383
FXUS63 KIND 171831
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
231 PM EDT Fri May 17 2024

.KEY MESSAGES...

- Isolated/scattered showers this afternoon and evening with some
  thunder possible

- Areas of fog again tonight

- Warmer with Mid 80s Sunday into early next week

- Increasing severe storm threat Tuesday night into Wednesday

&&

.SHORT TERM (This evening through Saturday)...
Issued at 231 PM EDT Fri May 17 2024

A positively-tilted trough axis is positioned northwest of us, and
weak shortwave perturbations embedded within are evident in water
vapor channel imagery passing through. The surface thermal, MSLP,
and flow pattern is fairly nondiscript, but the low levels are quite
moist. Even with clouds, only minimal surface diabatic heating will
be needed for isolated to scattered convection this afternoon and
evening. Movement will be slow given the weak midlevel flow (east-
northeastward at around 5-10 mph).

No displacement of the moist low level air mass and wet ground means
that fog will once again be possible tonight, and probably over a
broader area. The radiative component will be minimal but moisture
magnitude should be enough for areas of fog, some of which could be
dense.

Since IVT shows the deeper rich moisture plume shifting east with
the first perturbation, more widespread rain is not expected.
Stratus may be prevalent until the upper-level trough axis passes
midday Saturday, and isolated/scattered convection will be possible
until then, though peaking with diurnal cycle. By Saturday`s diurnal
peak the subsidence inversion will be strengthening and this should
suppress most convection, though isolated coverage is still
possible into the afternoon.

&&

.LONG TERM (Saturday night through Friday)...
Issued at 231 PM EDT Fri May 17 2024

The lower troposphere is expected to remain quite moist Saturday
night despite slightly deeper mixing Saturday afternoon and a slight
nudge of drier continental air. To offset this, moist ground, light
winds, and clear skies should still result in at least some patchy
fog.

Our highest temperature at IND so far this season has been 85 (on
May 2nd), and as ridging builds this weekend should cause us to
reach or exceed this value. Mid-80s for highs are about 10-15
degrees above mid-May climo, and should continue into the first part
of next week as western troughing deepens and ridging across the
east continues to amplify.

The aforementioned ridge/trough pattern will result in an
increasingly active convective regime across the Plains this weekend
into early next week. Flow is modest in our Region through early
next week, and probably incapable of sustaining a consolidated area
of convection this far east through Monday and into Tuesday, but
this may occur Tuesday night. By then, medium range models indicate
phasing and a stronger mid-latitude system moving northeastward from
the Plains into the Great Lakes region. How quickly this system
closes/occludes, and at what pace it moves northeastward is in
question as ensemble spread is still relatively large.

Rich moisture through a deep layer preceding this system is favored
in this pattern and indicated in the models, and by Tuesday night
and/or Wednesday thunderstorms appear likely. Mesoscale details will
dictate timing and magnitude of the severe threat. Scenario #1 is an
MCS Tuesday night or early Wednesday which may decrease with the
diurnal cycle, and scenario #2 with a more lagging system and/or
second subsequent shortwave trough (as is indicated but a subset of
GEFS members) which may lead to convection forced along the front
during the diurnal cycle Wednesday over central Indiana. Sufficient
instability and deep layer shear overlap are indicated to support
organized thunderstorms and a severe threat in either case. Ensemble
upper-end QPF outliers generally indicate limited flood threat with
this first system owing to its quicker progression.

It looks like continental air may nudge down to around the Ohio
River at least briefly Thursday in the wake of the aforementioned
system but should readily return shortly thereafter in response to
the next system in the westerlies. But, timing of this system and
its interaction with a moist/unstable environment for additional
convection is uncertain, since ensemble spread grows considerably by
this time period.

Day 8-14: Early indications for the weekend of the 25th and
subsequent week are for a continued synoptically-active period with
near or just above normal temperatures. There are indications of a
progressive short wavelength pattern, albeit not particularly
amplified, which should bringing periodic chances for convection and
associated hazards. Ensemble data generally shows positive
precipitation anomalies during this period.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 1248 PM EDT Fri May 17 2024

Impacts:

- Isolated showers this afternoon through early evening

- Areas of fog and IFR or lower visibility tonight

Discussion:

The air mass will not change much through tonight, so with wet
ground, residual low level moisture, and nocturnal cooling, fog will
likely form again. This could be locally dense. Coverage may be
greater tonight compared to last night. MVFR ceilings also may
occasionally occur. Isolated to scattered showers are expected later
this afternoon and evening. Brief visibility reductions could occur
if one impacts an airport, but this will be short-lived if it
occurs.


&&

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

$$

SHORT TERM...BRB
LONG TERM...BRB
AVIATION...BRB