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635
FXUS63 KIND 192246
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
646 PM EDT Thu Sep 19 2024

.KEY MESSAGES...

- Above normal temperatures continue into the weekend
- Conditional risk for isolated showers/storms Friday evening
- Cooler temperatures early next week with increasing rain chances

&&

.SHORT TERM (This evening through Friday)...
Issued at 121 PM EDT Thu Sep 19 2024

We remain on the northeast periphery of subtropical ridging with
above normal temperatures. The primary near-term focus has been
adjusting relative humidity for fire weather interests. Although
winds are too light for significant fire spread concern, antecedent
dry conditions and drought have resulted in increasing initial
attack-type fire activity, with fire magnitude requiring further
assets unlikely. Lower tropospheric moisture model errors are common
in these regimes where mixing processes are the primary driver for
RH. Even the 5th percentile of blended model guidance is
substantially more moist than a subset of high resolution models,
which have been too aggressive with drying through the diurnal
mixing process. So we`ve compromised with the official deterministic
forecast by splitting the difference. This seems like the best
decision given trends in ACARS vapor soundings of subtle lower
tropospheric moisture advection to offset drying through the mixing
process. We will continue to monitor observations through the rest
of the day.

A weak midlevel perturbation and associated warm advection will be
enough for an increase in midlevel clouds and perhaps virga later
tonight into early Friday.

There is a conditional convective scenario late tomorrow as trailing
weakly baroclinic surface boundary will progress into Indiana during
the evening. A modest deeper plume of moisture will arrive but the
details of this are where the greatest uncertainty lies regarding
convective potential. The lower troposphere may not moisten
sufficiently for true surface-based convection until after peak
diurnal heating, and/or result in relatively high-based convection
within narrow CAPE profiles. Whatever convection can develop at the
end of the diurnal peak may be able to sustain into the evening as
moisture advection increases.

In a reasonable worst case scenario, richer moisture supports
isolated to scattered convection along the front near the Illinois
border, generally near and north of I-74, that would drift further
east during the evening. Hodographs are elongated due to strong west-
northwest flow aloft and would favor storm-scale organization and a
marginal wind/hail threat. The most likely scenario is that
convection will struggle to develop due to modest moisture at best.
Regardless, the footprint of rainfall will be minimal and
insignificant relative to the needs given the ongoing drought
conditions.

Slight enhancement of southwesterly lower tropospheric flow coupled
with relatively strong mixing during the afternoon should again
result in ~11-13 degree positive 2-m temperature anomalies Friday.
Our normal high temperature is 78 degrees at IND. The record high
for the 20th of September is 94 set in 1940.

&&

.LONG TERM (Friday night through Thursday)...
Issued at 121 PM EDT Thu Sep 19 2024

Prograding subtropical ridge continues to influence our weather
through Saturday with ~10-12 degree positive 2-m temperature
anomalies. Flow at the crest of the ridge will become increasingly
perturbed the latter half of the weekend into early next week. This
could couple with a more established subtropical moisture plume for
some periodic rain potential during this period. Forcing is expected
to be fairly weak and mostly driven by broad/weak moisture regime,
so the multi-model mean rainfall totals are around 0.50-0.75 inches
late Sunday through early Tuesday. In the current pattern, models
have had a high bias, however. Temperatures are expected to be
closer to climatological normals early next week as the ridge
progresses and clouds and precipitation chances increase.

From mid-week onward, there is growing spread among ensemble members
on the synoptic-scale pattern. Clusters show fairly even
distribution of model camps and little confidence on if the western
ridge becomes more amplified or how progressive and deep the eastern
trough is. Regardless, it does appear that this period of the long
term forecast will feature generally dry conditions with mean
northerly flow and temperatures near or just below mid-September
climatology.

Day 8-14: There is a trend in the medium-range multi-model ensemble
mean for increasing high-latitude positive height anomalies. This
should generally translate to near or just below normal
precipitation for our region, along with above normal temperatures.

&&

.AVIATION (00Z TAF Issuance)...
-- Changed Discussion --
Issued at 645 PM EDT Thu Sep 19 2024

Impacts:

- Winds becoming southwest on Friday morning
- Isolated convection possible Friday afternoon

Discussion:

VFR conditions are expected through the period.

Any cumulus will dissipate early. Some mid and high clouds may
arrive overnight from dying convection to the west, especially at
the western sites. Additional cumulus will develop Friday, with
isolated convection possible Friday afternoon into evening.
Confidence in coverage is too low to mention in the TAFs that far
out.
-- End Changed Discussion --

&&

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

$$

SHORT TERM...BRB
LONG TERM...BRB
AVIATION...50