000
FXUS63 KIND 181916
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
316 PM EDT Fri Aug 18 2017

.UPDATE...
The SYNOPSIS...NEAR TERM and SHORT TERM Sections have been updated
below.

&&

.SYNOPSIS...
Issued at 316 PM EDT Fri Aug 18 2017

A weak front will move through central Indiana late tonight and
early Saturday with a few showers and thunderstorms. Dry weather
will return for the rest of the weekend with progressively warmer
and more humid conditions by early next week. A strong cold front
will track across the lower Great Lakes and Ohio Valley on
Tuesday...bringing a potential for severe weather. Drier and much
cooler weather will follow for the second half of next week.

&&

.NEAR TERM /Tonight/...
Issued at 316 PM EDT Fri Aug 18 2017

Surface ridging drifting across the Ohio and Tennessee Valleys this
afternoon keeping dry and less humid conditions across central
Indiana. 18Z temperatures ranged from the upper 70s north to mid 80s
south under partly cloudy skies.

Quiet weather will continue through the evening with potential for
scattered showers and perhaps a few thunderstorms late tonight as a
weak frontal boundary moves into the area from the west.

Cu field has developed this afternoon along an 850mb thermal trough.
As the trough axis progresses east into western Ohio...already
seeing coverage of the cu field diminish across western counties as
drier air expands into the region. This trend will continue through
the rest of the afternoon with skies becoming mostly clear for a few
hours during the evening.

Scattered convection is expected to develop in the vicinity of low
pressure and an associated weak frontal boundary moving east through
the mid Mississippi Valley. The low will track east ahead of an
upper level wave...gradually weakening as it moves into the forecast
area after 06Z. Axis of deeper moisture is thin and limited ahead of
the front...and overall forcing aloft is focused further north and
generally poor over the forecast area. Instability profiles are weak
and elevated above the remnants of a capping inversion. MLCAPEs will
likely remain at or below 500j/kg for most through the night. Result
will be scattered convection arriving into the region late tonight
in a weakened state with any thunderstorms being isolated at best.
highest pops will be generally north of I-70 between 06-12Z. The
take home here is that some will get rain...many will not.

Overall confidence is high in the forecast...lower in convective
coverage and timing.

Temps...an overall model blend worked nicely for tonight as lows
should hold in the mid 60s for much of the area.

&&

.SHORT TERM /Saturday through Monday/...
Issued at 316 PM EDT Fri Aug 18 2017

Forecast challenges are few through the period as overall quiet
weather resumes. Greatest interest will be Monday specifically with
respect to cloud cover potential for eclipse viewing.

The frontal boundary will be in the process of departing into Ohio
on Saturday morning but the upper wave trailing the surface features
will not fully clear the forecast area until closer to midday. Will
hang onto low chance pops for any scattered light showers that
persist over eastern counties through the morning. By early
afternoon...the entire forecast area will be dry as northwest flow
aloft and weak high pressure at the surface take over. Similar to
this afternoon...convective temperatures in the lower 80s and a
trailing low level thermal axis behind the upper wave will produce
conducive conditions for diurnal cu formation gradually diminishing
from west to east later in the day.

Upper flow will transition quickly to a quasi-zonal pattern by
Saturday night through the rest of the short term as central Indiana
remains on the periphery of a flattened ridge from the lower
Mississippi Valley east into the southeast states. High pressure
will remain the primary influence at the surface through Monday as
well with Gulf moisture riding up around the back side of the ridge
through the Missouri Valley into the upper Midwest. Sky conditions
will range from mostly clear to partly cloudy through Sunday night
with likely less cu during the day Sunday than that experienced
today or expected Saturday.

Southerly flow developing for Monday will gradually advect
progressively more humid air into the region with model soundings
suggestive of scattered cu for the afternoon. The overall cloud
forecast for Monday is of high interest to many with the eclipse
during the afternoon. It does appear at this time that cloud
coverage during the 17-20Z time period will likely not exceed
30-40% for most of central Indiana. Scattered cu will be possible
which should not impact viewing. Potential though for mid and high
level clouds drifting into the area from convection to the west
poses a bit more of a concern. Will monitor closely with
additional detail and higher confidence to come over the weekend
as we get within the 24- 48 hour window.

Temps...with subtle cool advection on Saturday...highs will hold in
the 80 to 85 range. The advection of warmer and more humid air
Sunday and Monday will enable temperatures to gradually warm to near
90 on Monday. Lows will remain in the 60s through the period. A
model blend matched low level thermals well and was used.

It should be noted that even though the path of total eclipse will
pass to the south and west of the forecast area...much of central
Indiana will experience 90% or greater coverage. This will likely
cause brief temperature drops on the order of a few degrees near the
peak coverage point between 18-19Z before the temps begin to rise
again as the eclipse ends. Impacts on temperatures will be greatest
over the lower Wabash Valley in closest proximity to the path of
totality through western Kentucky.

&&

.LONG TERM /Monday Night Through Friday/...
Issued at 214 PM EDT Fri Aug 18 2017

Models are close enough that the model blend initialization
was accepted for most items.

An upper trough will develop over the area early in the long term,
allowing colder air to flow into the area. A potent cold front will
move through Tuesday, bringing showers and thunderstorms. Will have
to watch for the threat of severe weather Tuesday, as the Storm
Prediction Center has outlooked part of the area for the potential
of severe storms.

High pressure will then build into the area for Wednesday through
Friday, bringing dry and cooler conditions. Temperatures will be
below average during this period.

&&

.AVIATION /Discussion for 181800Z TAF Issuance/...
Issued at 1230 PM EDT Fri Aug 18 2017

Brief restrictions possible in scattered convection early Saturday
morning...otherwise VFR conditions generally expected.

Quiet afternoon in progress with surface ridging over the region.
Diurnal cu field has formed with an 850mb thermal trough drifting
through the forecast area. Expect the cu to linger through the
afternoon with a diminishing trend from the west. Cu will fully
dissipate early this evening. W/NW winds near 10kts with briefly
higher gusts are possible this afternoon.

Clouds will begin to increase later tonight as a weak frontal
boundary drifts into central Indiana ahead of an upper wave.
Overall moisture profiles are not terribly impressive and forcing
aloft is generally lacking. Expect the potential for scattered
convection to briefly impact the terminals from the predawn hours
through Saturday morning. Instability is marginal at best with
KLAF likely to be the only site that may see a stray storm late
tonight. Will carry VCTS here with prevailing showers and a VCSH
mention at the other three terminals.

Model soundings and RH progs are suggestive of the possibility of
MVFR stratocu to move across the region immediately behind the
front for a few hours Saturday morning before more substantial
drying advects into central Indiana by the afternoon. Light and
variable flow tonight and Saturday morning will veer to northwest
by the end of the period.

&&

.IND WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SYNOPSIS...Ryan
NEAR TERM...Ryan
SHORT TERM...Ryan
LONG TERM....50
AVIATION...Ryan