Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 071949

Area Forecast Discussion
National Weather Service Northern Indiana
249 PM EST Thu Dec 7 2017

Issued at 248 PM EST Thu Dec 7 2017

A few lake effect snow showers and flurries will remain possible
into this evening, mainly north of US 20 in Indiana and Michigan.
Dry and continued cool conditions will follow into tonight and
Friday, with lows in the teens tonight and highs in the low 30s
tomorrow. Another system will bring widespread light snow to the
entire area later Friday night into Saturday morning. Most
locations will likely receive between 1 and 3 inches of snow. This
will transition to lake effect snow across northwest Indiana and
southwest Lower Michigan Saturday afternoon into Saturday evening
with locally heavy amounts possible.


.SHORT TERM...(This Evening through Friday)
Issued at 248 PM EST Thu Dec 7 2017

Leftover light lake effect snow showers and flurries, mainly north of
US 20 in far nrn IN/lower MI, will continue to diminish in coverage
and intensity late this afternoon into early this evening. This
will occur as inversion heights lower in response to drying/subsidence
and gradual low level backing. Little additional accumulations/impacts
are anticipated. Dry and continued chilly otherwise into tonight and
Friday...with sunny skies tomorrow morning giving away to increasing
afternoon clouds across nw IN/lower MI in advance of the next


.LONG TERM...(Friday Night through Thursday)
Issued at 248 PM EST Thu Dec 7 2017

An active northwest flow regime will remain in place into this
weekend and next week thanks to a highly amplified western NOAM
ridge/eastern NOAM upper trough +PNA configuration. The result will
be a continuation of below normal temps and several opportunities
for snow (and lake effect snow) as several vort lobes rotate
southeast through the Great Lakes and Ohio Valley.

The first mid level shortwave will impact the local area later
Friday night into Saturday morning. This feature continues to look
rather impressive as it drops almost due south from the Arctic Cycle
with pv intrusion deep to near 700 mb. Associated mid level
DCVA/100-150m 5H height falls will elicit a period of stronger
isentropic ascent (40 kts of cross isobaric flow) along and in
advance of system sfc trough later Friday night through mid
morning Saturday morning. A period of light snow will accompany
this forcing, though amounts should generally be limited to 1-3"
given limited moisture return and rather quick west to east
progression of this large scale synoptic forcing. Snow showers
will likely persist into the late morning and early afternoon
under the vort max, with heavier/brief bursts possible given
steepening low-mid level lapse rates under cold(er) pocket aloft.
Roads will likely be slick in many locations during this time.
Will continue to cover possible impacts through social media and
HWO for now.

Attention later Saturday morning through Saturday evening will turn
to possible significant lake enhanced/effect snow into our nnw
flow lake belts. A single band or a couple stronger multi-bands
look to set up set up as nnw flow parallels the long axis of the
lake resulting in strong frictional/thermal convergence over
eastern Lake MI, especially morning through early afternoon given
deeper layer synoptic support. Good omega and super- saturation
with respect to ice within the dendritic growth zone (per latest
fcst soundings) continues to hint at the potential for decent
flake size/snow-to-liquid ratios wherever shallow convection
organizes (possibly offset some by stronger winds). Brief Lake
Superior/long Lake MI connection and sufficient thermodynamics
(lake-800 mb differentials in the 20s/inversion heights 7-10
kft/0-2km theta-e lapse rates to -2 k/km) are also in place even
into the aftn/eve as any synoptic moisture/support exits off to
the east. So, the potential is definitely there for a headline
level event. However, the obvious uncertainties at this fcst range
are where exactly better banding materializes (pivots) and if the
diurnal component/disruption and stronger boundary layer flow
keeps activity more cellular. With that said think there is enough
confidence and model agreement to issue a Winter Storm Watch (>50%
confidence) for LaPorte/St Joe IN/Berrien counties later Friday
night through Saturday evening. Opted to leave surrounding
counties (Starke/Marshall/Elkhart/Cass MI) out of a watch for now
as not as confident in persistence of LES and warning level
impacts. Anticipate these areas needing at least a SPS or advisory
at some point.

The pattern remains relentless into next week as several
more clipper systems move through with snow chances (followed by
heavier lake effect snow?) and reinforcing shots of cold air. The
upper wave on Monday and another LES event in its wake Monday
night/Tuesday looks to be the next clipper system to watch.
The responsible pv anomaly appears to have a more pronounced low-
mid level thermal gradient and slightly better moisture to work
with around Monday. This could result in another 1-3" or more type
swath of snow (mainly along/north of US 30 per latest
guidance)...possibly followed by another LES event.


.AVIATION...(For the 18Z TAFS through 18Z Friday Afternoon)
Issued at 1245 PM EST Thu Dec 7 2017

Lake effect clouds and associated MVFR conditions expected to
prevail over the next several hours, with SHSN causing brief IFR
conditions at KSBN. Flow will become more westerly pushing SHSN
north of KSBN. Flurries still possible at KFWA through the
afternoon with improving conditions later this afternoon.

Once the lake effect shifts away, VFR conditions will prevail
through the remainder of the period.


IN...Winter Storm Watch from late Friday night through Saturday
     evening for INZ003-004.

MI...Winter Storm Watch from late Friday night through Saturday
     evening for MIZ077.

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



SHORT TERM...Steinwedel
LONG TERM...Steinwedel

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