this would support a few supercells capable of a tornado,
along with large (possibly significant) hail and damaging wind
gusts. We may also see an additional round or two of supercells
later into the evening that would pose a hail risk well into the
overnight hours in addition to the initial storms.

All in all, the SPC Enhanced Risk appears appropriate, though
there are certainly failure modes (lack of destabilization and/or
moisture advection) that would shift the severe threat out of our
area. As is always the case, observational trends will be
important to watch through the day today. However, this does
appear to be the best severe threat for the area this spring.

Meanwhile, across the I-25 corridor, today should be cooler with
highs in the low/mid 70s. With the best instability located to the
east, any showers or storms that form this afternoon would likely
be sub-severe but could pose a brief gusty wind threat. Elevated
fire weather conditions are possible across portions of the high
country as very dry air advects into those areas.

&&

.LONG TERM /Tuesday through Sunday/...
Issued at 208 AM MDT Mon May 20 2024

There will be a shortwave trough over the far northeast corner
Tuesday morning that will continue to provide forcing for
convection that continues from Monday night. These showers and
storms should depart Colorado by around 9-11am. There will be
plenty of dry hours especially over the urban corridor as the
morning through the early afternoon should be dry. However, a
secondary shortwave trough with colder air aloft will begin moving
across our forecast area during the mid afternoon. This will
provide strong forcing and widespread precipitation is expected to
form. This will fall as snow above 9-10 kft and some accumulation
is possible across the higher mountain ridges and slopes. Some
mountaintops could be coated with 3 or 4 inches of snow during
this event. Some of the higher mountain passes and Trail Ridge
Road will likely receive accumulating snowfall. There could be
travel impacts especially Tuesday evening across the mountain
passes but overall, the travel impacts will be limited to small
areas.

As a result of the rainfall Monday night, multiple cold fronts, and
low clouds Tuesday morning, high temperatures will stay in the low
60s or even upper 50s across the urban corridor and plains on
Tuesday. This will limit instability and most unstable CAPE will
struggle to get above 300-500 j/kg. Therefore, storms that form over
the higher terrain and move onto the plains will likely remain
below severe limits. However, the strong forcing and shear along
with good moisture could lead to strong storms that have hail and
gusty winds. Rainfall totals across much of the area will be
around a quarter to half an inch.

The shortwave trough will depart our area Tuesday night with ridging
building in on Wednesday. This will lead to warmer temperatures and
mostly dry conditions. There could still be a few isolated showers
and storms that develop.

Another shortwave trough will enter northwestern Colorado on
Thursday. There will be strong southwesterly flow ahead of this
trough and that will lead to warm conditions and possibly the return
of 80 degree highs across the plains. A cold front associated with
this trough will move across the northeast plains Thursday night
which could create a line of storms. There is a low but non-zero
chance that these storms could become severe and this threat will
need to be looked into over the next few days.

There will continue to be plenty of troughs that make their way
across the western US Friday through the weekend. Isolated to
scattered showers and storms are possible each day as a result of
these troughs.

&&

.AVIATION /12Z TAFS through 12Z Tuesday/...
Issued at 539 AM MDT Mon May 20 2024

Northerly push arrived close to on schedule this morning. Some
stratus has developed north of the terminals but satellite trends
don`t suggest that this will cause too many issues this morning.
There could still be a brief period of near MVFR cigs but not high
enough confidence to include in the TAF. Winds today will slowly
turn more to the east/southeast by the late morning or early
afternoon with a gradual increase in speeds as well.

Still quite a bit of uncertainty in the forecast late this
afternoon into tonight. Storm coverage again looks highest east of
the terminals but there`s a brief period between 21-00z today
where storms could develop this far west. PROB30 still seems
appropriate for this potential. Beyond that, a wind shift back to
the north is likely sometime this evening along with an increase
in low cloud cover. Poor model agreement with the timing and
amount of low clouds and the current forecast is close to a middle
ground. Potential is still there for several hours of MVFR/IFR
cigs overnight depending how moisture plays out.

&&

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

&&

$$

SHORT TERM...Hiris
LONG TERM...Danielson
AVIATION...Hiris