Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 100942

Area Forecast Discussion
National Weather Service Northern Indiana
442 AM EST Wed Jan 10 2018

Issued at 428 AM EST Wed Jan 10 2018

A mix of precipitation is expected through Friday with
temperatures soaring to near record highs on Thursday then
crashing into the single digits and even below zero next week.
Rain and fog will be possible through Thursday evening follwed by
freezing rain, sleet and snow late Thursday night into Friday.
Significant snow accumulations remain possible Friday night into
Saturday over far eastern Indiana and northwest Ohio but the
location of the heaviest snow remains uncertain.


.SHORT TERM...(Today and Tonight)
Issued at 427 AM EST Wed Jan 10 2018

Warm air advection continues over the region and surface temps
responding with slow warming in the southwest early this morning.
Meanwhile, northeast areas have seen temps fall into the upper
20s with influence of retreating high pressure helping keep winds
light and onset of mixing from warm air advection aloft just
getting underway. Problem this morning will be with potential for
a brief period of very light freezing pcpn where sfc temps lag
rising above freezing. Hires models showing low levels saturating
and most pcpn falling after 2m temps rise above freezing. However,
with colder start in the northeast coupled with another instance
of actual sfc temps being several degrees colder than measured 2m
temps, some glazing is possible where pcpn begins to fall before
warmer air aloft begins to really dominate. This will have to be
monitored through the early morning hours but will have chances
for light freezing rain or freezing drizzle through daybreak
central and mid morning northeast. Again, could see some brief
freezing on roads and sidewalks even where measured temps are
showing above freezing.

Areas of fog and drizzle/light rain possible remainder of today into
tonight with warm and moist air over cold ground and snowpack.
Degree of dense fog difficult to determine given rather strong
boundary layer winds so for now stayed with areas of fog.
Temperatures should rise through the day and even tonight with
strong warm surge and many areas likely to be near or above 50


.LONG TERM...(Thursday through Tuesday)
Issued at 427 AM EST Wed Jan 10 2018

...From Near Record Highs to Near or Below Zero Temps This Period...

Quite a bit of uncertainty remains in the long term with respect to
pcpn types Thursday night into Friday and possible winter storm for
eastern areas Friday night into Saturday. What is nearly certain is
continued warmth and near record highs on Thursday followed by
arctic cold once again for latter half of forecast period.

Upper level pattern expected to buckle once again with large scale
ridge building through central Canada and deep trough taking shape
over the Great Lakes through the weekend. This ridge does not look
strong enough for cross polar flow but still strong enough to
reach polar region and allow seasonable arctic air to surge back
into the eastern CONUS. Transition period of this occuring still
looks very interesting and full of uncertainty as 00z EC took a
different direction toward what was the outlier called the GFS for
the weekend storm system which is more progressive than past

First order of business will be with pcpn changeover Thursday night
into Friday. Deep moisture surge and rain expected Thursday into
Thursday evening ahead of sfc cold front with a coupled jet
structure aiding in large scale lift. Sfc temps expected to fall
quickly Thursday night with frontal passage as cold air undercuts
warmer air aloft. This should lead to a period of freezing rain and
some sleet before changeover to all snow through the night. The
question remains how fast this will occur and how much ice
accumulation will develop in the transition. Latent heat processes
from the rain and water in warm sector often limits initial ice
development in these situations but we must also consider a flash
freeze scenario given quickly falling near sfc temps. This in
itself could pose some travel problems with wet roads quickly
freezing. Still too early for any kind of advisory but this period
may certainly need a headline as details become a bit more clear.
For now will have some light ice accumulations per the forecast
builder program along with some snow accumulations.

Focus then shifts to southern wave lifting out of the south and
degree of phasing and timing with northern stream wave. As mentioned
the 00z EC has become more progressive and even faster than the GFS
which was considered an outlier. This has thrown more uncertainty
back into the mix for how fast this system will move through and how
deep it will become. Regardless, models still kicking out
significant pcpn for our far southeast and have stayed with a
Superblend/WPC blend for QPF and snow amounts. This yields the
POTENTIAL for 6-10 inches of snow in our far southeast but this is
far from certain and certainly expect a lot more changes in coming
model runs as jet energy gets sampled better. Will keep highlighting
in HWO and social media but confidence remains low on where this
heavy snow will actually fall. The changes in timing also has
impacts on the lake effect potential and setup later Saturday. For
now kept pops in the high chance category.

Saturday night through Tuesday

Lake response should be diminishing through Saturday night as winds
not only weaken, but flow backs to more of a westerly fetch and
eventually southwesterly by Sunday afternoon in response to the next
fast moving northern stream wave. With regards to the lake effect,
Not a lot of confidence as to exact timing and fetch with the noted
differences in GFS/ECMWF so will keep pops subdued somewhat with mid
to high chc for now.

A piece of energy (Alberta Clipper) is forecast to break off the
polar upper low and dive south into the northern Plains Sunday and
then progress into the upper Great Lakes by Monday evening. While
best dynamics will remain north with the low, still expecting a
period of light snow to develop ahead of the trough and associated
arctic front Sun Ngt into Monday. The clipper will have a
reinforcing shot of cold air with it that will linger into Tuesday
with at least some lake response expected in its wake.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday Night)
Issued at 1257 AM EST Wed Jan 10 2018

General LIFR cigs will persist through most if not all the TAF
period. KFWA has experienced a bit of drier air that has allowed
for some temporary mixing of the stratus. This may remain the case
the next couple of hours until the surge of much moister and
warmer air moves in towards 12Z. Greatest impacts with the warmer
air may reside at KSBN vs KFWA given sharp differences in
snowpack in terms of vsby impacts. Winds will be on the somewhat
stronger side, which would normally be a deterrent to lower
vsbys. However, release of snowpack moisture and significantly
warmer air temperatures typically offset this effect. While not
shown in current TAFs, vsbys to 1/4sm will likely be needed for
KSBN at some point.




SHORT TERM...Lashley
LONG TERM...Lashley/Fisher

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