966
FXUS63 KIND 241656
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1256 PM EDT Fri May 24 2024

.KEY MESSAGES...

- Isolated thunderstorms possible this afternoon with more
  widespread coverage this evening/overnight. A few storms may be
  severe with strong gusty winds.

- Showers and thunderstorms likely on Sunday. Severe thunderstorms
  are possible.

- Temperatures will return to near normal next week

&&

.FORECAST UPDATE...

No major changes to the forecast with conditions generally evolving
as expected.  As of mid-morning, our main focus areas continue to be
the gradually weakening line of convection as it crosses into
northern Illinois.  There are active warnings along the line, but
for the last 30 minutes, the number of severe reports continues to
lessen which lines up with the thoughts that as the line continues
to push to the east and exit the better shear associated with the
parent low pressure system it would weaken.  By the the time it
arrives in north central Indiana byre is a well established but
fairly weak cold pool which will help it to sustain through the late
morning hours, but think it will not survive long enough to impact
the forecast area this afternoon.

There is good model agreement that any thunderstorms that do arrive
would be towards the later afternoon and evening hours as convection
fires along the residual outflow boundary more into Missouri and
central Illinois which again would likely weaken as it moves out of
the more favorable environment.

The other potential concern for this evening/early overnight is a
residual MCV from yesterday`s convection across the Southern Plains
as it pushes further into Arkansas and Tennessee and gets ingested
into the LLJ and turns more northerly.  The NAM has a good handle on
this which brings convection into southern/southeastern Indiana by
around 5-7PM.  The tornado threat that often is enhanced by an MCV
doesn`t look to be the case this time around with observations not
showing a stronger easterly wind component compared to what would
normally be seen.

With both of these systems, the effective shear is marginal at best
with only 10-20kts of shear by the evening hours.  Damaging wind
gusts remain the most likely threat if either of these two systems
develop a more robust cold pool but that would be on the higher end
of the expected outcome and more likely to occur with the convection
moving in from the west compared to the MCV.

&&

.SHORT TERM (Today and tonight)...
Issued at 305 AM EDT Fri May 24 2024

Mostly quiet weather conditions are expected for much of the day
with ridging centered over the Ohio Valley. At the surface, weak
high pressure is aiding in optimal conditions for fog development
with mostly clear skies and light winds. Look for any patchy fog to
quickly mix out after sunrise. Increasing southerly flow ahead of an
approaching system will lead to warmer temperatures in the mid
80s.

While much of the day should remain quiet, increasing moisture could
support isolated diurnal thunderstorms across W/SW portions of
central IN. Forecast soundings show weak deep-layer shear in the
afternoon, thus severe weather is not expected. More widespread
convection is likely during the evening/overnight hours as most
CAMs show an MCS across eastern Nebraska propagating towards the
Great Lakes Region. The main portion of this MCS may stay to the
north of the area, but low-level convergence along the front will
be sufficient to allow for scattered thunderstorm development in
a very unstable airmass across IL. New convective development
should then grow upscale as it progresses eastward.

CAMs are still not in great agreement regarding timing of the storm
complex moving into the area. In addition, there remains uncertainty
in convective organization. The main reason for this being that
effective bulk shear will be relatively weak as the line of storms
outpaces the stronger mid-level flow over IL/WI. However, the
favorable thermodynamic environment combined with weak but
sufficient deep-layer shear for loosely organized storms supports an
isolated damaging wind gust threat. Locally heavy rainfall is also
possible. The convective evolution of this system across IL will be
monitored closely later today as there is still a chance for a more
organized MCS to develop.

Given the latest trends in CAMs having a weaker MCS/cold pool, the
timing of best rain chances appears to be this late this evening
into tonight. A stronger cold pool would lead to a sooner arrival of
precipitation so this may need to be updated in future updates.
Expect temperatures generally in the upper 50s to low 60s overnight.

&&

.LONG TERM (Saturday through Thursday)...
Issued at 305 AM EDT Fri May 24 2024

The main concern for the long term will be the potential for a
severe weather outbreak including all severe weather types and heavy
rain and localized flooding Sunday and Sunday night.

Saturday through Sunday night...

The long term will start off less ominous with a cold front
sweeping east into eastern Indiana by daybreak Saturday. Behind this
front, ridging will build in late in the day and night which will
bring a brief respite to the convection over western sections and
all of central Indiana Saturday afternoon and into the night.

However, a Northern Plains trough will dig southeast and spin up a
surface low that will lift northeast across the Missouri Valley
Sunday and into the western Great Lakes Sunday night. An associated
warm front will lift northeast across central Indiana Sunday morning
and a cold front will then sweep east across the area Sunday night.
Upstream storms, potentially in the form of an MCS will likely move
through Sunday morning near the warm front and then re-development
will likely occur Sunday afternoon and night in the warm sector,
along and ahead of the quickly advancing cold front. Exact timing is
still too difficult to pinpoint all the way out to Sunday but what
is known is the combination of a potent mid to late Spring System,
potentially very unstable atmosphere along with strong 0-3km shear
present a clear severe weather outbreak possibility Sunday and Sunday
night. Model soundings are suggesting sfc based CAPEs in excess of
2000 J/kg and 0-3km Bulk Shear of 40-50 knots along with long curved
hodographs and 0-3km SRH in excess of 500 m2/s2 by Sunday afternoon.
Any discrete storm, either early near the warm front or later in the
warm sector, ahead of the cold front, could potentially develop
rapidly and rotate. Upper level winds will also steer these storms
along rather quickly to near 40 mph.

In addition to severe weather, heavy rain and localized flooding
will also be a concern as PWATs could be as high as 1.8 inches,
which is the max moving average for late May for the nearby ILN
upper air site.

Went with 30 to 50 percent morning PoPs reflecting the possibility
some of the slower end models verify and also with uncertainty if an
MCS will in fact be fully in tact and then 70 to 90 percent PoPs for
the afternoon and night when instability will be increasing and
later the cold front will be getting closer.

The southerly low level flow will also result in temperatures some 5
to 10 degrees above normal with highs in the lower to middle 80s
looking reasonable.

Memorial Day through Tuesday...

A pair of upper waves will drop down from the northwest Memorial Day
and Tuesday. Both of these will have enough moisture and instability
to work with to be able to generate at least scattered convection,
especially during the peak heating afternoon hours. Temperatures
will likely be closer to normal in the wake of the Sunday night cold
frontal passage which translates to afternoon highs mostly in the
70s.

Tuesday night through Thursday night...

Look for mostly dry and seasonable weather the remainder of the long
term as ridging builds in overhead.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 1256 PM EDT Fri May 24 2024

Impacts:

-MVFR to IFR cigs 09Z to 14Z
-MVFR to IFR vsbys during showers/storms
-Best chance for TSRA from 22Z to 03Z

Discussion:

Convection is likely across much of central Indiana from the 22Z to
03Z timeframe with low confidence on the exact timing.  MVFR to
briefly IFR vsbys expected during periods of heavier rain but
remaining VFR outside of those short periods. Cigs will gradually
drop to MVFR during the late overnight hours with highest confidence
from 09Z to 14Z. Winds will occasionally gust to 20kts this
afternoon but otherwise will be light and variable at 4-8kts.

&&

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

$$

UPDATE...White
SHORT TERM...Melo
LONG TERM...MK
AVIATION...White