Area Forecast Discussion
Issued by NWS Indianapolis, IN

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000
FXUS63 KIND 180723
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
223 AM EST Sat Nov 18 2017

.UPDATE...
The LONG TERM Section has been updated below.

&&

.SYNOPSIS...
Issued at 332 PM EST Fri Nov 17 2017

A strong low pressure system will approach central Indiana tonight
and move through Saturday, bringing rain, chances for
thunderstorms and windy conditions to the area. Much colder air
will fill in behind the front for Sunday, but moderating
conditions return on Monday. A couple of frontal passages will
bring temperature swings during the week, but at this point these
appear to be mostly dry.

&&

.NEAR TERM /Tonight/...
Issued at 922 PM EST Fri Nov 17 2017

Sped up the showers up about an hour based on radar trends and High
Resolution Rapid Refresh reflectivity progs.

Previous discussion follows...
Issued at 332 PM EST Fri Nov 17 2017

Rain will overspread the area tonight from the west and northwest
as an upper jet and low level jet start to take aim at Indiana.
The best forcing will begin to move into northern parts of the
forecast area before midnight and then precip will spread south
from there. Elevated instability will start to advect into the
area on a strong low level jet and bring a slight chance for
storms to the western counties initially that will overspread the
forecast area as well during the overnight and increase to a
chance for storms before daybreak in the west. Low temperatures
will drop little from where they are currently with cloud cover
increasing and warm advection throughout the night. In fact, look
for increasing temperatures starting by midnight if not before. At
this point, the low level jet really amps up during the overnight,
with 50 to 70 kts possible, but enough of an inversion should set
up to keep these winds from mixing down.

&&

.SHORT TERM /Saturday through Monday/...

Issued at 332 PM EST Fri Nov 17 2017

Forecast focus is on potential for thunderstorms and the strength of
these, as well as on potential for strong wind gusts
Saturday/Saturday night.

A strengthening low pressure system will move through central
Indiana, pulling a cold front through the area on Saturday. The
thermodynamics with this system are lacking, but the dynamics are
impressive. A strong upper jet will be pointing toward the area
starting tonight and lasting through some of the day on Saturday
before it sinks south Saturday afternoon/evening. The low level jet
will also be in place over the area and quite strong as well.
Elevated instability will be plentiful, and this should allow
thunderstorm development in the warm advection ahead of the cold
front. During this pre-cold frontal passage time, though, NAM
soundings show a fairly stout inversion in place over the area. This
should inhibit these stronger winds from reaching the surface as it
should be difficult for them to punch through the inversion.
However, within showers and thunderstorms a few damaging gusts could
reach the surface by mixing down with the rain, and this goes along
well with a marginal risk for severe storms. Think widespread gusts
during this time will remain around 30 to 45 mph, with higher gusts
possible in showers and thunderstorms.

The highest winds in the low level jet will remain ahead of the cold
front, and winds aloft behind the front drop in speed.
However the pressure gradient increases across the area during that
time, and the inversion mixes out allowing for the winds aloft to
reach the surface. Thus think even though low level jet winds will
be higher ahead of the front, think surface sustained winds and
gusts will be higher after frontal passage. GFS is stronger than the
NAM but has some support from the 0z ECM, so think reality will wind
up somewhere in between. Think 30 mph sustained winds with gusts
over 45 mph are most likely between 4 and 8 pm, however, they could
be possible pretty much any time from mid morning until around
midnight. Thus used that timing for wind advisory issuance. Left out
the northernmost counties for now, though, as conditions there may
be a little less conducive to the stronger gusts due to earlier
timing of the cold frontal passage there. This area will need to be
monitored.

Storm chances will move out of the entire area by Saturday evening,
and much colder air will advect in. After Saturday highs in the
upper 50s to lower 60s, Saturday night lows will crash to the upper
20s to lower 30s. Sunday highs will probably not get out of the 30s.
Temperatures will rebound into the 40s to lower 50s Monday with some
sunshine.

&&

.LONG TERM /Tuesday through Friday/...
Issued at 219 AM EST Sat Nov 18 2017

Ensembles indicate a rather quiet weather pattern can be expected
during this period, with little threat for precipitation.

A short wave trough is progged to pass through the Great Lakes
region around Tuesday, but the better dynamics and deeper moisture
look to stay to the north of the forecast area. Will continue
with a dry extended.

&&

.AVIATION /Discussion for the 180600Z TAF Issuance/...
Issued at 1055 PM EST Fri Nov 17 2017

Aviation concerns are plenty for the TAF period with low level wind
shear, gusty winds, thunderstorms and low ceilings and visibilities
all possible at times. Good confidence in trends through the period,
but confidence in exact timing of any of these elements is not
great.

Short term models suggest a warm front, near the Mississippi River,
will lift northeast across central Indiana  through 12z. Occasional
s dowers will be moving over the terminals through the day.
Instability progs continue to show elevated instability, suggesting
there could be some lightning until the cold front passes 18z-21z.
So, went with VCTS in addition to widespread MVFR showers. Could
also see IFR conditions from time, but with GFS LAMP optimistic,
went no lower than 1K foot ceilings and ism.

40-45 knot jet will settle south and east across the terminals
overnight, so kept low level wind shear in through 10z-13z. After
that, winds will pick up especially after 18z as the front moves
through. Could see gusts as high as 35 to 40 knots toward 00z
Sunday.

&&

.IND WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 11 AM this morning to 1 AM EST Sunday for
INZ042>049-051>057-060>065-067>072.

&&

$$

SYNOPSIS...CP
NEAR TERM...CP/MK
SHORT TERM...CP
LONG TERM....JAS
AVIATION...MK



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