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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
301 PM CST Monday Feb 8 2016

Short term...
250 PM CST

Tonight through Wednesday...

Snow/snow shower trends and accumulations...Indiana Lake effect
snow...and colder temperatures are the main forecast concerns
through mid-week.

Deep closed upper low was over the region this afternoon within
large eastern North American upper trough. Several vorticity lobes
were rotating about this feature...and along steep low level lapse
rates due to strong cold advection will be the main modulating
factors in coverage and intensity of light snow/snow showers over
the next couple of days. One such wave was noted in water vapor
imagery currently moving southeast across central Illinois...with a
corresponding area of light snow across southern parts of the
forecast area this afternoon. Another axis of sheared vorticity
was over Lake Michigan and eastern Wisconsin with another batch of
snow showers. Flurries were likely occurring in most other areas
in saturated cold air mass. Light snow will likely continue to
affect southern parts of the forecast area for the remainder of the afternoon
as the central Illinois wave continues to move off to the
southeast...with light snow/snow showers expected to increase from
the north again this evening/overnight as sheared vorticity to our
north wraps south-southeast around upper low center which develops
across Northeast Indiana/Northwest Ohio. This upper low will
slowly lift out to the east-northeast on Tuesday...with another
short wave dropping into the trailing portion of the upper trough
and across especially eastern parts of the forecast area Tuesday
night. Model quantitative precipitation forecast amounts remain generally light...with non-uniform
coverage due to expected often showery nature of precipitation. still appears that widespread snowfall of an inch to
perhaps 2-3 inches are likely by the time measurable precipitation shifts
east of the area late Tuesday night...generally lowest far west
and higher across the east parts of the County Warning Area.

Within this weather regime...continued cold advection will result
in lake-effect snow showers developing across parts of Northwest
Indiana tonight...and most likely continuing into Thursday morning
at least in lighter fashion until low level winds back to the
west. Thermodynamics become favorable for lake snow showers this
evening... as lake surface to 850 mb Delta-T increases to around
20 c/km by Tuesday morning. Combined with with deeper forcing
provided by aforementioned vorticity lobe...the potential for heavier
snow showers looks to be from later tonight/early Tuesday...and
again Tuesday night into Wednesday. Low level convergence and
fetch continue to favor mainly northeast portions of Porter
County...with the axis of heaviest snow expected to be across La
Porte and Berrien counties to the east of the County Warning Area. Northeast
Porter County does have the potential for several inches of
accumulation during the period...depending upon location/intensity
of banding. Long duration of light/occasionally moderate snow
showers totaling perhaps 4+ inches does not appear to warrant
headlines at this point...though will continue to mention both the
lake effect and the potential for Tuesday morning rush hour impact
in an Special Weather Statement.

Temperatures very slowly falling into the low-middle 20s this
afternoon...and will continue to fall off into the teens in most
areas tonight. Little recovery is expected Tuesday as cold
advection persists...with cold air lingering through middle-week with
overnight lows in the single digit and highs in the teens by



Long term...
300 PM CST

Wednesday night through Monday...

Below normal temperatures to continue into the weekend as large
scale upper trough over eastern North America shifts slowly
east...but is reinforced over the weekend by another closed low
which develops over New England/Quebec. Eventually the trough axis
shifts far enough to the east for some moderation Sunday night
into Monday. Strong northwesterly flow aloft downstream of
western/central Continental U.S. Ridging will likely see several minor short
waves rippling through the flow. Differences remain in longer
range guidance with location of low level baroclinic zone to our
southwest...though the 00z/12z European model (ecmwf) have maintained continuity in
a minor disturbance tracking across the Midwest Friday with the
potential for some light snow. Depending on how quickly high
pressure exits in the day 6-7 period there is also a chance of
light snow as warm advection develops Monday. Until then... cold
is the rule with overnight lows in the single digits (and possible
around zero or below) and highs in the teens to low 20s before
slight moderation toward the end of the forecast.



for the 18z tafs...

Upper level low over Lake Michigan early this afternoon will
continue to slowly meander eastward across the Great Lakes today
and tomorrow with pinwheeling upper level disturbances bringing
periodic chances for light snow showers and flurries to the
region. One of these waves is now dropping south into central
Illinois which should allow a lull in the snow activity in its
wake through the rest of the afternoon. Another wave is prognosticated to
rotate across the terminals late this evening and tonight. Forcing
is fairly weak however low level moisture is focused within a
relatively deep snow growth zone which should allow for efficient
snow production. Confidence in the timing details remains fairly
low though given the rather benign forcing. During the day
Tuesday...cold air spilling into the region will result in
steepening low level lapse rates and once again prime the
atmosphere for decent snow production if we realize any
forcing...however any forcing continues to be weak resulting in
considerable uncertainty so for now will just introduce a prob30
for snow early to middle afternoon. Winds will remain out of the
northwest through the period...with gusts increasing late this
afternoon into early evening. Expect ceilings to remain MVFR through
the period.



223 PM CST

Low pressure centered near Lake Huron/Lake Ontario early this
afternoon will very gradually meander towards Lake Ontario by
tomorrow night with moderately strong north to northwest winds
expected to be in place across Lake Michigan. Winds are expected
to remain around 30 knots this evening through Wednesday
night...with a few gale force gusts possible mainly Tuesday into
Tuesday night. Freezing spray is also a concern Tuesday through
Wednesday evening. High pressure will build from the upper
Midwest Wednesday night into the Ohio Valley Thursday night
resulting a modest tapering of the winds as they back to the west.
Another cold front will sweep across Lake Michigan Friday turning
winds back to northerly and picking up once again. This front will
usher in a bitterly cold Arctic airmass which could result in
additional gales...particularly Friday night...and freezing spray
across Lake Michigan.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz743-lmz744-lmz745...midnight Tuesday
to 9 am Thursday.

Small Craft Advisory...lmz740-lmz741-lmz742...midnight Tuesday
to 3 am Thursday.



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