954
FXUS63 KIND 071859
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
259 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.

&&

.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 (18Z TAF Issuance)...
Issued at 146 PM EDT Tue May 7 2024

Impacts:

- Good confidence more PM thunderstorms will develop and
  move across before coverage decreases or ends and flying
  conditions improve

- General wind gusts to 25+ knots this afternoon

- Gusts to 50+ knots and large hail possible in severe storms this
  afternoon and evening

- Patchy fog possible overnight

Discussion:

Thunderstorm re-development is expected this afternoon. Strong and
severe thunderstorms are possible with this activity, and very poor
flying conditions are likely within or near any shower/storm.

Winds will become gusty and switch to the S and SW during the day
with gusts to 20+ knots away from convection up to 50+ knots in
severe storms.

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
shortly beyond the end of the current TAF period.

&&

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

&&

$$

SHORT TERM...Eckhoff
LONG TERM...Nield
AVIATION...Eckhoff