IND issues Area Forecast Discussion (AFD) at Aug 18, 9:51 PM EDT
972
FXUS63 KIND 190151
AFDIND
Area Forecast Discussion
National Weather Service Indianapolis IN
951 PM EDT Mon Aug 18 2025
.KEY MESSAGES...
- Isolated to scattered showers and storms this afternoon with
potential for greater coverage over northern counties late this
evening into the overnight
- Scattered strong storms Tuesday producing heavy rainfall
- Very warm to hot and humid conditions expected through Tuesday
- Dry and cooler late this week through early next week; Highs
falling into the upper 70s and low 80s by Sunday
&&
.FORECAST UPDATE...
Issued at 951 PM EDT Mon Aug 18 2025
The main focus for tonight is the evolution of a complex of
thunderstorms across Illinois as it makes its way southeastward
toward Indiana. Lower confidence exists in the overall longevity and
maintenance of these storms as they move away from better dynamics
further to the northwest. If the complex can develop its own cold
pool, then the possibility increases that they will hold together
into Indiana, but at the moment, radar data indicates the southern
flank of these storms have already become outflow dominant with
decreasing cloud top heights and upper level reflectivity. At the
moment, the threat for severe weather locally appears relatively
low, with the main threats of any storms being heavy rain,
lightning, and gusty winds.
With this said, an approaching weak mid and upper trough and surface
front overnight may provide just enough forcing within a warm and
unstable environment to keep scattered convection ongoing through
the night. A sharp west to east theta-e and instability gradient
exists across Indiana, so expect storms over Illinois to begin
turning to the southeast... following the gradient, but remaining on
the unstable side. Expect most storms to become elevated as a low
level nocturnal inversion develops, limiting the overall wind
threat... but a gust to 40 mph cannot be ruled out.
The greatest chances for storms overnight will be along and north of
the I-70 corridor...better chances as one goes further north. The
southern most storms within the upstream complex appear to be
following a path that would almost parallel I-74 to the IN/IL state
line. Timing for storms will be after midnight for Northwestern
portions of Central Indiana... then isolated to scattered convection
should remain across the area through the overnight hours.
There will likely be a lull in thunderstorm activity sometime mid
morning into the early afternoon hours as morning convection wanes
and the frontal boundary pushes toward I-70. Another round of storms
is likely by tomorrow afternoon, mainly for South Central Indiana as
storms fire along the frontal boundary.
&&
.MESOSCALE UPDATE...
Issued at 306 PM EDT Mon Aug 18 2025
Long-lived strong thunderstorm that has steadily crossed the region
from the Upper Wabash Valley earlier this morning...is currently
impacting westernmost portions of the Indianapolis Metro. Gusts at
both KIND and KEYE have been nearing 30 mph...with possibly slightly
higher winds in eastern Hendricks County, on a trajectory towards
northeastern Morgan County. Scattered areas of lighter rain
surround patches of moderate rainfall rates along the I-70 corridor
from near Cloverdale to the KIND airport, as well as eastern
Hendricks County. Rain will continue tracking to the south-
southeast while continuing to slowly weaken amid an overall capped
thermal profile.
Temperatures will likely fail to reach 90F for many points from
Indianapolis and to the north/west...courtesy of remnant clouds
and some lingering -RA towards the Lafayette area.
&&
.SHORT TERM (This evening through Tuesday)...
Issued at 223 PM EDT Mon Aug 18 2025
Mainly isolated convection continues across the northern Wabash
Valley in the vicinity of a boundary separating the oppressive and
more unstable airmass to the west.
The upper level ridge will expand into the region through the rest
of the day and into early Tuesday but will be shortlived as it
quickly retrogrades and reestablishes over the southern Rockies.
This will lead to a buckling of the mid level heights and enable a
frontal boundary currently over the northern Plains and upper
Midwest to shift south into the region on Tuesday. Model guidance in
general has sped up the southward progression of the boundary with a
growing confidence that it will be located across southern portions
of the forecast area by Tuesday evening.
The rest of the afternoon remains in a bit of flux with the presence
of the aforementioned boundary across the northwest portions of the
forecast area. Increasing instability will gradually shift east
across the forecast area this afternoo. Despite a strengthening of
the capping inversion...convection may persist from the Indy metro
west and northwest for the next few hours.
The remnants of a convective cluster over Iowa and Minnesota earlier
this morning continues in a weakened state over southwest Wisconsin.
Renewed development is likely along the remnant outflow across
northern Illinois by late day and this should progress downwind into
northern portions of Indiana this evening. Establishment of a more
mature cold pool will determine if the convection can make it as far
south as the northern Wabash Valley by late evening as wind fields
are much weaker with diminishing instability further south. At this
point...will carry precip chances into northern counties late
evening into the overnight but keep them confined primarily to the
northwest half of the forecast area. Convection should be below
severe levels by the time it arrives late this evening but a few
cells will likely produce gusty winds.
The remnants of the evening convection is likely to produce an
outflow boundary that will set up somewhere further south across the
forecast area and serve as a focal point for renewed development
Tuesday afternoon in addition to the frontal boundary. This is
likely to align more across the southeast half of the forecast area
by the afternoon. Modest instability with MLCAPE values near 3000
j/kg will combine with precip water values between 1.75 to 2 inches
to produce torrential rainfall as a primary impact. There is
potential for localized downbursts with stronger cells with DCAPEs
above 1000 j/kg and the strong heating expected ahead of the front.
Primary timing for storms will occur during the afternoon into the
evening.
Temps...A sticky afternoon ongoing with temperatures likely to top
out in the upper 80s and lower 90s in most locations with heat
indices up to 100 degrees in the Wabash Valley. After a night with
lows ranging from the mid 60s to lower 70s from east to west...
highs will rise into the upper 80s and lower 90s again for Tuesday
with an uptick in overall dewpoints into the mid 70s by afternoon.
This will create plenty of fuel for convection to fire off of later
in the day while also enabling heat indices to peak in the lower
100s over most of the forecast area. Have no plans at this time for
a Heat Advisory with much of the forecast area remaining below
criteria.
&&
.LONG TERM (Tuesday night through Monday)...
Issued at 223 PM EDT Mon Aug 18 2025
Wednesday through Sunday...
Dry and seasonable weather is expected on Wednesday through Sunday.
A few showers may be possible on Wednesday morning as Tuesday's cold
front may still be lingering across the far southern and eastern
parts of Central Indiana. After this departs, strong high pressure
aloft over the American southwest is expected to extend a ridge axis
to the Great Lakes. This ridge axis will progress across Indiana
through Friday and into Saturday as it reaches the Tennessee River
Valley. Surface high pressure is expected to accompany this feature.
Forecast soundings on Wednesday through Saturday show subsidence and
dry air within the mid and upper levels. These ingredients will not
be favorable for convective development. Thus a period of mostly
sunny days and mostly clear nights will be expected. The expected
dominate surface high pressure system through this time will be
found north of Central Indiana. This will result in surface flow
having a more easterly or northeasterly component, allowing for
seasonal highs in the middle to upper 80s and lows in the lower to
middle 60s.
Sunday and Monday...
Northwest flow aloft is shown to develop by Sunday and Monday,
allowing an upper level trough to push into Indiana. This system
appears to be accompanied with a surface cool front, which will
allow even cooler temperatures to arrive for next week. Given the
upper forcing and surface front expected to pass, this will be our
next best chance for rain in our forecast. Confidence is low for
rain as southerly gulf flow never really materializes. Moisture
availability remains a concern.
&&
.AVIATION (00Z TAF Issuance)...
Issued at 746 PM EDT Mon Aug 18 2025
Impacts:
- Isolated to scattered storms developing Tuesday
- Patchy fog possible primarily at KLAF predawn Tuesday
Discussion:
Mainly VFR conditions this evening over Central Indiana before
scattered showers and storms move back into the region from the
north and west. Convection currently in Northern Illinois will
likely be in a weakening phase as it enters Indiana later tonight,
with some guidance keeping most locations dry. While there is lower
confidence in the exact evolution of storms overnight, added VCSH to
KLAF during the overnight hours, as that site would have the best
chance sub-VFR conditions from storms. Low confidence exists in the
southward progression of these storms and if they will be able to
sustain themselves as far south as KIND. Added VCSH to the KIND TAF
after 10z to account for the chance and will update the TAF
accordingly based on trends overnight.
A frontal boundary slides south through the state by early Tuesday
and will serve as a focal point for renewed convective development
by late morning Tuesday into the afternoon, especially for areas
near and south of I-70. Best threat for afternoon storms will be at
KBMG. Lower confidence in the afternoon storm threat at KIND and
KHUF as the boundary may already be south of those locations. MVFR
or worse cigs and vis are possible under any storm.
Light southerly winds overnight become southwesterly at 8-12kts by
Tuesday morning ahead of the approaching front. Expect a wind shift
to the west, then northwest through the afternoon and evening hours
from north to south.
&&
.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
UPDATE...CM
SHORT TERM...Ryan
LONG TERM...Puma
AVIATION...CM
Source: IND issues Area Forecast Discussion (AFD) at Aug 18, 9:51 PM EDT (https://mesonet.agron.iastate.edu/p.php?pid=202508190151-KIND-FXUS63-AFDIND)
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