Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 070944

Area Forecast Discussion
National Weather Service Northern Indiana
444 AM EST Thu Dec 7 2017

Issued at 435 AM EST Thu Dec 7 2017

Lake effect snow is expected today, mainly near and north of US 20.
Accumulations of 1 to 2 inches are possible. Dry conditions are
expected tonight and Friday but another system will bring
widespread snow to the entire area on Saturday. Locations outside
of the lake effect snow belt region will likely see 1 to 3 inches
of snow while the snow belt could see 6 or more inches of
accumulation before the snow ends Saturday night.


.SHORT TERM...(Today and Tonight)
Issued at 435 AM EST Thu Dec 7 2017

Slower arrival of short wave and surface trough has delayed veering
winds and onset of lake effect snow. Regional radar mosaic showing
leading edge of pcpn now entering far northern forecast area as
winds gradually shift northwest. However...upstream obs indicate
winds already beginning to back over eastern WI with dew points
dropping into single digits. WNW trajectories and drier air
entrainment should help offset larger delta T values and modest
instability this morning. Shrinking window for optimum lake effect
snow should also help keep amounts in check with 1 to 2 inches
still possible today and maybe a localized 3 inch in extreme
northern areas of our Michigan counties. Flow slowly backs
westerly this afternoon with inversion heights dropping so snow
showers should become confined to Michigan with a weakening trend.
Clearing works in tonight away from flow off the lake. Gradient
winds to remain in the 5 to 10 mph range which will help keep
boundary layer mixed. Despite this low temperatures expected to
still drop into middle and upper teens with cold airmass in place.


.LONG TERM...(Friday through Wednesday)
Issued at 435 AM EST Thu Dec 7 2017

Active and cold pattern continues through the long term. Main
focus on clipper system Friday night into Saturday with lake
enhancement and transition to lake effect likely yielding
measurable snow for entire area and some heavier amounts closer to
Lake Michigan.

Blocked pattern continues with upper low remaining over Hudson Bay
through the weekend with staunch ridge over west coast and active
jet squeezed between the two. Pacific energy will top ridge Friday
and dive south towards the Midwest Friday night. Left front region
of 500mb 100 kt jet streak will provide good upper level divergence
while sharpening mid level trough pivots across the area Saturday.

Brief period of isentropic lift develops in warm air advection
regime late Friday night and this should help spread light snow
across western two thirds of the area before daybreak Saturday. As
weak surface low drops into Michigan Saturday morning, low level
flow will become northwest and period of moderate lake enhanced snow
expected in the northwest while light snow spreads across remainder
of forecast area. General accumulations of 1 to 3 inches look likely
away from the lake from synoptic system.

The snow belt region should see a good snowfall from this event
while long fetch and strong flow will likely yield several inches
well inland as well. Low level winds veer northerly Saturday
morning with convergent signature down axis of Lake Michigan with
upstream pre-conditioning off of lake Superior. Delta Ts not
extreme but do climb into upper teens and eventually low 20s by
Saturday afternoon. 0-2km theta e lapse rates are impressive
between -2 and -3 K/km Saturday over southern Lake Michigan while
inversion heights remain above 10kft most of day. Strong lift is
centered within saturated DGZ through the day as well and snow
ratios about 20:1 expected. These ideal factors should lead to a
rather intense dominant single band for much of the day with
snowfall rates of 1 to 2 inches easily attainable. Blowing snow
will also be a concern as momentum off the lake will bring 15-30
mph winds well inland. About the only questions...assuming models
continue with these solutions...are whether band will be
transitory and spread the snow around or remain more stationary
with concentrated amounts. Also some concern for the daytime
timing as we often see partial disruption to organization with
low daytime heating and mixing. For now have painted a rather
broad swath of heavier amounts on Saturday and carried these well
inland given strength of flow and length of fetch which
climatologically favor areas as far inland as Marshall and western
Kosciusko counties. These amounts will likely be narrowed down to
a smaller swath once higher resolution models come into play.

Expecting headlines will eventually be needed for this event, at
least closer to the lake where heavier amounts expected. Since this
will be the first widespread synoptic snow of season, advisory may
be needed away from lake as well. For now will play up in HWO and
weather story. Thankfully this snow will be falling on a Saturday
with fewer vehicles on the road. Travel conditions along interstates
over NW Indiana to see some significant impacts with combination of
heavy snow, low visibilities and strong winds.


.AVIATION...(For the 06Z TAFS through 06Z Thursday Night)
Issued at 1255 AM EST Thu Dec 7 2017

Lake delta T values will climb over 20C later today and become
very favorable for lake effect snow, but with a marginal fetch,
conditions at SBN will not be optimum during this TAF period for
lake effect snow showers. Kept the integrity of the ongoing/00Z
TAFs with the best window for lake effect snow from 12-16Z when
the fetch briefly becomes a little more northwest. Kept a TEMPO
IFR visibility during this period. Otherwise, conditions should
become VFR at SBN later this afternoon.


LM...Small Craft Advisory until 10 PM EST Friday for LMZ043-046.



SHORT TERM...Lashley
LONG TERM...Lashley

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