441
FXUS65 KBOU 201817
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1217 PM MDT Mon May 20 2024

.KEY MESSAGES...

- Severe storms, including a few tornadoes, are expected this
  afternoon and evening across northeast Colorado.

- Have multiple ways to receive warnings. Severe threat continues
  after dark for northeast Colorado plains.

- 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

&&

.UPDATE...
Issued at 1108 AM MDT Mon May 20 2024

...SEVERE STORM CONFIDENCE AND POTENTIAL FOR TORNADOES INCREASING...

We have increasing confidence in regards to our severe storm
potential this afternoon and evening. All stratus has burned off,
allowing diurnal heating to occur unabated.  VAD Wind Profiler
from the 88D shows strong southerly flow (35 kts at 9000 ft MSL).
While inversions are fairly sharp at the moment, we do expect
sufficient surface heating under strong solar insolation, and
weakening inversions through that and increasing/veering wind
profiles. The latest guidance, even the cooler NAM, has come
around to a warmer day with highs pushing into the lower to mid
70s over most of the plains, and upper 70s to near 80 in Lincoln
County. Thus, we have more confidence in convective initiation by
mid to late afternoon.

There`s still some uncertainty in where that occurs, but all
signs are pointing toward a developing Denver Convergence
Vorticity Zone (DCVZ) from southern metro Denver east and
northeast toward DIA and then onto the plains. That convergence,
in combination of weakening cap, will likely be enough to spark
convection by 2-3 pm. Given the strong shear profiles and enhanced
low level vorticity along the DCVZ/warm front, we think the
probability of at least one or two tornadoes is relatively high,
and this could even impact parts of the Denver metro area early
on (landspouts).

Those storms that develop are then expected to spread
east/northeast onto the plains very late in the afternoon and
especially through this evening. Updraft helicity swaths show high
(>70%) probabilities of UH exceeding 150 once we move east into
Washington County and points east and northeast, aligning with
HREF general averages of an impressive 0-1 km helicity of 100-150
m2/s2 and 0-3 km helicity over 300 m2/s2. Putting this all
together would suggest potential for stronger, longer lived
tornadoes than what we typically see in northeast Colorado!

HREF suggests not only that first round of storms firing along
the developing DCVZ/warm front and then organizing east/northeast
across the plains, but a second round of severe storms could also
originate in southeast Wyoming and clip through the northeast
plains late this evening in the 8 pm to midnight window.

One failure mode is the initial lack of low level moisture. That
said, even with CI at T/Td of mid 70s/lower 40s we still have near
1000 MLCAPE. Moisture advection starts to increase later this
afternoon with increasing southeast flow, and continues through
the evening also aiding that second round of severe storm threat.

It`s a good day and evening to stay weather ready, and be ready
to take action!

&&

.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 /18Z TAFS through 18Z Tuesday/...
Issued at 1215 PM MDT Mon May 20 2024

Main concerns surround thunderstorm potential late this afternoon
and early evening, and then lower ceilings and showers overnight.
Storm coverage still looks highest east of the TAF sites, but
there is a reasonable chance of storms still developing in the
area 21Z-00Z. Because of the TAF rules, we were forced to either
go VCTS or TEMPO for this period, and opted that probabilities
were high enough to hear thunder or have VRB winds at KDEN and
KAPA for TEMPO. That would be due to their proximity of a
convergence zone/warm front. KBJC is likely too far north of the
zone, so lower storm threat there.

Still some uncertainty with regard to ceiling heights overnight,
due to coverage of showers and storms, but odds would favor
IFR/MVFR ceilings late this evening (after 04Z) behind another
frontal surge and shallow, moist upslope. Those lower ceilings
could stick around through about 16Z. Then expect scattered
showers and storms Tuesday afternoon, with probably a little
higher coverage than today.

&&

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

&&

$$

UPDATE...Barjenbruch
SHORT TERM...Hiris
LONG TERM...Danielson
AVIATION...Barjenbruch