614
FXUS63 KIND 181052
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
652 AM EDT Tue Jun 18 2024

.KEY MESSAGES...

- Extended period of hot and humid conditions through next weekend

- Afternoon heat indices approaching 100 this week may be hazardous
  to sensitive and vulnerable groups

- Isolated to scattered shower and storms today, a few storms may be
strong to severe with brief periods of strong winds

&&

.SHORT TERM (Today and tonight)...
Issued at 302 AM EDT Tue Jun 18 2024

Another hot and humid day with isolated thunderstorms for Central
Indiana.

Early this morning, showers are still present on radar imagery,
mainly over the Wabash River Valley and into Southern Illinois. The
region still remains on the western edge of an anomalously strong
ridge centered over the Southern CONUS. Strong southerly return flow
through the column is bringing ample tropical moisture northward
from the Gulf into the Ohio Valley. Current PW observations show the
strongest axis of moisture advection extends from around the Yucatan
Peninsula northward through the Mississippi River Valley and into
portions of SW Indiana where PW values are at or above 2 inches! A
30-40kt nocturnal low level jet is working to enhance moisture
advection northward while also helping isolated areas of showers and
stratiform rain to persist through the night. Lightning activity has
decreased significantly over the last few hours, however areas of
stratiform rain and showers have increased across portions of SW
Indiana and along the IL/IN state line under the LLJ where the
narrow axis of 2"+ PW are observed. Satellite imagery shows cloud
cover increasing in this area and slowly pushing NNE. Expect
continued showers across Western Indiana through the early morning
hours while areas along and east of the I-65 corridor remain
relatively dry through the morning hours.

Short term guidance continues to show this narrow band of strong
tropical moisture advection remaining over the state through the
day, resulting in increased cloud cover and the chance for convection
through the day. This will likely keep highs around 3-8 degrees
cooler than yesterday, especially for portions of Central and
Western Indiana with thicker cloud cover and repeated shower
activity. Forecast soundings show a a very saturated atmospheric
profile likely with multiple cloud layers within the lower levels
under overcast skies in the upper levels. Despite increased clouds
today, enough solar heating should take place to at least heat most
locations into the mid to upper 80s with dew points in the lower 70s
once again. Best chance for highs in the lower 90s will be across
Eastern and Northeastern portions of Indiana where thinner cloud
cover and less convective activity will allow for more surface
heating. Nonetheless, it is going to feel very muggy with heat
indices in the mid 90s to near 100 for most locations.

Scattered showers across Western Indiana are expected to slowly
diminish in coverage after sunrise today, but still expect clouds to
persist for much of the day. Additional convection will likely
develop during the afternoon and evening hours across portions of
Central Indiana, with the threat for an isolated strong to severe
storm once again. Short-term guidance performed fairly well yesterday
highlighting Western and SW Indiana for the best potential for
storms. Today, short-term guidance highlights Central and
Southeastern Indiana for the best coverage of convection during the
afternoon and evening, likely due to differential heating boundaries
setting up as temperatures remain a few degrees cooler further west
under thicker clouds. A high instability, low shear profile will
likely lead to a similar scenario as yesterday with storms quickly
developing then collapsing on themselves resulting in a brief
damaging wind/microburst threat.

With such a humid airmass, do not expect much relief from the heat
during the nighttime hours, with overnight lows ranging from the low
70s to near 80 degrees in some spots!

&&

.LONG TERM (Wednesday through Monday)...
Issued at 302 AM EDT Tue Jun 18 2024

Wednesday through Next Weekend...

An extended period of heat and high humidity will continue into next
weekend, and potentially redevelop again next week. Although the
magnitude may be marginal for extreme heat, the duration and very
warm overnight lows are the primary concerns.

An anomalously strong upper ridge over east will slowly migrate
westward and become centered over the Ohio Valley by the end of the
week. Early in the period, models show 500-mb height anomalies over
northern New England and the Canadian Maritimes near or exceeding
June climatological maxima. This translates to the hottest surface
temperatures on the windward side of the ridge axis (over our
region) extending up to the crest of the ridge, well into eastern
Canada. A complicating factor for extreme heat this far west will be
a deep subtropical moisture plume with clouds and limited mixing.
However, this will provide the moisture element to bring heat index
values up to around 100 degrees.

Although the traditional 105F heat index criteria likely won`t be
met, the duration of the heat episode, warm nights (minimums in the
mid to sometimes upper 70s), and early onset in the season is enough
to continue messaging heat related illness potential, especially for
vulnerable populations.

While the heat and humidity have already reached very uncomfortable
to oppressive levels this week, some of the hottest temperatures of
this heat wave may be yet to come. Thursday into early next weekend
may feature temperatures reaching the upper 90s in some spots as the
ridge becomes centered directly overhead. The main axis of strong
tropical moisture advection during this period will be pushed
westward while subsidence increases with the ridge building
overhead. Sinking, drier air will result in less cloud cloud cover
while drier, sinking air heats up faster. In addition to the slight
change in the weather pattern, the area will already be on days 5-7
of the extended period of heat, with soil moisture continue to
decrease in many areas. This all leads to higher confidence in
hotter temperatures in the Thursday through Saturday timeframe. Even
with humidity values slightly lower (dew points in the 60s, rather
than lower 70s) feels-like temperatures will likely still be around
100 degrees during the afternoon and evening.

As mentioned above, the axis of strongest tropical moisture
advection shifts a little to the west for the latter half of the
week. This will likely lead to a lesser threat of isolated to
scattered single cell convection during the afternoon, with the best
convective coverage along more synoptic scale boundaries. Guidance
does suggest a weak, diffuse front extending from the St Lawrence
River valley to Lake Erie and through Northern Indiana. Will have to
watch where exactly that boundary sets up Wednesday through Saturday
as this will likely be there area where afternoon convection
develops.

Cluster analysis and spaghetti plots show a little more chaos among
the ensembles by the weekend but still a general signal of a
shortwave trough to our north nudging a cold front and potentially
more organized (albeit not widespread) convection by Sunday. Post-
frontal air mass will be more continental and drier into early next
week. The longer range pattern does favor temperatures at or above
normal for this time year year. So expect more 90 degree days
heading into late June and 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 (12Z TAF Issuance)...
Issued at 652 AM EDT Tue Jun 18 2024

Impacts:

- Scattered showers for KLAF and KHUF this morning
- Isolated thunderstorms possible at all sites after 16z
- Gusts near 20kts this afternoon

Discussion:

Mostly VFR conditions expected through the TAF period. Latest
satellite and radar imagery shows an area of showers moving north
along the IL/IN border with rain reported at both KHUF and KLAF.
This rain is light with VFR cigs and vis under the area of
precipitation. Best chances of showers and storms during the morning
will be at KBMG, KHUF, and KLAF. Expect convection to diminish in
coverage across Western Indiana in the 12-15z timeframe. Then later
this afternoon and evening expect additional scattered thunderstorms
to develop across portions of Central and South Central Indiana with
the best threat for storms at KIND and KBMG and points south and
east. Mainly VFR conditions expected with brief periods of erratic
wind directions and sub VFR conditions under the strongest storms. A
few storms may produce brief gusts over 40 mph and lightning.

Expect convection to diminish in coverage after sunset with drying
conditions and VFR cigs tonight.

&&

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

$$

SHORT TERM...CM
LONG TERM...CM
AVIATION...CM