731
FXUS63 KIND 141811
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
211 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 (Today and tonight)...
Issued at 309 AM EDT Sun Jul 14 2024

Two main forecast challenges exist in the short term period. Namely,
possible convection and associated hazards along with excessive heat
potential.

CONVECTIVE POTENTIAL AND HAZARDS

First, we`ll discuss the convective potential today. As of this
writing (230am) we have two upstream convective systems. The nearest
is located in the Chicagoland area and the further one is near
Minneapolis. Both of these systems have mature cold pools and are
generally propagating southeastward towards Indiana. Timing places
the first MCS near Lafayette early this morning, around
sunrise...and the second (should it survive) during the afternoon
hours today.

Our main challenges regarding these are: how far do they propagate
before dissipation, timing and hazards, and subsequent convective
redevelopment (if any). These challenges are particularly tricky
since, when dealing with cold pools, convective allowing models can
struggle. The HRRR, for instance, has been having a very difficult
time with these two complexes. Tossing CAMS to the side for now, we
can look at our mesoscale convective system (MCS) conceptual models
to get an idea on what may happen.

Downstream across Indiana, ACARS soundings out of IND show abundant
MUCAPE along with a 20-25kt west-southwesterly low-level jet (LLJ).
Aloft, we see rather little in the way of upper-level flow with only
a 25kt 250mb wind. A plane-view look at current conditions, we see a
diffuse theta-e gradient oriented north-south across western
Indiana. Models generally take this gradient and lift it
northeastward with time.

So what does this tell us? For one, there is plenty of instability
for these convective systems to work with. Second, the presence of
an instability gradient/advection along with a LLJ and lack of shear
tells us it may propagate preferentially southward more than
eastward (greater influence from upshear propagation than downshear
propagation). The lack of shear may also allow the system to become
cold pool dominant quickly which may mitigate severe potential. The
system may weaken through the morning hours as the low-level jet
diminishes after sunrise.

The first MCS, should it propagate southward enough, may strengthen
the theta-e gradient. This may allow the second MCS, near
Minneapolis, to propagate a bit further southwest than one would
expect. Again, this depends on how the first MCS evolves. Any
lingering boundaries/cold pools may also serve as a focal point for
later convective initiation. Model soundings show much more
instability developing by the afternoon along with large amounts of
DCAPE, and convection may gain the potential to produce damaging
winds along with marginal hail.

There are a lot of moving parts regarding today`s convective
threat...and a variety of things can happen. One or both MCSs
propagate further than modeled...one or both could dissipate
early...scattered afternoon convection develops independently of the
morning MCSs...or develops on their remnant outflow boundaries.
Today`s forecast will likely need updates as the evolution of
upstream convection becomes clear.

TEMPERATURES AND HEAT POTENTIAL

As mentioned above in the convective section, we have a 25kt low-
level jet which is coming from the southwest. The theta-e gradient
is associated with the advection of warm moist air from the
southwest. This air mass will allow for temperatures a few degrees
higher than yesterday, with most locations seeing highs in the lower
90s. Dew points will be high as well, with readings in the lower 70s
being common. Still, heat indices top out near 100 for the most part
which is below advisory criteria. Additionally, the convective
potential and uncertainty described above makes advisory-level heat
far from a given. The highest temperatures and heat indices look to
be in our southwestern counties, which are deeper into the warm air
mass and further from the convection approaching from the north.

&&

.LONG TERM (Monday through Saturday)...
Issued at 309 AM EDT Sun Jul 14 2024

Monday through Wednesday...

Hot and humid mid-summer will peak through the early portions of the
long term as southwesterly breezes gusting up to 20 mph maintain low-
70s dewpoints Monday, with perhaps mid-70s for many locations on
Tuesday.  These oppressive humidity values will hold morning lows
mainly in the mid-70s and combine with the warm advective flow to
bring afternoon highs in the 92-96F range Monday...with the same
maximums expected for at least the southern half of the region
Tuesday.  Less certainty surrounds the corresponding convective
threat that will be focused north of Indianapolis, which should
allow for only marginally hot conditions across the north on
Tuesday.  The high humidity will boost heat index values at least 5-
10 degrees above the temperature...with afternoon max heat indices
expected to range from generally 100-105F Monday, and 95-105F
Tuesday...although criteria heat indices will be more easy to reach
in southern river valleys.

Convective showers and thunderstorms will certainly be possible amid
what should be abundant PM instability, albeit with little wind
shear to help realize this potential.  Best shot at organized
strong/severe storms will be possible arrival of upstream MCS-type
cells forming along CAPE gradient near a line from Twin Cities to
Chicagoland.  This conditional threat would be greatest across
northern counties with stronger winds from collapsing cells the main
hazard.  Greater certainty in numerous to widespread showers and
thunderstorms crossing the region from northwest to southeast with a
solid cold frontal passage through PM hours Tuesday.  Less-
impressive, yet still ample CAPE will combine with mediocre mid-
level lapse rates and what will probably be inconsistent wind shear.
Expect a few strong storms, but severe potential for this time is
less certain and also conditional.  Expect lingering showers from
north to south into Wednesday under the passing frontal zone.

Thursday through Saturday...

Confidence is increasing in noticeably milder and less humid
conditions during the climatologically hottest week of the year. The
amplified western ridge will induce cooler surface high pressure to
plunge southeastward from south-central Canada through the mid-week.
What now appears may be a quasi-omega block across most of Canada,
should serve to slow the west to east progression of these lower
heights...with high pressure taking all three days to slowly cross
the Great Lakes, with northerly/northeasterly low-level flow
prevailing into central Indiana.

This will translate into at least 2 days with temperatures 5-10
degrees below normal amid low humidity levels for most locations.  A
modest north-south gradient in both values is expected...with
morning lows on the coolest overnight likely ranging from the mid-
50s in far northern zones to around 60 towards the Ohio Valley.
Expect rain-free conditions to be the rule under a column sporting
precipitable water values almost 50 percent below normal for mid-
July.

&&

.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...Eckhoff
LONG TERM...AGM
AVIATION...Ryan