623
FXUS63 KIND 042122
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
522 PM EDT Thu Jul 4 2024

.KEY MESSAGES...

- Scattered thunderstorms this evening, a few storms may pose a
damaging wind threat in South Central Indiana

- Drier overnight tonight with patchy fog

- Dry conditions, less humid this weekend...Monday-Thursday
  expected to exhibit seasonable warmth/humidity with a few rain
  opportunities

&&

.MESOSCALE UPDATE...
Issued at 522 PM EDT Thu Jul 4 2024

Have seen a gradual uptick in isolated to scattered convection as
subtle destabilization has occurred over the region and particularly
across the southern half of the forecast area. A broad area of
showers continues to pivot east across northern counties in an
environment lacking substantial instability in the vicinity of a
weak surface wave. Had a few reports of funnel clouds within this
area of showers earlier over central and northeast Illinois but have
not seen anything additional recently.

While the potential for severe is still present for another 3 hours
or so across south central Indiana...overall trends continue to hint
at convection largely remaining below severe levels. While
instability has struggled to increase...the presence of the surface
wave and remnants of an MCV in the region complicates the convective
potential somewhat as both features induce an infusion of low level
directional shear that any storms could tap into. Current ACARS
sounding at KIND shows this nicely with directional shear present
from the surface through 850-900mb.

There remains a zone from south of Terre Haute east through Rush
County and points south where the combination of slightly higher
instability on the northern flank of the gradient interacting with
the increased low level directional shear could enable a few cells
to briefly intensify to severe levels. Damaging winds with any
embedded microburst would be the primary severe weather risk but the
level of low level spin also is suggestive of a non-zero tornado
threat. There are caveats which support the trend for convection to
largely remain subsevere... mainly the presence of a subtle but
lingering warm nose aloft and overall very poor mid level lapse
rates. Along and north of I-70...convection will be weaker but still
expect a steady increase in localized downpours over the next couple
hours and perhaps gusty winds.

Confidence continues to rise that most if not all of the convection
shifting east of the forecast area near or just after 00Z as the
surface wave and remnant MCV move out of the region. This will set
the stage for dry but sultry conditions for outdoor activities
during the second half of the evening.

&&

.SHORT TERM (This evening through Friday)...
Issued at 300 PM EDT Thu Jul 4 2024

Latest satellite and radar imagery show convection associated with
this morning`s complex of showers and storms pushing off to the east
with widespread cloud cover in its wake. Weak low pressure noted on
surface observations continues to push eastward into Central
Illinois and will work its way into North Central Indiana by this
evening. Surface to 850mb thermal gradient exists just south of this
low from around Kansas City through Southern Ohio acting as a quasi-
warm frontal feature. However the low level environment on either
side of the front is very warm and moist. ACARS soundings from IND
and STL indicate stability along and north of this boundary with a
developing "warm nose" around 850mb, keeping the lowest 1 km agl
stable with a marginally unstable and drier environment above it.
This warm nose will likely inhibit substantial convective
development across Central and Northern Indiana over the next few
hours. Thicker cloud cover along and north of the I-70 corridor may
also further enhance the S-N thermal gradient across the area
through the evening hours. Satellite imagery clearly indicates
stability for the northern half of Indiana with a healthy cu field
developing in Southern Illinois and Indiana as the environment
destabilizes. KSDF ACARS soundings paints a very different picture
of the environment along and south of the aforementioned boundary
with little indication of an inversion and steep low level lapse
rates with CAPE approaching 2500-3000 j/kg. This strong thermal and
instability gradient across South Central and Southern Indiana may
be the area to watch for convective development going into the
evening hours. Southeast flow at the surface with westerly flow
aloft, 40-60 kt bulk shear, 30-50kt 0-3km shear, and 100-200 m2/a2
effective helicity may be sufficient for organized convection to
develop this evening with damaging winds, isolated tornadoes, and
flash flooding as main threats. Confidence is low as guidance
typically does not handle situations well that highly depend on
micro to mesoscale features. Best threat for these storms looks to
be along and south of the I-70 corridor; however if areas further
north can destabilize over the next few hours, cannot rule out a
stronger storm there as well. Central and North Central Indiana will
likely remain on the stable side of the boundary, with isolated, non-
severe convection possible, but not likely through this evening.

Tonight...

After dusk, any remaining isolated showers and thunderstorm across
Central Indiana should begin to wain as the atmosphere further
stabilizes. This should keep conditions fairly dry through the early
overnight hours. Guidance does show additional convection developing
along the boundary south of the region; however most of that
precipitation will likely also remain along and south of the Ohio
River. Central Indiana remains in a weird location, north of the
main boundary, but ahead of an approaching low and secondary front
to the north and west. Lower confidence exists in the northward
extent of convection by the early morning hours tomorrow. Lately
guidance has been way overdoing overnight convection and brining it
in too fast. So will go with consistency and keep the overnight
hours drier. Although with such a saturated airmass and a low level
inversion keeping moisture trapped at the surface, would not be
surprised to see patch fog again tonight and towards sunrise
tomorrow. Added patchy fog across the whole region to account for
this chance with the Wabash Valley and lower lying areas and valleys
having the best chance for fog development.

.Tomorrow...

There could be some showers and maybe a thunderstorm ahead of the
approaching low and associated "cold" front tomorrow morning. HREF
members and RAP guidance shows isolated showers and storms at best
during the mid to late morning hours with more widespread convection
developing east of the region later in the day. Expect a drying
trend through the afternoon hours across Central Indiana with peaks
of sun likely and temperatures rising into the low to mid 80s.

&&

.LONG TERM (Friday night through Thursday)...
Issued at 300 PM EDT Thu Jul 4 2024

Friday Night through Sunday...

Broad yet weak surface high pressure will cross from the central
Plains to the Mid-Atlantic this weekend ahead of a slowly-
approaching, pronounced mid-level long wave trough over much of the
central CONUS.  Dry conditions will therefore return to central
Indiana as precipitable water values aloft drop by almost two-thirds
below today`s near-record levels.  West-northwesterly light to
moderate breezes Saturday will bring the coolest day of the next
week, when widespread upper 70s are expected north of the I-74
corridor, perhaps at the cost of some lingering morning cloudiness.
Low to mid-60s minimums will be the rule for both Friday and
Saturday nights amid noticeably lower dewpoints.  Expect more
typical July conditions for Sunday as modest southerly flow returns
mid- to upper 80s and seasonably moderate humidity.

Monday through Thursday...

The next work week should return a touch more warmth and humidity to
continue the overall seasonable conditions from Sunday.  The central
US upper trough will continue to approach during the early week
while weakening and probably focusing more of a weak SW flow into
Indiana.  THis combination of WAA ahead of lower heights should make
for a very warm day Monday ahead of milder readings through the mid-
week.  Probably a few to at times scattered showers around the realm
during Monday-Tuesday while the trough is approaching, especially
Tuesday if a southern vort could phase into the trough axis while
crossing the Midwest.  Rainfall amounts would likely be lighter and
not contribute to further drought improvement for most locations,
but isolated t-storms/downpours would be possible.  A trend to
mainly dry conditions for the Wednesday-Thursday timeframe assuming
the trough pulls across the Midwest on schedule, with isolated
afternoon showers possible.  Since Indianapolis` last 90F+ on June
22, IND has only reported one day above 87F (88F, July 3)...and the
highest temp currently forecasted for the next 7 days is 88F on
Monday, 7/8.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 149 PM EDT Thu Jul 4 2024

Impacts:

- Breaks in clouds through this evening, cigs bouncing between VFR
and MVFR at times

- Scattered convection 19z-00z, but confidence in coverage and
location are low

- Potential for patchy fog and MVFR to IFR conditions late tonight

Discussion:

Low confidence forecast over the next 12 to 24 hours.

Current satellite and radar imagery show morning convection pushing
off to the east with widespread MVFR to low end VFR cigs in its wake
across Central Indiana and Illinois. Cigs will likely bounce around
between categories through the evening hours as a low level
inversion developing may keep moisture and lower clouds trapped near
the surface. Observations indicate a weak meso-low over Central
Illinois that may spark off isolated to scattered convection later
this afternoon and evening. Confidence in this is low as guidance
has a poor handle on the environment currently behind the morning
convection. Latest ACARS soundings from IND and STL do show a
developing low level inversion around 850mb which may inhibit
convection from developing. Will watch this closely as the afternoon
progresses. Best storm threat does appear to be near KBMG and points
southward.

Precip chances drop overnight; however with such a saturated airmass
and light winds, areas of fog may develop. Added fog and lower vis
and cigs to all TAF sites from around 05z-14z tonight. Do not expect
it to be foggy that whole time, but periods of fog and lower vis are
certainly possible with how moist the boundary layer is under the
inversion.

Winds will be light and variable through the period with ESE winds
through the afternoon, becoming light out of the NW this evening and
then potentially going calm tonight.

&&

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

&&

$$

MESOSCALE...Ryan
SHORT TERM...CM
LONG TERM...AGM
AVIATION...CM