Area Forecast Discussion
Issued by NWS Indianapolis, IN

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364
FXUS63 KIND 010213
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1013 PM EDT Sun Apr 30 2017

.UPDATE...
The NEAR TERM and AVIATION Sections have been updated below.

&&

.SYNOPSIS...
Issued at 256 PM EDT Sun Apr 30 2017

A potent cold front will move through the area tonight, bringing
more potentially heavy rain and strong to severe thunderstorms. Much
cooler air will then move in for the work week. Windy conditions can
be expected Monday and Tuesday. A low pressure system will bring
more rain chances Wednesday into Saturday.

&&

.NEAR TERM /Overnight/...
Issued at 1012 PM EDT Sun Apr 30 2017

Central Indiana enjoying a brief reprieve from the heavy rain with
no precipitation across the forecast area over the last hour or
two. That break is short-lived however with renewed development
being aided by an axis of deep moisture at the nose of a
strengthening low level jet over eastern Illinois. The cold front
remains further west over Illinois with a thin convective band arcing
from near the Quad Cities down to near Cairo.

The overall forecast is in really good shape. The main adjustment
was to slow down timing of the cold frontal passage by a few hours
late tonight based on some of the most recent hi-res guidance.
Heavy rain and flooding remain the primary threats overnight with
precip water values in excess of 1.5 inches and much of the area
already having seen 3 to 5 inches of rain since Friday. With that
being said...enough shear...storm relative helicity and
instability present to not entirely rule out a damaging wind gust
or brief spinner with stronger convection until the cold front
passes. The overall severe weather threat is low but also non-
zero. Expect the initial wave of activity over eastern Illinois
will eventually shift into the area over the next few hours with
the convective band along the cold front to follow during the
predawn hours. Still anticipate the front will be near or just
east of the forecast area by daybreak.

Tweaked hourly temps to better match current obs...otherwise
overnight lows still look attainable. Zone and grid updates out.

&&

.SHORT TERM.../Monday through Wednesday/
Issued at 256 PM EDT Sun Apr 30 2017

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

There may be some partly cloudy skies Monday morning as the area is
in the dry slot behind the cold front. However, during the afternoon
wrap-around moisture will move in along with colder air aloft,
allowing skies to become mostly cloudy. Along with the moisture
return, some weak forcing could lead to a few showers, so went
with some low PoPs during the afternoon.

Colder air will flow in with highs only expected to be in the 50s
most areas. However it will feel cooler thanks to windy conditions.
A tight pressure gradient and cold advection will allow sustained
winds over 20mph and wind gusts approaching 40 mph. These are below
wind advisory criteria but are close. Will have to keep a close eye
for a need to go with an advisory if expected speeds increase a
little more.

Cyclonic flow continues Monday night along with a surface trough
moving through. This will provide enough forcing to go with some low
PoPs most areas. Breezy conditions will continue.

On Tuesday high pressure and drier air will try and build in from
the west, and this will allow dry conditions and some sunshine to
return. However the pressure gradient will still be tight enough for
breezy conditions.

Another upper trough will approach the area on Tuesday night and
Wednesday. An area of convergence will set up across southern
Indiana. These should provide enough forcing for some chance PoPs
across much of the area by Wednesday afternoon.

Stayed with the model blend for temperatures. These will be below
normal, especially Wednesday when highs will only be in the 50s.

&&

.LONG TERM /Wednesday night through Sunday/...
Issued at 230 PM EDT Sun Apr 30 2017

Models and ensembles in general agreement that the long term will
start off with a Plains trough and a weak eastern ridge. Then, an
upper low will break off and move into the Tennessee Valley before
being absorbed again by an Ontario low within the upper trough as
the trough moves into the Appalachians. However, the 12z GFS briefly
stalls the Tennessee Valley upper low Thursday night and thus is on
the slow side of the ensemble means and even more so the 00z ECMWF
by 12z Friday. However, the GFS is not as slow as the 06z or 00z
versions. So, will lean toward a quicker ensemble mix.

A southern Plains surface system will move across the Tennessee
Valley Wednesday night and Thursday and the Appalachians Thursday
night and Friday. Deepening moisture and isentropic lift should
result in widespread showers with the potential for more heavy rain
Wednesday night into Thursday. Then, should only see a 709few
lingering light showers for the remainder of the work week.

MOS blend temperatures look to have a good handle on the cool
airmass that will be in place along with the extensive cloud cover
that is expected to be around starting Wednesday night. The result
will be well below normal temperatures with highs only in the 50s on
Thursday. As the system exits northeast, should see some sunshine
and moderating weekend temperatures.

&&

.AVIATION /Discussion for 010300Z TAF Issuance/...
Issued at 1012 PM EDT Sun Apr 30 2017

Minor adjustments to wind gusts and direction over the next few
hours...otherwise forecast in good shape.

00Z discussion follows.

Restrictions expected within rain and storms tonight otherwise
VFR conditions through late afternoon Monday.

Waves of rain and embedded thunder continues to rotate through the
area from southwest to northeast as deep Gulf moisture is
advected north into the area ahead of deep low pressure centered
over the mid Mississippi Valley. The low will track northeast into
the Great lakes overnight and enable a cold front to swing across
the forecast area after 06Z. Periodic chances for showers and
thunderstorms will continue to impact the terminals until the
front passes...with MVFR and possibly IFR restrictions within any
of the heavier rain. Southeast winds this evening will veer to
south immediately ahead of the frontal passage. Gusts to 25kts
will be possible. Low level wind shear will also be a concern this
evening.

Once the front moves through...should see clouds scatter with even
a brief period of clearing as a well pronounced dry slot passes
across the region from west to east. Clouds will begin to fill
back in through the course of the day Monday however as the cold
pool associated with an upper low spreads into the area. Model
soundings and RH progs indicate ceilings could slip into the MVFR
category by late day although confidence is low at this point. The
primary impact to aviators on Monday will be strong wind gusts
from the southwest...possibly approaching 35kts at times in the
afternoon. Could see a few light showers develop as well later in
the day but no restrictions are anticipated.

&&

.IND WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 8 AM EDT Monday for INZ021-028>031-
035>049-051>057-060>065-067>072.

&&

$$

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



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