336
FXUS65 KBOU 202101
AFDBOU

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

.KEY MESSAGES...

- A few severe storms, including potential for tornadoes, are
  expected late 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 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 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 /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.

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...AD
AVIATION...Barjenbruch