Area Forecast Discussion
Issued by NWS Indianapolis, IN

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000
FXUS63 KIND 042158
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
558 PM EDT Mon Jul 4 2022

.Mesoscale Update...
Issued at 558 PM EDT Mon Jul 4 2022

Convective cluster riding along an expanding instability gradient
continues to press southeast late this afternoon over east central
Illinois.  The strongest cells at 2130Z extend roughly west-east
across McLean...Ford and Iroquois Counties in Illinois.

With the instability gradient and theta-e ridge both slowly building
into the Wabash Valley from the west...it is expected that this area
of convection will drift southeast perhaps impacting the far NW
portions of the forecast area in the 23-01Z time period this
evening. The atmosphere becomes rapidly less favorable for
convective maintenance further east across central Indiana with
model soundings and surface obs both showing dewpoints noticeably
lower and a much drier airmass. May actually tend to see the most
intense of the storm cells migrate south into the deeper instability
and moisture as the whole complex drops southeast.

Model soundings do show steep lapse rates through in the surface to
3km layer  into the northern Wabash Valley...so even as the
convection likely weakens...the potential will be there for cells to
still drop gusty winds across the area. That combined with lightning
and heavy rainfall will be the primary concerns for outdoor plans
into the evening across the northwest portions of central Indiana.
Elsewhere across the forecast area...expect a dry evening for
activities.

The next mesoscale update will be issued later this evening.

&&

.Short Term...(This evening through Tuesday night)
Issued at 242 PM EDT Mon Jul 4 2022

...Stormy Pattern with Dangerous Heat...

Tonight.

The start of an active weather pattern begins during the overnight
hours tonight as a series of storm complexes move through the Great
Lakes Region with possible impacts to the northeastern portions of
the forecast area. Latest model soundings show little instability
across the this evening into the early overnight hours, but as the
night goes on, moist air will continue to advect into the area and
will create increased instability.

Current thoughts are that the initial complex of storms currently
ongoing in eastern Iowa will dissipate as it crosses into Indiana
this evening, but the second system originating further north into
Wisconsin will be able to tap into the increased instability and
make it into the northeastern portions of the forecast area late
tonight into early tomorrow morning.  A combination of fairly weak
shear and a near surface inversion will limit the damaging wind
threat with this complex.  The continued advection of warm and moist
air into the night will keep temperatures quite mild with lows only
dropping into the mid 70s.

Tuesday and Tuesday Night.

Not a lot of confidence in the forecast for Tuesday with conditions
highly dependent on how things evolve tonight.  Will discuss a
couple possible solutions and how it would impact the weather for
tomorrow.  Current thoughts are that the most likely solution will
be that a complex of storms will push into the northeastern counties
late tonight into tomorrow morning which will leave an outflow
boundary across central Indiana.  As temperatures rise into the mid
90s, convective temperatures will be reached and with the 3000 J/kg
of CAPE expected, additional storms will initiate along that
boundary around 5-7 PM and push through the area.

Another possible solution would be that if the storms don`t move
through the area late tonight, the loss of the potential cloud cover
during the morning hours would lead to even warmer temperatures and
a more extreme heat threat going through the daytime hours.  Still
think that storms would fire late in the day, but convection would
be more sporadic and less organized with the lack of shear and
guiding of a residual boundary.

Have elected to issue a Heat Advisory across the forecast area, but
the extent of the hottest temperatures remains somewhat uncertain
due to potential for morning cloud cover. Warm and moist air will
continue to advect into the area through the day with highs in the
low to mid 90s combined with dew points approaching the mid 70s will
create heat index values well over 100 degrees for much of the area.
Depending on how tonight`s weather evolves, this Heat Advisory area
may need to be shrunk by tomorrow morning.

Additional storms are possible late into the overnight hours Tuesday
night as the next ridge riding complex of storms moves in from the
northwest. This active pattern of overnight storms looks to continue
for the next several days which should provide relief from the
developing drought conditions.

&&

.Long Term...(Wednesday through Monday)
Issued at 242 PM EDT Mon Jul 4 2022

Highlights:

- Hot and humid with periodic thunderstorm chances through Saturday

- Severe thunderstorms, heavy rain and flooding possible at times

- Drying out over the weekend and not as hot and humid

Wednesday through Friday...

Models in good agreement on the main synoptic details for the long
term. A dome of high pressure will remain over the South through
Friday with trough over southeastern Canada and New England. This
will place central Indiana within a northwest flow regime on the
periphery of an upper jet. Meanwhile, at the surface, a wavy front
will be somewhere over the lower Great Lakes to northern parts of
central Indiana and to further complicate things, a Canadian front
may be just to the north of the aforementioned one. Any wave moving
through the northwest flow will interact with a sharp instability
gradient, deep moisture and jet induced diffluence and be well
capable of producing a series of mesoscale thunderstorm complexes
accompanied by the potential for damaging winds, large hail and
heavy rainfall/localized flooding. The devil will be in the
mesoscale details, which models struggle with especially beyond 24-
36 hours. Any leftover outflow boundaries and differential
heating boundaries will also be a big factor in daytime convection.
Thunderstorms will tend to move east and southeast with a fairly
sharp SW-NE instability gradient and with NW flow aloft. These tend
to effect especially NE parts of central Indiana and be nocturnal in
nature. Combo of deep shear and moderate or greater instability
would favor bowing potential with damaging winds possible. But as
mentioned earlier, heavy rainfall and flooding will be possible,
especially later in the week, over areas that see thunderstorms
repeatedly move through, the next few days.

In addition to the temporal and spacial forecasting difficulty
regarding these thunderstorm complexes, temperatures will also be a
challenge with the uncertainty in the degree that convective induced
debris will play. That said, it does not take too many breaks in the
clouds in July to allow temperatures to recover very quickly and
with the ridge just to the south, will stick with afternoon highs in
the 90s through at least Thursday. With the potential for the heat
index to reach 105 degrees or higher, will be issuing a Heat
Advisory through Wednesday.

Saturday through Monday...

The weather will be changing, at least briefly, this weekend as the
southern ridge retrogrades and builds across the Rockies. This will
cause the upper winds to veer and push a cold front through the
forecast area and bring in a much drier and seasonable airmass. Held
on to small PoPs on Saturday with some timing uncertainty, but good
confidence the convection will be to the south by at least Sunday.
Temperatures should only reach the middle or possibly upper 80s this
weekend with much more comfortable humidity levels. Would not rule
out a return to thunderstorm chances next Monday as a trough
approaches from the upper Midwest, but too much uncertainty on
timing. More confident temperatures and humidity will start to creep
back up in southerly low level flow ahead of this feature.

&&

.Aviation...(18Z TAF Issuance)
Issued at 1244 PM EDT Mon Jul 4 2022

IMPACTS:

-Southerly gusts to 20kts this afternoon and evening.

Discussion: Generally dry conditions expected through the TAF period
with VFR cigs and vsbys. Southerly wind gusts will occasionally gust
to 20kts this afternoon and evening before becoming more
southwesterly tonight.  There is a small chance for showers/storms
late tonight, but confidence remains too low for a mention in the
TAF at this time.  If storms do occur, best chances are at LAF and
IND.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from 1 PM to 9 PM EDT Tuesday for INZ021-028>031-
035>042-046>049-055>057-065.

Heat Advisory from 1 PM Tuesday to 9 PM EDT Wednesday for
INZ043>045-051>054-060>064-067>072.

&&

$$

Mesoscale...Ryan
Short Term...White
Long Term...MK
Aviation...White


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