448
FXUS63 KIND 031902
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
302 PM EDT Fri May 3 2024

.KEY MESSAGES...

- Potential for patchy fog and low clouds late tonight and early
  Saturday.

- Isolated to scattered showers and storms through this evening then
  again on Saturday.

- Multiple chances for showers and storms throughout most of the
  forecast period, with continued above normal temperatures.

- Potential for severe weather during the middle of next week,
  highest chances Tuesday evening and Wednesday night.

&&

.SHORT TERM (This evening through Saturday)...
Issued at 256 PM EDT Fri May 3 2024

Damp and mainly cloudy afternoon in progress across the forecast
area as a diffuse frontal boundary lingers over the southeast half
of central Indiana. Have seen some breaks in the lower stratocu
develop behind the boundary but tend to fill right back in with an
abundance of low level moisture lingering. Scattered showers have
redeveloped over southeast counties over the last hour or two. 18Z
temperatures ranged from the upper 60s to mid 70s.

Today marks the beginning of a more unsettled regime setting up over
the Ohio Valley for the next several days and not expected to reach
its zenith until the early to middle part of next week. However the
presence of lingering low level moisture at least across parts of
the region and the remnant boundary likely to settle near the Ohio
River tonight will bring at least a periodic threat for isolated to
scattered convection through late day Saturday and eventually beyond.

Through Tonight

Expect any isolated showers or thunder will focus in the vicinity of
the boundary and deepest portion of the moisture plume...focused
across the southeast half of the forecast area. The current ACARS
sounding highlights weak instability levels that have increased
slightly courtesy of diurnal heating despite the clouds. It should
be noted however that the actual primary windshift and dewpoint
boundary lags back to the northwest from the front...and that could
aid in additional isolated development early this evening focused
back near the I-70 corridor. Will hold onto isolated pops for a
short period as far northwest as a Terre Haute-Indy Metro-Muncie
line as a result. Could certainly see a few rumbles of thunder but
overall any convection over the next few hours will be disorganized
and weak in the absence of little if any BL shear and light winds
through the lower levels.

Broad area of clear skies and dry air noted on current satellite
imagery this afternoon back across Wisconsin and northern Illinois
associated with a high pressure ridge. This feature will drift east
into the Great Lakes tonight and while the bulk of the subsidence
and dry air will remain to the north of the forecast area...this
will sway enough influence locally to allow for clearing skies late
tonight focused across the northern Wabash Valley. Further south
across the forecast area...the dewpoint boundary will shift south a
bit before stalling overnight with much of the forecast area south
of I-70 remaining at 60 degree dewpoints or higher. While any threat
for a shower is minimal overnight considering little forcing aloft
and negligible instability...the presence of the deeper moisture
will keep the potential for lower stratus across the southern half
of the forecast area and may also bring patchy fog into play prior
to daybreak. While the air will be slightly drier in northwest
counties...there is an increased potential for some fog formation in
these areas as well with skies set to clear late.

Saturday

The aforementioned surface ridge will move northeast away from the
Great Lakes on Saturday...leaving a residual axis of drier air
extending southwest from lower Michigan into eastern Missouri. This
will be slowly gobbled up through the course of the day between the
deeper moisture plume focused across the Ohio Valley and our
southern counties and a frontal boundary tracking through the
Missouri Valley. The veering of low level winds to easterly and
eventually southeasterly by the afternoon will enable the deeper
moisture to advance back to the northwest across central Indiana and
interact with an increasingly unstable airmass as CAPE values climb
to 1000-2000 j/kg. While overall forcing aloft and shear remain
weak...the combination of the instability and moisture will be
enough to generate scattered convection again on Saturday with the
best focus once again over the southeast half of the forecast area.

Storms will be of pulse intensity and may see a few stronger cells
that can throw out localized higher wind gusts. Otherwise...
lightning and brief/localized heavy rainfall will be the primary
impacts to outdoor activities on Saturday from any convection.

Temps...lows will fall back into the middle 50s tonight across the
northern Wabash Valley but most of the rest of the forecast area
will remain closer to 60. Low level thermals support highs rising
into the upper 70s and lower 80s across much of central Indiana
Saturday with the warmest reading likely to be across western
portions of the forecast area where convective coverage will be
lower and more sunshine will be realized.

&&

.LONG TERM (Saturday night through Friday)...
Issued at 256 PM EDT Fri May 3 2024

We`re looking at a fairly active long range with precipitation
chances nearly every day. Severe weather is not expected through the
weekend, though this may change by early to mid next week.

SYNOPTIC OVERVIEW

The upper-level flow pattern across North America is rather simple.
A long ridge axis stretches up the eastern seaboard and a broad
trough exists over the northern Plains. Along the Mexican border, a
zonal but fairly fast subtropical jet extends into the Gulf. Further
west, over the Pacific Ocean, another trough can be found diving
southward out of the Gulf of Alaska. It`s this Pacific trough that
may become the driving factor for active weather next week.

Through the weekend, however, we`ll be dealing with occasional
showers and thunderstorms as a few impulses aloft pass by. The
first, covered in the short term, is more a part of the northern jet
while a second lifts northeast from the subtropical jet Sunday into
Monday. With no strong push of continental air, a warm and buoyant
air mass is likely to be in place through the weekend and into next
week. As such, any system passing by should have access to enough
instability for convective precipitation.

In fact, model soundings Sunday-Monday show essentially a moist
adiabatic profile. Low LCLs with high freezing levels may promote
more contribution from warm rain processes. Available moisture is
high (PWAT over 1.5 inches) and deep, so rainfall may be quite
efficient at times. This of course depends on how much forcing there
is for convection, and some model uncertainty remains regarding
where exactly the vort max tracks.

By Monday afternoon, the Pacific trough should be ejecting into the
Plains in a highly amplified manner. Guidance shows the trough
occluding quickly, with upper-level flow regaining a more zonal
orientation Tuesday and Wednesday. The occluded low meanders over
the northern Plains for a few days, with vort maxes occasionally
rotating around and progressing eastward over the Midwest. Guidance
is in good agreement regarding the larger-scale pattern but
disagrees with the smaller features (vort maxes). The placement and
timing of these vort maxes may play a critical role determining
severe potential Tuesday and Wednesday.

FORECAST CONFIDENCE

COnfidence is good with the large-scale pattern, but decreases with
regards to sensible weather. The primary driver of this uncertainty
is variations in track/timing of various vort maxes. Additionally,
convective regimes can have a cascading affect on subsequent
convective evolution...so uncertainty can be high even if the larger-
scale pattern is more certain.

That being said, a few things can still be said about our potential
for severe weather / flooding next week. As mentioned before,
ensemble guidance is in good agreement with the larger pattern.
Additionally, NAEFS/ECMWF situational awareness tables show
anomalous moisture through this weekend and much of next week.
Deterministic guidance, while showing more disagreement between
individual models, is still useful in visualizing how this moisture
interacts with passing waves. Periodic surges of richer/deeper
moisture advect northward ahead of each wave, signifying potential
for heavy rainfall in addition to severe weather through the period.

Regarding severe weather, CSU machine learning and CIPS analogs have
been honing in on the Tuesday-Wednesday period over our region. It
remains a rather broad area, and favorable synoptic conditions
(Occasional synoptic forcing, instability, shear) add confidence to
these signals. It`s difficult to downscale things any further at
this range since the final outcome will likely rely on features not
yet resolvable. Still, the potential exists for severe weather along
with heavy rainfall next week. Stay tuned for updates.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 100 PM EDT Fri May 3 2024

Impacts:

- Isolated showers and thunderstorms through early evening

- Fog possible overnight but depends upon how much MVFR stratus
lingers/expands late tonight and early Saturday

Discussion:

Low stratus persists in spots early this afternoon but some breaks
in the cloud deck have developed as well. Increasingly diffuse
frontal boundary draped across the region with higher dewpoint air
persisting over the southeast half of central Indiana will support
isolated convective development through the remainder of the
afternoon and into the evening. The only terminals that is likely to
be free from any potential rain is KLAF as drier air is already
expanding into the northern Wabash Valley.

Winds will eventually align to N/NE overnight as the remnant frontal
boundary settles near the Ohio River. Low level RH progs and model
soundings show plenty of leftover moisture within the boundary layer
that may expand back to the northwest towards daybreak Saturday as
low level winds veer to a more easterly direction. The potential for
fog will be dependent on the degree of clearing which remains
uncertain as there are hints that MVFR level stratus will encompass
the terminals near daybreak. Winds will veer further to southeast on
Saturday with lower clouds gradually decreasing in coverage.

&&

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

&&

$$

SHORT TERM...Ryan
LONG TERM...Eckhoff
AVIATION...Ryan