Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 190634

Area Forecast Discussion
National Weather Service Indianapolis IN
234 AM EDT Sat Aug 19 2017

The LONG TERM Section has been updated below.


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 /Rest of Tonight/...

Issued at 939 PM EDT Fri Aug 18 2017

Increased pops across the northwestern counties around Sat 06Z to
match current radar trends, otherwise no changes. Considered
lowering overnight lows a degree or 2, but held off due to
increasing cloud cover overnight.  Updated grids have been sent.

Previous Discussion...
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. 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 /Tuesday Through Friday/...
Issued at 232 AM EDT Sat Aug 19 2017

Ensembles are in good agreement with the upcoming weather pattern
during this period. Short wave energy is expected to drop southeast
through the Great Lakes on Tuesday, eventually carving out a long
wave trough over the eastern parts of the country for the rest of
the extended.

Will keep PoPs in the forecast for Tuesday and Tuesday night as the
upper trough and associated cold front move through. In the wake of
this front, it appears a drier and more stable air mass will move
into the area. Will go dry after Tuesday.


.AVIATION /Discussion for the 19/06Z TAF Issuance/...

Issued at 1152 PM EDT Fri Aug 18 2017

VFR conditions are expected to be the prevailing flight category
throughout the TAF period despite increasing chances for showers
and thunderstorms after Sat 06Z with a frontal boundary. The only
exception could be some MVFR fog formation around dawn at KLAF
dependant on moisture from any showers/storms that do go through
over the next several hours. Meanwhile, winds through the period
will be variable at 4 to 8 kts.




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