773
FXUS63 KIND 041749
AFDIND

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

.KEY MESSAGES...

- Thunderstrom complex expected to move through from mid morning
  to early afternoon. Isolateds damaging winds possible

- Expected lull in between rounds of storms during the mid afternoon
  and evening for 4th of July

- Dry conditions and less humid late Friday through Sunday

&&

.FORECAST UPDATE...

Lower confidence forecast today as an MCV to the west and leftover
boundaries from morning convection will greatly influence the
evolution of the local weather pattern over the next 6 to 18 hours.

Current analysis and observations show an MCV near Kansas city with
a quasi-warm frontal feature/low level temperature gradient
stretching eastward into South Central Indiana. A complex of
elevated showers and storms developed earlier this morning along
this boundary on the nose of a 30-40kt easterly LLJ. Convection has
already pushed into much of Central Indiana and will continue moving
east-northeast over the next several hours. Both SDF and IND ACARS
soundings do show that this convective activity is still slightly
elevated at this hour with MUCAPE values nearing 1000j/kg. Soundings
show deep saturation through the column, PW values near or exceeding
2.00", high freezing levels and surface dew points well into the
70s, and 0-3km and 0-6km shear in the 25-35 kt range, resulting in
an environment conducive for warm rain processes, heavy rain, and
continued convective activity through the rest of the morning and
afternoon hours. Not expecting any severe weather with this round of
precipitation as most of the convection is elevated; however if
enough surface heating does take place on the southern extent of
this convective complex within Southern Indiana, a few strong wind
gusts could reach the ground as storms become surface based in that
region. Main threat through early afternoon will be mainly lightning
and localized flooding in areas that see repeated rounds of heavy
rainfall.

With the MCV still well to the west and the back edge of the
convective complex and thicker cloud cover pushing eastward through
Central Illinois, warm sector destabilization may take place across
Central Indiana later this afternoon and evening. Confidence is
lower as even hi-res short term guidance does not handle mesoscale
MCVs and boundaries very well. However, with an MCV present and
moving eastward as well, the potential is there for afternoon and
evening convection to develop across Central Indiana as this region
will be within the warm sector of this mesoscale low with the quasi-
warm front draped across the region. Bulk shear values in the 35-50
kt range in addition to a destabilizing, very humid, tropical
airmass may lead to an environment capable of producing severe
storms. While lower level flow is weaker, there may be enough shear
present to support organized convection and a few supercells. While
the threat is low, a tornado or two cannot be ruled out, especially
as the quasi-warm front is in the region. Main threats will be
lightning, torrential rainfall, and damaging winds. Will be watching
how everything evolves through the day and continue to update the
forecast as necessary. Keeping 30-40 PoPs this afternoon and evening
to account for the thunderstorm threat.

&&

.SHORT TERM (Today and tonight)...
Issued at 227 AM EDT Thu Jul 4 2024

This Morning and Early Afternoon...

06Z IR satellite imagery is beginning to show the first glimpse of
the development on a nocturnal MCS over eastern KS into western MO.
This will continue to strengthen as it moves eastward supported by
diabatic pressure drops and an anomalously moist surface layer. This
MCS will progress relatively quickly, but will still take another 8-
10 hours before it reaches central Indiana tomorrow morning. Prior
to its arrival, upper level cloud cover will help mitigate some
diurnal cooling keeping the nocturnal inversion shallow.

As this system approaches from the west, deepening low pressure in
the MCSs wake will help increase southerly low level flow,
destabilizing the shallow CIN quickly. This increase in low level
flow (30-40kts of EFF Bulk Shear) along with elevated surface
moisture (72-75 dew points) will also aid in convective
organization, likely keeping the MCS fairly strong even in a non-
climatologically prone time period. There is still some uncertainty
on the specifics of where the MCS will pass through, but generally,
the axis of precipitation should begin somewhere between Terre Haute
and Evansville as it enters Indiana.

As mentioned, the shear and elevated moisture should be able to keep
the MCS organized, with any areas directly in its path under a
possible severe threat (Terre Haute to Evansville and locations
eastward). There is some potential for the severe threat to dive
south along the LLJ axis, with central Indiana left in the weaker
upshear component. If the MCS remains organized over central
Indiana, the main threat will be damaging winds within wet
microbursts, but an isolated QLCS tornado can not be ruled out due
to the increased near surface southerly flow (0-1km SRH 80-120
m2/s2). Worst case scenario would be a more widespread wind threat
along the UDCZ in the event surface temperatures and near surface
lapse rates are able to increase rapidly prior to MCS arrival.

Warm frontal like processes along the differential heating boundary
with a modest SW LLJ should also allow for a greater precip shield
north of the MCS, providing 0.5-1.0" of much need rain along and
north of the I-70 corridor. The MCS should exit central Indiana by
around 18Z +/- an hour.

One non-precipitation item of note, temperatures pre-MCS arrival
over far southern central Indiana may be able to get into the upper
80s late this morning into the early afternoon. If this does occur,
our daytime high will occur earlier than typical.

Late Afternoon through Evening...

Following the MCS passage, local pressure gains should induce subtle
ridging in the low to mid levels during the afternoon and evening
hours ahead of an approaching low over the Upper Midwest. With this
in mind, despite a frontal boundary near the Ohio River, mid level
drying and subsidence may work to inhibit convective development
during the latter half of the day tomorrow. Still, an isolated
shower or thunderstorm cannot be ruled out, if an updraft is able to
withstand the drying mid levels.

Generally, skies should remain cloudy over much of central Indiana
through the mid afternoon. As the mid levels begin to dry, a few
pockets of sunshine are possible in the early evening hours. Given
greater certainty of cloudy conditions, afternoon temperatures will
be kept in the low 80s throughout central Indiana, but areas with
greater evening sunshine may be able to reach the mid 80s.

Tonight...

After dusk, any remaining isolated showers and thunderstorm should
begin to wain as diurnal cooling related CIN takes over. This should
keep conditions fairly dry through the early overnight hours until a
short wave and associated low approaches from the west. Another
convective system is likely to develop in the warm sector of the
short wave this evening over the Missouri Valley, of which will be
the primary threat for rain over central Indiana late tonight. HREF
members are still widely varied on strength and location of this
MCS, leading to high uncertainty in the thunderstorm forecast for
central Indiana as of this forecast update.

&&

.LONG TERM (Friday through Wednesday)...
Issued at 227 AM EDT Thu Jul 4 2024

On Friday, stronger synoptic-scale forcing arrives as closed
midlevel low passes to our north and DCVA progresses across Indiana.
Lower tropospheric warm/moist advection will occur early in the
morning until the cold front arrives midday to early afternoon. A
couple scenarios are possible. 1. the warm advection regime
continues to feed convection that is ongoing to our south from the
previous diurnal cycle, tempering new development further north at
least somewhat. 2. an upstream convective cluster evolves and moves
into Indiana during the morning and may result in greater rain
coverage further north into central Indiana than scenario #1. Still,
the northward extent will be limited somewhat by model indicated
instability, which tapers with northward extent. Frontal convection
during the early afternoon should be across the eastern portion of
the area given currently modeled frontal timing. A post-MCS airmass
would be more overturned and less conducive than a scenario where
less convection is present during the morning. Refinements to the
forecast will occur later today as the mesoscale details become more
clear.

Cooler (around 5F negative anomalies) and slightly less humid
conditions are expected Saturday in the wake of the front. Midlevel
height rises and strengthening southwesterly flow ahead of the next
shortwave trough should bring temperatures back to near climatology
on Sunday, but the subtropical moisture band isn`t particularly
pronounced or anomalous. Forcing and moisture become sufficient for
some precipitation by Monday, but even then the warm conveyor belt
is somewhat fractured from deep subtropical moisture connection.
It`s possible models are under-representing moisture given Beryl`s
remnants at lower latitudes, augmenting trough strengthen and
moisture which models can struggle with fully capturing latent heat
release. So, if the forecast early next week trends, it may be
toward a wetter scenario.

Model spread grows a little into the middle part of next week, but
the general pattern favors very modest air mass change in the wake
of the shortwave trough. Deep moisture will be shunted away so rain
chances will be low, even with one or more perturbations rotating
through the broader trough. Temperatures should be near or just
below early July climatology which is max 85 and min 66 at IND.

&&

.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.
&&

$$

UPDATE...CM
SHORT TERM...Updike
LONG TERM...BRB
AVIATION...CM