431
FXUS63 KIND 141901
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
301 PM EDT Sun Jul 14 2024

.KEY MESSAGES...

- Showers and thunderstorms possible today and tonight, damaging
  winds and heavy rainfall are the primary hazards.

- Periodic shower and thunderstorm chances through Wednesday, with
  low confidence in one or more windows of potential severe weather

- Hot and humid Sunday through Tuesday, possibly nearing Heat
  Advisory criteria, particularly Monday

- Milder and less humid late this week into the weekend

&&

.MESOSCALE UPDATE...
Issued at 212 PM EDT Sun Jul 14 2024

- Primary strong to severe threat remains across east central
Indiana for the next 1-2 hours.

- Additional development possible further south/west, though more
  uncertain, and hydrologic concerns may take precedence.

Tail end of a small QLCS continues to push ESE across roughly the
northeastern quadrant of central Indiana at this hour. For the most
part, convection has been just sub-severe, with occasional pulses in
intensity. That said, the main line has weakened steadily, with
rapid incipient development occurring along and ahead of the cold
pool in east central Indiana.

Additionally, the tail end of the primary outflow boundary stretches
back into west central Indiana, nearing the I-69 corridor, while
another secondary outflow from the prior convection near LAF also
sags southward toward the I-70 corridor. Additional development will
be possible in the west/southwest this afternoon, as an uncapped
airmass with 1500-2500 J/kg MLCAPE exists per both SPC mesoanalysis
and ACARS soundings, though extent of this development is uncertain
given minimal shear further south and near term stabilizing effects
of the near-surface cold pool.

Given flow orientation largely along the west-east oriented outflow
boundaries, localized hydrologic/flash flood concerns may be the
primary issue with any late afternoon convection, as training and
slow storm motions are a concern. The degree of instability and a
slight drier layer centered around 700 mb suggests at least an
isolated downburst threat with the strongest cores.

&&

.SHORT TERM (This evening through Monday)...
Issued at 301 PM EDT Sun Jul 14 2024

Another complicated and lower confidence forecast this afternoon
with mesoscale features and thunderstorm complex interactions
driving the details.

Rest of This Afternoon...

End of the line of convection is becoming more east-west orientated
early this afternoon. The line may sink southward with outflow
controlling additional development, especially since areas south of
this line remain unstable.

850mb winds remain parallel to the boundaries which could lead to
some training of convection that does develop, so flooding will be a
potential this afternoon. Good instability but little shear across
the south may lead to pulse type storms with potential for some
downbursts with the strongest storms.

Will go with slight chance to chance category PoPs with the
boundaries around south. Across the north, some recovery will occur
later this afternoon in the west. Will have some slight chance PoPs
for any isolated development.

Hot and humid conditions will continue south ahead of any outflows.

Tonight...

Convection will initially diminish this evening with loss of
heating. However, another line of convection will approach from the
northwest late evening or early overnight.

Westerly 850mb winds will increase in speed once again, aiding in
keeping the line going for a while into at least the far northern
forecast area.

Uncertainty remains pretty high though on how far south this line
makes it into central Indiana as main forcing for the convection
remains north of the area. A stronger cold pool will allow it to get
farther southeast and a weaker cold pool will let it die earlier.
The storms could be strong to severe depending on cold pool
development.

Will go with high end chance category to low end likely category
PoPs north early in the overnight with lower PoPs south.

Flooding concern remains tonight with the storms, especially if they
can become east-west orientated parallel with the 850mb winds.

Monday...

There may be some clouds around from overnight convection, but these
should dissipate during the morning. Temperatures will climb
quickly, and dewpoints will remain up.

This should create hot and humid conditions, with afternoon heat
indices over 100 possible.

Although mid-level temperatures will warm, which will help limit
convection some, feel that leftover boundaries from overnight
convection plus any weak impulses into the upper flow could spark
off at least isolated convection. Will add some isolated PoPs to the
afternoon many areas to cover this. Given the instability, a strong
to severe storm cannot be ruled out.

Uncertainty in initial sky cover and potential convection, combined
with marginal heat indices for an advisory, will hold off on
issuing an advisory at this time.

&&

.LONG TERM (Monday night through Sunday)...
Issued at 301 PM EDT Sun Jul 14 2024

As the subtropical ridge weakens and western CONUS ridging
amplifies, an anomalously strong upper level trough will swing
through the Great Lakes mid week, driving a cold front southward
through the region Tuesday into Tuesday night. The additional upper
level and frontal forcing, combined with the antecedent hot and
humid airmass, will set the stage for development of numerous
thunderstorms, especially along and ahead of the boundary Tuesday
afternoon into Tuesday night.

Hydrologic concerns will be top of mind given several factors -
relatively wet antecedent conditions, slow frontal movement, a
significantly boundary-parallel component to modest deep-layer
tropospheric flow producing slow storm motions and potential for
training, in the background setting of very warm and moist, deeply-
saturated airmass, near climatological maximum precipitable water
values (in the time of year where this maximum is at its annual peak
to begin with), and seasonably high wet-bulb zero heights/deep warm
cloud layer promoting efficient warm rain processes. All of these
factors mean that storms that form will easily produce heavy
rainfall, and will pose a threat for flash flooding.

Some severe threat may exist, particularly Tuesday afternoon and
evening, though uncertainty remains elevated given likelihood of
some prior convection which could impact degree of destabilization,
and adequate but relatively modest deep layer shear. Damaging winds
would again be the primary threat, either in areas where cold pools
can organize or with localized wet microbursts. Current day 3
marginal is reasonable, and continues to be supported by a
consistent and robust (perhaps too robust given the aforementioned
uncertainties, which of course the ML cannot incorporate) signal in
experimental machine learning guidance.

Once the boundary sags south of the area by late Wednesday, a fairly
dramatic and pleasant change is expected, with temperatures and
dewpoints dropping as much as 10 to nearly 20 degrees late week and
into the weekend, as a strong surface high pushes into the region.
Thursday and beyond appear extremely nice, particularly for what is
ordinarily the hottest and most humid time of year.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 1227 PM EDT Sun Jul 14 2024

Impacts:

- Convection to impact KLAF and possibly KIND this afternoon

- Additional convection possible late tonight and early MOnday
across central Indiana

- Southwesterly wind gusts 20-25kt this afternoon

Discussion:

Medium to high confidence forecast over the next few hours but much
lower confidence thereafter as multiple convective clusters may
impact portions of central Indiana through Monday afternoon. Initial
line of storms moving through KLAF currently and may clip KIND over
the next few hours. Could see brief restrictions due to torrential
rainfall...and gusty winds from the W/NW will be possible for a
short time as well. Not out of the question that KBMG and KHUF may
be impacted by scattered convection later this afternoon but
confidence too low for a mention at this point.

Once these storms pass later this afternoon...should be relatively
quiet with only isolated convection into the first part of the
evening. Clouds should diminish late afternoon into the
evening...then likely increase again tonight with the expectation of
a secondary convective cluster developing over the upper Mississippi
Valley during the evening. Confidence in the details are low at this
point but should a complex grow upscale and mature...potential would
be there for it track across the forecast area late tonight and
early Monday potentially impacting one or more of the terminals.
Strong wind gusts and torrential rains would again be primary
concerns with restrictions likely associated with any storm.

Southwest winds may gust up to 20-25kts this afternoon outside of
any convection...with resumption of wind gusts possible again by
midday Monday.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

MESOSCALE...Nield
SHORT TERM...50
LONG TERM...Nield
AVIATION...Ryan