-- Discussion containing changed information from previous version are highlighted. --
310
FXUS63 KIND 190700
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
300 AM EDT Wed Jun 19 2024

.KEY MESSAGES...

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- Extended period of hot and humid conditions through this weekend

- Isolated storms possible this afternoon and evening
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&&

.SHORT TERM (Today and tonight)...
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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.
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&&

.LONG TERM (Thursday through Tuesday)...
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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.
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&&

.AVIATION (06Z TAF Issuance)...
Issued at 139 AM EDT Wed Jun 19 2024

Impacts:

- Isolated showers and thunderstorms possible this afternoon
- Patchy fog possible late tomorrow night

Discussion:

Latest satellite imagery shows a strong area of high pressure over
the Eastern CONUS with a fetch of tropical moisture on the western
edge of this ridge stretching from Texas through Indiana. Strong mid
to upper level moisture advection has kept VFR clouds and isolated
showers around the region over the past 24 hours and will continue
to do so through this afternoon. Ridging will nudge west later today
pushing the plume of tropical moisture westward as well, resulting
in clearing skies by this evening and tomorrow.

Potential is there for isolated showers and thunderstorms again this
afternoon and evening across Central Indiana. Added VCSH after 17z
in the TAFs to account for this. Mainly VFR conditions expected, but
brief MVFR to IFR cigs and vis possible under a rogue storms.
Confidence is too low to get into specific timing and locations of
storms, however this will be watched closely this afternoon.

With recent rainfall over the past few days and a very humid
airmass, potential is there for patchy fog toward the end of the TAF
period tonight. Vis may drop to MVFR levels or lower in a few spots,
with the best chances at KBMG, KHUF, and KLAF. Confidence is low on
this at the moment but still worth mentioning.

Winds will not be a concern through this TAF period remaining under
10 kts out of the south. Winds will go calm at times early this
morning and then again tonight.


&&

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

$$

SHORT TERM...CM
LONG TERM...Ryan
AVIATION...CM