086
FXUS63 KIND 241840
AFDIND

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

.KEY MESSAGES...

- Showers and a few thunderstorms continue at times through today,
  with a low chance for an isolated strong to severe storm

- 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

&&

.MESOSCALE UPDATE...
Issued at 224 PM EDT Tue Sep 24 2024

Diurnal heating has lead to instability over Central Indiana as SPC
CAPE analysis suggests values of 1000-2000 J/KG across our forecast
area. High surface dew points along with a favorable shear profile
along with continued heating late this afternoon will lead to
continued scattered shower and thunderstorm development,
particularly ahead of the trough axis. HRRR continues to suggest
shower and thunderstorm developing across our area within the warm
sector. Thus shower and isolated thunder chances will continue
across the area until 7-8PM, which coincides with the loss of
daytime heating and the exit of the trough axis with the departing
low pressure system to the north.

&&

.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)...
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 cetnral 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.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 1221 PM EDT Tue Sep 24 2024

Impacts:

- VFR to MVFR Conditions expected this afternoon. MVFR conditions
expected with passing showers and storms.
- VFR this evening transitioning to MVFR/IFR overnight in Fog.

Discussion:

Low pressure over eastern Illinois will push northeast to Michigan
today while an upper trough remains across the region. A warm and
moist air mass across Central Indiana will allow for scattered
showers and storm development this afternoon. However, confidence
for specific timing remains low. Thus a large window of VCTS will be
used through the afternoon hours.

As the low passes to the north this evening, heating will be lost
and instability thunderstorm coverage is expected to decrease as the
lower level flow becomes westerly.

Overnight, weak lower level flow along with high dew points near 60
and low dew point depressions should lead to fog development. MVFR
to IFR fog will be expected during the overnight hours, with the
lowest visibilities expected at BMG/HUF and LAF. Fog will quickly
burn off as heating and mixing resumes after sunrise on Wednesday.


&&

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

$$

MESOSCALE...Puma
SHORT TERM...Puma
LONG TERM...Updike
AVIATION...Puma