690
FXUS65 KBOU 210353
AFDBOU

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

.KEY MESSAGES...


- Severe threat continues mainly just over Washington continue for
  the rest of the evening.

- Another round of 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 951 PM MDT Mon May 20 2024

Severe convection is continuing over the east central CWA
at this time, mostly over Washington County.  There is other
convection over the rest of the eastern half of the plains.  Hail up
to 3 inches has been reported with the storms earlier this
evening. Upstream over eastern Utah and western Colorado, there
is a significant sized area of convection, mostly producing light
rain associated with an upper trough axis moving eastward through
Utah. I will make some pop, wind, sky and temperature grid
adjustments based on current conditions and some of the high
resolution models. I will make sure there are some pops over most
of the CWA overnight. Models continue to show temperatures about
15 degrees F below normals for Tuesday`s highs.

&&

.SHORT TERM /Through Tuesday/...
Issued at 300 PM MDT Mon May 20 2024

...SEVERE STORM POTENTIAL INCREASES THIS EVENING...

While confidence grew earlier in the day regarding severe storm
potential, there are still some uncertainties to deal with. First
off, surface temperatures north of the warm front/developing
Denver Convergence Vorticity Zone (DCVZ) were running a couple
degrees cooler than forecast. Thus, CIN was holding on a little
stronger, and instability was also weaker than expected, with
MLCAPE as of 2 pm only reaching 250-500 J/kg from Denver onto the
Palmer Divide. At the same time, it was noted very dry air aloft
working in from the southwest, with single digit/lower teens
dewpoints noted on the mountain tops through southwest Colorado.
This was serving as initial disruption of updrafts as well.

That said, we are now just starting to see convective development,
and there will be further warming along/south of the convergence
zone where there is more sunshine. Dewpoints were also starting to
rise, with mid 40s dewpoints now back into southeast Denver, and
near 50 dewpoints into pushing back already toward Fort Morgan,
Akron, and Limon. Thus, with further heating and low level
moisture advection (mainly along and south of the convergence
zone) we should be able to see MLCAPES grow to around 1000,
perhaps ~1500 J/kg. At the same time, shear is quite impressive,
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. Updraft helicity
swaths show high (>70%) probabilities of UH exceeding 150 once we
move east into Washington County and points east and northeast.
Putting this all together would suggest potential for stronger,
longer lived tornadoes than what we typically see in northeast
Colorado, IF we get to those values mentioned above.

Meanwhile, north of the DCVZ/warm front, temperatures will remain
cooler and we`re starting to fill in with clouds. As expected,
those areas roughly from Denver northward through Fort Collins
will likely only see weaker isolated convection at least for this
evening.

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. Those
could be undercut by cooler air and thus have mainly a large hail
threat, but still something to monitor closely.

For later tonight, there should be a round of showers and a
couple storms with upper level support still present. Then, there
will likely be a break of some sort tomorrow morning before
another round of scattered showers and storms develops Tuesday
afternoon across the mountains and spreads east over the plains.
This time, the threat of severe will be quite limited with MLCAPE
less than 500 J/kg.

Finally, with each passing wave we`ll see snow in the mountains
above 8,500-9,000 feet. A couple inches and some slush will be
possible on the high mountain passes with each passing wave.

&&

.LONG TERM /Tuesday night through Monday/...
Issued at 300 PM MDT Mon May 20 2024

Ongoing scattered to numerous showers are expected to continue
across northeastern Colorado Tuesday night. There a slight decrease
in expected QPF amounts along the eastern plains but generally, most
areas can expect totals between 0.30-0.80 inches. WPC Super ensemble
indicates 0.20-0.50 over the high country which should lead to light
snowfall for elevations above 9 thousand feet. Slick travel along
higher mountains passes will continue through Wednesday morning for
snowfall totals between 2-5 inches. A small amount of instability
exist in the urban corridor thus isolated storms could produce small
hail through Tuesday night.

Flow will return westerly overnight into Wednesday morning. Skies
should clear by Wednesday morning but lingering low level clouds
could occur depending on how much rain occurs. Afternoon highs on
Wednesday should increase a few degrees compared to Tuesday. The
urban corridor and plains increase between 63-72F. Mountains and
valleys increase near 47-60F. Moisture becomes sparse along the
eastern plains but GFS/NAM support MLCAPE values between 200-300
J/kg which could support an isolated shower or non-severe storm or
two through early Wednesday evening.

There is strong agreement amongst model guidance on the next upper
level trough arriving to southeast Wyoming Thursday afternoon but
given this system is too far north, our region remain dry through
the period. A cold front could sweep across the foothills and plains
by Thursday night keep our low temperatures slightly cooler than
normal. A series of shortwaves troughs from the northwest will
impact the forecast area bringing afternoon scattered showers and
storms Friday through this weekend. Temperatures will continue near
normal through the period.

&&

.AVIATION /06Z TAFS through 06Z Wednesday/...
Issued at 947 PM MDT Mon May 20 2024

Models show fairly strong northerly winds for DIA most of tonight.
Speeds are progged to relax Tuesday morning but continue upslope
in direction. Normal drainage wind patterns are not expected
tonight. With current observations and model cross sections in
mind, will go with BKN-OVC010-020 for ceilings from 06Z into
Wednesday morning around 15Z. There should not be any visibility
issues overnight in spite of the Stratus.

&&

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

&&

$$

UPDATE.......RJK
SHORT TERM...Barjenbruch
LONG TERM....AD
AVIATION.....RJK