Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 172053
AFDIND

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

.UPDATE...
The SYNOPSIS...NEAR TERM and SHORT TERM Sections have 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 172100Z IND Taf/...

VFR conditions are expected to continue the next few hours as
high cloud as seen on GOES16 continues to stream across the
Region. Added some LLWS tonight as the warm front passes.

Previous Discussion Below

/Discussion for the 171800Z Tafs/...

Issued at 1241 PM EST Fri Nov 17 2017

VFR Conditions are expected to deteriorate to MVFR tonight and to IFR
on Saturday Morning.

GOES16 continues to show clouds invading across Central Indiana.
Low pressure over the Plains states will strengthen and push
eastward night...allowing a warm front to surge northward across
Indiana. This will bring a round of showers to central Indiana
tonight...along with MVFR conditions. Overnight Cooling and rain
may lead toward some IFR condition near 12Z on Saturday...but a
return to MVFR will be expected due to mixing and marginal
heating within the warm sector on Saturday morning. More rain and
MVFR conditions will be expected as the cold front passes on
Saturday afternoon. Time heights show very saturated lower
levels late tonight through Saturday night with favorable lift.


&&

.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...JP



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