Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 161416

Area Forecast Discussion
National Weather Service Indianapolis IN
1015 AM EDT Wed Aug 16 2017


The Near Term and Aviation Sections have been updated below.


Issued at 216 AM EDT Wed Aug 16 2017

Broad high pressure over centered over West Virgina will exit the
area today allowing a warm and humid air mass to arrive in Central
Indiana on southerly winds. A warm front associated with Low
pressure over the western plains states is expected push toward
Central Indiana...bringing chances for showers and thunderstorms
this afternoon and tonight.

The low will strengthen and reach the western Great Lakes on
Thursday and drag a cold front across Central Indiana. This will
result in more rain chances.

Cooler and dry weather is expected on Friday as high pressure
builds across the area. More showers and storms will be possible
on Saturday...but dry weather is expected to return as the weekend


.NEAR TERM /Rest of Today/...
Issued at 1015 AM EDT Wed Aug 16 2017

For the update...radar mosaic shows scattered showers over southern
Illinois and southwest Indiana moving northeast toward the forecast
area. Temperatures are in the mid and upper 70s with dewpoints in
the upper 60s to lower 70s and light southeasterly winds. Based on
the aforementioned radar trends, will add some slight chance pops in
the southwest and south a few hours earlier than before, moving in
over the next couple of hours. Kept any chance for thunder out of
the area until the afternoon as instability will be increasing
enough then to warrant it, but observations now show no lightning in
the showers near the forecast area. Also f note is that the showers
are generally weakening/dissipating as they move eastward,
especially the northernmost ones. Nevertheless, a slight chance for
showers increasing to a chance for showers and thunderstorms this
afternoon across the south and southwestern counties is warranted.
Across the northeastern counties, just went to slight chance in the
afternoon since instability will be less there and the cap from the
high will be stronger there as well. Also increased dew points to
closer match current conditions. No other appreciable changes made.
Previous discussion follows...

Water Vapor this morning shows weak ridging and subsidence across
Illinois and Indiana...with a weak short wave over SW Missouri.
The GFS suggests this weak wave will push across Central Indiana
this afternoon as the ridge axis exits to the east. Forecast
soundings suggest a gradual increase of moisture within the
column through the day. BY late afternoon weak summer CAPE near
2160 j/kg is available with convective temperatures in the middle
80s and minimal capping. THus will start off the day dry at most
location...but include precip chances late this afternoon as the
peak heating and weak dynamics arrive. With plenty of heating time
available and generally a warm air advection pattern in place
will trend highs at or above the forecast builder blend.


.SHORT TERM /Tonight through Friday Night/...
Issued at 216 AM EDT Wed Aug 16 2017

GFs and NAm agree and set up a favorable pattern for precip
tonight and again on Thursday. Both models suggest a deepening
upper trough over the upper midwest will be pushing slowly
northeast through Thursday...providing a warm and moist southerly
flow to Central Indiana. Models suggest favorable upper support
associated with this feature will push across the forecast area
tonight and again on Thursday. Forecast soundings tonight show a
deeply saturated column. Within the lower level the 310K GFS
Isentropic surface shows good up glide with specific humidities of
6-7 k/kg. Thus confidence for pops tonight is high and will try
to trend toward the likely category.

More favorable conditions exist on Thursday as the upper trough
axis is suggested by the models to push across the area in the
afternoon...along with an associated cold front. Forecast
soundings suggest some dry air within the column during the
morning and should we see a few breaks of sun within the warm
sector....steep lapse rates should lead to afternoon convection
ahead of the cold front. Forecast soundings on Thursday afternoon
show a healthy 2800 J/KG of CAPE with attainable convective
temperatures. Thus confidence is highest for precip chances on
Thursday afternoon as these features come together. Again will
strive to use at least likely type wording.

Models suggest that the cold front will pass and exit the area on
Thursday night...however will need to continue to include some
pops for any lingering showers/storms past 00Z Friday as dynamics
exit the area and diurnal instability is lost.

Models suggest cyclonic lower level flow on Friday as the
departing low pressure system to the northeast exits through the
course of the day. Forecast soundings suggest a dry column...with
a mid level inversion. Thus some BKN CU will be expected in the
afternoon as convective temps are reached. Given the cold air
advection and the NW flow developing aloft...will trend highs at
or below the forecast builder blend. Finally on Friday
Night...high pressure continues its influence and it builds
across the region in the evening...however overnight the GFS has
changed...developing a secondary upper trough poised to push
across Central Indiana after 06Z. Fort the moment...forecast
soundings late on Friday Night trend toward saturation as the
upper support arrives. Thus will need to include at least some low
chance pops aft 06Z Saturday...but confidence here is low until
this trend persists in a few more runs.


.LONG TERM /Saturday through Wednesday/...
Issued at 318 AM EDT Wed Aug 16 2017

Upper trough swinging through the region on Saturday will
necessitate some low pops for potential showers or storms, but
much of the weekend should be dry as surface high pressure moves
into the area.

Another frontal system appears poised to impact the area early
next week, and will necessitate mid to high chance pops at times
for thunderstorms.

Temperatures through the period should be within reasonable
distance of normal, and blended initialization handled this well.


.AVIATION /Discussion for the 16/15Z TAFs/...
Issued at 1015 AM EDT Wed Aug 16 2017

Updated current conditions only. Previous discussion follows...

VFR through the period.

An approaching system may spark showers and a few thunderstorms
this afternoon, with chances increasing as the period wears on.
Thunder is a threat but too uncertain for explicit inclusion at
this time. Cannot rule out brief MVFR or worse conditions in
showers and storms but again, too uncertain and transient for
inclusion at this time.

Winds will be less than 10KT through the period, generally
southerly early, becoming more westerly late.





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