937
FXUS63 KIND 181828
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
228 PM EDT Tue Jun 18 2024

.KEY MESSAGES...

- Extended period of hot and humid conditions through this weekend

- Partly cloudy Tonight and Wednesday.

- Stray Shower or Thunderstorm possible through Thursday.

&&

.SHORT TERM (This evening through Wednesday)...
Issued at 227 PM EDT Tue Jun 18 2024

Surface analysis early this afternoon shows a large area of high
pressure in place just east of the middle Atlantic States. This was
resulting in warm and humid southerly flow across Indiana and the
Ohio Valley.  Aloft, water vapor showed a strong ridge of high
pressure over the United States east coast, while a moderate through
was found over the western CONUS. Water vapor also showed a plume of
tropical moisture streaming from the Gulf of Mexico across the
Tennessee river Valley to Indiana, OH and MI.  Radar this afternoon
shows an area of showers under the plume over southern IL and SW
Indiana. Surface winds were from the south and dew points were quite
humid, in the lower 70s.

Tonight...

The upper high pressure system to the east is expected to begin to
retrograde a bit west tonight. This will slowly start the western
progression of the tropical plume of moisture that has been our main
weather feature today. However, until that occurs, southerly winds
with high clouds will persist through the evening and overnight. As
daytime heating is lost, instability will be lost which should limit
shower chances, but due to the plume aloft and plentiful moisture
available, any weak disturbances within the plume could result in a
brief passing shower.  HRRR continues to suggest very light
precipitation possible under the plume through the overnight.
Confidence on specific locations and timing is low, thus a large
blanket of low pops will be used across the forecast area. Again,
there will be many dry hours and any precipitation amounts will be
light. Given the warm and humid air mass in place along with the
expected cloud cover, lows in the lower to middle 70s will be
expected.

Wednesday...

The upper pattern will become more dominated by the large upper high
pressure system on Wednesday. The high will retrograde westward,
steering any upper forcing dynamics well north of Indiana, to
Ontario. This should also effectively begin to block the tropical
plume, leading to increased sunshine as these mid and high clouds
are lost. Forecast soundings start the day with some high cloud
present, but these are lost as the day progresses. Furthermore,
forecast soundings show attainable convective temperatures in the
upper 80s with CAPE near 1000 J/KG. Thus as high clouds exit and
stronger heating is achieved in the afternoon, CU development is
expected and brief diurnal showers and storms will be possible. Once
again will include chc pops across most of the forecast area, mainly
for the afternoon for these light, isolated and brief diurnal
showers and storms. Highs should be able to reach the lower to
middle 90s.

&&

.LONG TERM (Wednesday night through Tuesday)...
Issued at 227 PM EDT Tue Jun 18 2024

Wednesday Night through the Weekend...

An extended period of high heat will continue through the weekend,
and likely will redevelop again next week. Although the magnitude
may not reach extreme heat indices of greater than 105, the extended
duration and presence of very warm overnight lows will provide
hazardous heat conditions across central Indiana during the long
term.

This stretch of above normal temperatures will be initiated by the
eastward progression of anomalously high heights aloft over the Ohio
Valley. This will allow for greater atmospheric expansion and
therefore higher temperature values throughout the entire lower 5km
of the troposphere. Greater lower tropsphere temperaures will be
accentuated early this weekend as the ridge axis positions directly
overhead, with highest temperatures expected to occur on Saturday
afternoon (93-97F).

In conjunction with this deep ridging aloft, there will be gradual
south/southwest flow due to surface high pressure to the east and
north. This will allow for elevated dew point temperatures to remain
over the Ohio Valley, generally in the mid to upper 60s. The
combination of highs in the 90s and dew points in the low 70s will
push heat index values into the upper 90s through Thursday and
possibly low 100s Friday and Saturday. As mentioned, this within
itself is not overly hazardous, but the fact this is expected over a
succession of 4 to 5 days with over night lows in the low 70s puts
an even greater risk to vulnerable populations.

There are a few potential mitigating factors for high heat and are
mainly related to the potential for increased cloud cover and
isolated rain chances over the middle of the week. If cloud cover
becomes greater than 70% over the afternoon hours, highs may be
suppressed lowering the heat risk. Still, overnight lows should not
be greatly impacted with dew points remaining in the mid to upper
60s. By Friday, the greatest moisture and lift axis should shift
eastward, lowering the threat of afternoon clouds and rain chances
for Friday through Sunday.

Forecast confidence decreases late this weekend through early next
week as a shortwave approaches from the north. High pressure over
the Carolinas should impede shortwave progression some, but there is
still high uncertainty in how far east a weak pressure trough and
associated lift/precipitation will reach. Even so, ridging will
remain aloft and any weak wave passage that does occur should only
briefly impact surface conditions before upper 80s to low 90s
surface temperatures return for the middle of next week. Even looking
at the extended range (>10days), the pattern continues to favor
general temperatures at or above normal for the next few weeks. With
this in mind, expect more 90+ degree days heading into early July.

One note about the HeatRisk graphics by WPC. This is an experimental
product and values in the current pattern are higher than
traditional criteria, and may not be perfectly calibrated to best
represent the risk at this time. Thus, we advise users to take this
into consideration when viewing the HeatRisk product.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 1253 PM EDT Tue Jun 18 2024

Impacts:

- Isolated showers/thunderstorms possible this afternoon

Discussion:

VFR conditions expected through the TAF period. GOES16 shows a plume
of tropical moisture streaming north across Central Indiana,
providing mid and high level cloud cover. Should any breaks occur,
VFR CU development and brief SHRA/TSRA will be possible. Shower
coverage on radar appears to be increasing upstream across southern
IL and SW Indiana. Confidence for timing and coverage remains low,
thus have used VCSH for most of the TAF sites this afternoon as
this feature passes.

Expect convection to diminish in coverage after sunset as heating
diminishes and the tropical plume is expected to become more
directed at points west of Indiana as an upper high builds across
the southeastern states.


&&

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

$$

SHORT TERM...Puma
LONG TERM...Updike
AVIATION...Puma