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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
843 PM CST Tuesday Feb 9 2016

833 PM CST

Light snow showers/flurries continue to slide east/southeast
across the western Great Lakes along the back edge of a departing
middle-level vorticity maximum. With a shallow dendritic growth zone...and
minimal moisture within the is more of a small flake
or high volume of ice crystals that are causing the reduction to
visibilities due to the cold temperatures in the lowest few 0-3km of the
atmosphere. Fortunately the back edge of the precipitation field is
already pushing into western Illinois...and will continue to progress
east through the remainder of the evening as anti-cyclonic flow
begins to back winds to the northwest/west and setup a lake effect
conds for areas downstream of Lake Michigan and perhaps northwest
in or mainly far northeast Porter County.

Temperatures overnight will continue to radiate...mainly after midnight
as drier air arrives and helps to scour the thicker cloud cover.
This will allow temperatures to dip into the single digits above zero in
the west/southwest and where clouds linger longer temperatures will be
around 10-14 degree overnight.



Short term...
254 PM CST

Tonight through Wednesday night...

Short term forecast concerns continue to be details of snow
shower coverage/timing and light snowfall accumulations...and
heavier lake-effect snow showers for Northwest Indiana and
primarily Porter County. Secondary focus is on cold temperatures
through the period and sub-zero wind chills during the overnight
and early morning hours.

High amplitude upper trough remains across the eastern half of
North America this afternoon...with a closed low centered over the
eastern Great Lakes and upper Ohio Valley region. North-northwest
flow was across the upper Midwest and western Great Lakes to the
west of the large scale trough axis. A series of sheared smaller
scale middle-level short wave disturbances moving through the flow
continue to be the focusing mechanisms for modulating areas/timing
of more concentrated light snow and convective snow showers...with
cold Arctic air otherwise supporting occasional flurries or very
light snow showers across the region.

12z upper air analysis...afternoon water vapor and current rap
initializations depict stronger short wave energy moving southeast
of the County Warning Area across the Ohio Valley. Weaker/sheared middle-level
vorticity axis trails across northwest in/northeast Illinois and the
Western Lake Michigan of WI...providing weak but persistent
forcing above nearly saturated low levels below about 700 mb.
Thermodynamic profile in this layer is nearly isothermal within
the -12 to -18c range which is optimal for producing
dendrites...thus not much ascent needed to squeeze out
flurries/light snow snow showers above unstable boundary layer
with steep surface-850 mb lapse rates. Current activity appears fairly
light per regional radar mosaic with returns generally 25 dbz or
less...though guidance is in general agreement that there will be
some increase in coverage/intensity by early this evening mainly
over northeast Illinois/Lake Michigan/Northwest Indiana as a more robust short
wave propagates southeast from the upper Mississippi Valley. High-
res rap/hrrr-exp guidance suggests these more organized snow
showers will primarily affect northeast Illinois through the evening
hours before diminishing from the northwest pre-dawn Wednesday
morning as the middle-level vorticity axis shifts east of the forecast
area. Model quantitative precipitation forecast is fairly low...only a few hundredths of an widespread accumulations of more than a few tenths is
not expected...though with the showery nature of the more
convective elements a few places in far northeast Illinois could pick up
an inch or so of new snow. Only a few flurries are likely during
the day Wednesday with low level lapse rates still steep and
thermal profiles in the snow-growth range... though no significant
large scale ascent.

As for lake-effect snow over Northwest Indiana...current rap
soundings indicate lake surface to 850 mb Delta-T around 18 c/km
with equilibrium levels 5000-5500 feet...with slight westerly
component to boundary layer wind field producing multi-band set-up
with the strongest convergent focus east of Porter County along
the Michigan shore. Rap/NAM soundings indicate that equilibrium
levels will rise this evening however with Delta-T eventually
around or slightly better than 20 c/km. Boundary layer
convergence...while maintaining a focus just east of Porter County
does strengthen overnight into early Wednesday. Thus lake-effect
snow showers are expected to strengthen across
northern/northeastern Porter County during the period with light
accumulations of potentially a few inches likely especially east
of the Porter/Chesterton areas. Convergence weakens during the day
Wednesday likely allowing for a decrease in intensity...though
some potential for light lake-effect snow showers looks to
continue into Thursday before low level winds back more westerly
and shift any lingering snow showers east of the area.

Cold northwest low level winds will maintain Arctic air across
the area through the near term period...with overnight lows in the
single digits to around 10 above in warmer spots and daytime highs
generally in the teens to near 20.



Long term...
254 PM CST

Thursday through Tuesday...

The large scale upper pattern continues to feature a deep long-
wave trough across eastern North America through the end of the
week... reinforced as another closed upper low drops southward
from Hudson Bay to southern Quebec and upper New England by
Saturday. Amplified ridging upstream across the western Continental U.S.
Flattens with time however as an upper trough propagates across
the Pacific northwest and the intermountain west. This eventually
brings some relief from northwest flow for the Midwest by the eastern trough finally drifts off to the east and
weak/flat upper ridge develops downstream from the approaching
western trough. Chilly Arctic air remains in place through early
Sunday however...before moderating back to/slightly above normal
by Monday/Tuesday of next week.

A quick-moving clipper type system will ride along the baroclinic
zone south of the County Warning Area Friday...and have indicated low chance probability of precipitation
for light snow during the day. Chances of light lake-effect snow
showers return to Northwest Indiana Friday night into Saturday as
surface high approaches and surface winds veer more northerly for
a time...though the remainder of the County Warning Area looks to be dry until
flow becomes south on the back side of the high with the approach
of the trough from the west with a chance of light snow looks to
be late Sunday-Monday. Faster west- northwest flow looks to
persist into Tuesday...with some evidence of a quick clipper-like
system also presenting some chance of light snow on Tuesday.



for the 00z tafs...

Main forecast concerns are with periodic snow showers with MVFR
visible this evening...and persistent northwest winds/gusts.

Radar imagery showing snow showers persisting across much of the
region at this time...with this snow still expected to impact the
terminals. However...snow and lowest visible of 3-5sm should be
periodic this evening before the snow exits closer to the midnight
time frame. Any additional snow this evening should be light
amounts...around a couple of tenths at most. VFR ceilings in
place will continue tonight before scattering out Wednesday.
Northwest winds will continue through the period but am expecting
slightly lower speeds tonight. Wind speeds will further diminish
wedesday into Wednesday evening.



218 PM CST

A moderately strong north to northwesterly gradient persists
across Lake Michigan between an area of low pressure over the
eastern Great Lakes and strong high pressure over the plains. Low
end gales are expected to continue mainly across the north half of
the lake through middle morning Wednesday. The combination of cold
temperatures over the lake and 30 to 35 knots winds will result in
some freezing spray as well. Winds briefly diminish and back to
west-northwest Thursday as the ridge noses closer and the gradient
relaxes...however another low is prognosticated to form over the northern
Great Lakes Thursday and will push another Arctic front across
Lake Michigan on Friday. Northerly winds once again increase back
to at least 30 knots and cannot rule out some gales Friday night into
early Saturday...along with freezing spray as bitterly cold air
moves across the region.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...Indiana nearshore waters until 9 am Thursday.

Small Craft Advisory...Illinois nearshore waters until 3 am Thursday.



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