808
FXUS63 KIND 191715
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
115 PM EDT Wed Jun 19 2024

.KEY MESSAGES...

- Extended period of hot and humid conditions through this weekend

- Isolated storms possible this afternoon and evening

&&

.FORECAST UPDATE...
Issued at 1015 AM EDT Wed Jun 19 2024

No significant changes required to going forecast this morning.

Strong tropical moisture transport noted along the western edge of
the ridge on water vapor satellite imagery this morning. This is
also apparent in ACARS soundings, which depict a fairly deeply
saturated column, with the exception of a layer around 600 mb where
a drier layer exists - still, column precipitable water values from
the ACARS data are in excess of 2 inches, which is at or above
climatological daily maximum.

The ridge will subtly build westward today with heights rising
slightly. Despite this increasing subsidence/lack of large scale
forcing for ascent, ample instability will develop with fairly
minimal inhibition today, with isolated-widely scattered diurnal
convection expected, primarily this afternoon into early evening.

The weak flow and drier midlevel air suggests a threat for pulse
severe storms with localized damaging downburst winds the primary
threat with the strongest/deepest convective cores today. Lightning,
heavy downpours, and localized flooding will be threats with any
storm.

The increasing ridge influence should help to thin out cloud cover
and allow some breaks as the day wears on, though uncertainty
remains with respect to timing and extent of the clearing.
Regardless, highs will have no trouble reaching into the upper 80s
at least, with lower 90s likely well within reach - perhaps warmer
if clearing is faster/more significant than expected. This and
dewpoints in the 70s will drive heat index values at least into the
mid to upper 90s, yet another hot and sticky day across central
Indiana. The prolonged heat and humidity will lead to increasing
potential for health impacts with time, and we continue to message
this in all available venues.

&&

.SHORT TERM (Today and tonight)...
Issued at 300 AM EDT Wed Jun 19 2024

After a brief period of heat relief from clouds and showers, hot and
humid weather returns by this afternoon and evening across Indiana.

Latest satellite imagery still shows a healthy fetch of tropical
moisture streaming in on the backside of ridge from the Gulf into
Indiana. Latest IND and SDF ACARs soundings indicate this moisture
advection is confined to the 500-400mb layer with drier air
advecting in through the mid levels. Ridging over the eastern CONUS
will continue to slowly build westward over the Ohio Valley through
the day and into tomorrow, working to further dry out the
atmospheric profile and push the best moisture advection west as
well. While regional soundings do show higher pressure nudging in
already, expect thick higher clouds to persist into the morning
hours then begin thinning out during the latter part of the day as
the upper level moisture fetch becomes disrupted. This will likely
have impacts to max temperatures today, depending on how fast cloud
cover erodes. Clouds should erode within the 17-22z timeframe this
afternoon. Areas that clear out quicker have a much better shot at
hitting 90 degrees while areas under thicker clouds longer will
likely top out in the mid to upper 80s. Lower confidence exists in
which portions of Central Indiana clear out first; however recent
guidance suggests Western portions of the state should begin to see
thinning clouds sooner. Nonetheless, recent rainfall will work to
make the boundary layer extremely humid today with dew points well
into the 70s. Whether temperatures remain in the 80s or hit the
lower 90s, it is going to feel rather oppressive and sticky with heat
indices in the upper 90s to near 100.

Once again, isolated showers and thunderstorms will be possible
today in such a humid and unstable environment. Forcing and shear
are still very weak, so do not expect widespread coverage of
convective activity. But any storm does have the potential to
produce brief periods of heavy rain, gusty winds, and lightning.
Convection will likely diminish after sunset as instability wanes
and high pressure continues build into the region.

Increasing subsidence under the approaching low and mid level ridge
may result in a subsidence inversion developing overnight tonight,
trapping low level moisture near the surface. This combined with a
very moist boundary layer, light winds, and clearing skies may lead
to patchy fog overnight, especially in the Wabash River Valley and
areas that observed storms over the past day or two.

&&

.LONG TERM (Thursday through Tuesday)...
Issued at 300 AM EDT Wed Jun 19 2024

The primary focus for the extended is on the higher heat expected
through much of the weekend as an anomalously strong upper ridge
sets up and and centers over the Mid Atlantic region west into the
upper Ohio Valley with mid level heights pushing 600dm. The ridge
will eventually shift south by late weekend as the band of
westerlies pushes south from Canada aided by a stronger upper wave
set to track through the Great Lakes on Sunday. In its wake...broad
ridging aloft will expand into the Ohio Valley from the southwest
part of the country with a return to hotter temperatures anticipated
by the middle of next week.

Thursday through Sunday

As mentioned above...the hot temperatures remains the primary focus
through much of this period. The tropical moisture plume that has
been anchored across the region since late Monday will already be in
the process of retrograding in response to the westward expansion of
the higher mid level heights. By Thursday with the deeper moisture
now focused to our northwest...expect less cloud cover and a
resumption of the hotter temperatures experienced on Sunday and
Monday. The core of the ridge will retrograde across the region
Friday into early Saturday before shifting south...and expect
hottest daytimes highs for both days primarily in the mid 90s with
the potential for few spots to make a run at the upper 90s.

The arrival of the ridge over the region for late week will promote
deeper mixing during peak heating which will ultimately lead to
lower dewpoints during the afternoons and evenings focused primarily
in the mid and upper 60s. This should limit max apparent
temperatures to around 100 during peak heating as a result. While
that will still be uncomfortable...these heat indices will not be
overly hazardous. That being said...the extended period of heat
combined with overnight lows remaining above 70 will produce an
elevated risk to vulnerable populations.

By late Saturday night into Sunday...the flattening of the ridge
courtesy of the stronger upper wave moving into the upper Midwest
will shift the band of westerlies aloft into the region. A surface
wave will track through the Great Lakes and eventually pull a
weakening cold front into the forecast area by Sunday afternoon.
Overall confidence is low considering most of the deeper forcing
associated with the system will focus to the north of central
Indiana...but the late Saturday night through early Monday period
remains the primary timeframe for more organized convection to
impact parts of the forecast area.

Monday through the Middle of Next Week

High pressure will reestablish once the late weekend system passes
with temperatures trending hotter by mid week as the upper level
ridging returns to the region in some manner. At this stage...it
does not appear that the ridge will be as strong as what is expected
over the next 3-4 days...and that leads to decreased confidence in
the details especially in daily convective chances. There are hints
in the 7-10 day period of the ridge to retreat back south and west
which would open the door for increased risks for rain and
thunderstorms. Highs will slip back briefly into the mid and upper
80s on Monday before a likely return to the lower to mid 90s through
mid week.

One note about the HeatRisk graphics by WPC. This is an experimental
product and values in the current pattern are higher than
traditional criteria, and may not be perfectly calibrated to best
represent the risk at this time. Thus, we advise users to take this
into consideration when viewing the HeatRisk product.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 115 PM EDT Wed Jun 19 2024

Impacts:

- Isolated thunderstorms this afternoon into early evening.

- MVFR or worse fog possible late tonight.

Discussion:

Predominantly VFR conditions expected much of the period under the
influence of a ridge aloft. Transient restrictions will be possible
at times this afternoon into this evening in any isolated
thunderstorms that happen to impact a terminal, and again late
tonight in potential fog.

A hot and humid airmass remains in place across the region, and
towering cumulus is already developing across portions of the area.
The odds of any one TAF site seeing a thunderstorm remain low,
however, and cannot justify an explicit mention at this time - will
continue VCSH mention from earlier package. Transient restrictions
to visibility/ceilings will be possible anywhere a storm impacts.
This threat will wane this evening.

Winds will go light to calm overnight tonight, and the lingering low
level moisture may allow fog to develop. LAMP guidance hints at MVFR
at the outlying sites and possibly at IND - will go TEMPO MVFR at
IND and prevailing MVFR at the outlying sites, with a BCFG mention
at all, as anywhere receiving rain this afternoon could easily see
patchy dense fog develop tonight. This fog should develop beginning
in the 07-09Z time frame and mix out in typical diurnal fashion
after 12-13Z Thursday.

Winds will be below 10KT throughout the period, generally southerly
or southwesterly much of the period, with some variability expected.

&&

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

&&

$$

UPDATE...Nield
SHORT TERM...CM
LONG TERM...Ryan
AVIATION...Nield