Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

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

Short term...
334 am CST

Through Thursday...

The forecast theme of light snow with occasional moderate snow
showers remains on track for today with additional light
accumulation. Attention also is on Porter County Indiana where
temporary but more appreciable lake enhanced snow showers are
expected at times today and tonight. At this time no headlines are
planned to be issued as only minimal impacts are expected at this
time and some uncertainty on duration. An area wide Special
Weather Statement was again issued and one more focused for Porter
County. Also on track is the cold which looks to provide area wide
subzero wind chills to start Wednesday morning and probably
Thursday too.

The impressively amplified trough across eastern North America is
ever so slowly inching eastward. With a vigorous 150kt upper jet
on its western edge...there remains numerous embedded short waves
over the western Great Lakes. These disturbances and their lift
with continued low-level cold advection and steep lapse rates will
help the snow to continue today. The lapse rates will allow for
occasional snow showers which will bring sharply reduced
visibility and swaths of slightly higher snowfall amounts. A
general west-to-east fade in this synoptic activity is expected
late this afternoon through mid-evening. High temperatures will
rebound little if at all today with middle teens to middle 20s expected
from west to east with wind gusts to 25 miles per hour adding an extra chill
to the air.

As for lake effect snow in Northwest Indiana...the cold advection
in the low levels...characterized by 850mb temperatures dropping
to -15c or colder by early afternoon...will help strengthen lake-
induced instability. Modified rap and NAM soundings for middle 30s
surface temperatures provide cape of 50 j/kg much of today. Both
of these models indicate transient but strong low-level Omega into
Porter County today and early this evening. The activity is likely
to be somewhat disorganized due to a lack of a confluent
focus...but still enough of a duration to provide 12-24 hour
snowfall totals of 2-3 possibly 4 inches today and tonight...with
the higher end in northeast Porter. The better signal for the
confluent flow remains just east of our area where advisories will
go into effect from adjacent National Weather Service northern Indiana during the middle-

The strong cyclonic flow still in place tonight should warrant
clouds though some breaks could happen in northern Illinois by
daybreak Wednesday. The 4km Storm Prediction Center WRF column condensate field which
often does well with cold air advection stratocu keeps clouds around a good part
of the night as well...lending to a little higher confidence.
Temperatures will still fall within this regime and single digits
are expected in most locations by daybreak Wednesday. The
confidence in cloud cover drops quite a bit Wednesday and
Wednesday night though do anticipate a general diminishing trend
which will allow for Wednesday night to be a chilly one...with
some outlying near or even subzero lows possible.

Snow is not expected for the area on Wednesday outside of some
continued lighter lake effect snow showers in northern Porter
County...although the boundary layer flow will be gradually
backing so it is not anticipated to be much.



Long term...
345 am CST

Thursday night through Tuesday...

Winter continues going forward into the weekend and early next
week in an unsettled pattern. Global guidance continues to show a
sheared short wave diving over or near the area in northwest flow.
Most models and even ensemble members indicate light quantitative precipitation forecast with
this. It is a few days away yet but it has the look of one of
those clipper-like systems that could produce a narrow swath of
possibly a couple inches of snow given favorable thermal profiles
for snow growth and light low-level winds.

The northern stream of the jet again buckles behind the
aforementioned wave with an Arctic high of 1040mb + expected
across the upper Mississippi Valley by Friday night. The synoptic
regime continues to resemble one that could support mesoscale lake
effect snow...possibly even heavy...with convergence pointed
toward some of the County Warning Area. At this time that looks to be more
Northwest Indiana. Again far too early for something of that scale
to have big confidence in...but did increase snow chances in far
Northwest Indiana Friday night into Saturday.



for the 18z tafs...

Expect light snow showers/flurries to continue off and on through
the remainder of the afternoon though at least for the next few
hours expect variable high end MVFR to VFR conditions with lower
chances for IFR. Rap/hrrr models continue to hint at an uptick in
activity this evening so adjusted timing of the tempo to fit the
best window of IFR or near IFR conditions. Forecast soundings show
a deep saturated layer within the snow growth zone meaning the
atmosphere will be primed to produce decent sized dendrites...
however there is no significant source of forcing. That
said...probably wont take much to squeeze out some snow this
evening and an upper level disturbance is prognosticated to drop into the
northern Great Lakes region this evening with modest middle level
height falls as far south as chicagoland. Keying in on this to
time the possibility for conditions to lower again...however
confidence in the details remains low at this time given the very
benign forcing. Winds are expected to remain out of the northwest
through the period with gusts to around 20 knots much of the time.



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.



Visit US at http://weather.Gov/Chicago (all lowercase)

Follow US on facebook...twitter...and youtube at:

National Weather Service Glossary of Abbreviations