-- Discussion containing changed information from previous version are highlighted. --
597
FXUS63 KIND 201712
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
112 PM EDT Thu Jun 20 2024

.KEY MESSAGES...

- Hot and humid conditions persists into the weekend
- Next chance for storms arrives Sunday morning
- Heating back up into the 90s next week

&&

.FORECAST UPDATE...
Issued at 945 AM EDT Thu Jun 20 2024

No significant changes required to going forecast this morning.

A bit of high cloud is present across portions of central Indiana
this morning, but this should not be enough to significantly hamper
insolation, even with a bit of fair weather cumulus later today.
While winds will be light throughout the troposphere under the
influence of the ridge, a thermally-driven, modestly-mixed boundary
layer should develop underneath the dry, subsident layer aloft,
helping to reduce afternoon dewpoints slightly, though this will
improve afternoon heat index values only modestly over what they
would have otherwise been, and the lighter winds will make the heat
even more stifling.

&&

.SHORT TERM (Today and tonight)...
Issued at 301 AM EDT Thu Jun 20 2024

Hot and humid conditions continue today but with little to no
afternoon storm chances. Upper and lower level ridging retrograde
westward becoming centered over the Ohio Valley by this evening.
500mb heights reach 597dm locally, indicating an anomalously strong
ridge and warm atmospheric profile, even for this time of year.
Increasing subsidence directly under the ridge in addition to dry
air advection in the mid levels should keep any convective activity
at bay today, with the main area for storms located on the periphery
of the ridge north and well west of the region. Latest IND ACARS
sounding confirms the mid level dry air advection in the 850-500mb
layer. Potential for deep mixing into this dry layer may result in
drier air mixing down to the surface this afternoon, lowering dew
points from the low 70s to mid to upper 60s. While this isn`t much,
it will feel slightly less humid today with max heat index values
under 100 degrees, in the mid to upper 90s. Less cloud cover will
result in better boundary layer heating with highs maxing out in the
91-94 degree range. Conditions continue to remain under advisory or
warning levels for Central Indiana; however this type of heat still
may be hazardous to more sensitive and vulnerable groups.

Daily nightly lows continue to be anomalously warm as well, despite
a good radiational cooling set up, with some locations failing to
drop below 75 degrees. Expect lows tonight to slowly fall into the
low to mid 70s region-wide, with urban areas remaining the warmest.

&&

.LONG TERM (Friday through Wednesday)...
Issued at 301 AM EDT Thu Jun 20 2024

The primary focus remains on the hot temperatures through Saturday
as an upper ridge centers over the Ohio and Tennessee Valleys. The
ridge will buckle south this weekend as a strong upper level
stronger upper wave set to track through the northern Plains
Saturday and eventually across the Great Lakes on Sunday. The
passage of the upper wave in tandem with a surface cold front on
Sunday will likely serve as the best opportunity for rain and storms
across central Indiana through the first part of next week.

Friday through Sunday

The core of the ridge will be positioned over the Ohio Valley to
start Friday shifting south into the weekend as the band of
westerlies aloft dips into the area. Both Friday and Saturday will
be hot but for slightly different reasons. Friday will be the day
with diurnal cu especially minimized by broad subsidence under the
highs in the mid upper ridge. By Saturday as heights fall back on
the north side of the flattening ridge with a broader cu field
expected across the region and potentially even a subtle chance for
a few isolated cells developing with less subsidence and dry air
present in the mid and upper levels versus Friday. While that would
in theory lower daytime highs slightly...the to mid component of
compressional heating ahead of the aforementioned cold front within
a strong southwest flow pattern could be the equalizer to Friday
with respect to temperatures. Ultimately...expect both days to top
out at 93 to 96 with a few spots potentially sneaking into the upper
90s.

Deeper mixing during peak heating is anticipated under the ridge
which will lower dewpoints during the afternoons and evenings
keeping readings mainly in the mid and upper 60s. This should limit
max apparent temperatures to the low 100s 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
continue to produce an elevated risk to vulnerable populations.

Robust convection is expected to develop Saturday afternoon and
evening in the vicinity of the approaching cold front across the
upper Midwest south into the Missouri Valley. The front will likely
work into the forecast area by Sunday morning with the potential for
remnant convection to migrate into central Indiana as well. While
the front presents the best chance to see rain for the forecast
area...model trends with timing of the boundary lends to a lower
confidence in convective coverage over the area with a plausible
scenario being that the convection that develops to our northwest
late Saturday is in a weakening/diminishing state by the time it
arrives Sunday morning but cloud debris lingers across the region
hampering further development later on Sunday. Still a lot to iron
out in the details but considering the above possibilities...will
keep pops scattered at this point Sunday and Sunday night. Highs in
the upper 80s and lower 90s are expected Sunday.

Monday through Wednesday

High pressure will reestablish once the late weekend system with the
ridge attempting to build back into the area from the southwest
through midweek. With the core of the ridge displaced to the
southwest...height rises will not be as extreme which leads to
decreased confidence in the details especially in daily convective
chances potentially returning as early as late Tuesday into
Wednesday with the Ohio Valley sitting on the periphery of the upper
ridge. Highs largely remain in the upper 80s on Monday before a
likely return to the lower 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)...
-- Changed Discussion --
Issued at 112 PM EDT Thu Jun 20 2024

Impacts:

- MVFR fog possible late tonight, primarily at BMG.

Discussion:

Primarily VFR conditions expected throughout the period under the
influence of surface and upper level ridging.

Scattered-broken VFR cumulus based around 4-5KFT can be expected
throughout the afternoon, dissipating with loss of mixing/heating
this evening.

With mostly clear skies and calm winds, and based on latest hourly
guidance and persistence/little change in airmass, expect some
patchy MVFR fog tonight, primarily at BMG, which is one of the more
climatologically fog prone sites. Cannot rule it out entirely at the
other sites, but aforementioned factors suggest BMG has the highest
probability. Any fog will mix out in typical fashion after daybreak.

Winds will be below 10KT throughout the period, generally
southwesterly or westerly, though the ridge will promote variability
at times.
-- End Changed Discussion --

&&

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

&&

$$

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