545
FXUS63 KIND 200511
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
111 AM EDT Mon May 20 2024

.KEY MESSAGES...

- Isolated showers and storms across parts of the northern forecast
  area this afternoon and Monday afternoon/evening.

- Unseasonably warm through Tuesday, highs in the mid to upper 80s.

- Increasing threat for at least 1-2 rounds of strong/severe t-
  storms late Tuesday through Wednesday.

- Pattern remains active going into the weekend with storm chances
  Friday night through Sunday.

&&

.FORECAST UPDATE...
Issued at 917 PM EDT Sun May 19 2024

- Thunderstorms ending far northwest; Otherwise Mostly clear and
warm.
- Lows in the low to mid 60s; Warm

Surface analysis late this evening shows a broad yet poorly
organized area of high pressure stretching from NY State, across
Indiana and Ohio and then to points southwest including LA and
Mississippi.  The remnants of a weak cold front was still found over
far northwest Central Indiana. This front along with what is left of
diurnal heating was still allowing some thunderstorms to be present
over the Kokomo and Delphi areas. Weak southerly surface flow
remained in place across Central Indiana and dew point temps were in
the lower 60s.

As heating continues to be lost over the next few hours any
lingering convection in the northern parts of the forecast area is
expected to diminish and end. This should lead to dry weather across
the entire forecast area by 11pm-12a. HRRR is on board with this,
showing our forecast area precipitation free by midnight. Forecast
soundings overnight continue to show a dry column overnight and dew
point depressions are suggested to remain at 3F or greater. Thus the
fog threat overnight does not appear to be as great as the previous
nights. However, a caveat tonight will be area across the northwest
part of our forecast area that have just received new rainfall.
Additional lower level moisture in these are may all for the
development of fog with lower dew point depressions that might be
suggested. Thus will include at least a patchy fog mention at those
locations. Overall ,with light to calm southerly winds in place
across the area lows in the lower to mid 60s appear reasonable.

&&

.SHORT TERM (This evening through Monday)...
Issued at 256 PM EDT Sun May 19 2024

Weak ridging within a broad split flow pattern will promote rather
benign weather through the period. Our one exception may be a few
isolated showers or storms this evening near our northern CWA border
where a weak boundary resides. There is little in the way of flow
through the depth of the column. As such, storm-scale organization
beyond single/pulse cell is not expected. Nevertheless, the column
is quite dry so a core collapse leading to a brief burst of gusty
winds and small hail is not out of the question. Overall the threat
appears very low (under 25 percent).

Any convection is expected to diminish as night falls, leaving us
with a few lingering mid to high-level clouds. Winds at the surface
may go light and variable as the boundary layer decouples. As
mentioned before, the column is quite dry...so any fog overnight
will likely be very patchy in nature and mainly ground fog. With dew
points in the low 60s and no advection our low temperatures likely
remain in the 60s as well.

Monday will be quite similar to today, including a low risk for
showers and storms in our far north/northwest. Shower and storm
chances are again diurnally driven within a low-flow atmosphere.
Anything that develops will likely go through its life cycle within
an hour. The thermodynamic profile also remains similar to today, so
a low risk of a brief downburst (gusty winds and small hail) exists
in our northwestern counties.

As for temperatures, there is little to no change at the 850mb
level. However, displacement of the ridge to slightly east of us may
allow for a more brisk SSW wind (10kt) to develop during the
afternoon Monday. This may in turn lead to deeper mixing than today
allowing temperatures to be a degree or two higher.

&&

.LONG TERM (Monday night through Sunday)...
Issued at 256 PM EDT Sun May 19 2024

Monday Night Through Wednesday.

Another mild night is expected Monday night with lows only falling
into the mid to upper 60s as southerly flow keeps dewpoints above
normal for this time of the year.  Winds will be weakest across the
southern counties so will have to keep an eye out for fog as those
areas are also where we expect the sparsest cloud cover, but with a
few days of drying out fog will likely be limited to the river
valleys. Focus then quickly shifts to the potential for strong to
severe storms as a strong upper level low pressure system moves from
the Dakotas into the Upper Midwest and a surface cold front
stretches down into the Ohio Valley.

Storm chances begin as early as late Tuesday night as the surface
cold front pushes into northwestern Illinois and a weakening MCS
looks to potentially push out ahead and impact western Indiana.
There remains a fair amount of uncertainty as to if the MCS can
survive that late into the night with a lack of robust instability,
but if the cold pool matures it may last longer than the models
suggest.  Location of where this could impact is very uncertain but
a best guess right now would be along the I-70 corridor.

The better severe weather threat looks to set up Wednesday evening
into the early overnight hours along and just ahead of the frontal
passage. With dewpoints near 70, surface based CAPE values may be
well into the 2500-3500 J/kg range with models showing robust
moisture transfer in the 850-700mb layer. Shear is a bit more
marginal with the 500mb jet more towards western Illinois keeping
the effective bulk shear more towards 30kts which would still be
more than enough for severe weather, but may limit the higher end
threat. Surface winds will also be generally southwesterly which
will limit the curve in the hodograph.  Another factor working
against the overall severe threat is a weak cap at the top of the
boundary layer, but lift right along the front should be enough to
overcome the cap but would mean that the convection would generally
be limited to just along and not ahead of the front.

With all that in mind, do think that there is a solid shot for
isolated to scattered severe weather Wednesday evening into the
early overnight, especially across the southern half of the state
with a storm complex along the surface cold front. With several
unfavorable factors there is a potential for the lower end severe
threat, but lift with the front will help to overcome some of the
more limiting of the factors.

Thursday Through Sunday.

The surface front is then expected to stall just south of the Ohio
River Thursday into Friday with additional storm chances south of
the forecast area.  Confidence that the front settles to the south
is fairly high so will limit the afternoon POPs compared to previous
forecasts. By Friday afternoon there is a small chance that the
front pushes just far north to impact the southern counties, but by
then the front will be fairly diffuse and less impactful overall.
There does look to be a weak wave that may generate another storm
complex, but again that is currently expected to be more of an issue
along and south of the Ohio River.

Confidence in the forecast going into the weekend is fairly low with
significant model spread on the broader synoptic scale, but the
pattern looks to remain active with at least one round of storms
likely between Friday night and Sunday.

&&

.AVIATION (06Z TAF Issuance)...
Issued at 110 AM EDT Mon May 20 2024

Impacts:

- MVFR fog at KLAF overnight

Discussion:

With rain haven fallen earlier, fog has already developed at KLAF.
With just some passing high clouds and light winds, this fog should
persist overnight. Cannot rule out below MVFR conditions at times,
but not confident enough to add at this time.

Otherwise, VFR conditions are expected with some mid and high clouds
along with a few cumulus during the afternoon. Will watch how things
develop today to see if convection has to be added to KLAF for late
afternoon onward.

&&

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

$$

UPDATE...Puma
SHORT TERM...Eckhoff
LONG TERM...White
AVIATION...50