-- Discussion containing changed information from previous version are highlighted. --
847
FXUS63 KIND 200543
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
143 AM EDT Thu Jun 20 2024

.KEY MESSAGES...

- Isolated thunderstorms possible through this evening
- Warm and humid Summer weather Friday through Wednesday
- Friday will be the Hottest Day
- Best rain chances and cooler on Sunday

&&

.FORECAST UPDATE...
Issued at 906 PM EDT Wed Jun 19 2024

No significant changes required to going forecast this morning.

There is still some weak flow aloft out ahead of the ridge axis,
primarily over N/NE portions of Indiana, and has allowed for
isolated initiation as we remain above convective temperatures.
However, this will quickly end over the next few hours as diurnal
cooling removes near surface instability.

The increasing ridge influence as the axis pushes eastward should
help to thin out cloud cover with only 20-30% cover expected after
diurnal cu dissipates. With flow calming beneath the upper ridge,
near surface temperatures will efficiently cool overnight. This
could allow for pockets of fog to develop, especially where
rainfall occurred this evening. Still, dew point depressions
greater than 10 as of 01Z, with some lowering of dew points
overnight should keep fog formation from becoming widespread.

&&

.SHORT TERM (This evening through Thursday)...
Issued at 245 PM EDT Wed Jun 19 2024

A hot and humid airmass remains in place across central Indiana this
afternoon, and areas of cumulus congestus are noted visually and on
satellite imagery as earlier cloud cover erodes and insolation
continues to push temperatures toward or past the 90 degree mark.

Expect isolated thunderstorms to develop as the afternoon wears on,
with this threat persisting into the evening hours in a setting of
modestly weak effective shear and ample mean layer CAPE values of
1000-2000 J/kg. Precipitable water values on area ACARS soundings
are near or above climatological maximum, and these same data show a
midlevel dry layer around 600 mb that will help to promote an
isolated microburst/damaging wind threat due to evaporative cooling
and precipitation loading with the strongest/deepest convective
cores today. All storms will carry a lightning threat (by
definition, of course), and a threat for heavy downpours and
localized flooding given slow/meandering storm motions/cell
propagation and aforementioned anomalously moist column.

The threat for storms will wane late this evening as diurnal heating
is lost and gradual drying and a modest subsidence inversion begin
to develop aloft in response to the slow retrogression of the upper
level ridge centered over the mid-Atlantic states late tonight into
Thursday.

Low temperatures tonight should be near to slightly below
persistence values, or roughly the low to mid 70s, per low level
thicknesses and continued high dewpoints, with less cloud cover
expected. At least some light fog is likely given the light to calm
winds, and patchy dense fog will be possible before daybreak
Thursday, particularly in favored areas, such as river valleys and
those areas receiving rain today and in recent days.

As the ridge retrogrades/expands, the aforementioned subsidence
inversion should keep a lid on convective activity on Thursday,
though a rogue shower would not be entirely shocking in a continued
hot and humid airmass. Convective temperatures may be difficult to
reach, but expect at least a few cumulus to develop during the day.
Ample insolation, however, and 850 mb temperatures as warm as 19C
should push highs well into the 90s tomorrow, with heat index values
peaking around 100 in spots.

Will continue to message heat safety tomorrow and beyond, despite
falling short of strict criteria. Jump into hot weather was rather
abrupt this season. Experimental heat risk levels will be 3/4 on
Thursday - major to extreme - with impacts likely to anyone with
overexposure or without access to adequate cooling, particularly
given continued overnight warmth/humidity, and duration of the
current heat episode.

&&

.LONG TERM (Thursday night through Wednesday)...
Issued at 245 PM EDT Wed Jun 19 2024

Friday and Saturday...

Models suggest the upper pattern will be dominated by a strong upper
high pressure system settled over the Ohio Valley and the
southeastern states. This will keep any forcing dynamics confined to
points well north of Central Indiana. However, a very warm and humid
air mass will remain in place at the surface under that upper high.
850MB temps each day will be around 18-20C, amid a warm
southwesterly flow in place.  700MB temps near 10C suggest a weakly
capped environment and CAPE is shallow each afternoon. Thus given
very warm and humid air mass, a stray, rogue, convective afternoon
shower may be possible, but coverage will be quite limited. Thus may
use very low pops to account for this. Given the ample sunshine
expected as forecast soundings show dry mid and upper levels, highs
in the middle 90s are expected with heat index values around 100 on
Friday.

Sunday...

Models suggest a quick moving upper wave pushing out of the northern
plains in toward the Ohio Valley on Sunday. This feature is expected
to break down the flattening upper high as it was slowly evolving
into nearly a zonal flow.  Upper forcing with this wave appears to
arrive on Sunday and mid-levels respond with a band of saturation
passing across Indiana through the day. Forecast soundings reveal a
more saturated column as these features pass, with pwats near 1.74
inches present. Thus with the warm and humid air mass and the
passing forcing along with a weak cold front, our best chances for
rain during this period will be on Sunday. At this point an all day
rain does not look likely, but only a few hours of rain appear more
likely as the cold front passes. Clouds and rain will result in a
cool down in temperatures as highs in lower to middle 80s are
reached.

Monday through Wednesday...

Models suggest that upper level high pressure will re-establish over
the American southwest early next week. This will lead to the return
of northwest flow across the upper Midwest, spilling southeast into
the Great Lakes and Ohio Valley.  Models disagree with forcing
dynamics passing over the ridge toward Indiana at this time. During
this time surface high pressure passes across Central Indiana and
allows a warm and humid gulf flow to arrive. Thus more summertime
heat and humidity is expected. Overall, confidence is low for rain
and upper forcing appears uncertain. The least likely day for
precipitation may be Tuesday as the surface high passes. Above
normal temperatures in the upper 80s to near 90 are expected.

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 (06Z TAF Issuance)...
-- Changed Discussion --
Issued at 143 AM EDT Thu Jun 20 2024

Impacts:

- Patchy fog possible through 13z

Discussion:

Predominantly VFR conditions expected much of the period under the
influence of a ridge aloft. Winds have dropped to 5 kts or less
across the entire region with relatively clear skies. Potential is
there for patchy fog to develop in the 09-13z timeframe, so have
added BCFG to the TAF sites. Do not think fog will be particularly
widespread. Confidence is low in exactly where fog will develop,
however the typical fog spots in the Wabash River Valley likely have
a better chance. Kept conditions VFR in the TAFs, but if fog
develops over a TAF site there is a chance for brief MVFR conditions
or worse.

Any fog should dissipate in the 13-14z timeframe with VFR conditions
expected for the rest of the period. Winds will remain light under
10 kts today.
-- End Changed Discussion --

&&

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

$$

UPDATE...Updike
SHORT TERM...Nield
LONG TERM...Puma
AVIATION...CM