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000
FXUS63 KIND 171405
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1005 AM EDT Wed Apr 17 2024

.KEY MESSAGES...

- Gusty winds between 30 and 40 mph today.
- Isolated showers and storms this afternoon mainly northeast of
  Indianapolis. A severe storm is possible.
- Severe storms and locally heavy rain are possible late Thursday
  into Thursday night.
- Colder but dry for the weekend.

&&

.MESOSCALE UPDATE...
Issued at 1005 AM EDT Wed Apr 17 2024

Low stratocu continues to hamper warming and destabilization to some
degree this morning but are starting to see some mixing out of the
lower deck ahead of the approaching cold front over east central
Illinois. Just since 1330Z beginning to see convective initiation
from near the Chicago metro south along the Illinois-Indiana border
to north of KDNV. Temps as we approach 14Z were generally in the
upper 60s and lower 70s with dewpoints into the lower 60s as well.

The focus for the remainder of the morning into the afternoon is on
further convective development along and ahead of the cold front and
the conditional risk for severe storms over the next 3-5 hours
locally. Not much has changed with respect to earlier thinking and
now utilizing trends in the mesoscale to determine convective
evolution over the next few hours.

ACARS sounding analysis already showing increasing instability with
a nice thermodynamic structure with a moist boundary layer and as
the mid levels cool in tandem with the upper low pivoting through
the Great Lakes. Model soundings going forward into early afternoon
remain favorable for at least the threat for severe weather focused
especially across the northeast part of the forecast area where the
combination of stronger shear and helicity parameters will coincide
with the axis of deeper moisture and instability ahead of the
boundary. The shear and low level helicity will be maximized to our
northeast with increasing instability as heating maximizes during
the afternoon...with the best focus for severe convection set to
focus to our northeast. But locally...areas roughly along and east
or north of a Kokomo to to Fishers/Noblesville to New Castle line
are likely to remain in play for more robust convection through
about 18-19Z.

All types of severe weather are in play over the next several hours
with decent directional shear present through the boundary layer and
potential for steepening lapse rates as the low levels cool further.
Large hail and damaging winds would be the primary concerns but the
risk for a tornado or two remains as well with 0-1 km SRH values
over 100 m2/s2 and a favorable right turning hodograph through the
lowest 3km indicative of the directional shear present. The lack of
more substantial forcing aloft which remains displaced to the north
will likely be a limiting factor in more extensive convective
coverage but enough is present to warrant a close monitoring across
N//NE portions of the forecast area over the next few hours as
storms organize.

One last point to highlight is the concern for stronger gradient
wind gusts surrounding the frontal passage with a tight pressure
gradient. Will continue the Special Weather Statement which covers
the threat for occasional gusts to near or above 40mph through late
afternoon.

Zone and grid updates out.

&&

.SHORT TERM (Today and tonight)...
Issued at 301 AM EDT Wed Apr 17 2024

A large and occluding low pressure system resides over southern
Minnesota. A band of showers and thunderstorms have swept through
much of Indiana earlier this evening and are now exiting into Ohio.
These have been fairly weak, as they have outpaced the main axis of
instability feeding northward into the low. Another round of showers
and storms is possible this afternoon as the system`s cold front
passes through along with the increasing instability. Latest trends
in guidance have been increasingly pessimistic about our rain
chances, however, and it now looks that most of the CWA will remain
dry today.

As mentioned before, showers and storms are exiting the area and
have outpaced the axis of instability that helped them develop. This
axis stretches roughly through the state of Illinois as of 06z, and
is slowly spreading eastward into Indiana. The primary effect that
this will have on our weather through the morning hours will be seen
as increasing low cloud coverage. With richer moisture spreading
northward on the back of a potent low-level jet, these cloud may
only be a couple thousand feet off the ground. Additionally, as low-
level lapse rates improve and the MSLP gradient tightens...gusty
winds to as high as 30-35 knots will be possible through the morning
and afternoon hours. Initially out of the south, these winds will
gradually become westerly as the system`s front passes by.

Timing of the front will range from 13z-15z on our western border to
about 18z near IND, and then 21z closer to the Ohio border. Given
the rich moisture and instability ahead of this boundary, showers
and thunderstorms may develop as it translates eastward. However,
guidance shows the front losing definition somewhat with southward
extent. A minimal wind shift at FROPA from IND southwestward may not
induce enough lift for any showers and storms to develop. The wind
shift across northeastern portions of the area may be sharper, and
consequently this is where most showers and storms are depicted in
the latest model runs. Even here, trends have been towards a drier
solution.

Should a thunderstorm develop it has the potential to become strong
as model soundings show deep but skinny CAPE, over 40-50kt shear,
and modestly steep lapse rates (6-7 C/km). Strong wind gusts appear
to be the primary hazard as enough dry air may allow for potent
downdrafts. Marginally severe hail is possible too, but skinny CAPE
and only modest lapse rates may limit the potential. Again this is
all conditional on a thunderstorm developing at all, in which
guidance has become increasingly pessimistic.

By tonight, the front will be well east and our weather should quiet
down. Generally clear skies and lessening winds can be expected
through the end of the period. Cold air advection behind the front
may bring lows back into the low 50s and perhaps some upper 40s up
north.

&&

.LONG TERM (Thursday through Tuesday)...
Issued at 301 AM EDT Wed Apr 17 2024

Thursday and Thursday night...

Surface low pressure moving into Illinois will bring a warm front
north into central Indiana during the day on Thursday. This front
won`t have much upper support with it as upper ridging will just be
exiting the area as the front moves north.

Thus, expect much of the day Thursday to be dry. Temperatures will
return to the mid 70s to lower 80s. Depending on the timing and
final location of the warm front, readings could get even warmer per
ensembles.

Late on Thursday and into Thursday evening, as the surface low moves
into northern portions of central Indiana, forcing will increase
across the area. Moisture will be plentiful with continued southerly
flow. Thunderstorms will have initiated well west of central Indiana
earlier in the day and will move into the area late Thursday
afternoon or possibly Thursday evening.

CAMs are showing odds favor the storms having congealed into a line
by the time they reach central Indiana, but isolated cells could be
ahead of the line.

Given the expected low pressure track, best chances for severe
storms will be across the southern forecast area, with higher
chances south and west where best instability will be. Primary
threats will be damaging winds and large hail, but tornadoes cannot
be ruled out given the parameter space. Heavy rain is also possible.

Uncertainty remains on the path and timing of the surface low, which
would impact where and when the best chances for severe will occur.

Friday through Monday...

Colder air will flow in behind Thursday`s system, and central
Indiana will remain on the southern edge of a large upper system. At
the surface though, high pressure will remain in control. The high
will keep the area dry.

Low temperatures will dip into the 30s Saturday night and Sunday
night. Patchy frost may occur in the north Saturday night if winds
are light enough and cloud cover is lower.

Monday night and beyond...

Uncertainty is high on how upper waves will move through the area
during this time, leading to lower than average confidence in rain
chances. Will keep PoPs low. Temperatures will return to closer to
normal as the core of the colder air moves northeast.

&&

.AVIATION (12Z TAF Issuance)...
Issued at 625 AM EDT Wed Apr 17 2024

Impacts:

- Isolated showers and thunderstorms possible near IND between 14z-
  19z.

- South winds becoming SSW this evening then westerly during the
  afternoon.

- Wind gusts to 25kt increasing to 30-35kt after 14z.

Discussion:

Expect lowering ceilings as moisture advects in from the south.
Ceilings may dip into MVFR territory this morning at all
terminals. Improvement can be expected in the afternoon/evening as
dry air wraps in behind a cold front expected to pass through
early in the day.

The aforementioned cold front will bring a slight chance of showers
and storms, mainly from IND northeastward. South winds will become
westerly after frontal passage, and occasionally gust 25-35kt before
diminishing Wednesday night.

Expect high clouds to increase towards the end of the TAF period
as the next storm system approaches. Rain and thunderstorms are
likely later Thursday beyond the end of the period.

&&

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

&&

$$

MESOSCALE...Ryan
SHORT TERM...Eckhoff
LONG TERM...50
AVIATION...Eckhoff