Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 130248
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
848 PM CST Fri Jan 12 2018

.UPDATE...
848 PM CST

Convergence has set up across Lake County Indiana and into far
eastern Illinois this evening with NNW on the west side of Lake
Michigan and NNE winds at Burns Harbor. Across NE Illinois
therefore expect that accumulations will remain limited to maybe
another tenth as the band continues to shift, that and the snow
will have a hard time accumulating other than in piles as the wind
is whipping the snow around. Forecast soundings depict the
moisture will be coming to an end away from NW Indiana in the next
hour or so, and the convergence axis will shift as well. It will
linger a bit longer overnight there. Expect with the cold ground
and this band now a bit more organized that accumulations will
continue with some areas possibly getting up to an inch of snow as
winds will slacken some overnight.

No big changes to the forecast, the message is still the same
with some slick roads, visibility reductions likely across the
snow belt.

KMD

&&

.SHORT TERM...
244 PM CST

Through Saturday...

Primary forecast concern in the short term is the lake effect
snow. Lake effect snow flurries and isolated light snow showers
and have been steadily streaming into northeastern IL today and
have shown a bit of an uptick in coverage and a slight uptick in
intensity in the past few hours.

Bufkit soundings from southern Lake Michigan this afternoon show a
sharp frontal inversion just below 5kft, which is likely limiting
the intensity of the snow showers. Some cooling is forecast
between 5-10kft tonight which should allow equilibrium levels to
rise a bit. As the cooling occurs, temperatures in the convective
cloudiness should cool some and probably result in a bit better
flake production. While there looks to be some improvement in
parameters tonight, the set-up overall continues to look marginal
and given the expected transient nature of the band would expect
accumulations in NE IL to be in the coating to half inch range
before it shifts east into IN this evening. NW IN stands a better
chance to seeing amounts fluff up to locally around an inch or so
before the lake effect shifts east of Porter County Sat night.

Elsewhere, look for clearing skies and some relaxation in the
magnitude of the winds tonight. The gradual easing of the winds
should do little to help moderate the wind chills as temps fall
into the single digits over most of the area, the exceptions being
downtown Chicago and northwest IN where lake cloudiness will
provide a bit of a blanket.

- Izzi

&&

.LONG TERM...
244 PM CST

Saturday night through Friday...

Long term forecast focus is on the snow chances Sunday night into
Monday as another lobe of very cold air breaks off the polar
vortex and meanders south into the Great Lakes region early next
week.

Numerical guidances remains in fairly good agreement in phasing a
pair of shortwave troughs over Canada and digging them virtually
due south into the Upper Mississippi Valley and western Great
Lakes region Sunday into early next week. This energy is then
forecast to close off a deep tropospheric circulation that should
slowly move east. A very cold air mass associated with this upper
low should result in another bout of below to much below average
temps much of next week.

A Manitoba Mauler (eastern cousin of the Alberta Clipper) will
dive south into MN then turn east to northern IL or southern WI
Monday. Not a classic track for accumulating snow locally,
however a north-south oriented warm front will extend south from
the sfc low with respectable isentropic ascent to the east of the
front combined with DCVA ahead of the approaching upper low likely
to produce an area of mostly light snow. Forecast time-height
cross sections show a fairly deep DGZ (dendritic growth zone), but
fairly low in the troposphere (below 10kft). The strongest and
majority of the ascent and snow production looks to take place
above the better snow growth zone, which would suggest that
despite the cold temps SLRs (snow:liquid ratios) could end up
lower than would be expected with a system and air mass like the
one in place. Generally ran with about 15:1 SLR in the grids,
which is probably a bit generous given the forecast profiles.
Having said that, if guidance trends lower with the stronger
omega or higher up with the better DGZ, SLRs could certainly end
up higher. At this point, it looks like a longer duration 1-2" to
1-3" snow across the area.

Once the synoptic snow ends, we will probably continue to see
occasional flurries or light snow showers through Tuesday as
steep low level lapse rates beneath the upper low provide a
favorable environment for flurries/light snow showers in the very
cold air mass. In addition to the steep lapse rates (which lake
induced instability will further steeped) the upper low will also
result in high equilibrium levels and a very favorable LES (lake
effect snow) environment early next week in the cold air advection
regime behind the Mauler. Where the lake effect targets will
depend highly on the precise wind regime, which is hard to pin
down this far out. At this point, models and climo would certainly
favor the lake effect event being a IN/MI problem more so than
IL, but certainly bears watching.

Temperatures should remain very cold and well below average Monday
through Wednesday night. If we lay down a respectable few inch
snow cover with the Sunday night/Monday system, then temps could
end up being a bit colder than forecast. But would like to see how
much snow falls, before making changes to the blended model
initialization.

Cold upper low is forecast to get shoved east as high amplitude
upper ridge builds east into the area mid-late week. While models
are in good agreement in this evolution, it is worth noting that
closed upper lows like this have a habit of being more stubborn
than forecast and moving out more slowly. Once it does move east,
the upper ridge and strong warm air advection should result in a
big moderating trend late in the week. The depth of snow cover
will play a role in temperatures as initially the warmer air will
be dry and not particularly effective in snow melt. Once the snow
melts, the temperatures could once again get unseasonably warm,
though a lot of uncertainty this far out, so again maintained the
model blended temperatures unchanged.

- Izzi

&&

.AVIATION...
For the 00Z TAFs...

528 pm...Only forecast concern this period is lake effect snow
showers.

Light lake effect snow showers will continue across far northeast
IL as a more concentrated band takes shape this evening and moves
into northwest Indiana. Prevailing visibilities are expected to
remain vfr at ord/mdw with a brief heavier snow shower possible at
mdw this evening. As the focus shifts into northwest Indiana...
visibilities are likely to drop to 2sm or less at gyy but overall
confidence for how intense this lake effect band becomes is low.
Eventually it will shift east of gyy early Saturday morning. Mvfr
cigs with these snow showers will also slowly shift east overnight.
Northerly winds gusting 25-30kts this evening will slowly turn
north/northwest overnight as speeds/gust diminish. Winds will turn
more northwest Saturday with speeds relaxing to 10-15kts and
eventually well under 10kts with sunset Saturday evening. cms

&&

.MARINE...
244 PM CST

Northerly gales will subside tonight and no changes were needed to
the gale warning end times tonight. Freezing spray looks to
continue though nomogram suggests the threat of heavy freezing
spray is low so no headlines appear necessary for the freezing
spray. Winds should gradually back over the weekend and lock in to
a period of fairly strong southerlies Sunday night in advance of
the next system. At this point, it looks like the gradient won`t
be terribly strong in the wake of the Monday system, so haven`t
included gales in the grids in the cold air in the wake of the
system, but it will be something to keep an eye on for Monday
into Tuesday.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Lakeshore Flood Advisory...ILZ014 until 4 AM Saturday.

IN...Lakeshore Flood Advisory...INZ001-INZ002 until 4 AM Saturday.

LM...Gale Warning...nearshore waters until 6
     AM Saturday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...6 AM Saturday to 3
     PM Saturday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...6 AM Saturday to 6
     AM Sunday.

     Gale Warning...LMZ777-LMZ779 until 3 AM Saturday.

&&

$$

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