Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 011737

Area Forecast Discussion
National Weather Service Northern Indiana
1237 PM EST Thu Dec 1 2016

Issued at 336 AM EST Thu Dec 1 2016

Lake effect rain and snow will continue along and north of the toll
road through Friday but relatively warm temperatures will limit
accumulation and impacts to the roads. Otherwise...just cloudy
conditions with temperatures generally hovering in the 30s. Dry
weather expected on Saturday but another chance for light rain and
snow will arrive on Sunday. Little to no snow accumulation expected
with that system either.


.SHORT TERM...(Today through Friday Night)
Issued at 336 AM EST Thu Dec 1 2016

Primary midlevel vort max currently moving overhead, supporting lake
enhanced precip to our north and a few flurries/drizzle away from
the lake. Activity south of US-6 expected to shift eastward and
dissipate by later this morning but lake effect precip will
continue. Latest forecast soundings indicate inversion heights
remaining around 6-8 kft today with a slightly veered wind (around
270 degrees). NAM12 theta-e lapse rates remain around -1 C/km with
dry air upstream remaining too limited to significantly impact lake-
induced instability. Therefore, likely PoPs easily warranted in our
MI counties and even bumped them up slightly higher than previous.
Chances south of the toll road appear limited by westerly fetch but
maintained some slight chance buffer. Despite high PoPs, QPF totals
will be light given only modest instability/moisture depth and
limited fetch. Latest consensus T/Td suggests primarily rain during
the day, especially near the lake. Could see some snow mix in at
times in heaver bands away from the lake but not expecting any snow
accumulation or impacts given marginal temps, relatively warm
ground, and limited precip rates.

Flow continues to slowly veer tonight through Friday with a minor
drop in 850mb temps to around -8C. This suggests more of Indiana
will get in on the action but this also corresponds to inversion
heights steadily lowering to around 5 kft overnight and down to 3
kft by late Friday. Dry air advection/entrainment also steadily
increasing through Friday with upstream surface dewpoints eventually
dropping into the low 20s. This will steadily choke off measurable
precip by late Friday with just flurries/drizzle thereafter. Still
have a general decreasing trend in PoPs/QPF with mid-range chance
tonight and just low chance on Fri. Did speed up the drier PoPs a
bit based on latest guidance, though. Slightly colder temps will
allow for a bit more snow than rain tonight but again, QPF remains
light and just a tenth or two at best on grassy surfaces. Ice
nucleation increasingly a concern by late Fri but temps warm enough
for just rain during the day and doubt cloud depths will be enough
for hydrometeors to reach the surface by early Sat morning when
temps drop below freezing.

Outside of lake effect, main story for the rest of the CWA will be
cool and cloudy conditions with high temps stuck in the upper
30s/near 40F. Overnight lows generally in the low 30s but some upper
20s possible Friday night, especially if there are any breaks in the
clouds in our SW zones.


.LONG TERM...(Saturday through Wednesday)
Issued at 336 AM EST Thu Dec 1 2016

A relatively busy long term period for the area as multiple waves
eject out of the western CONUS in fairly rapid succession. After a
very brief return to dry/quiet weather on Sat, the first wave will
arrive on Sunday. Decent midlevel CVA noted, along with good cross-
isobar flow on the 285K surface. Moisture depth and magnitude are
somewhat lacking but sufficient for some light precip. Went with
likely PoPs but fast-moving system and lackluster forcing/moisture
will limit QPF. Thermal profiles continue to suggest a rain/snow
mix, though suspect that evaporative cooling may lead to a bit more
snow vs. rain than currently anticipated. Some light accumulation
may be possible on grassy surfaces but poor diurnal timing/marginal
surface temps will prevent any efficient accumulation and warm
ground/roads should limit impact.

Slight warmup anticipated Monday into Tuesday as downstream ridge
building occurs over the eastern CONUS and SW flow envelops our CWA.
Second ejecting shortwave will then arrive on Tuesday. Another
vigorous wave with better moisture this time. Will be warm enough for
just rain with this wave though as high temps climb into the mid 40s
and lows only in the mid 30s. Third, and potentially strongest, wave
then arrives late Wed/Thu. Models continue to struggle with details
of this system (as expected being a week away) but do expect a good
chance of rain at some point during this period with potential for
changeover to snow. GFS and ECMWF still showing much colder air in
the wake of this system with both models indicating 850mb temps near
-20C late Thu/Fri.


.AVIATION...(For the 18Z TAFS through 18Z Friday Afternoon)
Issued at 1232 PM EST Thu Dec 1 2016

Persistent MVFR cloud deck still expected to persist this period
with cyclonic flow and winds off of Lake Michigan. Low level winds
will veer some tonight and should see light rain and snow showers
move back south toward KSBN. Inversion heights lowering along with
decreasing moisture depth so reductions to VIS do not look likely
unless within an isolated shower. Will carry VCSH for now at KSBN.
KFWA remains within trajectories/cloud plume off of lake but with
shallow moisture and inversion doubtful much pcpn can make it this
far inland so no shower mention here.


LM...Small Craft Advisory until 4 AM EST Friday for LMZ043-046.




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