Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 231848

Area Forecast Discussion
National Weather Service Indianapolis IN
248 PM EDT Fri Mar 23 2018

The LONG TERM Section has been updated below.


Issued at 406 AM EDT Fri Mar 23 2018

After a dry day today...a significant late season winter storm will
impact central Indiana beginning late tonight and continuing
Saturday. Heavy snowfall will be possible over parts of the area.
High pressure will return for Sunday...but unsettled weather will
return for the first half 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/...
Issued at 408 AM EDT Fri Mar 23 2018

Quiet chilly morning ongoing as the region resides under high
pressure. A weak disturbance passing by just southwest of the area
was producing light rain over southern Illinois and extreme
southwest Indiana. 08Z temperatures ranged generally from the mid
20s to the mid 30s.

The main story is today will be the quiet before the wintry weather
arrives later tonight and into Saturday. Much of the day will be
influenced by the high pressure ridge as it gradually shifts east.
Clouds will be on the increase from the southwest ahead of a warm
front moving into the Tennessee valley by late day. The lower Wabash
Valley may deal with clouds for much of the day in closest proximity
to the approaching warm front while most of the rest of the forecast
area will see sunshine into the afternoon at least. Introduce low
chances for rain by late afternoon over southwest counties as the
initial shot of moisture and lift arrive.

Temps...expect temperatures to rise easily with the dry airmass
over the region and strong March sun. Leaned towards warmer end of
guidance as most areas warm into the upper 40s to around 50.


.SHORT TERM /Tonight through Saturday Night/...
Issued at 408 AM EDT Fri Mar 23 2018

The primary focus will be the potentially significant late season
winter storm poised to impact central Indiana beginning late tonight
and continuing into Saturday evening. There are still details to be
ironed out as we move into the 24 to 36 hour timeframe and multiple
items to consider that could drastically change precip type and
amounts in any one location. Lot to look at and consider.

The first order of business to address is the headline situation.
After discussion with neighbors and with confidence levels not yet
quite where would like to see them...have chosen to continue with
the Winter Storm Watch in its present state with an addition of
counties to the southwest based on a southward shift to the
potential for heavier snowfall off the 00Z model suite. With impacts
in the late second period and...more likely in the third period...
think there is still time to fine tune details and determine where
headline adjustments to warning and/or advisory would best fit. In
addition...the 12Z model suite should be able to capture the system
with the full RAOB network as it has comes ashore on the west coast
over the last several hours. That should hopefully iron out some of
the differences that still exist between various model solutions
which we will discuss below.

The upper wave will lift up and over the ridge centered over the
southern Plains later today and tonight before winging into the
region Saturday and gradually becoming absorbed by the deep upper
trough centered over the Canadian Maritimes and New England states
by late weekend. A surface low will accompany the upper
wave...ejecting out of the Rockies this evening then moving E/Se
into the lower Ohio Valley by late day Saturday. Overall model
agreement is decent regarding the main features with the exception
of the NAM which is a noticeable outlier to the north. Overall
ensemble guidance is tightly clustered and aligned with the op GFS
and ECMWF with the GGEM tracking slightly further south.

Rain will be overspreading the lower Wabash Valley during the
evening...with precipitation slowly expanding east across central
Indiana into the overnight as the initial surge of moisture and lift
contends with a notable layer of dry air through the lowest 10kft
courtesy of increasing easterly flow. As surface temps fall and
evaporative cooling commences in response to the saturation of the
boundary layer...will see precipitation transition to sleet and more
likely snow along the northern flank of the precip shield after
06Z...with the transition zone aligning somewhere near or just north
of a Terre Haute-Bloomington-Seymour line. Northeast of this
line...snow should become the predominant precip type through
daybreak with the potential for a light accumulation. This
transition line is likely to shift northeast slightly during the day
Saturday. Within the mix area...freezing rain is certainly a
possibility...but expect sleet will be the more prevalent precip
type outside of snow or rain based on model soundings.

The primary show however really looks to focus during the day
Saturday as the upper wave approaches with more substantial
deformation and frontogenesis develops and moves across the region.
The potential for a period of or bursts of heavy snow will come
during the day Saturday and should align in a NW to SE axis along
where the most intense deformation occurs just to the left of the
700mb low track.  Potential is there for a burst of more widespread
snowfall during the morning then after a brief break...more locally
heavy snow during the afternoon with the possibility of a convective
component. 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 and a potential for a few rumbles of
thunder within the heaviest snow bands will exist. Further to the
southwest where rain remains the primary precip isolated
thunder mention is warranted. impressive wind field
present through the boundary layer wrapping around the system will
lead to higher wind gusts Saturday that may peak at 30-40mph. This
will make the day even more miserable and in areas where a snow or
mix is ongoing...create greater impacts.

While all of the above presents a growing confidence in a
heavy...wet and impactful snow...low level thermals will offer a
challenge as they often do with late season winter storms. Surface
temps are likely to rise into the mid 30s over the northeast half of
the area with a warmer layer aloft that should warm advection be
slightly stronger than currently progged...could cause precip type
changes and/or limit heavier snowfall rates. The heaviest snow
falling during daylight hours highlights the complexity of the low
level thermal profiles even further. That being said based on the
forcing aloft and factors mentioned above...feel that rates within
the heavier bands will overcome marginal low level thermals and
cause problems at times. Precip will diminish as the system moves
away Saturday evening with skies clearing overnight.

So to summarize...the potential for significant winter weather
primarily in the form of snow remains high but some details still
need to be worked out. Would like to see an overall better model
consensus on the axis of heaviest the NAM remains
north and the ECMWF and GGEM have shifted much further southwest on
00Z runs. The overall consensus without the NAM does support a
slight shift to the southwest and have followed suit. As it stands
at this point...think the axis of heaviest snowfall is likely to set
up either side of I-74 by 20 miles or so. The axis will in reality
probably end up being tighter but would like to see better
consistency before committing to a narrower axis. Will continue with
snowfall amounts of 3-7 inches within the heaviest bands with a
potnetial for localized higher amounts which are common in banded
snow scenarios.

One thing to mention...warmer ground temps will likely lead to
compaction of snowfall relatively fast...especially once rates
diminish. Actual snow depths will likely be lower than snowfall
amounts for this reason. This will be cement block snowfall folks.

Temps...largely prefer the cooler end of guidance through the
period...especially on Saturday as temperatures are likely to be
kept down by precipitation and especially in areas where snow or
some form of wintry mix is prevalent.


.LONG TERM /Monday Night Through Friday/...
Issued at 247 PM EDT Fri Mar 23 2018

Ensembles indicate a slowly progressive nature to the long wave
features during this period. Mean upper trough initially over the
Rockies will gradually shift into the Midwest by the later parts of
next week.

Unsettled weather is expected for most of the long range period, as
occasional disturbances eject out of approaching mean trough. Will
continue to broadbrush PoPs from Monday night through Thursday
night. It appears by next Friday the main moisture plume will get
shunted off to the southeast, so will go dry by then. Progged
precipitable water into the midweek period continue to look very
high for this time of year, so heavy rain may become an issue.


.AVIATION /Discussion for the 23/18Z TAF Issuance/...

Issued at 147 PM EDT Fri Mar 23 2018

VFR conditions will start the TAF period. Clouds will start to
move in tonight and ceilings are expected to reach MVFR &
visibilities could reach IFR for Saturday morning. IFR conditions
are possible again Saturday afternoon.

A frontal system is moving into the area which will bring in
precipitation starting late tonight and lasting through Saturday
night. Confidence is high that there will be precipitation but
confidence is still low on timing and the type of precipitation,
whether it will be rain, snow, or a mix of the two. It will depend
on where the rain-snow line, oriented from NW to SE, sets up
across central Indiana. The system will bring MVFR and IFR
ceilings and visibility Saturday morning and there is a potential
for IFR conditions in the afternoon as well. Humidity was near 30
percent as of early Friday afternoon which will delay the timing
of precip reaching the surface as it has to overcome the dry layer

Winds will begin to increase overnight and gusts up to 25 to 30
kts are expected for Saturday.


Winter Storm Watch from 2 AM EDT Saturday through Saturday
evening for INZ021-028>031-035>049-054>057-063>065-072.



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