Area Forecast Discussion
Issued by NWS Northern Indiana

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000
FXUS63 KIWX 101758
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
1258 PM EST Sun Dec 10 2017

.SYNOPSIS...
Issued at 1255 PM EST Sun Dec 10 2017

It will be mainly dry today with a mix of clouds and sun with
highs primarily in the lower to middle 30s. There will be just a
chance of light snow showers north of the toll road through
tonight. Lows by Monday morning will dip into the lower to middle
20s. A fast moving clipper system will bring renewed snow and
lake effect snow chances Monday through Tuesday night. Several
inches of accumulation are possible this week, especially in and
near the lake effect snow belt region.

&&

.SHORT TERM...(Today and Tonight)
Issued at 450 AM EST Sun Dec 10 2017

Relatively quite short term period with a couple weak short waves
moving through before stronger wave arrives Monday.

Northwest flow aloft continues over the area today and tonight.
Several subtle short waves embedded in this flow will cross the
western Great Lakes. First wave to quickly dive southeast this
morning. Regional radar mosaic has shown returns upstream over WI
but diminishing trend noted as moisture starved weak wave runs
into drier atmosphere per 00z regional soundings. Not expecting much
more than flurries across the north as this wave passes.

Slightly stronger short wave and PV anomaly to dive south later this
afternoon and evening. Left front quad of 80-90kt speed max passes
over the north while main sfc low stays well north. Associated
surface trough will sweep across the local area and allow winds to
veer northwest while weak cold air advection follows. Delta Ts
briefly increase to mid teens following trough passage with a west
northwest flow off the Lake. Model soundings not very impressive
with DGZ remaining dry and inversion heights stay around 5kft or
less. Enough low level instability and moisture flux off the lake to
possibly generate some flurries or light snow showers. Not expecting
much if any accumulation but did add some low chance pops to better
match northern neighbors.

Low level winds quickly back late this evening and overnight as next
clipper system approaches. Warm air advection and weak isentropic
lift begin to develop late but expect most pcpn to remain west of
our area until closer to 12z Monday so only some low chance pops in
the northwest late tonight.


&&

.LONG TERM...(Monday through Saturday)
Issued at 450 AM EST Sun Dec 10 2017

Fast moving clipper expected to drop through the western Great
Lakes Monday but surface low will stay to our north. Isentropic
lift increases Monday morning ahead of this wave but moisture
remains limited with GOMEX still cutoff. Expect light snow to
develop in warm air advection mainly north of US30. Subtle
increase in intensity likely Monday afternoon in far north ahead
of main upper trough with increased convergence near surface
boundary. Will likely see an inch or less across northern Indiana
into NW Ohio but closer to two inches in Michigan.

Attention once again turns to potential headline worthy lake effect
snow event beginning Monday night and likely extending into Tuesday
night and possibly Wednesday morning. Arctic intrusion for this
event several degrees colder than most recent invasion and will
generate extreme delta Ts. Sfc-850mb reach low to mid 20s while
sfc-700mb eclipse the magical 30 differential which is found in
many heavy lake effect events. Nam12 0-2km ThetaE lapse rates
concur with values reaching -2.5 k/km Tuesday. This event will
feature more of a northwesterly component to winds compared to
last event but upstream conditioning across Lake Superior being
indicated in low level wind trajectories. Since airmass is colder
it is also relatively drier and forecast soundings off bufkit
indicate lower inversion heights between 7- 10kft. DGZ does look
to be saturated with good lift for much of the period, especially
Tuesday. Still a bit early to be getting into the details and
will have to see how models handle these variables. For now though
have significantly increased Pops over superblend init and went
with categorical wording in favored NNW fetch locations. Once
again wind speeds and trajectories will be favorable for bands to
penetrate well inland and this being picked up already be several
models. While confidence is high on this event occurring, the
details of wind direction and band orientation and movement still
uncertain to some extent. Did rely on SREF trajectories and PoP
orientation for this package. Several inches of snow appear likely
but still a bit early for headlines.

A couple fast moving clippers expected later Wednesday into
Thursday but models have significant differences on timing and
location. Looks like conditions do become favorable again briefly
for another round of lake effect behind these systems late in the
week. Moderating temps still on track for Saturday as flow becomes
less amplified and more zonal. Will have to watch for a potential
system late next weekend or early next week. Strong pacific jet
energy will likely be coming onshore and less amplified flow
should allow GOMEX to become available for any southern stream
energy to tap into and develop over the central CONUS.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday Afternoon)
Issued at 1244 PM EST Sun Dec 10 2017

Primarily MVFR cigs through the forecast period, yet should hold
above fuel/alternate criteria, per upstream obs and ceiling
forecast guidance. Chance of lowered vsby due to light snow
midday/early Mon afternoon at KSBN associated with arrival of
fast moving clipper system dropping southeast through IL by end of
forecast period.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...Small Craft Advisory until 10 PM EST this evening for LMZ043-046.

&&

$$

SYNOPSIS...Murphy
SHORT TERM...Lashley
LONG TERM...Lashley
AVIATION...Murphy


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