Area Forecast Discussion
Issued by NWS Indianapolis, IN

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617
FXUS63 KIND 210837
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
336 AM EST Wed Feb 21 2018

.UPDATE...The SYNOPSIS...NEAR TERM AND SHORT TERM sections have been
updated below.

&&

.SYNOPSIS...
Issued at 336 AM EST Wed Feb 21 2018

A cold front will slowly drop southeast across Kentucky and
Tennessee today. The front will stall and return north over central
Indiana Thursday night and then another cold front will sweep
through late in the weekend. The fronts will interact with a plume
of tropical moisture and result in periods of rain and flooding
through the weekend.

Save a small chance of rain or snow over the area Monday night, high
pressure will provide a relief from the rain Sunday afternoon
through the middle of next week.

Temperatures will be falling through the 40s and 30s today behind
the cold front. But then, they will return to slightly above normal
by Thursday and well above normal by Friday.

&&

.NEAR TERM /Today and Tonight/...
Issued at 336 AM EST Wed Feb 21 2018

The main issue today will be POPs and temperatures.

Radar was showing linear convection along a cold front that extended
front around Anderson to Indianapolis to Loogootee at 150 AM.
precipitation shield behind the front extended west into central
Illinois. Temperatures were falling fast behind the front. LAF was
all the way down to 40 degrees whereas pre-frontal temperatures are
in the upper 60s to around 70. Expect the rain to take awhile to
move through, but the heavier rain after 12z should be over southern
sections, closer to the departing front. That area has been mostly
dry the past couple of days. So should be able to leave the Flood
Watch expire. Blend POPs range from categorical over southern
sections to slight or no chance far north. With the front moving
away, this looks reasonable.

Models in good agreement that an impulse, currently over
southeastern Texas, in the persistent southwest flow aloft, will
lift northeast over central Indiana tonight. With tropical moisture
plume still over the area, should see more decent rains, mainly
south of Interstate 70. Blend QPF is bringing up to three quarters
of an inch to those areas tonight. Also of concern, is areas north
of Terre Haute, Indianapolis and New Castle late today and tonight,
where model soundings support a mix with freezing rain or snow. That
said, warm ground and less low level moisture there should make it a
little to no impact event there.

Some thought about issuing a Flood Watch for the southern counties
tonight, but with them being dry of late, should be able to hold off
for later in the week and weekend, when more high QPF is expected.

High confidence temperatures will fall through the 40s and 30s today
in the wake of the front in good low level cold air advection setup.
Blend overnight lows in the 30s look good as well with clouds around.

&&

.SHORT TERM /Thursday through Friday night/...
Issued at 336 AM EST Wed Feb 21 2018

POPs will be the main focus for the short term.

Should see a brief break in the rain Thursday afternoon and evening.
Then, a wave will move along the cold front and allow it to return
north over the area as a warm front. With tropical moisture and
upper waves in southwest flow still in play, should see more
widespread rain starting overnight Thursday night through Friday
night and into the long term. Will likely need more flood headlines
for this period.

With the front moving north, confidence is high temperatures will
warm back to well above normal by Friday.

&&

.LONG TERM /Saturday through Tuesday/...
Issued at 327 AM EST Wed Feb 21 2018

The wet and active pattern will continue through Sunday morning
before high pressure and a relaxed flow regime aloft provides a
much needed break with dry and mild weather for early next week.

The heavy rain and flooding threat will continue Saturday as the
frontal boundary lays out across central Indiana with surface wave
rippling along the boundary. A final and more significant surface
wave is poised to swing through the area Saturday night into
Sunday morning. The op GFS continues to hold serve with an
intensifying low pressure system aided by a strengthening
negatively tilted upper trough and while it remains the strongest
with the surface wave Saturday night...it is gradually gaining a
bit more support from other models. This would bring a greater
convective potential Saturday night and possibly the threat for a
low topped squall line with stronger winds along and ahead of the
cold front overnight Saturday.

Potential for an additional 1 to 2 inches of rain with locally
higher amounts Saturday and Saturday night with precip water
values once again rising to climatological maxes for late
February. Taking into account expected rainfall through Friday this
will only exacerbate flooding concerns into the weekend...
especially for rivers and streams...and in lowland and poor
drainage areas.

Drier weather will build in behind the front Sunday with high
pressure taking over across the Ohio Valley and lower Great Lakes
through the first half of next week. Temperatures will remain mild
for early next week with highs primarily in the 50s.

&&

.AVIATION /Discussion for the 210900Z TAF Update/...
Issued at 327 AM EST Wed Feb 21 2018

Early morning updates focus on near term adjustments as the cold
front and widespread rain impact the terminals. Seeing IFR
conditions and lower develop within heavier rainfall. Winds have
veered to northwest at all terminals as of 08Z.

06Z discussion follows.

Rainfall has been slow to arrive at TAF sites tonight, only
impacting KLAF currently. However, it should start spreading into
the remaining TAF sites over the next few hours as a cold front
moves into central Indiana. Conditions will quickly deteriorate
to MVFR and then IFR levels with onset of rain, and there will be
little improvement until late in the TAF period when VFR
conditions return with strengthening high pressure after Wed 18Z.
Meanwhile, winds will remain gusty out of the southwest over the
next several hours with a low level wind shear threat. They will
gradually veer to the west and then northwest this morning with
frontal passage.

&&

.IND WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 7 AM EST this morning for INZ021-028>031-
035>049-051>057-060>065-067>072.

&&

$$

SYNOPSIS...MK
NEAR TERM...MK
SHORT TERM...MK
LONG TERM....Ryan
AVIATION...TDUD/Ryan



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