Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 240815

Area Forecast Discussion
National Weather Service Indianapolis IN
415 AM EDT Sat Mar 24 2018

The SYNOPSIS...NEAR TERM and SHORT TERM Sections have been updated


Issued at 415 AM EDT Sat Mar 24 2018

A significant late season winter storm will impact central Indiana
through this evening. Heavy snowfall will be possible over parts of
the area...especially along and just south of the I-74 corridor
where several inches of snow are possible. High pressure will
briefly return for Sunday...but unsettled weather is expected for
much of next week as a slow moving frontal boundary moves across the
region. Rain and a few thunderstorms will be possible at times as
more seasonable temperatures return.


.NEAR TERM /Today through This Evening/...
Issued at 415 AM EDT Sat Mar 24 2018

Precipitation gradually continues to expand east into the region
early this morning...slowly eating into a stout dry layer present
through the lowest 10kft aided by easterly flow. 08Z temps range
from around 30 over eastern counties to the lower 40s in the lower
Wabash Valley.

Where to begin this morning. The focus remains on the approaching
system which will be bring a variety of precipitation types to the
area through this evening including the potential for a heavy...wet
snow on the northern flank of the precip shield. There are factors
for and against the potential for periods of heavy snow bands
developing later today that make this a much lower confidence
forecast than desired as we near event onset. Going to discuss in
order below...updated model guidance and mesoanalysis...factors for
heavy snowfall...factors against heavy snowfall...and ultimately
expected snowfall totals and headline changes. This has been a
headache inducing and exceptionally difficult process to be kind.

With event onset rapidly approaching...we have the benefit of
observations and the CAMs being able to capture most or all of the
event into this evening. This has provided a nudge up in confidence
levels yet still not where would like to see it at this late stage.
Low pressure currently moving through eastern Kansas early this
morning with an upper level wave over the northern Plains.
Convection has broken out north of the surface low over the Missouri
Valley...while locally heavy snow with sleet occasionally mixed in
has grown in coverage across eastern Iowa into western and central
Illinois. A band of enhanced precipitation with a combination of
rain...sleet and snow has moved into the Wabash Valley as of 08Z and
appears to be the main catalyst in saturating the boundary layer.
This will progress east across the forecast area through daybreak.

The surface low and upper wave will track E/SE through the course of
the day...with the surface wave tracking into the Tennessee and
lower Ohio Valleys this evening while the upper wave moves into the
area and weakens overnight as it shifts east. Overall model guidance
remains solid on the main features including the track and strength
of the surface low and upper wave. The NAM remains an outlier to the
north compared to most all other model data which considering the
strength of the dry airmass to the northeast of the not
preferred. Most CAMs guidance along with the GGEM...op GFS and most
of the ensemble data have zoned in on an axis generally 1-2 counties
wide near and just south of the I-74 corridor where the greatest
chance to see heavier snow and higher amounts will exist.

More substantial deformation and frontogenesis develops and moves
into the region later this morning with the approach of the upper
wave. The potential is there for bursts of heavy snow to come in
waves through the day with the strong forcing aloft present. Still
think a more widespread snowfall later this morning will be followed
by a brief break where snowfall rates diminish before more locally
heavy snow arrives this afternoon with convectively induced precip.
Cross sections continue to show a few hours from late morning into
the afternoon where strong vertical velocities and moisture interact
with the dendritic growth zone in the presence of conditional
instability. With the deformation axis likely to be nearby at the
same time...this is the time period where the heaviest snow rates
are likely to occur. Similar to ongoing convection early this
morning in eastern Iowa...the potential for a few rumbles of thunder
within the heaviest snow bands will exist later today which will
only accentuate rates. Further to the southwest where rain remains
the primary precip isolated thunder mention remains

While all of the above supports the potential for a heavy and
impactful snow...low level thermals remain a challenge as they often
do with late season winter storms. Surface temps within the zone of
heaviest snow potential are likely to rise no higher than the mid
30s. Model soundings continue to show a warmer layer aloft that
should warm advection be slightly stronger than currently
progged...would cause precip type changes that could disrupt heavier
snowfall rates. The most likely scenario here would be sleet mixing
in with the snow and cutting down on accums across the board. This
is a possibility that cannot be ignored. The heaviest snow falling
during daylight hours highlights the complexity of the low level
thermal profiles even further. And even with surface temps in the
mid 30s along with clouds and precip...road temps are likely to warm
into the 40s in spots based on the higher March sun angle alone. The
warmer ground temperatures will likely lead to faster compaction of
snowfall...whenever fall rates slow. Actual snow depths will likely
be lower than snowfall amounts for this reason.

Taking all of the above into consideration...cannot ignore the
impressive dynamics that will likely exist in that narrow zone just
north and east of the transitional precip area. Still feel that
rates within the heavier snow bands will overcome marginal low level
thermals and cause problems at times.

So to summarize...the potential for significant winter weather
primarily in the form of snow remains high and confidence levels
have risen enough to employ additional detail. Think the axis of
heaviest snowfall is likely to set up near and just south of I-74
with a zone of snowfall at 4-6 inches with locally heavier amounts
which often take place in mesoscale banding scenarios. The bust
potential remains higher than desired for the mitigating factor
mentioned...mainly the low level thermals and mixing concerns. At
the same time...the amount of moisture and forcing will likely
enable locally heavy snowfall to occur within this zone.

After a lot of thought and discussion with neighboring weather
offices...have chosen to introduce a Winter Storm Warning within a
narrow band in this zone near and south of I-74. While amounts will
likely not meet criteria throughout the whole zone and confidence is
nowhere near the desired level for the warning...potential is there
for pockets of 6"+ snowfall. The addition of easterly wind gusts
in excess of 30mph and locally heavy snowfall rates tips us in the
direction of introducing the warning as well from an impact
standpoint. This will include the southern and western portions
of the Indy metro and Marion County. Based on amounts...also
expanded the Advisory on the southern flank with Clay...Owen...
Monroe and Jackson Counties. These counties may see a substantial
gradient in snow or slop from near nothing southwest to a couple
inches northeast. Also extended headlines through 03Z tonight as
forcing aloft does not diminish completely until closer to

Temps...utilizing CAMs and raw model data to lean towards the cooler
end of guidance for highs today across much of the area. Coolest
temperatures will likely be found within the zone where heaviest
snowfall is likely in the vicinity of the I-74 corridor.


.SHORT TERM /Overnight Tonight through Sunday Night/...
Issued at 415 AM EDT Sat Mar 24 2018

Forecast challenges will be much less through this the
system today moves away from the region tonight and is replaced by
high pressure building south.

Low chances for precipitation will linger into the overnight over
southern counties as the upper wave shears out and gradually becomes
absorbed by the deep upper trough centered over the Canadian
Maritimes and New England. A strong high pressure ridge will expand
south from James Bay on the back side of the upper trough and into
the region for Sunday. Skies will steadily clear late tonight and
early Sunday from the north as much drier air and progressively
deeper subsidence advects south. Wind gusts will drop off overnight
as the surface wave moves away...but a decent surface pressure
gradient will remain on Sunday with the likelihood of a brisk E/NE
wind continuing. The return of sunshine and warmer temperatures
should go to work on the destruction of the snowpack immediately on
Sunday. Clouds will begin to increase again on Sunday night as
moisture return commences on the back side of the high pressure
ridge. overall model blend appeared reasonable for lows through
the period with an undercut in Sunday highs.  Should see highs
recover into the lower and mid 40s Sunday in areas with snow cover
with potential for slightly warmer temps elsewhere.


.LONG TERM /Monday through Friday Night/...
Issued at 404 AM EDT Sat Mar 24 2018

Central Indiana will remain under the influence of high pressure
through Monday morning, resulting in dry conditions.  However,
rain will increase from west to east by Monday afternoon across
the forecast area as a low pressure system approaches from the
Central Plains.  Latest blended initialization continues to trend
toward significant Gulf moisture and warm air filtering into
central Indiana ahead of this system, resulting in heavy rainfall
and embedded thunderstorms in the warm sector on Monday night and

Rain will linger into Wednesday, then taper off by Wednesday afternoon
in the wake of an associated cold front. Nonetheless, rain will
linger across the southern counties on Thursday, then fill in
across the remainder of central Indiana again by Thursday night as
an upper low enters the area. A few snow showers could mix in as
well on Thursday night with light accumulations.

Temperatures through the period will be above normal due to strong
warm advection with highs in the upper 50s and low 60s at times.
Meanwhile, overnight lows will be in the 30s and 40s.


.AVIATION /Discussion for the 24/06Z TAF Issuance/...
Issued at 141 AM EDT Sat Mar 24 2018

Conditions are currently holding at VFR levels. However, they
will quickly deteriorate to IFR or worse at times as rain and snow
overspread the TAF sites early this morning. There could be brief
periods of improvement to MVFR at times during lighter
precipitation, but MVFR will not become a prevailing flight
category until this evening when precipitation tapers off.
Meanwhile, winds will gust out of the east up to 30 kts today.


Winter Storm Warning until 11 PM EDT this evening for INZ028-

Winter Weather Advisory until 11 PM EDT this evening for INZ029-



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