706
FXUS63 KIND 211743
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
143 PM EDT Tue May 21 2024

.KEY MESSAGES...

- One more unseasonably warm day with SSW breezes up to 20-30 mph
this afternoon and evening

- Strong storms with a conditional severe risk tonight and again
  Wednesday afternoon and evening.

- Heavy rain possible Wednesday afternoon and night.

- Additional chances for rain and storms through the weekend.

&&

.FORECAST UPDATE...
Issued at 950 AM EDT Tue May 21 2024

The latest forecast is on track as the hottest temperatures of the
season so far are expected this afternoon.

Latest surface analysis shows a 995mb low over SE Nebraska placing
Central Indiana firmly within its warm sector in a sub-tropical,
unstable, humid airmass. IND ACARs soundings still indicates a
weakening low level inversion at the surface; however once this
breaks, steep lapse rates up around 800mb/2km agl will likely result
in deep mixing bringing drier air and gusty winds aloft down to the
surface. RAP soundings are initializing well this morning compared to
recent local ACARs soundings showing the deeper mixing. An
environment like this will support highs in the upper 80s this
afternoon, with a few locations potentially pushing 90 degrees! With
dew points expected to remain in the lower 60s, feels like temps
should not exceed 90 degrees; however for the first very hot day of
the year, sensitive and vulnerable populations may need to take
extra precautions with the heat. NWS Heat Risk tool highlights the
majority of the region in a moderate risk for heat related impacts;
further confirming the need for safety precautions for sensitive
groups and those working outside for extended periods of time.

Aside from the heat, once the inversion breaks, expect southerly
winds to increase to 10-15 mph with gusts upwards of 20-25 mph at
times during peak heating of the day.

&&

.SHORT TERM (Today and tonight)...
Issued at 309 AM EDT Tue May 21 2024

Today...

The third and final day of very warm, summer-like temperatures will
be felt across central Indiana today as the region basks in the
western, warm-advective portion of a broad, amplified subtropical
ridge...which will actually continue to build slightly ahead of
deepening surface low pressure lifting from Kansas to Minnesota.
Increasing winds amid the gradient between the broad surface ridge
centered near the Carolinas and the deepening system to our west
will be the main story today, especially for locations in/near the
Wabash Valley which will be closest to the cyclogenesis and tighter
portions of the gradient.  Light southerly winds early today will
increase through the morning while veering slightly to south-
southwest...with maximum flow on display this afternoon with
sustained winds around 10 to 20 mph and gusts as high as 15 to 30
mph.

Ample sunshine below a bit of high cloud crossing the upper ridge
will be found between FEW/SCT afternoon cumulus.  Humidity will more
closely match expectations for mid-summer than late spring with the
brisk flow delivering Gulf moisture and dewpoints seeing a further
bump up into the mid-60s over many locations.  Can`t rule out a
stray shower or two with a very subtle warm-frontal type trough
extending out southeastward from the passing low to our west,
although further ridging aloft will deter any organization of rain
showers.  Temperatures starting out near 65-68F will reach the low
80s by noon...and amid the well-mixed boundary layer top out by late
day 85-89F.  The record high at Indianapolis is 92 (1941).

Tonight...

Overnight will see a transition from the early week`s very warm and
mainly dry pattern to the beginning of convective potential as
forcing form the central CONUS storm system, or more namely its
trailing cold front, approach and begin to cross the CWA.  Robust
breezes will continue after sundown through the evening as the
gradient following the 988 mb low drags northward across the western
Midwest into the southern Great Lakes.  Precipitable water values
that had been around a very moist 1.25 inches Tuesday will get a
further boost to 1.50-1.75 inches, as western Gulf flow along the
approaching boundary is focused into the Wabash Valley.  CAMs are
continuing to show somewhat disorganized showers and probably a few
strong t-storms cross Illinois through the evening...arriving in
western counties around midnight.

Confidence in the organization of any cells is low with areas of
stronger wind shear likely patchy if not temporary.  Yet, 500-1000
J/kg of nocturnal CAPE and at times decent mid-level lapse rates
should help to sustain some storms, especially along the Upper
Wabash Valley where convection should arrive when better instability
is still lingering following the warm day.  Rain chances should
shift to south/east counties during pre-dawn hours...ahead of what
should be a slowly-crossing cold front somewhere west of the I-69
corridor.  Isolated flood potential may exist under strongest storms
given solid deep moisture and relatively slow storm motions.
Rainfall expectations are mainly under an inch with greater totals
under heaviest downpours. Clouds and SSW surface winds sustained
around 10-15 mph will keep readings up overnight, with lows in the
upper 60s to around 70F. Indianapolis` record high minimum of 70F
(1884, 2004) may be in jeopardy, with mild conditions perhaps
lasting through Wednesday evening.

&&

.LONG TERM (Wednesday through Monday)...
Issued at 309 AM EDT Tue May 21 2024

Into Early Afternoon Wednesday...

The initial line of convection should be east of the forecast area
by 12Z Wednesday. In its wake, clouds will cover the area, and
soundings indicate a good deal of low level clouds could be present.

The actual cold front will slowly work its way into the northwest
area during this period, ending the threat of significant convection
behind it. The front won`t have much support with it during this
time, as the surface low and main upper energy will be well
northwest of Indiana.

Clouds will likely keep instability down, so not expecting much if
any convection into mid-afternoon. Will keep PoPs low. With limited
instability, severe threat will be low to non-existent.

Mid-afternoon Wednesday into Wednesday night...

The lower clouds will break up some, allowing instability to build.
A lot of uncertainty remains on just how much of the clouds will
break and thus how much instability is available. There will be some
shear around, but nothing impressive. The cold front will continue
to slowly work southeast, and during the mid to late afternoon this
will be the primary source of forcing.

Thus, expect scattered convection to develop mid to late afternoon
Wednesday, with the convection confined mainly to the southeastern
half of the forecast area ahead of the front. If plentiful
instability can build, severe storms will be possible with damaging
winds and large hail the threats.

Wednesday night, an upper level wave/MCV will bring additional
forcing. This will interact with the front (now across the
southeastern forecast area). Severe storms will remain possible into
early evening, again mainly southeast and if good instability is
present. Heavy rain and flooding will also be possible as the front
slows and becomes nearly parallel to the upper flow, setting up the
possibility of training convection.

Farther northwest, the upper wave will provide lift over the surface
front and will bring elevated convection to the area.

Will go likely or higher PoPs most areas many areas Wed evening,
shifting southeast overnight Wed night.

Thursday into Monday...

The upper wave and stalled surface front will keep showers and
storms around the south on Thursday morning with lower chances
northwest farther from forcing. Lower instability should keep severe
weather at bay, but locally heavy rain remains possible.

Another upper wave will move in the southwest flow into the area on
Friday, bringing more showers and thunderstorms, especially across
the southern two-thirds of the area. Timing remains a bit uncertain
so will have to watch and refine PoPs.

Yet another upper wave will bring more rain around Sunday. However,
timing and specific path of these upper waves at this far out in the
forecast remains uncertain, and the period and location of highest
PoPs may shift. For now will stick with the model blend of likely
category PoPs on Sunday with low confidence.

PoPs will linger into Monday as the system exits and with the
uncertainty in timing of the system.

Tuesday and beyond...

Models are trending toward a cooler and damp pattern with perhaps an
upper low moving through around Tuesday.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 143 PM EDT Tue May 21 2024

Impacts:

- Southerly wind gusts around 18-26 kts through this evening
- Showers and a few storms possible 04z-08z
- MVFR cigs possible 05z-16z tomorrow


Discussion:

Mainly VFR conditions will prevail through this evening as central
Indiana sits under the tightening gradient between broad high
pressure well to the east and a surface low deepening from Kansas to
southwestern Ontario.

Main impacts this afternoon and evening are gusty SSW winds up to 25
kts at all TAF sites. Expect gusts to diminish around and after
sunset; however sustained winds will likely remain elevated
overnight in the 10-15 kt range.

An approaching, weakening line of storms will enter Indiana from the
west around 04z and track eastward through the state 04z-08z.
Potential for a brief period of lightning with convection along the
leading edge of the line. Highest confidence at the moment of seeing
thunder and lightning is at KLAF and KHUF. Lower confidence in
storms maintaining strength to produce lightning at KIND and KBMG;
however still mentioning VCTS in those TAFs in the 05-08z timeframe.
Brief periods of lightning, heavy rain, and erratic wind directions
and speeds possible with the line of storms. Expect MVFR cigs to
develop on the backside of the line through the early morning hours
before daytime heating helps the cloud deck to lift after 16-18z
tomorrow.

&&

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

$$

UPDATE...CM
SHORT TERM...AGM
LONG TERM...50
AVIATION...CM