Area Forecast Discussion
Issued by NWS Indianapolis, IN

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000
FXUS63 KIND 172253
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
552 PM EST Fri Nov 17 2017

.UPDATE...
The AVIATION 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 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 /Monday Night through Friday/...

Issued at 220 PM EST Fri Nov 17 2017

ECMWF suggests High pressure departing on Monday Night before a cold
front crosses the state on Tuesday Afternoon and Tuesday night.
ECMWF suggests good forcing at that time...yet limited moisture.
Forecast builder has responded by not including any pops which is a
little surprising. Thus confidence is low.

High pressure looks to return to the area on Wednesday and Thursday
as NW flow is show aloft with subsidence. ECMWF shows another weak
short wave and frontal boundary passing on Friday...yet once again
no pops included by forecast builder presumably due to a lack of
moisture. Thus again confidence is low.

Look for temps at or below seasonal normals as mainly a NW flow will
be in place next week.

&&

.AVIATION /Discussion for the 180000Z TAF Issuance/...
Issued at 552 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, just west of the Mississippi
River will lift northeast across central Indiana 03z-12z Saturday.
Showers will be overspreading the terminals after 03z, and
instability progs are showing elevated instability, suggesting there
could be some lightning toward 06z. So, went with VCTS 03z-19z in
addition to widespread MVFR showers. Could also see IFR conditions
from time, but with GFS LAMP more optimistic than the SREF, just
left it low end MVFR for now.

40-45 knot jet will settle south and east across the terminals
tonight, so went with low level wind shear through 12z. After that,
winds will pick up especially after 18z as the front moves through.
Could see gusts as high as 35 to 40 knots.

&&

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

&&

$$

SYNOPSIS...CP
NEAR TERM...CP
SHORT TERM...CP
LONG TERM....Puma
AVIATION...MK


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