383
FXUS63 KIND 250204
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1004 PM EDT Mon Jun 24 2024

.KEY MESSAGES...

- Thunderstorms possible Tuesday morning
- More thunderstorms possible Tuesday PM into early Wednesday;
  severe storms possible with this round
- Growing high heat threat for SW central Indiana tomorrow
- A brief respite from heat and humidity late in the week

&&

.FORECAST UPDATE...

Upstream convective initiation along an advancing boundary continues
to trend slower and further north, leading to greater uncertainty on
when precipitation will reach central Indiana tomorrow. There are a
few signals towards isolated to scattered convective development
along a weak low level wave out ahead of the boundary late
tonight into tomorrow morning. Any rain associated with this
initial round will likely remain over northern portions of the
region, with the bulk of the central Indiana thunderstorm threat
now likely to be delayed til the early afternoon tomorrow.

This delay in thunderstorm onset could cause a few forecast changes
for tomorrow. First, later onset may allow for greater surface based
instability to develop and therefor better chances for a damaging
wind threat with DCAPE values higher. Second, the decreased cloud
cover and lack of evaporative cooling could allow for temperatures
over S/SW portions of central Indiana to push into the mid 90s. If
temperatures do reach the mid 90s, dew point values over SW central
Indiana in the low 70s will create dangerous heat conditions. Due to
the uncertainty of precipitation onset, no headlines will be issued
at this time. However, in subsequent updates, a close eye will be
kept on this growing high heat concern.

&&

.SHORT TERM (This evening through Tuesday night)...
Issued at 225 PM EDT Mon Jun 24 2024

A challenging convective forecast starts tonight and lasts through
the short term period, with one or more MCSs or MCS remnants
potentially impacting central Indiana. Mesoscale details will
dictate timing and coverage of thunderstorms. See analysis below
including potential scenarios.

Models show moisture surging eastward into the area late tonight.
NAM-based guidance appears a little overzealous with the magnitude
of the ~850-mb moist layer/warm nose, hence the very strong buoyancy
depicted. In reality, moisture advection should drive at least
modest buoyancy such that convection would sustain, albeit probably
in a weakened state. The upstream buoyancy magnitude later today is
quite impressive and almost certainly to result in a cold pool-
driven system. Shear magnitudes are marginal but sufficient. It
would probably be the upshear propagation component along the
western flank of the cold pool that would give central Indiana the
best chance of being impacts late tonight through Tuesday morning.
Uncertainties on how this evolves are linked to subtleties with the
lower tropospheric thermodynamics, and cold pool depth/magnitude,
primarily. Thus, forecast updates will be needed once upstream
convection matures and the picture becomes more clear later this
evening.

Another note about convective potential tonight. There is a minor
signal within high-resolution models of warm advection-driven
convection occurring at the leading edge of aforementioned moisture
surge. A strong capping elevated mixed layer will likely prevent
deeper robust convection from occurring, but weaker convection
rooted in the midlevels is possible during the predawn hours. Again,
coverage and intensity with this should be limited, but it would be
separate from the conditional remnant MCS mentioned above.

There is a conditional chance of heat indices approaching Heat
Advisory thresholds across southwest portions of central Indiana
tomorrow afternoon. The marginality, along with uncertainties of
remnant cold pool and midlevel clouds, and/or diurnal convective
redevelopment preclude highlighting this concern at this time.

By Tuesday afternoon, a couple scenarios are possible with regards
to convection. 1. Diurnal redevelopment would favor any residual
cold pool/differential mixing zones spawn from morning convection.
These storms would occur in fairly weak shear but potentially strong
buoyancy and could pose a damaging downburst threat. 2. Morning
convection continues along residual cold pool flanks and may tend to
propagate upshear, westward. This may be limited by warm capping
elevated mixed layer. In a reasonable worst case scenario this could
result in localized heavy rain and flooding.

The next shortwave trough is still well west during the peak diurnal
cycle, but should drive another MCS. It may be late in the evening
or overnight before it reaches Indiana. We`ll still be on the
southern periphery of stronger westerlies and higher shear values,
but instability should be supportive of at least a low end severe
threat with wind being the primary hazard. Details on this evolution
and associated convective risks will be refined in subsequent
forecasts.

Quick disclaimer about use of CAMs in this pattern. Oftentimes, CAMs
struggle with weakly forced but unstable environments. The details
on the magnitude and footprint of the cold pool may not be modeled
well, and this is a primary driver in MCS intensity and longevity,
as well as forward propagating speed. Use individual CAM output
carefully and consider the spectrum of outcome that the
environment could support.

&&

.LONG TERM (Wednesday through Monday)...
Issued at 225 PM EDT Mon Jun 24 2024

Wednesday`s forecast is highly conditional on impacts of Tuesday
night convection. The shortwave trough axis doesn`t cross until
later in the day, but richer moisture and instability could be
overturned by robust convection Tuesday night leaving relatively
cool/stable conditions Wednesday. In this scenario, diurnal
regeneration of convection would be to our south. Nevertheless, even
in this scenario, robust convection may still be ongoing during the
diurnal minimum, fed by a westerly low-level jet and moisture
influx. We would have to watch this scenario closely during the
morning as tendency for upshear propagation along a composite
surface boundary within such a moisture rich environment could lead
to localized flooding.

Multi-model ensemble mean shows a dampening trend with prograding
mid-upper ridge to our south for the latter half of the week. So,
unlike this last recent trough/ridge progression, once cooler and
drier cP air mass arrives late Wednesday, it should last into early
Friday with below-climo temperatures along with less humidity.

Temperatures will increase to near or just above late June
climatology Friday afternoon into Saturday as southwesterly flow
strengthens preceding the next shortwave trough. The window for rain
chances with this next system is between Friday night and Saturday,
with some timing differences in deterministic models and individual
ensemble members. The pattern looks similar to the last couple of
systems, however, with primarily frontal forcing amidst typical
summertime instability south of stronger westerlies. One or more
clusters of convection may impact central Indiana during the above
mentioned time window.

Another push of continental air is expected Sunday into early next
week. During the week of July 1st, the pattern doesn`t change
substantially. One or more progressive low-amplitude shortwaves will
pass just to our north and we`ll be on the periphery of the ridge.
Some indication within the ensemble mean data of a building ridge
and more amplitude pattern during this period supporting above
normal temperatures.

&&

.AVIATION (00Z TAF Issuance)...
Issued at 733 PM EDT Mon Jun 24 2024

Impacts:

- Conditional chance of thunderstorms KLAF/KIND early Tuesday
- Additional low thunderstorm chances at all sites Tuesday afternoon
  and evening

Discussion:

Shallow fog is not expected to redevelop tonight given stronger
winds and mechanical mixing. VFR conditions should prevail outside
of thunderstorms. Thunderstorm coverage and timing confidence is
low. There is a low-mid range chance of thunderstorms approaching
KLAF and KIND Tuesday morning. These storms may stay northeast of
TAF sites and redevelopment may occur during the afternoon (end or
just after TAF period) near the TAF sites. Expect amendments given
the uncertainty that is typical with weakly forced convective events
in the summer. For now, handled low potential with PROB30.


&&

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

$$

UPDATE...Updike
SHORT TERM...BRB
LONG TERM...BRB
AVIATION...Updike