Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 131139

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
539 AM CST Wed Dec 13 2017

351 AM CST

Through tonight...

Another clipper system will bring the potential for some light
precipitation to the region again today, with the greatest
potential for any light snow accumulation mainly across the far
northern suburbs of Chicago. Winds turn northerly and gusty behind
the departing system tonight, with lake effect snow showers
expected to affect northeast IL with some accumulation potential
this evening, and eventually northwest Indiana later tonight into

Early morning GOES water vapor imagery shows a pair of mid-level
short waves digging southeast from the northern Plains and
Canadian prairies. A fairly stout 995 mb surface low was analyzed
over western Minnesota at 09Z, and guidance is in good agreement
in tracking this low southeast across far northeastern Illinois
this afternoon. Warm advection was noted across the area per RAP
analysis, with westerly 30 kt winds inducing isentropic upglide
across the western Lakes. Regional radar depicts mid-level returns
over parts of WI and Lake Michigan at this hour, though very dry
air in low levels has initially been preventing much of this from
reaching the surface south of the IL/WI border overnight. Slightly
better saturation appears to be occurring with band across
western WI, and high-res guidance generally develops this
southeast and clips far northeastern IL around sunrise. Thus a
short 1-2 hour period of light snow is possible mainly east of
Rockford and north of Chicago early this morning, with perhaps a
dusting possible. Forecast soundings then dry things out by mid-
morning, before the main mid-level vort and surface low approach
the area late morning. Again, forecast soundings indicate a
struggle to really saturate the column south of the IL/WI border,
though it does appear at least some spotty precip is possible
across the area during the midday and afternoon hours as the low
and cold front move through the area. Precip type may be an issue
as warmer, above freezing air is pulled north across the area
ahead of the low, with a mix of rain/snow or just rain along/south
of I-80. Similar to Monday, temps will likely warm into the lower
40`s south of I-80, and the mid-30`s farther north. Quick cooling
of the column then occurs with the cold frontal passage later
this afternoon, which should turn everything over to snow showers.
Again, some light accums of less than an inch are possible this
afternoon, mainly north of Chicago where precip has the best
chance of being primarily snow along/north of the surface low

Winds look to increase substantially behind the low/cold front by
this evening, with gusts of 35 mph as cold air surges back into
the region and steepens up low level lapse rates. Several models
turn winds north-northeast for a time during the evening,
suggesting the potential for lake effect snow showers to affect
northeast IL. Lake to H8 delta-T`s approach 20C overnight, though
a fairly stout subsidence inversion around 800 mb would limit
equilibrium levels to <7000 ft. Thus have fairly high confidence
in snow showers, but accumulations may be rather modest, perhaps
an inch if the band remains transient. Boundary layer flow
gradually backs more north-northwest overnight, shifting focus
into northwest Indiana after midnight, with slightly better
accumulation potential.



344 AM CST

Thursday through Tuesday...

Overview...Lake effect snow continues over northwest Indiana
Thursday morning likely impacting the Thursday morning commute. An
upper level wave may force light snow or flurries Thursday night
and Friday. Temperatures rise to around 40 Saturday and the next
chance of precip arrives Sunday.

While the low pressure system continues to the east coast, the
lake effect snow engine will continue to run. Delta T`s are
forecast to be in the low 20s, and lake induced equilibrium levels
will be around 800 mb. Forecast boundary layer winds direct the
plume over northwest Indiana and slowly shift the band east
through the morning. I currently only have 1-2 inches of snow
forecast in NW IN, due to low confidence in exact band placement
and how quickly it will shift east. However, if the band remains
stationary, snow totals could be higher. Local guidance suggests
totals could be up to 6-8 inches especially if the band sits over
an area for awhile.

An upper level wave moves over the region Thursday night and
Friday. Guidance differs on how much snow the system will produce
so I kept precip chances at chance or lower. Forecast soundings
show saturation in the snow growth zone, so I also included
flurries outside of where the best chance for light snow
showers/accumulating snow may occur. Breezy conditions are also
possible as a deepening low passes over the northern Great Lakes.

Temperatures warm into the 40s over the weekend. The next chance
for precip is Sunday. The GFS keeps the system mainly to our
southeast while the ECMWF brings it over eastern Iowa and southern
WI. The two models have very similar upper level patterns with a
positively tilted trough extending from the upper Midwest through
the desert southwest. If precip does occur, rain looks like the
most likely precip type.

Normal to slightly above normal temps are then expected through
early next week.



For the 12Z TAFs...

Primary aviation concerns are with potential for light snow or
snow showers (and attendant cig/vis issues) this afternoon, and
lake effect snow showers tonight. Winds become gusty from the
south this morning, shifting northwest this afternoon behind a
cold front with gusts near 30 kts possible into this evening with
winds becoming more northerly.

Strong low pressure system was near KMSP early this morning, and
will track southeast across far northeastern IL this afternoon.
Patchy light snow has developed in warm advection ascent across WI
overnight, and a brief 1-2 hour period of light snow is possible
north of Chicago terminals around sunrise this morning, but is not
expected to impact the terminals directly. South winds will
gradually increase, with some gusts above 20 kts possible as the
low approaches. Dry air in low levels should maintain VFR
conditions through the morning however.

Surface low/cold front moves through midday/early afternoon, with
winds veering to the northwest, and becoming gusty 25-30 kts.
Period of light snow/snow showers appears likely behind the front,
though confidence in coverage is fairly low given rather dry low
levels. MVFR cigs 1500-2000 ft expected in cold air behind the
front and with precip. Winds turn more northerly this evening,
still gusty, and become north-northeast just above the surface.
This looks to provide the threat of lake effect snow showers into
the Chicago terminals during the mid/late evening hours, with the
snow bands gradually shifting southeast into northwest IN
overnight. Fairly high confidence in snow showers developing,
though uncertainty remains in details of timing, and degree of
cig/vis impacts for ORD/MDW. Winds eventually turn more northwest
again by Thursday morning, decreasing, with VFR conditions



344 AM CST

Ended the current Small Craft Advisory for NW IN, but I did not make
any changes to the next Small Craft Advisory that begins later this

Low pressure over Minnesota will pass over southern Lake Michigan
this afternoon. South winds will increase to 30 kt over the southern
half of the lake ahead of the low this morning. Winds diminish and
display a counterclockwise circulation as the low moves over the
lake, and east winds increase to 30 kt over the northern half of the
lake. It looks like there may be a very brief period of gales over
the far southern end of the lake this evening. I only included
occasional gales in the GLF due to the short time frame and medium
to low confidence I have in gales.

The low continues to the east coast while winds gradually back to
the west.  Low pressure forms over Lake Superior Thursday night and
deepens as it reaches Lake Ontario Friday. West winds to 30 kt are
expected behind the low.

Winds become southeast as high pressure passes over Ontario
Saturday. While guidance differs on when the next low will arrive, a
weak low will pass over the lake early next week. Winds are either
west or northwest to 30 kt through the remainder of the forecast as
a low pressure system passes over central Canada.



LM...Small Craft Advisory...nearshore waters
     until noon Thursday.




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