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Area forecast discussion 
National Weather Service northern Indiana
555 am EST sun Dec 28 2014

issued at 333 am EST sun Dec 28 2014

Canadian high pressure will bring cool and dry conditions to the
area today. High temperatures will remain in the middle 30s with
overnight lows in the low to middle 20s. There is a chance of lake
effect snow Monday night into Tuesday as much colder air filters
into the region.


Short term...(today and tonight)
issued at 333 am EST sun Dec 28 2014

Little to discuss this short term period as subsidence/high
pressure build into the region in the wake of passing jet
streak/vorticity filament and associated low level front. Marginal
lake response seen in radar imagery and surface observation across
southwest Michigan as cold air advection ramps up and capitalizes on some
residual moisture. A few flurries may clip our northern counties
early this morning but dry air advection will quickly eat away at
any lake induced convection and entire area should be precipitation-free
from late morning through the overnight hours. Stubborn stratus
deck will likely persist through the early morning as well but
suspect we will see some sun by afternoon with strong dry air
advection and almost due westerly flow. Latest satellite imagery
shows northern Illinois is already clearing out. Notable cold air advection will
prevent any diurnal recovery though. 850mb temperatures are dropping to
around -7c...which will keep afternoon highs not much different
from current values. Lowered overnight minimums by a few degrees.
Clouds will be increasing by early morning as flow becomes more
northerly but should still see enough radiational cooling to allow
temperatures to drop into the low to middle 20s...still slightly above
normal for late December.


Long term...(monday through saturday)
issued at 333 am EST sun Dec 28 2014

Lake effect snow shower potential for Monday night-Tuesday
night...trend to colder conditions through middle week...and then
potential next system to affect the area toward the end of the
period will continue to be the primary forecast challenges this

Upper level ridge amplification will be ongoing Monday across
eastern Pacific with a series of low amplitude waves expected to
track across the region through middle week. Eastern Pacific jet will
carve out broad western Continental U.S. Negative upper height anomaly Monday
with guidance now being fairly consistent from run to run in
depicting a pair of low amplitude short waves to affect the Great
Lakes/middle MS valley Monday night into Tuesday. 1050 mb high will
seep down Lee of rockies Monday night into Tuesday...with persistent
low level cold advection across much of the upper Midwest to the
Ohio Valley.

Northerly flow on eastern periphery of this high will set up
potential of some lake effect snow showers for Mon-Tue. More
favorable trajectories initially on Monday may favor areas just west
of local area into Northwest Indiana/far northeast Illinois but
southward progression of this anticyclone should provide backing low
level flow to the north by Tuesday morning. Lake enhanced thermal trough
should provide some focused low level convergence by Tuesday morning
as colder air continues to advect southward...although local hires
guidance and suite of NCEP models vary in the placement of enhanced
surface troughing. Depth of lake induced instability to
increase...especially in the 06z-15z timeframe on Tuesday with NAM
modified buffer soundings suggesting lake induced equil. Levels to
around 8k feet by 12z Tuesday. Northerly low level fetch...increasing
depth of instability...and some weak synoptic enhancements from
aforementioned low amplitude short waves should allow threat of lake
effect snow showers to shift eastward into far northwest portions of the
area late Monday night into Tuesday morning. Synoptic setup may
yield some degree of shear in cloud bearing layer however...and
duration of any periods of deeper moisture still of low confidence
based on timing these short waves. Have continued idea of previous
forecast with a continued increase in probability of precipitation focused especially late
Monday night/Tuesday morning where at least some light accumulations
appear possible. Transition to more of a west-northwest fetch later Tuesday
along with strengthening middle level subsidence should tend to
diminish lake effect snow shower chances by Tuesday night...although
scattered lake effect snow showers may persist into Wednesday before winds
back more southwesterly.

Outside of lake effect snow showers...did add some slight chance -SW
probability of precipitation across much of County Warning Area late Monday night/early Tuesday associated
with weak synoptic forcing/strong baroclinicity. Wednesday is still
shaping up as the coldest day of the period as next northwest flow wave
brings low level cold core across the Great Lakes. No change made to
previous forecast with highs generally in lower 20s for Wednesday.

Attention for the remainder of the forecast will then shift to
cut-off pv anomaly across southwest Continental U.S. And its impacts on local
area as it gets ingested back into northern stream flow some time
late this week or next weekend. A very large spread in deterministic
models and large inter/intra gefs/ec ensemble differences offer
continued low confidence at this forecast distance. Much of these
differences may Stem from complicated upper flow pattern over Gulf
of Alaska providing a wide range of possibilities regarding details
of northern stream flow that will be important in how the cut-off
southwest Continental U.S. Upper low eventually phases. 00z ec has actually
sped up northeast progression of the cut-off trough with better
phasing more inline with GFS...but hesitant to fully buy into this
given above uncertainties with northern stream...and a good deal of
Gem/gefs individual ensemble members more blocky in nature.
Potential does exist for leading surge of warm air advection/isentropic
lift/moisture advection over cold dome across the area and no
changes made to previous high chance probability of precipitation by Friday night/Saturday.
Have kept predominately snow precipitation type...but retreating cold dome
and uncertainties in track could complicate ptype issues down the
Road. Temperatures should moderate back to seasonable levels toward the end
of the period...although prefer colder ec MOS at this time.


Aviation...(for the 12z tafs through 12z Monday morning)
issued at 555 am EST sun Dec 28 2014

Postfrontal stratus deck will gradually erode later this morning
as dry air advection and subsidence increase. Latest satellite
imagery shows clearing line already moving through Chicago and
forecast soundings/MOS guidance suggest clearing in our area
around 15z. Ceiling heights may flirt with fuel alternate criteria
during the next few hours but will hold with high end MVFR for now.
VFR conditions then expected for the rest of the taf period as low
level ridge moves overhead.


Iwx watches/warnings/advisories...
Lm...Small Craft Advisory until 11 am EST this morning for lmz043-046.



short term...agd
long term...marsili

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