Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 101751

Area Forecast Discussion
National Weather Service Northern Indiana
1251 PM EST Sat Dec 10 2016

Issued at 1239 PM EST Sat Dec 10 2016

Low pressure moves northeast from the Southern Plains towards the
region with snow breaking out tonight and extending through Sunday
night. Once the low pressure system is closer to the region, the
snow is expected to mix with or possibly change over to rain south
of Route 30 Sunday night before ending. Significant snow
accumulations are possible Saturday night through Sunday night for
parts of the area. High pressure follows for the beginning of next
week with calmer, but cool conditions. Thereafter, even colder air
will move into the Great Lakes region behind a cold front expected
to move through around the middle of next week.


Issued at 1048 AM EST Sat Dec 10 2016

No changes this morning to current winter weather advisory in
effect for tonight-Sunday. However, will be examining incoming
data for possible headline changes early to mid afternoon. The
next upper level PV anomaly tracking across the Pacific Northwest
this morning will emerge in lee of Rockies tonight with a
coincident downstream amplification of low level jet from the
southern Plains into the mid MS valley this evening. Warm
advection snow should rapidly break out across northern Illinois
this afternoon/early evening and expand eastward across the area
tonight. Snowfall rates are not expected to be all that
significant this evening with generally light accums. Isentropic
lift to pick up tonight and especially toward daybreak Sunday.
Isentropic signal should peak in the 12Z-18Z time frame with
likely mass convergence/low-mid level fgen response to augment
isentropic lift. Given recent trends in 12Z guidance and strength
of signal in the 09Z-18Z timeframe, may need to transition a
portion of the winter weather advisory to a winter storm warning
for particularly northern third of the area, and possibly extend
winter weather advisory a bit southward.


.SHORT TERM...(Today into Sunday Night)
Issued at 417 AM EST Sat Dec 10 2016

Forecast not a whole lot clearer for the short term as models
continue to vary on how fast saturation occurs later today into
tonight and northward extent/speed of brief window of warmer
temperatures Sun afternoon/eve. Models at least generally agreeing
on the track of the sfc low to the nw of the area. Now to the
finer details.

Passing mention to start things out with light lake effect
snowshowers/flurries persisting across far southern Lower Mi.
Expecting little/no accumulation with these with rapid decrease in
coverage near/after 18z as increasing flow ahead of the next
system cuts off any activity. NAM remains the fastest and most
aggressive not only in arrival of snow late afternoon/early
evening but also the degree of warm air surging north. GFS/ECMWF
has a slightly slower onset of light snow (more after 00z).
Elevated mixed layer on KILX 00Z sounding should be starting
saturation as we speak with mid level increase in moisture
arriving. This should hopefully allow precip to reach the ground
rather quickly as condensation pressure deficits in the 285 to 290
K surface drop below 20 mb and then bottom out on Sunday as stout
burst of isentropic lift occurs with main low level jet
impingement. With the further NW track of the surface low a surge
of above freezing temperatures well north through the area would
normally be a slam dunk. However timing of this area and shallow
cold surface temps may complicate things further.

So....with regards to headlines and accumulations...precip will be
all snow tonight into at least the first part of Sunday with the
potential for several inches over roughly an 18 hour period. At
the point QPF potential is highest is when the warmer air begins
to arrive, potentially reducing amounts across the board as snow
ratios drop, especially south of US 30. Extensive collaboration
took place with surrounding offices and resulting in hoisting of a
Winter Weather Advisory generally either side and north of route
6. Advisory type snow amounts are still gridded up further south
to the US 24 corridor, but warmer surface temperatures will likely
keep travel hazards minimized. If the precip remains as snow north
of Us 6 for the bulk of the event, then 6 to 9 inches would be on
tap for these areas. Little in the way of wind is expected so no
concerns for blowing/drifting snow. If warmer air is faster/more
pronounced as depicted in the NAM, then snow amounts across the
board could be reduced by several inches, especially south of
US 30.

Any mixed precip will quickly end Sunday night as colder air moves
back in and deeper lift/moisture depart to the east.


.LONG TERM...(Monday through Friday)
Issued at 417 AM EST Sat Dec 10 2016

Few changes to long term with focus on pending winter event next
36 hours or so. Piece of arctic air still in store for mid to late
week with what appears to be limited lake effect potential as
fetch may end up more westerly and deeper moisture/dgz interaction
may be limited. Coldest temps will take place Thursday with lower
to maybe middle teens for highs and lows into the single digits
above zero. Wind chills Thursday night may drop into the zero to
10 below range.

Next system will arrive late in the period but have held with slgt
chc/chc pops for now with plenty of time to sort out.


.AVIATION...(For the 18Z TAFS through 18Z Sunday Afternoon)
Issued at 1236 PM EST Sat Dec 10 2016

Wind direction fluctuating between more westerly and southerly has
allowed for SBN and FWA to get into some more lake effect snow with
MVFR conditions. With wind direction changing to more westerly soon,
only kept these conditions temporarily. Elsewehere, snow in the
Central Plains is moving into the Midwest and looks to reach SBN
around 02z tonight. Clouds moving into the region may drop CIGs to
MVFR around 01z at SBN first before moving to FWA. Expecting there
may be some early onset snowfall evporation occurring, but then IFR
conditions take over as the snow intensifies.


IN...Winter Weather Advisory from 9 PM EST /8 PM CST/ this evening to
     4 AM EST /3 AM CST/ Monday for INZ013-015-017-018-020-

     Winter Storm Warning from 9 PM EST /8 PM CST/ this evening to 4
     AM EST /3 AM CST/ Monday for INZ003>009-012-014-016.

MI...Winter Storm Warning from 9 PM this evening to 4 AM EST Monday
     for MIZ077>081.

OH...Winter Weather Advisory from 9 PM this evening to 4 AM EST
     Monday for OHZ004-005-015-016-024.

     Winter Storm Warning from 9 PM this evening to 4 AM EST Monday
     for OHZ001-002.



LONG TERM...Fisher

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