005
FXUS63 KIND 150414
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1214 AM EDT Wed May 15 2024

.KEY MESSAGES...

- Widespread showers and isolated-widely scattered thunderstorms
  through tonight into Wednesday.

- Widespread 1-2 inch rainfall likely, with highly localized 3-4+"
  amounts and flash flooding possible.

- Rain and storm chances return at times late Thursday and beyond.

- Above normal temperatures late week onward.

&&

.FORECAST UPDATE...
Issued at 1000 PM EDT Tue May 14 2024

The northern portions of a short wave disturbance will continue to
slowly cross the Midwest tonight, as corresponding surface low
pressure nudges eastward along the Ohio Valley.  A moist adiabatic
profile will feature a local precipitable water maximum of 1.25-1.50
inches over central Indiana overnight...promoting areas of numerous
rain showers.  Embedded heavier downpours, especially in isolated
TRWs across the region`s southern zones will present the risk of
localized flooding...although steady light to moderate rain over
central/northern zones could also present this threat by later
tonight for any locations with steady rain.  Winds will be steady
around 10 mph while slowly backing from ENE to NE.  Dewpoints will
guide temperatures dropping a couple degrees overnight...with lows
expected to range from the upper 50s across the northern tier, to
low 60s elsewhere.

&&

.SHORT TERM (This evening through Wednesday)...
Issued at 308 PM EDT Tue May 14 2024

A large, vertically-stacked low pressure system will very slowly
drift eastward through the lower Ohio Valley through tonight and
continuing into Wednesday.

Light to moderate rain has gradually overspread much of the area
today, and expect showers to become a bit more numerous overnight as
the low slowly approaches in a very moist airmass.

Though widespread cloud cover has significantly inhibited insolation
and resultant destabilization, some convective development has been
able to occur to our west/southwest today, and CAMs suggest
development of additional showers and isolated-widely scattered
thunderstorms over central Indiana later this afternoon and into
tonight. Instability is very weak but at least minimally present to
support this threat. While the coverage has been a bit aggressive at
times, there is a consistent signal for localized heavy rainfall
tonight.

Precipitable water values are hovering around 1.2-1.4+" - which is
90th percentile or greater climatologically. While wet bulb zero
heights/warm cloud depth is not incredibly deep, it is still in
excess of 10kft across the area per ACARS data, and profiles are
deeply saturated, with moist adiabatic lapse rates throughout the
depth of the troposphere, suggesting fairly efficient rainfall
rates, particularly in the presence of any convective enhancement.
There is also the presence of a remnant stationary boundary to our
north and the approaching frontal structures within the main low,
and suggestion that associated frontogenetic forcing could enhance
rainfall across some portion of the area this evening into tonight.
Weak flow fields also are suggestive of slow cell/band motions
tonight, contributing to potential for training/heavy rain amounts.

There is signal in both global/parameterized models and high
resolution/convective allowing models for locally enhanced rainfall
amounts and an attendant flash flood threat. Local probability
matched means suggest an isolated threat for 3-4+ inches, which
would exceed flash flood guidance.

Confidence is lower in placement of this heavy rain potential and
this will depend significantly upon evolution of any convection over
the next several hours, thus a Flash Flood Watch is likely a bit of
overkill, but will message this threat through a variety of other
available means for tonight.

As the low begins to depart tomorrow, expect rainfall to come to an
end from northwest to southeast. Some local thunder threat may exist
during the midday/afternoon hours, also gradually diminishing from
northwest to southeast.

Diurnal cooling tonight will be inhibited by the widespread cloud
cover and elevated surface dewpoints, and have leaned warmer than
blend numbers with lows in the upper 50s to low 60s. Early rain and
clouds on Wednesday will limit warming a bit, though some afternoon
breaks in the clouds should allow highs to get into the upper 60s to
low 70s.

&&

.LONG TERM (Wednesday night through Tuesday)...
Issued at 308 PM EDT Tue May 14 2024

A progressive upper-level pattern looks to bring a few rain/storm
chances through the long range.

SYNOPTIC OVERVIEW

Our synoptic pattern is currently quite progressive, with an active
polar and subtropical jet. Phasing between these two has not been
regular, and each branch has remained largely separate. As a result,
deep poleward moisture transport hasn`t been easy to come by. Still,
embedded waves within the progressive flow have provided enough lift
for rain today.

This pattern should continue through at least the weekend. In fact,
another wave very similar to the one we`re seeing today looks to
arrive Thursday or Friday. Like the current system, rain and storms
are possible with little threat for severe weather. A stronger
thunderstorm is possible on Thursday, however, as a mid-level speed
max passes nearby to our south. This feature may provide enough
shear for storm-scale organization but the overall dynamics involved
are not terribly impressive. Heavy rain may become the primary
hazard as model soundings show deep moisture with a large LCL-
Freezing Level spread, signifying the potential for efficient
rainfall production.

Guidance then transitions the overall flow pattern to something more
akin to split flow. Global teleconnections generally trend negative
going forward, so a more blocky/complicated flow pattern makes
sense. Concurrently, ensemble guidance shows broad troughing taking
shape over the western US with surface ridging over the Southeast.
This may allow for more efficient moisture/instability transport
northward over the Plains and Midwest. West coast troughing and
associated fast southwesterly flow aloft may allow for multiple days
of upstream convective activity. As such, our forecast may depend a
bit on how this activity evolves and propagates downstream. There is
a signal for enhanced precipitation during the 6 to 14 day time
frame.

FORECAST CONFIDENCE

Overall forecast confidence remains average, with the greatest
confidence now through the weekend. Thereafter, ensemble guidance
loses coherency but retains a weak enough signal to say that west
coast troughing is favored beginning next week. How this plays out,
and how it affects Indiana remains to be seen...but the larger-scale
pattern is beginning to reveal itself. Determining our day to day
weather, especially when convection is involved, is quite difficult
at this range. Temperatures look to remain around average this week,
with a warm up next week as flow turns more southerly.

&&

.AVIATION (06Z TAF Issuance)...
Issued at 1214 AM EDT Wed May 15 2024

Impacts:
- IFR to MVFR cigs through 20Z
- MVFR vsbys through 16Z due to -RA

Discussion:

Periods of IFR to MVFR cigs are expected through the rest of the
overnight hours as rain continues across central Indiana.  Cigs
should gradually improve to consistently MVFR after 12Z before
scattering out around 20Z. Rain coverage will gradually decrease
after 12Z but occasional showers are expected through the early
afternoon.  Winds will generally remain easterly to northeasterly at
5-10kts through the TAF period.

&&

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

$$

UPDATE...AGM
SHORT TERM...Nield
LONG TERM...Eckhoff
AVIATION...White