929
FXUS63 KIND 261431
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1031 AM EDT Sun May 26 2024

.KEY MESSAGES...

- Thunderstorms likely later this morning through early afternoon
  and again this evening.

- Some storms will be severe with damaging winds the primary threat.

- Low rain chances continue through Tuesday with dry conditions
  expected for mid-late in the work week.

&&

.MESOSCALE UPDATE...
Issued at 1031 AM EDT Sun May 26 2024

Bottom line up front: Line of convection moving into central Indiana
may still strengthen some as it moves through, with gusty winds,
small hail, lightning, and heavy rain the threats.

Not a lot has changed in thinking with this initial line of storms.
Observations in Illinois have shown wind gusts around 35 mph with
some reports of small hail. Steep mid-level lapse rates remain ahead
of the line. Also, some heating continues ahead of the line, albeit
limited thanks to cloud cover, and low level moisture continues to
push north.

Thus, while some strengthening may occur, storms will still
generally produce small hail and perhaps some wind gusts around 40
mph. An isolated stronger storm remains possible.

More stratiform rain with embedded thunder will linger behind the
initial line of storms.

&&

.SHORT TERM (Today and tonight)...
Issued at 259 AM EDT Sun May 26 2024

We will attempt to further refine timing of one, potentially two,
rounds of thunderstorms and explain forecast caveats, uncertainties,
and potential alternate scenarios. These situations are complicated
with mesoscale processes that are difficult to resolve even at this
close time range often dictating the outcome. Current thinking is
round #1 of thunderstorms will arrive around or just before noon and
exit by around 4pm, and round #2 of thunderstorms would arrive
around 8pm and continue until around 1am. See analysis below for
more details.

Phasing of a northern stream shortwave trough with an active
subtropical jet has occurred in the central Plains early this
morning, resulting in extensive convection within a reservoir or
strong instability. This seems to be the origin of the lead
shortwave trough shown in the model PV and vorticity fields arriving
in our region early today. These convectively-induced perturbations
are sensitive to model errors with their parent convection, most
notably when cold pools are not adequately modeled. This explains
some of the variance on timing and magnitude of an MCS or remnant
MCS that is expected to reach Indiana late morning/midday.

There appear to be two possible scenarios with the first round:

Scenario #1. A mature MCS that is able to sustain during the diurnal
minimum given downstream reservoir of at least modest instability.
In this scenario a cold pool-driven forward propagating convective
system would move into Indiana potentially earlier than the
aforementioned window, perhaps as early as mid-morning. An attendant
strong/damaging wind threat would accompany this system, along with
heavy rain and lightning.

Negatives for this scenario to occur include: (a) MCS may outpace
the midlevel wind maxima and stronger shear values resulting in cold
pool dominance and weakening (c) a tendency for upshear propagation
to dominate over downshear propagation resulting in a more
southeastward path, given expected southwesterly oriented low-level
jet.

Positives for this scenario to occur include: (a) models` tendency
to underestimate cold pool strength, especially with convection
spawn in such an impressive thermodynamic environment upstream in
the Plains, (b) magnitude of moisture that is expected to surge
northward later tonight as a result of mass response from the
upstream trough driving higher instability than typical during the
diurnal minima.

Scenario #2. A weaker band of remnant convection driven by
warm/moist advection that could be delayed until the nose of
stronger midlevel jet arrives, possibly not reaching Indiana until
midday/early afternoon. This scenario would not be driven by a
substantial cold pool and convection would likely be rooted above
the surface and not particularly impactful aside for rain and some
lightning threat.

Latest observational data shows that the modified continental air
mass that moved in yesterday wasn`t particularly intrusive into
lower latitudes. Mid-60 to low-70 degree surface dew points are
located near or just south of the Ohio River and will return
northward early this morning in response to strengthening
isallobaric flow in response to the upstream mid-latitude system. In
fact, this may result in some advection fog, albeit likely not
widespread or significant.

A developing MCV is now apparent across northwest Missouri with
extensive trailing convection into northeast Oklahoma. Although this
trailing convection is intense, it doesn`t appear to have formed a
consolidated cold pool yet and a mature cold pool-driven system is
at least a few hours from forming, most likely. We will continue to
monitor trends closely to our west to assess the placement and
magnitude of a forward propagating system in Missouri, or whether
upshear propagation begins to dominate as mentioned above, causing a
more southeastward track into southern Illinois and west Kentucky.

A reasonable worst case scenario is a mature forward propagating MCS
moving into Indiana mid-late morning with an attendant damaging wind
threat. 0-3-km shear vector orientation and magnitude would support
QLCS mesovortices near and north of the apex of any localized
surges, and the low-level thermodynamic environment is sufficient
for a nonzero tornado risk.

The most likely scenario is an MCS moving into Indiana by late
morning and progressing through with strong to locally damaging
winds, but perhaps weakening and progressing slower as shear becomes
weaker.

The CAMs that support the most intense MCS also indicate a fairly
strong wake mesohigh (i.e., a couple recent runs of the HRRR), so
we`ll need to watch for a potential enhancement of gradient winds
for a brief period as the MCS departs if the stronger scenario does
play out.

We are fairly confident in a period of wake mesoscale subsidence
before additional thunderstorms become possible by evening. What is
not currently clear is the duration of any trailing stratiform rain
during the midday into the afternoon. Trailing stratiform rain
becomes more likely when the cold pool dominates and shear magnitude
is relatively weak, and this appears to be a plausible scenario.
Given the difficulty of the models in handling the convective system
to begin with, rain cessation during the afternoon this is one of
the greatest uncertainties for today`s forecast, and we`ll need to
monitor trends and refine precipitation probabilities throughout the
day.

Unlike some mid-summer scenarios where tropospheric flow is weak and
a convectively-overturned air mass can eliminate subsequent
convective development, there should be fairly swift mass movement
with this system. Depending on the degree of overturning of the
upstream environment, a ~40-50-kt low-level jet should be effective
at replenishing moisture and instability by evening across at least
a portion of our area. A second shortwave trough is expected to
approach by then resulting in a period of ascent and convective
development both along its advancing front, and probably also along
differential mixing zones that may be along the edge of remnant cold
pools. Most of this redevelopment should occur to our southwest, and
perhaps into the southwest portion of our forecast area, and then
move into at least the southern portion of our area. This part of
the forecast is also uncertain and will depend on how the first
round of convection plays out. Nonetheless, the parameter space
should become supportive of severe storms with the second round.
Veered low-level flow and relatively straight hodographs with shear
vectors aligned nearly parallel to the advancing front would suggest
cell mergers and quick upscale growth over Illinois, with only a low
chance of supercells ahead of the frontal convection into Indiana.
Large hail and damaging wind would be the threats, and in a
reasonable worst case scenario any cells that can interact with
modifying remnant cold pools would have some potential for
tornadoes. This is most likely southwest of our area. Our area may
be impacted more after peak convective intensity into the mid-late
evening hours, before convection weakens and shifts east by the
predawn hours Monday morning.

Localized flooding is possible where multiple heavy rain-producing
thunderstorms occur. 1-hour flash flood guidance is around 1.5-2.0
inches.

&&

.LONG TERM (Monday through Saturday)...
Issued at 259 AM EDT Sun May 26 2024

Monday through Wednesday...

Broad upper troughing will remain across the eastern CONUS through
the first half of the extended with multiple shortwaves expected to
move across the region. This will keep low rain chances in the
forecast at times. Moisture return should remain limited ahead of
these disturbances as a cold front settles across the southeast so
look for QPF amounts to remain light. Isolated light showers or
drizzle may be ongoing during the first half of the day Monday with
broad cyclonic flow still in place, mainly across northern portions
of the area.

A better chance for precipitation is then expected towards the
evening and overnight hours as an upper level impulse approaches.
Subtle moisture return and daytime heating may provide a narrow
corridor of modest instability for isolated thunderstorms from
central IL to west/southwest portions of central IN during the
evening. Forecast soundings also show sufficient deep-layer shear
which could support a organized storms containing strong sub-severe
wind gusts. Severe weather appears unlikely at this time due to
limited amount of instability.

A second shortwave is expected to provide another opportunity for
precipitation late Tuesday. The best chance for rain will be across
N/NE counties closer to the upper wave where stronger forcing should
be in place. Weak moisture return will keep rainfall amounts light.
A cold front associated with this system will result in a brief
cooldown midweek. Lows are expected to fall well into the 40s
Thursday morning. Ensemble guidance is in good agreement that
surface high pressure begins to build in Wednesday providing quiet
weather conditions.

Thursday onward...

Upper ridging and surface high pressure will likely provide quiet
weather conditions for Thursday and Friday. Surface high pressure
then begins to slide east on Saturday with the weather pattern
expected to become more unsettled as an upper trough moves into the
region. Other than the brief cooldown midweek temperatures are
expected to generally remain near seasonal through the extended.

&&

.AVIATION (12Z TAF Issuance)...
Issued at 619 AM EDT Sun May 26 2024

Impacts:

- First round of storms 15-20z; second round 00-05z

- Southwest winds will gust to 25 knots Sunday; higher near storms

Discussion:

There is still a low probability of brief MVFR ceilings or
visibility early-mid morning as low-level flow strengths and
moisture returns. Otherwise, thunderstorms are possible at any TAF
site late morning/midday and again this evening. Timing is uncertain
and coverage appears limited enough to handle with VC and PROB30 for
now. Gradient wind gusts may peak around 25 knots at times today
from the south-southwest.


&&

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

$$

MESOSCALE...50
SHORT TERM...BRB/Melo
LONG TERM...Melo
AVIATION...BRB