235
FXUS63 KIND 132300
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
700 PM EDT Sat Jul 13 2024

.KEY MESSAGES...

- Periodic shower and thunderstorm chances through Wednesday, with
  one or more windows of severe threat possible, though uncertain

- Hot and humid Sunday through Tuesday, possibly nearing Heat
  Advisory criteria, particularly Monday

- Cooler and much less humid late week into next weekend

&&

.SHORT TERM (This evening through Sunday)...
Issued at 245 PM EDT Sat Jul 13 2024

Very warm afternoon ongoing as weak surface high pressure tracks
through the region. A subtle upper feature passing just south of the
forecast area is generating scattered convection near KEVV otherwise
it was dry across the Hoosier state. 18Z temperatures were in the
mid and upper 80s.

Lot of uncertainty remains with respect to future progression of one
of more convective clusters set to develop over the Upper Midwest
and eastern Dakotas this evening then track quickly southeast
overnight in the northwest flow regime aloft. Potential is there for
convection in a weakened state to be approaching northern counties
near or after daybreak Sunday but overall this remains a much lower
confidence forecast for Sunday.

This Afternoon and Tonight

For the rest of the afternoon...despite the surface high...SBCAPE
values in the 2000-2500 j/kg via the KIND ACARS sounding and
mesoanalysis but overall low level moisture is lower than the
previous couple afternoons. Within a low shear environment...cannot
rule out a stray shower or storm through early evening but the lack
of more appreciable moisture and forcing aloft will largely keep a
cap on rain chances for the rest of the day. The best chance to see
a shower may end up over far southern portions of the forecast area
in closest proximity to the weakening upper wave and in an area with
deeper moisture. Current satellite imagery shows a more enhanced cu
field as well over the lower Wabash Valley and likely have a few
small showers between Vincennes and Washington. Any cells will pulse
in intensity and be short lived. Locally heavy rain and gusty winds
would be primary impacts from isolated convection.

Any isolated convection along with the diurnal cu will diminish
prior to sunset leaving mostly clear skies through the evening.
Dewpoints will begin to creep back up this evening as well making
for an increasing muggy feel to the airmass overnight with light
winds. Localized fog is a possibility towards daybreak Sunday but
the potential for expanding mid and high level clouds associated
with convection over the Great Lakes may largely mitigate fog
concerns.

Sunday

As mentioned above...high uncertainty exists for Sunday due to the
convective cluster(s) set to develop and dive southeast across the
Upper Midwest overnight. Model trends today have slowed progression
of the convection and focused it further north through daybreak
Sunday keeping activity mainly north of a Chicago to Detroit line.
Expect the leading edge of the cloud shield associated with the
convection to expand into northern parts of the forecast area Sunday
morning but remain skeptical in any organized convection making it
as far south as north central Indiana.

Of greater interest from a convective standpoint is the back edge of
the MCS likely to be over Wisconsin in the morning. With strong
surface heating taking place further south even with some amount of
convective cloud debris through the morning...think there is a valid
possibility of the southwest flank of the overnight MCS
restrengthening by early afternoon and riding the instability and
theta-e gradients which are poised to sit from northern Illinois
southeast across northern Indiana and into northwest Ohio. If storms
can redevelop a mature cold pool...could see convection drift into
the northeast part of the forecast area through the afternoon
carrying a damaging wind threat. Additional isolated convection
would be possible further south where BL shear is weaker but within
a highly unstable atmosphere.

There still remains multiple moving parts to the Sunday forecast and
the initiation and evolution of convection to our northwest tonight
will be critical in determining rain chances...cloud coverage and
subsequently temperatures.

Temps...lows tonight will fall into the upper 60s and lower 70s from
east to west. Even with morning clouds...should be able to get
temperatures into the upper 80s and lower 90s by the afternoon. Any
convection that can build south into the forecast area could impact
that. With dewpoints back into the lower 70s over much of the
area...max heat indices may rise as high as the upper 90s and lower
100s. While this will be oppressive and uncomfortable...these heat
indices will remain below Heat Advisory criteria. Experimental NWS
HeatRisk product suggests level 3 of 4/major heat health impacts
possible across parts of the area Sunday...though it is important to
remember that the product remains experimental and is simply one
tool to quantify the threat for heat health-related impacts.

&&

.LONG TERM (Sunday night through Saturday)...
Issued at 245 PM EDT Sat Jul 13 2024

Model guidance continues to advertise a fairly stark difference
between the early part of the coming week and the latter part of the
week into next weekend.

Early in the week, a broad, seasonably strong upper level ridge
centered over the western CONUS, combined with a subtropical ridge
over the southeastern CONUS, will produce broad south/southwesterly
low level flow, helping to maintain a hot and uncomfortably humid
airmass over the region.

While airmass convection early in the week is likely to be
significantly inhibited by warm temperatures aloft (700 mb temps
generally 10-13C), will need to carry some low PoPs in the north
Sunday and Monday nights, respectively, as, given the position of
central Indiana on the periphery of the larger ridges aloft,
upstream organized "ridge-riding" convection may be able to get into
the area in these periods, though uncertainty is relatively high
given the significant dependence upon prior convection in such
regimes.

As we get into Tuesday and Wednesday, a shift will occur, with the
weakening subtropical ridge helping to allow an anomalously strong
(as low as the first percentile on GEFS model climatology) upper
trough to dip into the Great Lakes region, driving the tail end of a
cold front well south into the CONUS mid to late week. The addition
of this frontal and upper level forcing will require higher
thunderstorm chances, with likelies merited late in the day Tuesday
into Tuesday night as the front itself pushes through the area.

Some severe threat will be present at times, primarily in the form
of damaging wind risk with any organized convection that can sneak
into northern portions of the area late Sunday and late Monday
nights, with perhaps a slightly more substantial severe threat
Tuesday into Tuesday night as the front passes, given the presence
of seasonably strong instability and adequate deep layer shear. SPC
outlooks are reasonable, and experimental machine learning guidance
has maintained fairly good signal for this threat as well for
several days.

Hydrologic concerns will require monitoring as well, particularly
Tuesday into Tuesday night as the front gradually moves through the
area. Antecedent conditions will be relatively wet given recent
rainfall (and the potential for additional storms at times early in
the week). Precipitable water values will again rise to 2+ inches,
which is at or above climatological maximum, even given the fact
that we are entering the yearly climatological peak in terms of
PWAT. The front will be making steady but not quick progress through
the area, and a significant depth of tropospheric flow will be
mostly boundary-parallel, setting the stage for storm training risk.

Post frontal passage, welcome relief is anticipated, as a
significantly cooler and less humid airmass accompanies an
unseasonably strong surface high pressure system (GEFS
climatological 95-99th percentile for SLP and sub-10th percentile
for PWAT). Dry and pleasant weather appears likely late in the work
week into the coming weekend.

Temperatures are likely to get into the 90s across all or parts of
central Indiana Monday and Tuesday (especially Monday), though
convection potential introduces some additional uncertainty in this
regard. Low temperatures will generally be in the low to mid 70s.

Experimental NWS HeatRisk product suggests level 3 of 4/major heat
health impacts possible across the area Monday and perhaps in the
south on Tuesday, though it is important to remember that the
product remains experimental and is simply one tool to quantify the
threat for heat health-related impacts - that said, if current
forecasts were to verify, traditional heat index criteria will be
approached Monday afternoon, with maximum apparent temperature
values ranging from 100-105.

Post-frontal passage, a drop of 10-20 degrees is expected for both
highs and lows, with equivalent decline in dewpoint temperatures as
well, making for unusually pleasant midsummer conditions late week
into the weekend.

&&

.AVIATION (00Z TAF Issuance)...
Issued at 700 PM EDT Sat Jul 13 2024

Impacts:

- Low but nonzero chances for fog overnight

- Low confidence in a few showers or storms Sunday morning

Discussion:

Isolated showers and storms will diminish early this evening.
Coverage is too low to mention in TAFs.

High clouds from convection to the northwest plus continued drying
should prevent fog overnight, but cannot rule out some fog at the
outlier sites. Will keep it out of the TAFs.

Dying convection could be approaching KLAF Sunday morning, but odds
are too low to include. Additional convection could develop
later Sunday, but once again odds/coverage are too low to put in the
TAFs.

&&

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

$$

SHORT TERM...Ryan
LONG TERM...Nield
AVIATION...50