Special Weather Statement, NWS Indianapolis, IN

Area Forecast Discussion
National Weather Service Indianapolis IN
100 AM EDT Sat Jul 29 2023

.Mesoscale Update...
Issued at 1259 AM EDT Sat Jul 29 2023

A convective complex continues to barrel through central and northern
IL, now on the doorstep of central Indiana. Infrared imagery shows
some cloud top cooling on the southern half of the system, likely
leading to an overall decline in severe potential for western and
southern portions of central Indiana (outside of the watch area).

On the northern extent, currently over the SE suburbs of Chicago,
cloud tops are much higher, indicative of the better shear/cold pool
balance seen on radar. This will be the primary area of concern for
damaging wind gusts and isolated tornadoes (Highlighted by the
current Watch). The overall parameter spacing is still fairly potent
with a narrow corridor of 4000+ J/kg MUCAPE and a developing 35-40kt
LLJ. This corridor is on the southern edge of a prior evening line
of warm advection induced thunderstorms, and is where a better shear
balance may exist as this system moves S/E. With that said, a
cooling PBL and eventual elevation of the complex should counteract
some of this and eventually remove the threat of isolated tornadoes.

As the system continues to push through, stabilization of the PBL
and a separation from better upper level shear should lead to
weakening and dissipation around and after 08Z. At this point the
severe threat will diminish rapidly, but 30-40mph wind gusts and
localized flooding will still be a threat.

&&

.Short Term...(Today and tonight)
Issued at 1259 AM EDT Sat Jul 29 2023

* Severe Storms Possible Friday Night

* Quieter weather for much of the area Today

Currently an ongoing MCS is pushing ESE through NE IL and Northern
IN. This will continue along an instability axis created by warm
advection induced thunderstorms from earlier in the evening.

MORE DETAILS ON STORM THREAT TONIGHT IN MESOSCALE DISCUSSION
_______________________________________________________________

Synoptically, this overnight system is driven by a broad upper level
trough and jet axis, along with a goldy-locks zone of highly warm
and moist air near the surface and just enough cold air aloft for a
potent amount of instability. This has allowed the system to continue
to develop even after peak diurnal heating ended and now cooling has
begun. This system is expected to pass through central Indiana
between 05-09Z overnight, with some potential for residual weak
showers and storms over the area through the morning. This will
greatly depend on where any left over boundary reside and the
overall potency of the MCS an d associated cold pool as it becomes
elevated over the area. The stronger the cold pool, the more
stabilization and subsidence will occur in its wake, and there for
less shower coverage.

Otherwise, a mostly mundane pattern is in store for central Indiana
tomorrow. The main forcing should be pushed far enough south to keep
conditions mostly dry through the day. Temperatures will be closer to
seasonal as NW flow in the wake of the MCS helps keep warmest
temperatures to the S. The current expectation is for highs in the
mid to upper 80s, with a W to NW wind of around 8-12mph and dew
points in the upper 60s; a reprieve from prior day conditions.  The
only caveat to this quieter forecast is the location of the meso-low
from the prior mentioned MCS. If this is too far north, parts of S
and SE central Indiana may stay within the quasi-warm sector where
greater unstably and lift could lead to afternoon thunderstorms.
Confidence in the afternoon forecast will greatly increase by
morning as models ingest more accurate initial conditions.

Tomorrow night should be much of the same, with dry and mostly clear
skies. Patchy for will be a threat over rural areas to the north and
river valleys to the south that get any evening rain. With clear and
mostly calm conditions a few areas may see dense fog, but confidence
in where this could occur is very low.

&&

.Long Term...(Sunday through Friday)
Issued at 1259 AM EDT Sat Jul 29 2023

The general synoptic-scale theme in the long term will be a
deepening trough in the east and amplifying ridge in the west. We
will remain in northwesterly flow and on the periphery of deeper
more anomalous moisture. This is a pattern where mesoscale details
will ultimately dictate timing and coverage of precipitation (i.e.,
compact midlevel perturbations, residual cold pools, gravity waves,
etc.). One or more organized convective systems will be possible
given stronger momentum aloft on the periphery of the ridge
contributing to adequate deep layer shear. A limiting factor may be
the tendency for weak instability owing to a southward nudge of
continental air Sunday into early next week. Preceding this on
Saturday night, an active convective regime may be in progress from
the previous diurnal cycle given residual deep moisture,
instability, frontal convergence and upstream shortwave trough. This
trough should nudge a slightly cooler/drier cP air mass in on
Sunday.

A more amplified shortwave trough on Thursday should push anomalous
precipitable water and higher instability into Indiana increasing
coverage of convection at some point Thursday into Thursday night.
Although ensemble spread grows toward the end of the week, there is
still enough of a signal to indicate at least slightly cooler/drier
cP air mass nudges into central Indiana by Friday.

&&

.Aviation...(06Z TAF Issuance)
Issued at 1140 PM EDT Fri Jul 28 2023

Impacts:

- Best timing for thunderstorms with brief MVFR and worse conditions
  along with near-severe to severe wind gusts is 06z-10z. Less of
  chance KHUF sees activity

- Convection could linger after 10z and at times later today but
  confidence too low to put in TAFs

Discussion:

A thunderstorm complex will drop southeast across central Indiana
overnight with the potential for strong to severe wind gusts and
brief heavy rain and MVFR and worse flying conditions. Additional
storms are possible later this morning with leftover boundaries and
lingering moisture and instability. Best chance of thunderstorms
looks to be from 06z-10z at KLAF, KIND and KBMG. The complex may
miss KHUF as it moves southeast.

Mostly VFR flying conditions are expected away from thunderstorms.

A cold front will drop southeast over the terminals this afternoon
resulting in a wind shift SSW to NW. Wind speeds will generally be
below 10 knots except briefly much higher related to the gust front
of the convective complex.

&&

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

Mesoscale...Updike
Short Term...Updike
Long Term...BRB/Updike
Aviation...MK

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