389
FXUS63 KIND 181719 AAA
AFDIND

Area Forecast Discussion...UPDATED
National Weather Service Indianapolis IN
119 PM EDT Sat May 18 2024

.KEY MESSAGES...

- Dense fog early this morning

- Isolated showers and storms this afternoon

- Unseasonably warm Sunday-Tuesday with highs in 80s...ahead
of increasing strong t-storm threat late Tuesday and Wednesday

&&

.MESOSCALE UPDATE...
Issued at 119 PM EDT Sat May 18 2024

Visual observations from the NWS office indicate cumulus has some
vertical depth. Day Cloud Phase satellite RGB shows some enhancement
of the cumulus field along a north-south oriented differential
heating boundary, from near Kokomo to Indianapolis, but no
glaciation has been noted yet. Across northern Indiana, at the
periphery of stronger wake subsidence behind the departing upper
level trough, cumulus is even more vertically developed, and a few
brief weak radar echoes have occurred just north of our forecast
area. Latest IND ACARS sounding shows a dry subsident capping layer
about 3-4-km, limiting vertical growth of convection. The shallow
vertical updrafts that are occurring are experiencing very weak flow
and thus almost no shear in the convective layer, so they`re slow-
moving and short-lived. So, isolated showers should have ~5-8%
coverage at most today. If a convective cell can overcome the
capping layer and dry midlevel entrainment, a few lightning strikes
would be possible, but this seems like a very low probability, and
if it occurs would be brief as individual cell lifespan will be
limited.

&&

.SHORT TERM (Today and tonight)...
Issued at 259 AM EDT Sat May 18 2024

Early This Morning...

Mostly clear skies, light winds, and moist low level atmosphere and
ground have lead to some fog formation, especially across the
southern forecast area. Satellite and surface observations show that
dense fog is not widespread, but larger areas of thicker fog are
developing in the far south.

Based on the satellite trends, and little change in the lower
atmosphere, have issued a Dense Fog Advisory for the far south and
southeast portions of the area. Will issue a Special Weather
Statement farther north and west and will watch trends for possible
expansion of the Advisory.

Today...

Fog will mix out after sunrise, but stratus will take longer to do
so in the southeast where thicker fog was. Cumulus will pop up
during the later morning and afternoon, so partly cloudy looks good
for overall sky cover today.

The area will remain in some broad upper troughing, but best forcing
will be south of central Indiana, closer to an upper low in the
Tennessee Valley.

In addition, there may be some lingering old surface boundaries
across the area, based on convection seen Friday night.

Given these, and some weak instability this afternoon, will keep an
isolated shower/thunderstorm mention across the southeast two-thirds
of the area. Confidence is not high though given the weak forcing.

Temperatures will warm to around 80 degrees most areas.

Tonight...

What, if any, convection that develops this afternoon should be gone
or below mentionable coverage by 00Z. Will continue with a dry
forecast this evening.

Overnight, a front will approach the area from the northwest.
However, an upper ridge will have moved in. The front would have
limited moisture to work with anyway, and with the ridge aloft, feel
that central Indiana will remain dry.

Wouldn`t be surprised if some patchy fog returns in the south given
the light winds and continued low level moisture.

Low temperatures will be around 60.

&&

.LONG TERM (Sunday through Friday)...
Issued at 259 AM EDT Sat May 18 2024

Sunday through Monday Night...

Unseasonable warmth and rather humid conditions will be the rule
through early next week as heights build into Indiana from the
Mexican Highlands ahead of a slowly-plunging trough over the western
CONUS.  Broad yet very weak surface high pressure aligned from the
Great lakes to the Mid-Atlantic will oversee light winds Sunday that
should trend to light to moderate southerly breezes by late Monday.
Can`t rule out a stray shower, especially late Monday...yet
confidence was too low to deviate from the Blend`s 14/less POPs.
Dewpoints in the 60-65F range will guide overnight minimums...while
afternoon highs reach the mid to upper 80s both days courtesy of
ample sunshine.

Tuesday through Friday...

Central Indiana`s proximity in the very warm and unseasonably humid
warm sector of a slowly-approaching cold front...for what should be
at least 3 periods into Wednesday...will present favorable
conditions for at least a couple rounds of strong to severe storms.
Diurnally driven instability possibly exceeding 2000-3000 J/kg both
Tuesday and Wednesday should combine with ample mid-level lapse
rates and increasing wind shear.  Uncertainty lies in embedded short
waves in the southwesterly flow from the central High Plains to
Upper Midwest which will influence lift and placement/timing of
forcing aloft.  Wednesday should present the greater threat of
stronger storms as the front approaches/crosses the CWA...although
any prior MCS around the Tuesday night timeframe (and subsequent
cloudiness) could dampen Wednesday`s potential.  This dynamic and
variable set-up will be refined in further updates into early next
week.

Behind the mid-week frontal passage...a few showers cannot be ruled
out amid modest westerly flow.  The next weather system should track
into the Upper Midwest as the workweek ends, with POPs increasing
again as it`s cold front approached around the end of the long term.
Both max and min temperatures will approach record levels into
Tuesday...before several days of seasonable conditions follow.

&&

.AVIATION (12Z TAF Issuance)...
Issued at 642 AM EDT Sat May 18 2024

Impacts:

- IFR and worse conditions early in the period

Discussion:

Fog and stratus will persist early in the period across all sites.
There remains some uncertainty on how fast fog and then stratus will
mix out, so timing in the TAFs only has low to medium confidence.

Cumulus will then form later this morning and persist into the
afternoon. An isolated shower or thunderstorm is possible this
afternoon, but odds are much too low to include in the TAF.

Additional fog may form overnight tonight.


&&

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

$$

MESOSCALE...BRB
SHORT TERM...50
LONG TERM...AGM
AVIATION...50