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897
FXUS63 KIND 242241
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
641 PM EDT Tue Sep 24 2024

.KEY MESSAGES...

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- Showers and a few thunderstorms ending this evening

- Seasonable temperatures for much of this week with highs in the
  70s and lows in the upper 50s and 60s

- Potential for more rain late week into the weekend from tropical
  remnants. Many uncertainties remain on location and amounts of
  rainfall
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&&

.MESOSCALE UPDATE...
Issued at 447 PM EDT Tue Sep 24 2024

Central Indiana remains in a broad warm sector ahead of a deepening
surface low this afternoon. Broad lift and 1000-2000 J/kg of
instability within this warm sector has allowed for scattered
showers and thunderstorms to develop. This has primarily been
focused along two advancing boundaries; one over far eastern Indiana
and one along the I-69 corridor.

Overall shear has been supportive of updraft development, but
greatest effective shear has remained over southern Indiana and KY.
This could move northward with time though as a dry slot has been
noted via visible satellite imagery, of which could help deepen the
instability layer allowing for storms to tap into greater shear
aloft. 0-1 km shear has slowly been increasing as well, as the
surface low deepens increase low level backing of the winds. This
low level shear has a greater concentration over N/E IN and OH.

All of this said, central Indiana still looks to remain in a
marginal threat for isolated downbursts within organized updrafts,
with a slightly greater threat east of central Indiana. If the
greater deep shear is able to match up with the low level shear,
an increased threat for isolated tornadoes will be possible, but
this looks to be more likely over portions of OH. After the line,
currently over the I-69 corridor, passes, the severe threat
should also end (around 7-8 PM EDT).

&&

.SHORT TERM (This evening through Wednesday)...
Issued at 224 PM EDT Tue Sep 24 2024

Synopsis:

Surface analysis early this afternoon shows low pressure in place
over northwest IN. This was resulting in cyclonic lower level flow
across Central Indiana. Our forecast area resided within the warm
sector with south to southeast surface winds across the area. A weak
surface trough axis was found pivoting around the low over western
Central Indiana. Radar this afternoon shows the development of
instability convective showers and storms across Central Indiana.
Dew points remained moist, in the mid 60s to near 70. Aloft, a deep
upper trough was in place over the northern plains, extending south
to the southern plains states. This was resulting in southwest flow
over Central Indiana aloft.

Overnight -

Fog will be expected to develop overnight. Light winds are expected
to arrive, and lower level  residual moisture will remain present.
Dew point depressions overnight are expected to fall to 0-2F.
Furthermore, there is a lack of an arrival of dry air as dew points
only fall to around 60 overnight. Thus fog development is expected,
particularly away from the urban heat island of Indianapolis. Expect
overnight lows to fall to the upper 50s to lower 60s.

Wednesday -

Forecast soundings suggest good subsidence an drying within the mid
levels on Wednesday, however they do suggest to saturation during
the afternoon hours as convective temps are reached. Meanwhile
aloft, the upper trough over the plains is expected to push east
toward the Ohio valley and surface high pressure tries to build over
Indiana. All of this could lead to some shallow rain shower
development and CU development. Confidence for partly to mostly
cloudy skies due to CU development is high, however, confidence for
rain showers is low and any precip should remain rather light.  HRRR
hints at a few showers across the area, but mainly to the northwest,
perhaps lake enhanced, coming off Lake Michigan. Thus will include
slight chances for afternoon shower across much of the area.
Confidence in this is quite low. Highs on Wednesday should reach the
middle to upper 70s.

&&

.LONG TERM (Wednesday night through Tuesday)...
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Issued at 224 PM EDT Tue Sep 24 2024

Synoptic Pattern:

Currently, central Indiana is positioned upstream of an amplified
trough, with an associated week low pressure system. This will
allow for marginal baroclinicity in the low levels, and a weak push
of cooler air following its departure.

In the upper levels, the ridge over the Inter-Mountain West will
amplify, creating strong AVA and height rises over the Great Lakes
region. In return, the aforementioned low will become "cut-off" from
the polar jet and become stationary over the Ozarks/Mid-Mississippi
Valley. This should place central Indiana in a rather mundane, cool
pattern through mid-week, with the majority of the forcing from the
cut off low staying south/west of central Indiana.

The pattern will begin to shift late week as a tropical system
plunges into the CONUS. With the cutoff low over AR/MO, and a high
over the Mid-Atlantic, steering flow should push the warm-core low
north into the Tennessee Valley late Thursday, where it will begin
extra-tropical transition. As this process begins, it will also
interact with the cut-off low, creating a Fujiwara effect as the
vort maxes collide. This should push the remnants of the tropical
low westward into the Ohio Valley. There is still a lot of
uncertainty on the impact of the antecedent high pressure over the
Great Lakes and strength of the tropical low leading to low
confidence in the overall impacts to central Indiana.

Temperature/Cloud Cover/Wind:

With the low becoming cutoff, very little air mass movement will
take place mid week, keeping 850mb temperatures essentially
unchanged. This, along with mostly cloudy skies will lead to
deamplified diurnal swings with highs in the low to mid 70s and lows
in the upper 50s to low 60s.

As the tropical low nears later in the week, dew points and 850mb
temperatures are expected to increase, but surface temperatures will
likely stay in the 70s with continued cloud cover and increasing
rain chances. Pockets of 80s cannot be ruled out, but most of the
region should stay in the 70s through Sunday.

A tightening pressure gradient on the northward portion of the
tropical low should lead to greater overall winds over the Ohio
Valley Friday and potentially Saturday. Current expectation is for
sustained between 15-20mph with gusts upwards of 30mph, but this
could change depending on the tropical low`s strength and
location.

Precipitation:

As mentioned, central Indiana will be positioned in the threshold
between low pressure to the SW and high pressure to the north. This
should limit moisture transport northward and keep central Indiana
mostly dry through Thursday. Still, with  the low positioned close
enough to the Ohio Valley, a few stray/isolated light showers cannot
be ruled out.

This changes for Friday onward as the tropical low nears from the
S/SE. There is still a lot of uncertainty on timing and amounts, but
trends are increasing overall chances for rainfall Friday and
Saturday. Precipitation chances continue through Monday of next week
with generally low pressure remaining over the region, but
uncertainty is too high Sunday onward for confidence in rain
occurring.
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&&

.AVIATION (00Z TAF Issuance)...
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Issued at 641 PM EDT Tue Sep 24 2024

Impacts:

- Ceilings dropping to MVFR tonight and possibly into IFR early
Wednesday morning
- Fog possible overnight through Wednesday morning
- VFR ceilings returning by Wednesday afternoon


Discussion:

A cold front has passed to the east of all of the terminals early
this evening with most of the lingering showers moving away and
diminishing. Drier air will briefly filter in through the evening
but expect low clouds to settle back in as deep moisture lingers
within the boundary layer through the night. The combination of
light W/NW flow and the low level moisture also supports the
development of fog overnight through daybreak Wednesday but
confidence is lower in just how widespread and low visibilities may
get. Keeping visibilities mainly in the 2-4SM range predawn through
the first half of the morning Wednesday.

Weak high pressure builds into the area Wednesday but low level
moisture will persist with cooler air aloft. Expect ceilings to lift
to a decent VFR cu field for the afternoon. Northwest winds will be
less than 10kts on Wednesday.
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&&

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

&&

$$

SHORT TERM...Puma
LONG TERM...Updike
AVIATION...Ryan