461
FXUS63 KIND 150429
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1229 AM EDT Mon Jul 15 2024

.KEY MESSAGES...

- Showers and thunderstorms possible 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

&&

.FORECAST UPDATE...
Issued at 946 PM EDT Sun Jul 14 2024

Central Indiana has remained convection free this evening, so have
removed evening PoPs.

Thunderstorms are organizing as expected across northern Illinois
this evening. This complex will move east and then southeast,
following the instability gradient, entering central Indiana after
midnight.

Unfortunately, questions still remain on how far southeast the
complex will get before it weakens and eventually dissipates. Will
still have to wait to see how well its cold pool develops.

For now have continued (with some tweaks) the previous forecast of
likely PoPs in the far north, tapering down to slight chance or no
PoPs across the southern half of the area.

Severe storms with damaging winds and heavy rain remain possible,
especially if the cold pool becomes organized. Will still have to
watch for training of storms if the westerly 850mb flow causes some
redevelopment along the west flank of the complex overnight before
they weaken.

&&

.MESOSCALE UPDATE...
Issued at 1228 AM EDT Mon Jul 15 2024

- Thunderstorms moving in after 3 AM from the northwest

Raised PoPs a bit and took them further southwest based on radar,
mesoscale and CAMs trends. MRMS radar reflectivity was showing an
MCS from Lake Michigan to northern Illinois. The northeast part of
this complex was moving more east while the southwest elements were
showing a southeastward push into the better instability as LAPS
data was showing a bullseye of 3400 J/kg SB CAPE over east central
Illinois. In addition, there was good moisture convergence over the
upper Wabash Valley. May need to raise PoPs even more as the night
goes on as it becomes clearer if a decent cold pool develops within
the modest deep shear. Both the NAM nest and HRRR are similar in
bringing the bulk of the convection into the upper Wabash Valley as
points northeast see diminishing convection.  Overall confidence is
low whether or not any convective elements will retain a severe
threat, but it can not be ruled out.

&&

.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 (00Z TAF Issuance)...
Issued at 653 PM EDT Sun Jul 14 2024

Impacts:

- Potential for convection after 05Z for the northern sites
- Additional convection possible Monday, mainly late
- Some gusts around 20kt Monday

Discussion:

Only low to medium confidence exists for convection evolution
tonight. Complex of storms may move into the northern sites after
05Z and continue through 09-10Z before dissipating. However, there
are alternative scenarios in timing and sites affected. For now have
just kept continuity with the previous TAFs as that for now is the
most likely solution.

Otherwise mainly some mid and high clouds overnight, with some
cumulus on Monday. Winds may gust over 20kt on Monday, especially
during the afternoon.


&&

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

$$

MESOSCALE...MK
UPDATE...50
SHORT TERM...50
LONG TERM...Nield
AVIATION...50