471
FXUS63 KIWX 201025
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
625 AM EDT Thu Jun 20 2024

.KEY MESSAGES...

* Hot and humid conditions will continue through Saturday. Peak
  afternoon heat indices will approach 100 degrees in many
  locations, hazardous to sensitive and vulnerable groups. The heat
  headlines have been extended through early Saturday evening.

* Isolated to scattered thunderstorms are possible this
  afternoon and again Friday afternoon, mainly north of US Route
  30. Showers and thunderstorms become likely Saturday night
  into early Sunday.

&&

.DISCUSSION...
Issued at 340 AM EDT Thu Jun 20 2024

Very little change from previous forecast as stagnant synoptic
pattern allows for continuation of dangerous heat and humidity and
peak afternoon heat indices around 100 into Saturday. With
consistent signals of heat indices in upper 90s to around 100, have
extended the current heat headlines through Saturday evening.

Upper level longwave ridge will exhibit some slight
retrogression over the next few days which will keep heat dome
entrenched across most of the Ohio Valley. One difference in
today`s weather will be a very weak, shallow sfc trough
dropping across southern Lower Michigan/Lake Michigan vicinity
with some light northwest winds developing. This weak onshore
component will provide some relief from the excessive heat for
the Lake Michigan shoreline areas. This weak boundary also
should help to focus isolated to scattered afternoon showers and
thunderstorms. Have maintained inherited gradient in PoPs from
south to north as retrograding upper level ridge will limit
instability magnitudes across southern half of the forecast
area. Some slightly warmer mid levels today, and perhaps
slightly lower low level dew points should result in peak
afternoon instability magnitudes generally weaker than
yesterday. However, with surface based CAPES on order of 1000
J/kg and steep low level lapse rates/mid level theta-e minimum
in place, cannot discount potential of a few storms producing
primarily sub-severe gusts of 40+ mph. A few storms may persist
into this evening before exhibiting a similar diurnal
diminishment as yesterday evening. The best chances of reaching
peak afternoon heat indices around 100 today will be in the
Excessive Heat Warning area, while elsewhere generally mid to
upper 90s heat indices are expected.

For Friday, a continued southwest drift of upper level anticyclonic
circulation center into southern Indiana/western Kentucky is
expected. With a similar thermodynamic environment in place for
Friday would expect similar high temperatures in comparison to
Thursday. Sfc boundary which washes out across the southern Great
Lakes today, will continue to lose its integrity into early Friday.
This may lesson coverage of diurnal pop up showers and storms on
Friday with low level moisture quality also somewhat in question in
terms of overcoming weak mid level warm nose.

For Saturday, some slight suppression of upper level ridge
continues, but emergence of more substantial upper level short wave
forcing from the lee of the Rockies will allow for some better
southwest low level return flow during the day Saturday. This should
result in a more uniform mid 90s high temperature distribution
across the area. Sfc dew points should be held in check in this
better mixing environment, which will result in only modest
differences of apparent temperatures from air temperatures.
Nonetheless, peak afternoon heat indices should top out around
100 across much of the area on Saturday. With confidence medium
to high in minimal impacts from clouds/isolated convection, did
go ahead and extend the current heat headlines through Saturday
afternoon with primary focus on cumulative multi-day effects of
heat of this magnitude.

Better chances of showers and thunderstorms arrive Saturday night
through Sunday as stronger mid/upper level short wave tracks across
the western Great Lakes. Still some notable differences in how
a few of these upstream waves phase across the Upper
Midwest/Great Lakes for this period which will have an impact on
sharpness of frontal convergence and associated moisture
pooling. This setup does appear to feature limited instability
magnitudes though, given weak mid level lapse rates and late
timing of this frontal forcing Saturday night into Sunday.

Slightly cooler air arrives for Sunday (mid 80s to around 90),
which should continue into Monday (mid 80s) before warmer low
level air rapidly advects eastward downstream of next northern
tier CONUS upper trough. This should result in high temps back
into the lower 90s for much of the area by Tuesday. Additional
chances of showers and thunderstorms will be in place for
Tuesday and Wednesday as decent instability gradient could set
up from the Mid MS Valley to the southern Great Lakes, although
temporal fine tuning of these chances will be needed over next
several forecast cycles. Passage of this associated front should
take temps back to more seasonable levels toward the end of
this forecast valid period.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 624 AM EDT Thu Jun 20 2024

VFR conditions are expected to persist this forecast period. A
weak sfc trough dropping across the southern Great Lakes will
have a wind shift to the northwest associated with it by
midday/early afternoon. Winds are expected to remain at or below
10 knots through this forecast valid period. As was the case
yesterday, will need to watch for potential of isolated to
scattered showers and thunderstorms this afternoon, particularly
in vicinity of the weak sfc trough across far northern Indiana.
The potential of these isolated showers and thunderstorms should
peak in the 20Z-23Z timeframe, but should quickly diminish
toward 00Z.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Heat Advisory until 8 PM EDT /7 PM CDT/ Saturday for
     INZ005>008-012>015-020-022-023-103-104-116-203-204-216.
     Excessive Heat Warning until 8 PM EDT Saturday for INZ009-017-
     018-024>027-032>034.
     Air Quality Alert until midnight EDT /11 PM CDT/ tonight for
     INZ020-103-203.
OH...Excessive Heat Warning until 8 PM EDT Saturday for OHZ001-002-
     004-005-015-016-024-025.
MI...Heat Advisory until 8 PM EDT Saturday for MIZ078>081-177-277.
MARINE...None.

&&

$$

DISCUSSION...Marsili
AVIATION...Marsili