Flood Watch 3, NWS Indianapolis, IN

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Area Forecast Discussion
National Weather Service Indianapolis IN
207 PM EDT Mon Jul 15 2024

.KEY MESSAGES...

- Thunderstorms with damaging wind potential late this evening and
  tonight

- Flash flooding possible across portions of the area tonight
  through tomorrow

- Pleasant weather with cooler temperatures and lower humidity Thursday
  into the weekend

&&

.SHORT TERM (This evening through Tuesday)...
Issued at 207 PM  EDT Mon Jul 15 2024

An active convective period continues with challenges regarding
thunderstorm timing and intensity, inherent to uncertainty with
mesoscale details. It appears that thunderstorm formation this
afternoon will be limited or non-existent across central Indiana,
and a round of thunderstorms will arrive late this evening with
damaging wind potential into the night. We are also concerned about
flash flooding potential tonight through part of tomorrow. See
meteorological analysis for more details.

On the synoptic scale, we remain on the southern periphery of
midlevel westerly flow. Perturbations within the flow are likely
significantly augmented by diurnal convective blow-ups. Modeling of
this process requires mesoscale precision in the initial conditions
that isn't always possible, and can spiral into chaos quickly. This
is the reason for the diversity we're seeing among the CAMs with
regards to placement and timing of convection. It is important not
to focus on any one specific model in this pattern.

Our three main concerns as far as weather impacts are:

1. Dangerous heat
2. Severe thunderstorms with damaging wind
3. Flash flooding

In regards to the near-term heat, through early evening temperatures
and dew points will be high enough to meet traditional Heat Advisory
criteria (105F heat index) across portions of central Indiana. In
the latest ACARS vapor sounding and short term model guidance, it
appears that moisture depth is sufficient to preclude mixing of
drier air and meaningful fall dew points through the diurnal peak.
Southwesterly breeze and some cloud cover from diurnal cumulus are
components that will keep the Wet Bulb Globe Temperature risk
category contained slightly, though still "high".

Thunderstorm development today should be isolated at most given
residual subsidence-enhanced capping. Evidence of this can be seen
in laminar flat look to cumulus field in visible satellite imagery
all the way back into Illinois. Water vapor channel imagery shows
core of the subsidence moving east early this afternoon. It's
possible the stronger capping may exit before peak diurnal period
sufficient for a few convective cells, especially near any residual
mesoscale boundaries, but this is low probability.

Upstream over Iowa and closer to the influence of a weak midlevel
impulse, diurnal convection will develop later this afternoon. Given
the magnitude of instability, thunderstorms should become quite
robust. This will occur at the southern periphery of stronger
westerlies aloft and sufficient shear values for a forward-
propagating MCS to evolve. The downstream environment would favor
approximately 30-40 mph east-southeasterly system motion. Moderate
shear would probably support trailing stratiform rain quickly
evolving and enhancing the cold pool, perhaps more than modeled.
Thus, confidence in an MCS impacting a good portion of central
Indiana late this evening and overnight is fairly high. Wind damage
is the primary threat and there is at least a low chance of
significant wind damage, especially earlier in the night before it
begins to outpace the better parameter space. Northwest portions of
central Indiana, including Newport, Lafayette, Crawfordsville, and
Kokomo have the highest potential for wind damage, tapering somewhat
with southeastward extent into the Indy metro and eastward later in
the night.

Flash flood concerns today and tonight are multi-faceted. Given
recent rainfall (1-2 inches and locally higher over the past 72
hours) even a quick-moving forward-propagating MCS with limited
duration of heavy rates could have some flash flood concern. These
systems tend to be quite efficient rainfall producers in a short
amount of time, and could easily exceed latest RFC 1-hour flash
flood guidance of around 1.50 inches across the northern third of
central Indiana. This would yield marginal/low-end flooding or flash
flooding, most likely.

Of greater concern is a relatively low probability scenario later
tonight on the western flank of MCS cold pool, and how it interacts
with the low level jet. It appears back-building and training are
possible across a focused area that is difficult to pin down at this
time. Warm cloud depth is seasonally large with model indications of
rich moisture in this layer. In fact, overall precipitation
efficiency may be enhanced by an east-west band of mid-upper level
moisture. Uncertainties remain on where a mesoscale convective flash
flooding scenario could potentially evolve, but the potential
magnitude despite spatial coverage combined with aforementioned more
widespread low-end threat justifies a Flood Watch.

Uncertainties grow further tomorrow on the severe and flooding
potential. This will largely depend on where the remnant MCS cold
pool resides during the afternoon once diurnal destabilization
occurs. The synoptic front will still be lagging to the northwest so
another active convective day seems likely, largely driven by
mesoscale details that we can't resolve with precision at this time.
There may be an ongoing flood/flash flood threat early with residual
convection, given indications of modest instability remaining and
the aforementioned reasons for potentially locally heavy rainfall
amounts. In addition to flooding, moderate instability developing
later in the day but with marginal deep layer shear should mean
sporadic damaging wind potential from loosely organized multicell
clusters is the main severe threat. Again, this is conditional on
mesoscale details, as one plausible scenario would hold this threat
primarily to our south.

There is some potential for dangerous heat across far southern
portions of our forecast area tomorrow. We will not issue a Heat
Advisory at this time due to uncertainties on how far south the cold
pool from overnight convection will be, residual midlevel clouds,
and potential ongoing convection or development before peak heating.
Will reevaluate tonight and early tomorrow based on trends with
convection.

&&

.LONG TERM (Tuesday night through Monday)...
Issued at 207 PM EDT Mon Jul 15 2024

Wednesday, it appears that the synoptic front will be across our
south or just south of the area. We'll continue low probabilities
but confine them a little further south than the previous forecast.
Drier/cooler continental air mass will lag some until the amplifying
synoptic trough axis moves through late Wednesday. The air mass
change will be most noticeable Thursday continuing into the weekend
before gradually modifying Sunday into early next week. Moisture
returns then, but the synoptic flow pattern is rather quiescent.
Organized precipitation appears unlikely but we may enter a scenario
next week with diurnal isolated to scattered convection daily. After
a period of 5-10F negative temperature anomalies relative to mid-
July climo, temperatures should return to near climo next week. For
the day 8-14 day period there's not a strong signal of above or
below average precipitation or temperatures at this time.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 1247 PM EDT Mon Jul 15 2024

Impacts:

- Southwest winds gusting to 20-25kts this afternoon

- Restrictions possible within convection late this evening through
early Tuesday

- MVFR ceilings possible Tuesday morning

- Southerly wind gusts to around 20kts Tuesday

Discussion:

Pockets of VFR cu with mid level clouds lingered over parts of
central Indiana early this afternoon with gusty southwest flow. A
mix of sun and clouds will persist through the afternoon with
occasional gusts up to 20-25kts before diminishing this evening.

Confidence continues to grow in a substantial convective cluster
developing over the upper Mississippi Valley early this evening and
tracking E/SE overnight. While the details are not fully in focus
yet...these storms are likely to impact the terminals from north to
south overnight with strong winds from variable directions and
torrential rainfall that will produce restrictions through early
Tuesday.

High uncertainty for Tuesday morning as convection may be ongoing in
parts of the area. Model soundings support potential for MVFR
ceilings to develop through the morning with wind gusts resuming
from the south.

&&

.IND WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM EDT this evening for INZ021-028>031-
035>049-051>057-060>065-067>072.

&&

SHORT TERM...BRB/Ryan
LONG TERM...BRB
AVIATION...Ryan

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