126
FXUS63 KIND 201903
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
303 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

&&

.SHORT TERM (This evening through Friday)...
Issued at 303 PM EDT Thu Jun 20 2024

Heat and humidity remain the story through the short term, with
conditions stifling and uncomfortable but remaining well short of
advisory levels. Experimental HeatRisk continues to highlight level
3 and 4 levels, primarily driven by duration of the recent heat and
fairly warm overnight lows, though it remains important to remember
that the product is, in fact, experimental, and may not best
represent the risk profile at this time.

That said, especially given the jump from quite cool to quite warm
conditions, and the duration, there will at least be continued risk
to sensitive and vulnerable populations. We have and will continue
to emphasize and message this threat via multiple venues.

Scattered-broken diurnal cumulus have developed today, which has
slightly slowed warming, though we continue to creep toward or just
past the 90 degree mark at this hour, on our way to the lower 90s
most areas. This cloud cover will dissipate this evening, with
mostly clear conditions expected overnight.

The ridge remains overhead tonight, thus, expect winds to go light
and variable to calm most areas. This should allow for solid
radiational cooling conditions, and have cut blend numbers a few
degrees toward MOS consensus, which tends to handle such situations
more adequately. Some patchy fog may occur late tonight, though
given the fact that little occurred last night in a similar
situation, significant fog appears unlikely aside from some patchy
dense fog in favored terrain areas.

The ridge axis drops southward slightly tomorrow, though it still
appears that peripheral convective activity should remain outside
the area, though some more cumulus should develop. Winds will remain
light, but some continued mixing down of dry air aloft will help to
limit dewpoints a bit. Blend numbers have been running hot most of
the week, and this appears to be the case tomorrow as well. Bias
corrected grids should be catching onto the pattern by now, and the
BC MOS consensus appears to support highs a bit cooler than NBM
tomorrow, and were leaned on significantly. Still, given the lower
dewpoints and solid insolation, low to mid 90s will be in play
tomorrow, with afternoon peak heat index values into the upper 90s
in spots.

&&

.LONG TERM (Friday night through Thursday)...
Issued at 303 PM EDT Thu Jun 20 2024

We continue to focus on heat in the long term period. Although our
traditional Advisory or Warning criteria are not met, there are
other factors that make the heat dangerous, especially to vulnerable
populations (e.g., duration of the heat wave and warm overnight
lows). So, we encourage our partners in media and emergency
management to continue promoting precautions from heat-related
illness. We are currently using the Special Weather Statement to
convey concern over the heat and related impacts. Modest relief will
arrive Monday. See analysis below for more details.

NAEFS and EPS mean 500-mb heights Saturday morning will be close to
climatological maximum for June just to our south. The ridge will be
retrograding at that time and also dampening as a northern stream
shortwave trough approaches Indiana late Saturday. This
reorientation will continue to push more concentrated subtropical
moisture within the mid levels to our north, so it should be a less
cloudy pattern. Also, the approaching shortwave and associated
shifts to the MSLP pattern will result in 10-15 mph winds, helping
to keep wet bulb globe temperature out of the extreme category.
Saturday has the potential to be the warmest day of the heat wave
given veering low-level flow and more favorable trajectories of
advection from semi-arid and hotter air mass to our west. This also
could result in slightly more mixing of dry air downward during the
afternoon, though the PBL in general is still quite moist and this
probably won`t be very noticeable. All this considered, peak heat
index values around 100F are likely Saturday.

Mid-70s for minimums Saturday night/Sunday morning are likely though
there may be a slight warm bias in blended and raw model guidance.
Statistical guidance (MOS) is comparatively lower, and NBM
percentile guidance is starting to recalibrate to recent warm bias
with a cooler 50th percentile than NBM deterministic. Thus, will
modify low temperatures slightly lower. This is still high enough
for some concern for vulnerable populations especially, as overnight
lows are an important component of overall heat risk.

A ribbon of rich moisture through a deep layer ahead of the
shortwave trough and attendant surface front will move into the area
Saturday night into Sunday. There should be a clear diurnal maximum
noted with convective coverage and intensity. Saturday night`s
upstream convection may struggle to make it deep into Indiana, but
the deep subtropical moisture plume may help maintain at least weak
convection late into the night and into Sunday morning. The diurnal
peak Sunday should result in reinvigoration of convection again
Sunday afternoon either across the southern portion of the area or
just to our south. Precision on frontal placement is tough at this
range and we`ll be able to better refine precipitation probabilities
in subsequent forecasts.

Continental air mass that is drier and slightly cooler will arrive
Monday behind the front. Rich PBL moisture is shown in the models to
quickly return Monday night into mid-week. Veered flow aloft and
approaching shortwave may lead to steeper midlevel lapse rates
favoring moderate to strong instability. Stronger flow aloft and
resultant shear for storm organization may be focused to our north
precluding a more substantial threat, however. Nevertheless, likely
largely driven by instability magnitude, CSU machine learning severe
probabilities pick up on this with low probabilities.

Behind the midweek front, another push of continental air will lead
to pleasant conditions the later half of the week. There is some
signal for at least periodic return of above average temperatures in
the 8-14 day period, but the signal in the model suite isn`t
particularly strong for impactful or long-duration heat at this
time.

&&

.AVIATION (18Z TAF Issuance)...
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.


&&

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

$$

SHORT TERM...Nield
LONG TERM...BRB
AVIATION...Nield