032
FXUS63 KIND 180505
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
105 AM EDT Sat May 18 2024

.KEY MESSAGES...

- Areas of fog again tonight

- Warmer with Mid 80s Sunday into early next week

- Increasing severe storm threat Tuesday night into Wednesday

&&

.FORECAST UPDATE...
Issued at 913 PM EDT Fri May 17 2024

- Lingering showers/storms end
- Partly Cloudy with Fog overnight

Surface analysis late this evening shows weak area of high pressure
over MI with disorganized southerly flow across Central Indiana. A
weak boundary was found across northern parts of Central Indiana as
a few late blooming showers or storms developed there this evening.
GOES16 shows decreasing cloud cover across Central Indiana as
diurnal cooling has begun. Water vapor imagery shows best
moisture aloft across the area has shifted a bit farther east,
stretching from Arkansas and LA to KY and SRN Indiana before
reaching points in the northeast. Dew points across Central
Indiana remained rather high, in the upper 50s and lower 60s.

Little overall change is expected overnight with the ongoing weather
pattern. Weak, poorly organized southerly flow looks to  remain
place as Indiana remains caught between two more organized area of
low pressure, the first being over the northern plains and the
second off the Atlantic Coast. Thus as heating is lost this evening
clouds and any lingering showers/storms will continue to diminish.
Light flow, clear skies and high dew points will be a recipe for
fog. Models suggest dew point depressions to fall to 1-2F. Thus will
expect patchy fog once again overnight, with areas of dense fog
possible late.

Lows will once again fall to the upper 50s and lower 60s.

&&

.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 (06Z TAF Issuance)...
Issued at 105 AM EDT Sat May 18 2024

Impacts:

- Areas of fog and IFR or lower visibility tonight

Discussion:

The combination of mostly clear skies at most sites along with light
winds and low level moisture will lead to fog development overnight.
There remains some uncertainty on how widespread fog will be. Will
continue to forecast IFR and worse at the sites, but with lower
confidence. The exception in lower confidence will be KBMG where IFR
has already been observed.

Fog will mix out after 13Z or so leaving behind VFR conditions. An
isolated shower is possible, but coverage will be too low to mention.

&&

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

$$

UPDATE...Puma
SHORT TERM...BRB
LONG TERM...BRB
AVIATION...50