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442
FXUS63 KIND 080117
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
917 PM EDT Tue May 7 2024

.KEY MESSAGES...

- Severe storms expected today, especially during the afternoon
  and evening. All forms of severe hazards are possible.

- Additional severe storms are possible Wednesday afternoon and
  evening. All forms of severe hazards are again possible.

- Heavy rain is possible at times through Wednesday night. Localized
  flooding will also be possible.

- Periodic chances for showers and a few storms late week into early
  next week.

- Normal to slightly below normal temperatures from Thursday
  into early next week.

&&

.MESOSCALE UPDATE...
-- Changed Discussion --
Issued at 917 PM EDT Tue May 7 2024

A general east-west axis of mixed mode convection continues mid this
evening S and SE of Indy, with a tendency for some new cells to
develop along the southern outflow boundary of the existing complex.
Besides continued strong/severe weather potential into late evening,
heavy rain amounts and rates have materialized due to some training
cells, with radar rainfall estimates of 1.5 to 2.5 inches in some
areas. As a result, a flood advisory is in effect til midnight EDT
for parts of Johnson, Shelby, Rush, and northern Decatur counties.
Use caution and watch for significant ponding of water on roads and
low-lying areas and near creeks and streams.

Eventually CAMS take the current activity ESE and out of central IN,
while a new potential focus may occur over parts of southern IN into
KY. Overnight, storms should end across our region as drier and more
stable air temporarily infiltrates central IN. Then renewed
destabilization and additional convection should occur Wednesday
afternoon and evening in our area, especially southern half.

Issued at 753 EDT Tue May 7 2024

Convective mode has become a little messier over last hour or two,
with a mix of supercells and now a more linear segment across Shelby
and Johnson counties. The line segment evolved due to outflow
mergers creating a local cold pool which has pushed east across
Shelby, increasing the potential for some strong/severe wind gusts.
Meanwhile, the back portion of this line over Johnson has evolved
more into a heavy rain/local flood threat as cells train east across
the county. However, a new line segment now over southern Johnson is
moving more normal to its own axis, increasing the local wind and
hail threat in that part of the county.

Several supercells are still in play over central IN, one warned in
Rush County, others in Randolph that appear to be weakening last
couple radar scans. Additional cells expected this evening along and
south of current activity as unstable SW inflow continues to impinge
on convective outflow boundary from ongoing cells. Besides
continuing severe threat, additional heavy rain could result in more
ponding of water and localized flooding.

Issued at 634 EDT Tue May 7 2024

Scattered supercells continue to track E and NE over central IN. The
earlier cell that was approaching southern Indy metro failed to
pulse up as cell mergers occurred more in front flank of cell which
may have limited unstable inflow into updraft zone. Nevertheless it
remains a core entity as it passes across SE Marion Co. at this time
and will continue to monitor as it moves east.

Other supercells not showing much low-level rotation at this time.
This could be due to slightly more veered boundary-layer flow into
the storms, limiting some streamwise vorticity ingestion into
updraft zone. Nevertheless, we are getting some good hail cores with
occasional 1 inch reports still happening with strongest cells.

Most concern over next 1-2 hours would be the storms along/near the
I-70 corridor which have unimpeded axis to unstable low-level inflow
and reside on the northern gradient of the maximum instability over
far SW IN on southward. Storms also are riding along their own
created outflow boundary which could possibly promote better
convergence and tilting/rotation into the updraft. Will continue to
monitor.

Issued at 540 PM EDT Tue May 7 2024

Persistent supercell continues to track east across southern and
eastern Putnam Co. at this time. Storm has history of 1 inch
diameter hail, with isolated report up to ping pong in Putnam along
with funnel cloud as relayed by EMA. Still healthy rotation aloft
but less so in low-levels. Still not seeing really any cell or
shower mergers into southern inflow flank of storm yet although
radar now showing some cells developing just to its south and moving
NE toward it. Such mergers can enhance low-level streamwise
vorticity and rotation potential, subsequently leading to better
tornado potential (but not always). WoFS data suggests that some
mergers and some cell strengthening may occur as it continues to
propagate east toward southern half of Indy metro over next hour,
although rain from downwind part of storm will affect Indy area
shortly. Will continue to monitor trends on this storm closely.

Elsewhere, we are continue to monitor supercell structures over
Boone, Clinton, and Tipton counties at this time.

More updates to come.
-- End Changed Discussion --

&&

.SHORT TERM (This evening through Wednesday)...
Issued at 259 PM EDT Tue May 7 2024

Our focus in the short term is the severe weather potential this
afternoon.

THIS AFTERNOON

Synoptic Overview

A potent trough and associated jet streak are beginning to edge into
Indiana. We`ll find ourselves in the right exit region of the jet
streak. Typically, the right exit region isn`t usually considered
the best place for large-scale lift. However, we`re dealing with a
cyclonically curved jet streak which maintain favorable lift in both
the right and left exit regions. The best vorticity advection looks
like it will pass to our north. Still, enough divergence exists
within the jet streak for synoptic-scale lifting this afternoon.
500mb height falls signify that this large-scale lifting is ongoing.

At the surface, a cold front across northwestern Illinois will
advance eastward. This feature will provide another source of lift
in addition to the dynamics above. A potent low-level jet (35-45kt
at 850mb) will advect warm buoyant air back northward eroding what
remains of a cold pool/CIN. Rapid destabilization of the planetary
boundary layer (PBL) is expected as we head into the afternoon
through a combination of advection and solar insolation. Additional
lift will arise from the buoyant forces near the surface. As such,
thunderstorms are likely once again both along the boundary and
within the open warm sector.

Convective Mode

Now that we`ve reasoned why thunderstorm development is once again
expected this afternoon, we can take a look at convective mode and
hazard types. Latest RAP analysis shows plenty of low-level shear,
and this is corroborated by latest satellite imagery showing
billows/waves in the warm sector stratus (17z). RAP soundings depict
0-1km shear over 20kt, with effective bulk shear over 50kt.
Hodographs are long, with some curvature in the lowest 3km
(especially as one heads further east). The thermal profile shows
modest lapse rates with tall CAPE profiles and abundant low-level
moisture. NAM/GFS soundings are quite similar. The jet streak aloft
is generally perpendicular to the warm sector and cold front.
Therefore, multicell clusters and supercells appear to be the most
likely storm mode today. Reason being is we have a lot of shear with
the best large-scale lift is passing to our north. Additionally,
faster flow may still be a bit further west and storms could have
poor anvil-level relative outflow (which may promote multicell over
supercell). The jet streak will be deeper into the region later in
the day as well, so supercell mode may become more prominent after
6pm or so. Linear mode may be possible in the evening as the
front/pressure trough becomes a bit more parallel with the shear
vector.

Hazard Type and Timing

In terms of hazard type, we have an "all of the above" situation
this afternoon. Abundant buoyancy and shear in the lowest 1km should
promote tornado potential, with the possibility of a strong tornado.
Additionally, tall CAPE profiles with fast flow through the Hail
Growth Zone upward will promote a large to very large hail threat.
Dry mid-level air may lead to DCAPE values over 1000J/kg allowing
for efficient downward momentum transfer and a damaging wind threat.
These hazards are most likely within supercells that develop. In
terms of timing, given the lack of a cap in the model soundings...it
seems probable that convection fires up rather early in the
afternoon. As of 17z, low- level stratus still has a wavy
appearance signifying low-level stability...though this is
beginning to change. Satellite trends show convective rolls and a
few towers beginning to develop over northern Illinois. The most
probable timing of convective redevelopment is between 18-21z.

TONIGHT

High-res guidance generally shows convective activity exiting the
area before 06z. As mentioned above, some linear convective mode is
possible in the evening...and if the line orients itself parallel to
the shear vector then storm training may occur. This may promote a
heavy rainfall threat. This seems most probable in the south and
eastern portions of our CWA. Surface winds diminish quickly after
sunset and another round of overnight fog is possible as PBL
inversion develops.

Wednesday`s severe weather threat will be covered in the Long Range
Discussion below.

&&

.LONG TERM (Wednesday night through Tuesday)...
Issued at 259 PM EDT Tue May 7 2024

WEDNESDAY/WEDNESDAY NIGHT...

BOTTOM LINE UP FRONT - Another day of potentially significant severe
weather is likely Wednesday, with all hazards possible, though the
corridor of highest severe potential may be further to our
south/southwest as opposed to today.

Central Indiana will begin the day on the cool side of the effective
warm front, which will likely lay somewhere along a Kansas City -
Paducah line and points east/southeastward by daybreak, though the
eastward extent becomes a bit more uncertain as this portion of the
boundary may be convectively reinforced.

All models, including CAMs, show this boundary pushing steadily back
northward through the day on Wednesday, though there is decent
variability in how quickly this occurs and what that means for
destabilization and evolution of tomorrow`s rounds of convection.

The rough consensus appears to be that the front will get as far
north as the I-70 corridor or thereabout by late afternoon, though
convection will very likely be in progress by this time, as minimal
inhibition is expected within the warm sector. Convection may then
push eastward and perhaps even east/southeastward as potential
upscale growth occurs, cold pools become established and storms
begin to ride the instability gradient along the boundary, which may
be held up on its northward lift by this activity, which is a
failure mode for severe episodes we`ve seen in this area several
times before. The lack of capping may also contribute to a variety
of convective modes, with discrete storms ahead of any possible QLCS
segments, though all hazards look to be on the table - with relative
threat of each hazard depending significantly upon these modes.

Shear will not be an issue, as deep layer and low level bulk shear
numbers are more than adequate for organized storms across the area,
especially along the boundary - and significant instability will be
present within the warm sector. It appears that convection will fire
fairly early in the day over western Missouri into NE Oklahoma/NW
Arkansas, initially moving eastward or even ENE-ward with the mean
flow, with a gradual shift to eastward motion and then ESE-ward with
time as convection organizes and begins to modify the environment.

Focus area over the last 24 hours has shifted south somewhat, and
given the CAM guidance and uncertainties with respect to warm
frontal progress/airmass recovery further northward into central
Indiana, this appears reasonable. If QLCS mode becomes predominant
early in the day, this activity may well skirt across the southern
portions of central Indiana in late afternoon/early evening as it
moves eastward and then begins to develop southeastward along the
instability gradient. However far north the warm front manages to
progress, any convection near and along it, particularly discrete
cells, will have the potential for tornadoes, possibly significant,
though again, the most likely corridor for this appears to be just
off to our south/southwest at this time.

Significant rainfall potential will exist with this activity, and a
hydrologic threat will require monitoring tomorrow into tomorrow
night, dependent upon storm track and evolution.

THURSDAY - TUESDAY...

BOTTOM LINE UP FRONT - Cooler with periodic shower/storm chances
through the rest of the forecast period.

Lingering shower and storm chances will accompany a secondary cold
front and the upper level trough axis as it passes through the
region on Thursday, and periodic shower chances will be necessary
throughout the remainder of the long term as individual waves within
the larger upper level cyclonic flow impinge upon the region.

Temperatures will be cooler, perhaps significantly so, late in the
week and into the weekend, though some recovery back into the mid to
perhaps upper 70s appears likely by early next week as the large
scale upper trough pulls northeastward and heights begin to build
locally/flow becomes quasizonal.

&&

.AVIATION (00Z TAF Issuance)...
Issued at 732 PM EDT Tue May 7 2024

Impacts:

- Thunderstorms continue to impact KIND through 0-1z. KBMG could see
impacts through 2-3z.

- Patchy fog possible overnight.

Discussion:

Thunderstorms remain mainly east of a line from Kokomo to Terre
Haute. Strong and severe thunderstorms remain possible for the next
few hours with very poor flying conditions within or near any
shower/storm, which at this point should not impact KHUF or KLAF.

After storms exit the area, skies should clear and some patchy fog
may form overnight. Winds may go light and variable for a time.

Wednesday will feature increasing clouds and a renewed storm threat.
The best storm chances appear to be across southern Indiana, with
lesser chances north. Will leave out thunder mention for tomorrow
for now without good confidence on timing and location but include
some convective cu. Winds become southerly around 10-14 kts
Wednesday afternoon.


&&

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

$$

MESOSCALE...Funk
SHORT TERM...Eckhoff
LONG TERM...Nield
AVIATION...CP