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Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois
332 am CST Wed Dec 7 2016

Short term...
328 am CST

After our mild fall some may not believe winter is really here to
stay, even after our first accumulating snowfall Sunday. Well
today begins our reminder in the temperature department that
indeed it is winter across the region. And if you are still
doubters, we have another round of accumulating snowfall on tap
for the weekend, and then even colder weather in the long range
horizon.

First with brief some good news, we got to see the sun for a brief
time Tuesday, and the next few days will be precipitation free
for many areas. Upper level low pressure across the upper Midwest
and adjacent south central Canada. The upper low will meander
eastward across Lake Superior. High clouds will stream in from the
southwest today ahead of a disturbance across the base of the
large scale upper trough axis. Meanwhile, wrap around lower clouds
across Wisconsin will likely invade northern Illinois today and
into Thursday as the low continues east through Quebec. We could
even see a few flurries late today but the better chance will be
in Wisconsin. The bigger story will be the cold, as highs top off
in the mid 20s north to the low 30s far south as cold advection
continues. A brisk west wind will blow with gusts in the mid 20
mph range, making it feel like the mid teens to lower 20s.

There is a better chance for a few flurries or maybe brief snow shower
Thursday as the broad trough will transit the area, and
temperatures in the at least broken cloud deck will be into the
better snow growth zone. With more clouds in place and even colder
air moving in on northwest winds, high temps continue their
downward spiral with low 20s expected in most places. And while we
may get more cloud breaks Friday, the cold air remains entrenched
across the region for lows again in the low 20s to possibly some
upper teens. High pressure will move overhead Friday night into
early Saturday, so expect likely the coldest night of the week
Friday night with lows in the teens, approaching the single digits
in some areas. The high will also bring an end to lake effect snow
across the southern tier of Lake Michigan.

Each of the next few nights, wind chills will reach the single
digits.

Kmd

&&

Long term...
328 am CST

Saturday through Wednesday...

The long term story is on accumulating snowfall for Saturday PM
into and Sunday. Nothing Earth shatteringly different with the 0z
suite of guidance with the GFS, ec, and Canadian models generally
staying in their respective camps this go around, continuing
considerable uncertainty on storm evolution. What do have to note
that the energy driving this system is still spinning off the
coast of Alaska, so it has some time go before this portion of the
storm will be sampled by radiosondes. So let's at least discuss
what they have in common for messaging purposes. The baroclinic
zone separating the warmer air to our south and our colder air
mass will lay out across Iowa and northern Illinois Saturday
afternoon. Strong southwest lower level flow will feed into this
frontal boundary as the upper flow becomes more cyclonic. Upper
level jet energy will also aid in lift and expect a quick ramp up
in snow during the afternoon and even more so in the evening. The
models do also largely agree that precip type for most if not all
areas will be snow. The Canadian model is about 6 hours faster
then the ec and 9 hours ahead of the GFS in starting
precipitation, and even gets things going Saturday morning, though
it looks like there may be some initial dry air to overcome. The
ec while lighter on precip amounts initially lingers the snow
longer. This snow appears to be a bit fluffier than our last one.

Where things differ is in the strength of the upper flow pattern and
whether a surface low will completely develop over our area as
the GFS/Gem say, vs remaining as an inverted trough axis like the
ec before the surface low develops to our east. This leads to
significant differences in the strength and timing of forcing and
therefore associated qpf, with the GFS double the amounts of the
ec (more in some places), and spread in gefs ensembles of 0.2 to 1
inch of quantitative precipitation forecast. The GFS ensemble mean is around 0.55 and the Euro
ensemble mean is around 0.3. Therefore while forecasting amounts
are futile at this juncture, no matter how you paint it,
accumulating snowfall of at least several inches is expected
during this time.

The back side of this system will not be like our last snow where we
stayed mild. This will be the more typical one-two punch of snow
and then brisk and cold. Even colder air will invade the region
mid to late week (see the CPC 8 to 14 day outlook, which has our
entire area with well below normal conditions). To give you a
hint, even the model blend has highs in the mid single digits to
low teens, which is very likely overdone in an anomalously cold
situation with a snowpack in place.

Kmd

&&

Aviation...
for the 06z tafs...

Main aviation concerns will remain breezy westerly winds through
the taf period, along with the potential for development of MVFR
ceilings Wednesday into Wednesday night.

Surface low pressure continues to pass slowly north of the Great
Lakes late this evening. Blustery west winds gusting around 20 kts
at times will continue across the terminals through the period, as
the forecast area remains in the southern periphery of the
cyclonic flow around the slow-moving low. Cold air will continue
to spread in from the west/northwest, which will help to maintain
a well-mixed boundary layer with persistent gusts. Winds will
likely back to around 250 degrees Wednesday, before returning to
the 270-280 deg range by Wednesday evening, with gusts near 20 kts.

An area of MVFR cigs has passed just north of the terminals this
evening, though forecast model guidance suggests redevelopment
during the day Wednesday with bases in the 1500-2000 foot range.
This cigs look to linger into the evening, with some lowering of
the inversion allowing cigs to lower a few hundred feet from
earlier in the day. While an errant snow flurry is not impossible,
no substantial precipitation is expected.

Ratzer

&&

Marine...
308 am CST

With low pressure to the north and an associated cold front now
well east of the lake, strong westerly winds are in place over all
of the lake this morning. Although a brief window of gale force
gusts did occur over the south half, this has ended with to 30 knots
winds expected today. Over the north half, west/southwest winds
have increased to gales with gales to 40 knots currently being
reported. Do think these gales to 40 knots will continue this morning
but possibly see some slight lowering back to gales to 35 knots this
afternoon. These windier conditions will persist into this evening
but it's looking like gales will continue longer than previously
thought. Now have gales into very late tonight, and have extended
the current Gale Warning. Strong westerly winds will continue for
the nearshore as well, with hazardous conditions for small craft
now expected at least into early Friday and possibly longer. This
will especially be the case for the Indiana side where higher
waves may persist.

Rodriguez

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...nearshore waters
until 10 am Wednesday.

&&

$$

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