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596
FXUS63 KIND 060146
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
946 PM EDT Sun May 5 2024

.KEY MESSAGES...

- Confidence continues to increase for severe weather potential both
  Tuesday and Wednesday. All hazards are possible, with damaging
  wind and large hail as the primary threat.

- Heavy rain threat Monday through Wednesday. Flooding possible due
  to multiple rounds of convection.

&&

.FORECAST UPDATE...
-- Changed Discussion --
Issued at 946 PM EDT Sun May 5 2024

Forecast is in good shape this evening.

Showers and some thunderstorms are moving north across western
Kentucky, associated with an upper system. These will continue to
push north and east overnight, moving into southern portions of
central Indiana. Based on latest hi-res model guidance and trends on
radar, tweaked PoPs upward a bit in the south later tonight.

Low temperatures look good, so only adjusted hourly forecasts based
on latest trends.
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&&

.SHORT TERM (This evening through Monday)...
Issued at 254 PM EDT Sun May 5 2024

Gradual erosion of the morning stratus deck has taken place the last
few hours...but the most recent ACARS sounding from KIND shows the
inversion remains and the result has been a steady transition into a
robust cu field. This has mitigated temp rises slightly from
previous thinking...with 18Z readings ranging from the mid 60s in
the north to lower 70s south.

High pressure currently moving across the Great Lakes will be the
primary influence on weather across central Indiana for the rest of
the day and into the first part of the overnight...but the approach
of a surface wave late tonight into Monday will transition to a more
unsettled pattern for Monday. The bigger impact however will be
lifting a warm front into the Ohio Valley during the day Monday that
will become a focal point for heavier rainfall and multiple
opportunities for severe weather across the region for Tuesday and
Wednesday. More will be discussed on those threats in the Long Term
discussion below.

Rest of This Afternoon and Tonight

The deeper subsidence aloft present with the high pressure over the
Great Lakes has served to subtly strengthen the low level inversion
and that has been the reason clouds have been extremely sluggish to
mix out. Model soundings do hint at the drier air and subsidence
working down into the boundary layer through late afternoon which
should enable a weakening of the inversion and a decrease in cu into
the early evening. But the initial high clouds associated with
convection over the lower Mississippi Valley north into the Ozarks
are already spreading into the region from the southwest. There will
be some sun into the evening but clouds will largely remain in
abundance.

The leading edge of rainfall will lift into the Ohio Valley this
evening and should arrive across the lower Wabash Valley around or
shortly after midnight. Further expansion northeast will be slow and
fighting against a progressively dry flow of air from the E/NE that
will keep the boundary layer from moistening up. Expect rainfall
will struggle to maintain its coverage as it lifts across southern
counties during the predawn hours with amounts remaining very light
at under a tenth of an inch.

Monday

The aforementioned surface wave will track along the Ohio River on
Monday with a slow but steady saturation occurring as moisture
overruns the warm front across the southern half of the forecast
area. The easterly flow will remain a problem for achieving deeper
saturation...particularly across the northern half of central Indiana
where showers will struggle to expand north as they run into the
drier boundary layer air. An increase in isentropic lift over the
front during the afternoon will lead towards widespread light
showers mainly south of I-70 but with generally light amounts no
greater than 0.10 to 0.25 inches.

Cannot entirely rule out a few rumbles of thunder as well with model
soundings hinting at weak amounts of elevated instability. As
mentioned above however...the greater impact will be to move the
pieces into place and essentially table set for the more significant
convective impacts poised for Tuesday and Wednesday.

Temps...decent temperature gradient expected overnight with lows in
the upper Wabash Valley nearing 50 while much of the southern half
of the forecast area hovers near 60 under increasing clouds and with
rain developing. Nudged highs back a bit from previous forecasts for
Monday with abundant cloud cover and periodic light showers.
Generally anticipate lower 70s for most of the region.

&&

.LONG TERM (Monday night through Sunday)...
Issued at 254 PM EDT Sun May 5 2024

The primary forecast challenge through the long range continues to
be severe weather potential Tuesday and Wednesday. Active weather
looks to continue through the remainder of the week and possibly
thereafter as troughing takes hold over the east coast.

SYNOPTIC OVERVIEW

A trough is currently working its way through the intermountain
west. As the trough ejects into the Plains on Monday it will trigger
rapid cyclogenesis. The resulting surface low then occludes over the
Dakotas and the parent trough flattens considerably. Multiple
waves/vort maxes should rotate around the broader trough Tuesday and
Wednesday. These waves will determine the timing and location of
potential severe weather.

The overall pattern is quite favorable for repeated convective
activity. While occasional weak cold fronts have infiltrated the
Midwest, none have been strong enough to displace the warm buoyant
air far to the south. As such, warm humid air is expected to stream
northward ahead of the main trough. The first round of convection
develops over the Plains Monday afternoon, and progresses rapidly
eastward into Tuesday morning. Guidance has been trending more
towards the idea of this activity maintaining coherence as it
crosses Indiana. That means a round of showers and thunderstorms is
becoming increasingly likely early Tuesday. This could have
implications for later development in the afternoon and evening.

Guidance has trended a bit further south in recent runs regarding
convective redevelopment Tuesday afternoon. Additionally, the
initiating vort max appears a bit weaker. Parameter space remains
supportive of severe weather as ample shear and instability are
present. The morning convection may alter the location of best
instability, as mentioned above. Trends will need to be monitored.

By Wednesday, a more potent wave looks to rotate around the base of
the trough and initiate another round of storms. Guidance depicts
surface cyclogenesis occurring as the wave works its wave into the
Midwest. Poleward transport of warm buoyant air looks to be quick as
the surface low pressure develops. The northward flowing air mass
should allow an instability gradient to set up somewhere in the
region. This may become the focused corridor for convective
potential over time.

Heavy rainfall is possible, especially on Wednesday, as model
soundings show deep moisture and large LCL-Freezing Level spread.
This could lead to rather efficient rainfall production.

FORECAST CONFIDENCE

Confidence continues to increase regarding severe potential Tuesday
and Wednesday, yet many important factors still need to be ironed
out. Namely, placement and timing of initiating vort maxes/waves.
Additionally, evolution of Monday`s convection and how it may impact
Tuesday`s activity.

CSU machine learning and CIPS both continue to show strong signals,
but with a slight southward trend in the past 24 hours. The area
remains rather broad, and will narrow with time as guidance
continues to come into consensus. Overall confidence is greatest
with the larger-scale pattern and decreases with regards to the vort
maxes and their timing/placement.

&&

.AVIATION (00Z TAF Issuance)...
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Issued at 656 PM EDT Sun May 5 2024

Impacts:

- MVFR conditions developing around 12Z Monday across most sites
- Showers will impact the southern sites the most late tonight
  and Monday

Discussion:

VFR and quiet conditions can be expected through the first 9 hours
or so of the TAF period, then lower clouds will increase from south
to north. MVFR ceilings with perhaps some MVFR visibility in rain
will then spread northward to most sites. IFR cannot be ruled out,
especially at KBMG, but odds are not high enough to include at the
moment.

Winds will become northeast by 06Z and then east around 12Z.
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&&

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

$$

UPDATE...50
SHORT TERM...Ryan
LONG TERM...Eckhoff
AVIATION...50