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346
FXUS63 KIND 202129
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
529 PM EDT Fri Sep 20 2024

.KEY MESSAGES...

- One more hot day Saturday

- Isolated to scattered thunderstorms late this afternoon and evening

- Turning cooler and more active with greater rain chances Sunday
through early next week

&&

.MESOSCALE UPDATE...
-- Changed Discussion --
Issued at 529 PM EDT Fri Sep 20 2024

A few thunderstorms have developed across the far northern forecast
area late this afternoon, on the nose of an axis of higher
instability. This area is near but not quite in some better shear as
well.

One storm has taken advantage of the available instability and shear
and is likely producing some small hail as it moves across Howard
County. Gusty non-severe winds remain possible with the storm as
well.

Upstream, scattered storms are developing in the higher instability
in Illinois. As this axis shifts east into the evening, additional
isolated to scattered storms should move into central Indiana.
Initially, some hail and gusty winds will remain possible with
stronger cores.

See the Short Term discussion below for the caveats against more
widespread storm development.
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&&

.SHORT TERM (This Evening through Saturday)...
Issued at 146 PM EDT Fri Sep 20 2024

A broken line of convection continues to develop along a weak
frontal boundary across the lower Great Lakes. To this point...
development southwestward along the boundary back into northwest
Indiana and northeast Illinois has been a struggle...with warm air
aloft and a capping inversion between 850 and 700mb serving as
deterrents. Despite clouds moving across the forecast area from
earlier showers...18Z temperatures had warmed into the mid and upper
80s over much of the region.

It has been two weeks since there has been any sort of convective
threat to talk about...and the aforementioned frontal boundary to
our north will provide at least the opportunity for some locations
across central Indiana to see some much needed rain later this
afternoon into the evening. Remain a bit pessimistic on how
extensive the convective coverage will look and will discuss this in
greater detail below.

This Afternoon and Evening

Short range trends through the rest of the afternoon will be
critical in determining how convection evolves into the forecast
area through the evening. While the signal is there in most of the
model suite for scattered convection later...the CAMs have not been
overly helpful in the details so far today. While the presence of a
narrow axis of deeper low level moisture and an overall increase in
BL shear support convective development...the negative factors also
present are likely the main reason for the struggles from the
individual CAMs in addition to the mishandling of low level
moisture. These negative factors...

1) An abundance of dry air in the mid levels and a lingering cap in
the 850-700mb layer
2) poor lapse rates
3) impacts from dry antecedent surface conditions

In a nutshell...still feel the overall model suite is overdoing
convective coverage for late afternoon into the evening. The
available instability and shear alone is suggestive of convective
development...but anticipate that storms will be in a messy...
chaotic multicellular mode that likely remain scattered at best due
to the dry air aloft and the poor lapse rates. A rise in pops late
this afternoon into the evening is warranted but have a hard time
justifying much higher than 30-40 pops at this point and focused
primarily across the northern half of the forecast area. Lightning
and brief heavy rainfall will be the primary impacts to Friday
evening activities but there remains the possibility for locally
gusty winds with any collapsing cells.

Late Tonight through Saturday

The frontal boundary will migrate south tonight and eventually wash
out across the southern part of the state by Saturday morning.
Cannot rule out a few showers accompanying the boundary south
overnight but dry conditions are expected across the forecast area
prior to daybreak Saturday. Model soundings are supportive of a
sharp nocturnal inversion developing in the predawn hours and with
light flow and the potential for an infusion of moisture from any
rain later today in the near surface layer...the fog potential is
elevated towards daybreak Saturday and focused especially across
northern parts of the forecast area.

Any fog will burn off quickly Saturday morning with ridging aloft
and a weak surface high reestablishing over the region. Expect yet
another hot day with dry conditions and mostly sunny skies in tandem
with light E/SE flow. Mid and high level cloud coverage may increase
subtly late day Saturday...signaling a welcome transition to a more
unsettled and cooler regime Sunday into the first half of next week.
More on this in the Long Term discussion below.

Temps...lows will slip down into the lower 60s over northern
counties where a light northerly flow will develop in wake of the
front. Further south expect mid and upper 60s. Low level thermals
support upper 80s and lower 90s on Saturday but this will put a wrap
on the recent stretch of hot weather with cooler temps to follow
into early next week.

&&

.LONG TERM (Saturday Night through Friday)...
Issued at 146 PM EDT Fri Sep 20 2024

Saturday Night Through Monday.

The pattern continues to trend more active for late this weekend
into early next week as a frontal system associated with a broad
trough across southern Canada pushes through the Midwest.  Latest
model runs and ensemble guidance suggests that the frontal passage
is likely late Sunday into Sunday night with scattered to numerous
showers out ahead of the front. A secondary wave just behind the
main wave then looks to move through on Monday with the potential
for heavier showers.  The QPF for the initial event is likely a bit
overdone with the very dry antecedent conditions but with saturation
from the first event, some appreciable rainfall looks likely by
Tuesday but there remains quite a bit of uncertainty as to where the
higher axis of rain will fall.

There is high confidence in much cooler air behind the front with
highs in the upper 70s to 80 on Monday followed by the low to mid
70s through the rest of the week.

Tuesday Through Friday.

Forecast confidence begins to fall off by Tuesday as models struggle
on how quickly to push the Monday system through with at least a
decent chance for wet conditions to continue into Tuesday before
conditions begin to dry out for the middle of the week.  There will
remain some low chances with weak impulses within the northwesterly
flow aloft, but the best forcing will be well gone by late Tuesday
into Wednesday.

Focus then shifts to the low potential for more appreciable rainfall
towards the end of the work week with models showing there may be a
seasonably strong closed low to the west of Indiana which would
advect significant amounts of Gulf moisture into the Ohio Valley.
This scenario looks unlikely based on how ensembles are handling the
low, but there is a decent number of individual members that show
the heavy rain outcome and will need to be monitored going forward.
Looking beyond Friday into the weekend, tropical moisture looks to
continue to advect northward which could keep the active pattern
going into the following week.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 1221 PM EDT Fri Sep 20 2024

Impacts:

- Isolated to scattered convection late afternoon and evening
possibly impacting KIND and KLAF

- Fog developing late tonight through daybreak Saturday...especially
at KLAF

Discussion:

Axis of mid and high level clouds diminishing as it drifts across
central Indiana early this afternoon. Skies will quickly shift back
to mostly clear for the afternoon before an increase in clouds
coinciding with scattered convective development expands in from the
northwest by late afternoon. Trends continue to support potential
for convection to potentially impact KIND and KLAF in the 21Z to 01Z
timeframe ahead of a weak frontal boundary. Confidence remains lower
in any convection at KBMG and KHUF and will keep any precip mention
out at those sites at this time. W/SW winds will generally be around
10kts through early evening but could see briefly gustier winds
within any convection that passes over or near a terminal.

High pressure and dry air will reestablish overnight with light and
variable flow likely to go near calm in the predawn hours. There is
support for fog development at the outlying terminals and especially
at KLAF that will likely last through daybreak. Expect mostly sunny
skies Saturday with light southeast flow developing.


&&

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

$$

MESOSCALE...50
SHORT TERM...Ryan
LONG TERM...White
AVIATION...Ryan