Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 121058

Area Forecast Discussion
National Weather Service Northern Indiana
558 AM EST Tue Dec 12 2017

Issued at 432 AM EST Tue Dec 12 2017

Heavy lake effect snow will continue to increase through this
morning and continue into this evening as very cold air spreads over
the region. Several inches of accumulation are expected along with
blowing and drifting snow especially north of Highway 30. The snow
should diminish Tuesday night but another fast moving clipper will
bring more snow chances mid week followed by another round of lake
effect snow showers. Warmer temperatures are expected this weekend
with highs in the mid to upper 30s expected.


.SHORT TERM...(Today and Tonight)
Issued at 432 AM EST Tue Dec 12 2017

Welcome to lake effect land. A few changes to the forecast this
morning, mainly adjusting area of maximum snowfall and adding
LaPorte County, IN to the advisory. The front has passed into the
southeast portion of our CWA as of 9z, and it will continue to sink
southeastward through the day. As planned, lake effect has commenced
as deeper moisture continues to make it southward across Lake
Michigan. Additionally, northwest winds are gusting to around
25-30 knots inland, and up to 35-40 knots on Lake Michigan. Thus
it looks like the forecast is largely on track for our event, and
most of the headlines are in good shape. While snow amounts in
LaPorte county will not reach more than 3 inches (in far north-
northeast parts), with the wind and low visibility due to blowing
and drifting, opted to bump them into a winter weather advisory.
In contrast to the other headlines, this one drops by 00z. Later
shifts can drop if it things clear out faster than anticipated, as
winds will eventually shift slightly, forcing bands further

The only fly in the ointment for the forecast is the inversion
heights do not appear to be as high as originally planned, which
will lower the intensity of the snowfall somewhat. Models have
continued to trend a little lighter with QPF as well, and shifted
the highest amounts just to the northeast a bit, which lines up well
with the best band currently observed on radar. Despite slightly
lowered amounts across counties on the southern fringes of the
warning/advisories area, we`ll still see periods of intense snowfall
rates (though maybe not as impressive as we initially thought), and
blowing/drifting snow due to wind gusts of 30-40 mph, with worst
conditions along Lake Michigan. Things taper off later tonight as
winds become more southwesterly again. I left the remaining
headlines the same.


.LONG TERM...(Wednesday through Monday)
Issued at 432 AM EST Tue Dec 12 2017

Our parade of clipper systems continues through the long term, with
several chances for lake effect snow interspersed between these
systems. This will be the focus of the long term discussion today.

After a brief break in precipitation once lake effect diminishes on
southerly flow Wednesday morning, we`ll see another system swing
through. There are two camps of model guidance for this system. The
GEM/ECMWF keep the low track a bit further south than the GFS/NAM,
so this will impact where the greatest precipitation amounts are
with the system, with the GFS/NAM having higher QPF (around 0.25"
liquid) to the north of our CWA, and the ECMWF/GEM having it largely
over our north and east CWA starting Wednesday after 15z and
continuing through Wednesday night. If the further southwest tracks
verify, this puts around 1-3" max in the higher QPF areas. If not,
it will be less. For now went middle of the road until we get a
better handle on the system. Highs Wednesday will be in the low 20s
far NE, and the low 30s in the southwest. Lows will dip into the
teens and 20s. Winds don`t look to be as strong with this system as
they were with the former one.

As for lake effect snow in the wake of this system Wednesday into
Thursday, 850mb temps drop to around -13 to -14, so there is some
instability from the lake, in addition to a northerly fetch across
Lake Michigan. Delta theta-e values also support some instability
developing, with perhaps enough moisture and some synoptic support
(shortwave) aloft. There are limits, though as to the drier air that
moves in quickly, and the flow backs around to the west by Thursday
afternoon, which suggests a north-south orienting band developing
across the long axis of Lake Michigan, which then meanders eastward
as flow switches around. Kept pops lower at this point given
uncertainty, but think there is a chance we see some additional lake

The next clipper moves in on Friday, though this time with a
northern track across eastern Upper Michigan and lower michigan.
Models are in agreement with the low center for the most part, and
have the system drifting eastward into southern Ontario and Quebec
by Sunday afternoon. This will bring a cold front through our area,
with largely westerly flow throughout the period. Expect lake effect
pops along the shore accordingly for west-wind favored regions, and
a brief pump of system snow during the day Friday along and ahead of
the front, then focusing along the shore and north of US 30 by
Saturday morning.

Things may taper off by Saturday, though it depends on which model
you pick, as the GFS brings an amplified ridge into the CWA for
Saturday afternoon ahead guessed it...another clipper. The
catch is, the ECMwF has a much weaker system and less developed
500mb trough, whereas the GFS has a better developed system and
shunts another system to the southwest out of our area. It takes the
clipper across Upper Michigan/northern Michigan, and out of the
area. The ECMWF, because it has a weaker system up north brings the
southwestern system out of Missouri and into the Ohio River
Valley...which will bring us much more precipitation by Sunday
night. Long story short, will keep in line with consensus thinking
for now, but we`ll have to keep an eye on this one. Milder
conditions are expected by this period as well, with highs Saturday
into Tuesday climbing into the upper 30s and even a few areas south
of US 30 in the low 40s (Sun-Tue). This means rain will mix in with
snow at times, or completely in southern portions of the area.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday Morning)
Issued at 553 AM EST Tue Dec 12 2017

Lake effect ongoing and will continue into this evening. Regional
radar mosaic shows several dominant bands all with upstream
connectivity to Lake Superior. These will continue to stream
southeast today with bands gradually shifting to the east and
winds back through the day. Terminals to continue with highly
variable vis and cigs much of the day as squalls move inland.
Will have to stay with generic tempo groups to cover this
variablity. Conditions improve through the night as drier air
moves in and flow continues to back, eventually shifting all snow
east and north of terminals.


IN...Winter Weather Advisory until 7 AM EST Wednesday for INZ014-

     Winter Storm Warning until 7 AM EST Wednesday for INZ004>006-008.

     Lakeshore Flood Advisory until 3 PM CST this afternoon for

     Winter Weather Advisory until 6 PM CST this evening for INZ003.

MI...Winter Storm Warning until 7 AM EST Wednesday for MIZ077>079.

     Lakeshore Flood Advisory until 4 PM EST this afternoon for

LM...Gale Warning until 7 PM EST this evening for LMZ043-046.




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