339
FXUS63 KIND 031731
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
131 PM EDT Wed Jul 3 2024

.KEY MESSAGES...

- Hot and humid Today; Highs near 90 with dew points in the low 70s

- Rain and thunderstorm chances Wednesday through Friday, A few
strong to severe storms are possible on Wednesday and Thursday.

- Uncertainty increases for timing of thunderstorms on Thursday
  and Friday

&&

.FORECAST UPDATE...

The main areas of focus today are tracking a weakening complex of
showers and storms over MO and Southern IL and how it may impact
Central Indiana as well as watching for additional convection to
develop later this evening.

Current satellite and radar imagery shows this complex of weakening
showers and non-severe storms moving eastward into South Central
Illinois. Lightning activity is steadily decreasing as the complex
interacts with drier air over Indiana. Latest ACARS soundings from
around the region do confirm drier through the column currently
across most of the Ohio Valley; however most guidance still shows
strong moisture advection through the day as a plume of higher
moisture values, 2"+ pw, advects into the region.

Energy from this dying MCS in addition to lift associated with the
right entrance region of the jet and an approaching weak front
should be sufficient to spark off convection this afternoon. With
moisture increasing through the day, the atmosphere should be
sufficient for warm rain processes and heavy rainfall from any
storm. Increasing instability in addition to a weak 20kt LLJ
overhead should be enough to allow for convection to organize
resulting in a damaging wind threat as well. At the moment, nothing
of concern has developed. Currently watching the 18z-03z timeframe
for convective development with areas along and south of the I-70
corridor having the best threat for storms. Will update the forecast
and discussions as needed based on latest trends today.

&&

.SHORT TERM (Today and tonight)...
Issued at 216 AM EDT Wed Jul 3 2024

Tuesday Night into Wednesday Morning...

Radar and satillite imagery showcase a healthy MCS currently over
Missouri, that should progress into western IL over the next few
hours. As this MCS moves eastward it will encounter abnormally dry
airmass, seen through KIND ACARS soundings with 10-15 degree
dewpoint depressions through the entire 0-10km column. This dry
layer will errode some as the MCS pushes eastward, but all that
should be left as this system reaches central Indiana is a broken to
overcast 3-6km cloud deck. For now isolated shower chances have been
kept over far NW and W central Indiana, given some uncertaity on the
swiftness of MCS decay. Outside of this area, temperatures should
remain fairly mild as the PBL struggles to decouple completely
underneath increasing upper level clouds and a steady W/SW low level
flow. Current low temperature forecast is between the upper 60s and
low 70s across central Indiana.

Wednesday and Wednesday Night...

The 0-3km layer should quickly moisten post dawn as the high breaks
down beneath upper level diffluence, resulting in an increase of
S/SW winds. By 14Z, most areas south of I-70 should have dew points
arleady in the upper 60s to low 70s. The remants of the previous
mentioned MCS is expected to leave a weak stationary boundary over
central Indiana, of which will be the primary focus for convective
intiation in the afternoon.

Temperature wise, central Indiana will be split in two, with areas
north of I-70 likely remaining in the mid to upper 80s benath steady
cloud cover and weaker surface flow. Futher to the south, sun light
should trickly in by mid to late morning, aiding in temperature
gains. Most areas south of I-70 should reach or exceed 90, with some
locations approaching 92/93 in far southern Indiana. A few CAMs have
a well mixed boundary layer this afternoon, but given consistant
flow through the lower 1km, this solution looks unlikely. The
current expectation is for a closer to climatelogical moisture to
temperature vertical distribution, with dew points remaining in the
low 70s this afternoon over southern Indiana. The combination of
temperautres in the low 90s and dew points in the low 70s will push
heat index values near to just above 100. An SPS has been issued to
highlight the heat risk for this afternoon.

As mentioned, the stationary boundary will be the focus for
convective development later this afternoon over central Indiana.
Current HREF members are varied on where this boundary will be
positioned, but given the pattern and consistent surface flow, the
current forecast leans towards further north solutions, keeping the
I-70 corridor in >40 PoPs this afternoon. Eventually, the CAPE
gradient will push south, increasing convective coverage this
evening over southern Indiana.

In regards to thunderstorm severity, bulk sheer values are generally
not very supportive of sustained, and organized updrafts, but the
thermodynamic profile does hint at an increased damaging wind threat
this afternoon and evening; 00Z HREF suite shows a broad area of 50-
100 J/kg 0-3km CAPE and DCAPE values 1000-1200 J/kg. Small hail
cannot be ruled out, but a large hail threat looks highly limited
given elevated freezing levels and modest updraft strength. This
overall threat will be for all areas south of the aforemtioned
boundary, but far southern central Indiana has the greatest risk
given higher thunderstorm coverage expected this evening.

Around dusk, evapoative cooling and a lack of consistent moisture
return. should rapidly increases local surface based CIN and limit
convective development. A few isolated thunderstorms can not be
ruled out for tonight, but most areas should remain dry. A secondary
low from the SW will begin to develop late Wednesday into Wednesday
night, helping re-establish a moist warm sector over southern
portions of the region. This should not lead to any rainfall pre-
dawn, but dew points and temperauters should remain elevated. Expect
overnight lows to range from the mid 70s over far SW central Indiana
(Knox Cty), to upper 60s over NW central Indiana (Tippecanoe Cty).

&&

.LONG TERM (Thursday through Tuesday)...
Issued at 216 AM EDT Wed Jul 3 2024

The forecast for Thursday (July 4th) and Friday continues to be
complicated with below average confidence. Mesoscale details are
unclear due to convection in a weakly forced unstable regime. How
the convection evolves tomorrow evening will have some implications
on potential Thursday, and likewise the potential for Friday will be
dependent on convection Thursday. We`ll explain these details and
list some possible scenarios in the analysis below. Bottom line, we
will continue to encourage lightning safety messaging, as well as
the dangers of thunderstorm-driven wind gusts, as these will both be
the primary hazards through Friday. We encourage our media partners
and emergency managers to do the same, given the juxtaposition of
numerous outdoor events with these potential thunderstorm hazards.

On the synoptic scale, mid-upper level ridging will continue to
weaken as one shortwave trough departs. This will result in falling
midlevel heights over our region and a southward shift of a weakly
baroclinic front. Some models are more nondescript with the MSLP
pattern than others, and show the front will be masked by residual
convective cold pools. It appears as though convection may continue
into Thursday morning mainly to our south along the northern
periphery of a modest low level jet and associated weak low-level
moisture advection. There may be a tendency for convective elements
to propagate upshear and thus move further south. How much this
convection augments mesoscale features is unclear at this time. It
does not appear there will be a noticeable push of continental air
into our area, so even north of the front once diurnal heating
commences isolated to scattered convection may reform anywhere
across central Indiana, since this pattern doesn`t support
meaningful capping.

One caveat to the above is the potential for a MCS or remnant MCS
that may initiate from where the warm advection signal is strongest,
over Missouri. This could be fostered by increasing large-scale
ascent from approaching deepening shortwave trough over the northern
Plains. Rich moisture and at least modest instability will support
convective regeneration, and although the lower tropospheric flow is
quite weak, westerlies aloft are strong enough to contribute to 30-
40-knots of effective shear. In this scenario, a fairly slow-moving
weakly organized MCS or cluster of convection may enter western-
central Indiana during the morning or midday (uncertain timing),
resulting in more widespread rain coverage. A tendency for at least
some element of upshear propagation given the modeled kinematic
fields may favor stronger convective elements to our south and
perhaps some limit on how far north central Indiana will be
impacted. With the most intense convective cells, damaging wind
gusts is the primary threat, given the moisture distribution,
instability, and shear values expected.

The best chance of a dry period Thursday would be post-MCS (if one
moves through), where overturning and mesoscale subsidence
temporarily limits convective development. As closed midlevel low
approaches late Thursday evening and night, lower tropospheric flow
should strengthen and replenish moisture perhaps driving either (1)
some warm advection convection, or (2) maintaining another upstream
MCS. It appears by midday or afternoon Friday, the cold front will
be east and diurnal convection that develops along it would be
mostly in Ohio, but perhaps in our far east.

Obviously with all the caveats of convective timing, the temperature
forecast has a fairly high degree of uncertainty Thursday and into
Friday. Post-frontal air mass will be cooler and drier this weekend,
but probably not to the magnitude seen with the most recent
continental air mass. Nevertheless, this should lead to a dry period
with the next opportunity for rain being Monday and Tuesday.
Probabilities during this period are low/mid-range and spread across
a broad period for now, due to increasing ensemble spread and
uncertainties about how this front will/won`t potentially interact
with deeper subtropical moisture surge.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 131 PM EDT Wed Jul 3 2024

Impacts:

- SCT t-storms along and south of I-70 18z-03z

- Potential for MVFR cigs and vis near sunrise with additional rain

- Southwesterly wind gusts up to 20 knots through this evening

Discussion:

A weak front just north of I-70 will be the focus for shower and
thunderstorm development this afternoon and evening across Central
Indiana. Convection has already been observed along this front in
North Central Indiana but has since moved away from any TAF sites.
Latest satellite imagery does show a cumulus field developing along
and south of the I-70 corridor. Expect continued growth of cumulus
clouds over the next couple hours with showers and a few storms
developing along I-70. Coverage is expected to increase later this
evening and push southward toward KHUF and KBMG. Brief periods of
heavy rain, lightning, gusty winds, and MVFR cigs and vis are
expected with any storm. Outside of any storm, southwesterly to
westerly winds may gust up to 20 kts at times south of the front,
with winds becoming WNW behind the front at 10 kts or less.

Expect convection to wane shortly after sunset with a lull in the
precipitation overnight. Potential is there for patchy fog to
develop as winds subside overnight and surface temps fall near the
dew point. Not expecting anything widespread, but MVFR or lower cigs
are certainly possible, especially in areas just north of the front,
mainly KLAF. Hi-res guidance does indicate another complex of
showers and storms possibly pushing in during the 09-12z timeframe
along and south of the I-70 corridor once again. Lower confidence
exists as guidance tends to poorly handle timing and placement of
multiple rounds of convection, especially more than 12 hours out. But
will still put the threat for showers and MVFR cigs in the TAFs
during this period as lower clouds and patchy fog may still develop
even if this second round of convection holds off.

&&

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

$$

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