NWS Indianapolis, IN wrote its last area forecast discussion before the first event at May 14, 1:42 PM EST, 3 hours 26 minutes earlier. Its key messages:
Area forecast discussion, May 14, 1:42 PM EST, 3 hours 26 minutes before
Area Forecast Discussion
National Weather Service Indianapolis IN
242 PM EDT Wed May 14 2025
.KEY MESSAGES...
- Chances for showers and storms into tonight. A strong storm
possible this evening
- Conditional severe storm threat for Thursday, especially late day
into Thursday night. Additional severe threat on Friday, primarily
south of I-70.
- Near record highs possible Thursday and remaining very warm on
Friday
&&
.SHORT TERM (This evening through Thursday night)...
Issued at 241 PM EDT Wed May 14 2025
Rest of This Afternoon....
Clouds from this morning's convection have thinned, allowing
temperatures to rise into the 70s. Where sunshine has been present
all day, temperatures were around 80.
In between, a weak pressure trough was moving slowly northeast.
Isolated showers have been developing along this trough. This will
continue as the trough drifts northeast. Thunderstorms will also be
possible as instability grows, but not expecting any severe weather
given the expected parameter space.
Warm and humid conditions will persist.
Tonight...
Warm advection aloft will kick in tonight, with the leading edge of
the stronger advection moving southwest to northeast during the
night. The boundary layer will cool, but instability will remain
aloft. Forcing from the warm advection will work with the
instability aloft to create some scattered showers and thunderstorms.
Instability will be high enough that some potentially severe hail
will be possible in a few storms. The best instability will remain
across the southwest forecast area, so that is where the main threat
will be.
As the forcing moves away from the best instability in the
southwest during the night, coverage may decrease. Will have highest
PoPs (chance) southwest with lower PoPs northeast.
Thursday...
For the severe threat, please see the section below.
A cap should keep much of the day dry. Can't rule out something
breaking through though if an area of the cap is weaker than the
rest, so will keep some slight chance PoPs in the afternoon.
The air will be very warm for mid-May. Given the expected lack of
convection, there will be enough sunshine to boost temperatures to
near record levels. However, some areas that saw a lot of rain
Wednesday may have a harder time reaching these levels thanks to the
wet ground. Will go mid 80s to around 90 for highs.
Forcing will be enough for chance PoPs all areas Thursday night.
---Severe potential on Thursday---
A pronounced and pristinely maintained elevated mixed layer capping
a warm/moist PBL will lead to strong instability across our region
tomorrow. This EML is being carried eastward from its source region
in the central Rockies by strengthening westerlies at the base of
closed/deep low migrating across the northern Plains. There is about
a 75-mile spread in the position of the attendant Pacific front that
will serve as an initiating boundary and this is one element of
uncertainty in tomorrow's severe thunderstorm risk. The other is
whether or not the trough's geometry, position, and timing will be
enough to effectively lift the capping EML this far south. If
convection does form, the parameter space would support severe
thunderstorms, including supercells with the potential for
significant severe. Scenarios, and reasonable best- and worst-case
scenarios are covered below.
Scenario #1 Little/no convection forms this far south precluding a
convective hazard threat.
Subjective estimate of occurrence: 30%
Analysis: Either, (1) diurnal destabilization and frontal
convergence are not a match for the magnitude of the capping EML,
and/or (2) trough lags and is less amplified (there has been a
slight multi-run trend toward this).
Scenario #2 Convection initiates, likely well west of our area near
the Mississippi River, and sustains eastward into central Indiana.
Subjective estimate of occurrence: 55%
Analysis: Once convection initiates, as midlevel thermal ridge moves
eastward and slight ascent/cooling overspread the area, residual
strong instability into the evening would sustain convection. There
may be a tendency for storm mode to be less organized and rooted
within moist/ascent layer aloft, atop the EML, thereby limiting the
severe magnitude some. With any convection that can overcome
inhibition from strong capping EML and remain deeply rooted in the
PBL, a more substantive severe threat would be possible with very
large hail the primary threat, though all hazards are possible.
Scenario #3 Intense/mature supercells sustain into central Indiana
during the evening with accompanying significant magnitude severe.
Subjective estimate of occurrence: 15%
Analysis: If strong capping EML weakens sufficiently and supercells
are more easily able to deeply root in the PBL traversing central
Indiana well into the evening, all severe hazards are possible,
including a tornado threat. Elongated middle portion of the
hodograph favor very large hail in this scenario. This is a less
likely scenario since ascent needed to lift the capping EML appears
insufficient given the characteristics and timing of the parent
trough and so the higher-end scenario will likely be confined to the
Great Lakes region, north of our area.
In scenario #2 or #3, convection may persist into late evening or
even overnight along the trailing and decelerating Pacific front
across roughly the southern one half to one third of Indiana. Later
in the night, convection should mostly or completely diminish as
forcing for ascent isn't particularly robust and some capping may
remain. Further, the front will become ill-defined as low-level flow
veers, and there is a weak warm/moist advection signal at best.
There is a low probability scenario of warm/moist advection being
just sufficient enough to interact with trailing boundary keeping
some convection going over southern Indiana into the pre-dawn hours.
&&
.LONG TERM (Friday through Wednesday)...
Issued at 241 PM EDT Wed May 14 2025
Friday and Friday Night...
Forcing from an upper low will interact with lingering moisture and
instability to produce the threat for showers and thunderstorms once
again Friday. Timing remains uncertain, but later Friday afternoon
into Friday evening look best. With rain not likely not arriving
until later in the day, highs in the mid 80s look reasonable.
--- Severe Potential Friday ---
Models generally agree on a slow-moving closed low moving toward
Lake Superior by late in the day. The mean westerlies on the
equatorward side of this low are about +2-sigma in magnitude and
particularly broad, contributing to strong deep layer shear across
our entire region. When coupled with at least moderate instability,
the threat for severe thunderstorms is present with significant
severe a possibility, particularly if an organized MCS/wind threat
evolves. Some of the models are more suppressed with the warm sector
and focus a potential intense MCS and high-end wind event further
south closer to the Ohio River, though this evolution and placement
is uncertain. Even then, some severe threat would occur this far
north with all convective modes and severe types possible. Once the
characteristics/timing of the mid-upper trough are more clear, and
its influence on the warm sector's northward return, we will be able
to convey more certainty.
Saturday and Sunday...
Cooler and drier air will move in behind a cold front on Saturday,
and then high pressure will build in for Sunday. This should keep
the area dry for the weekend and bring seasonable temperatures.
Monday into Wednesday...
Uncertainty increases for early next week as an upper low ejects out
of the southwestern USA and moves into the Upper Mississippi Valley.
Questions remain on the strength and timing of the upper low as it
moves northeast, as well as the development of its associated
surface low pressure system. This leads to lower confidence in PoPs
through the period.
On Monday the surface warm front will try to move north, but upper
ridging will building in aloft. Will have some low PoPs, but the
ridge may be able to squash any convection that tries to form.
PoPs will continue Tuesday and Wednesday as the system itself moves
in and through. Right now it looks like Tuesday night has the best
chances.
Temperatures will depend on the coverage of rain and timing of the
warm front, but highs in the 70s look reasonable.
&&
.AVIATION (18Z TAF Issuance)...
Issued at 1259 PM EDT Wed May 14 2025
Impacts:
- Scattered SHRA and isolated TS this afternoon mainly KLAF/KIND and
then again tonight most sites
- Brief MVFR ceilings possible this afternoon
- Potential for more MVFR ceilings Thursday morning
- Gusty winds Thursday
Discussion:
A weak trough will move north this afternoon and bring more
scattered showers and isolated storms, mainly for the northern
sites. This evening, another boundary will move north and bring
another threat for convection to most of the sites.
The day Thursday should remain convection free. Some MVFR ceilings
are possible in the morning with another boundary, then gusty winds
will develop.
&&
.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&
SHORT TERM...50/BRB
LONG TERM...50/BRB
AVIATION...50