Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230252

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
952 PM CDT Thu Mar 22 2018


948 PM CDT

No changes to going Winter Storm Watch. An early look at the
latest available guidance largely supports the current Watch
counties. The 00z NAM features very significant 1"+ liquid
equivalent amounts in a good chunk of the watch counties, which
would indicate potential for very heavy snowfall rates and
widespread 8"+ snowfall amount should things evolve close to what
is depicted. It also portrays an exceptionally sharp cut-off over
the northeast third of the CWA due largely to low level dry air.
This cut-off has been well advertised.

However, have some concern off this run alone that the magnitude
of forcing, especially mesoscale from mid-level frontogenesis, and
resulting strong lift, would cause heavy enough snow aloft to
saturate the low levels in parts of the area under the dry air
cut-off, especially sufficiently inland of Lake Michigan. This
would include portions of the western/southwest Chicago metro on
Saturday morning. A good way to visualize this potential is with
the simulated Composite Reflectivity, compared to 1km AGL
simulated reflectivity or the actual QPF output, which shows
strong 25+ dbZ echoes well north into the CWA, but little/no
reflectivity at 1km level and little/no QPF for part of area under
high reflectivity aloft/Comp Ref.

With the above being said, the verbatim outcome of the 00z NAM did
bring it into better agreement with the other operational guidance
after the 18z runs of the 12km and 3km NAMs were well north of
the consensus. On one final note, the 21z SREF did adjust
northward and increase mean QPF/snowfall from previous run (was
highest mean of previous 4 runs), but with still very significant
spread, especially among NMB members. All the above goes to say
that there is still uncertainty in the outcome of this event, such
as exact placement of the likely very sharp cutoff somewhere over
the northeast CWA, along with top end snowfall amounts in the
heavy snow swath. The current watch counties remain the highest
confidence for significant snowfall and impacts. The hope is that
full sampling by the 12z RAOBS of the southern stream short-wave
entering southern California will clear up lingering discrepancies
for the 12z guidance suite should there still be fairly large
spread with remainder of 00z suite.



308 PM CDT

Through Saturday...

Quiet weather will continue for roughly the next 24 hours across
northern Illinois and northwest Indiana under the influence of high
pressure, but winter returns Friday evening through Saturday with
some locations expected to experience heavy snow while others may
see a wintry mix.

A strong wave rounding the base of a deep upper trough over the
eastern Pacific this afternoon will move onshore over SoCal late
this evening then emerge over the central Great Plains Friday. In
response, surface low will deepen over Kansas causing the baroclinic
zone to tighten from the mid-Missouri Valley to the lower Ohio
Valley serving as the focus for the heavy precipitation to come.
Increasing southerly flow ahead of the low will transport moisture
rich air from the western gulf into portions of the Midwest. The one
inch PWat contour is still expected to move north into central
Illinois with values just under an inch creeping into our western
and southern CWA counties.

Synoptic scale forcing looks to peak late in the overnight hours
into early Saturday morning with periods of mesoscale enhancement
quite likely into mid morning Saturday. Models advertise coupled
left exit/right entrance regions of the jet stream moving
overhead during this time. Meanwhile, isentropic ascent ramps up
from the southwest late in the evening with forecast soundings
indicating saturation occurring around or after midnight in the
southwest counties and slowly spreading northeast during the
overnight hours. Dry easterly low level feed of air may result in
several hours of virga and continues to cause a source of
uncertainty in start time. Confidence is highest for snow up to a
Rockford to Rensselaer line by around daybreak Saturday and may
only spread another 50-75 miles northeast through the day.

Models continue to advertise a corridor of strong f-gen developing
over the forecast area, topped with steep mid/upper level lapse
rates along with slant-wise and, at times, very modest upright
instability. Certainly couldn`t rule out an isolated rumble of
thunder or two within the stronger banding. Thermal profiles show
saturation through a deep region of plate growth but also cutting
through a wide temperature range. Continue to call for sub-climo
snow ratios, but there may be some times of more efficient snow
rates accompanied by larger aggregates.

Forecast challenges continue to center around likely areas for
mesoscale banding, sharp cut-off on the north end of the precip-
shield and wintry mix potential in the far south. No major changes
in thinking with latest forecast package. Major global models
continue to remain in reasonably good overall agreement though
ensemble spreads reveal ongoing uncertainty in the specifics.
Subtle differences in the thermal profiles persist, but favor
cooler solutions given the low track to our south, poor
insolation, along with the combination of dynamic cooling and wet-
bulbing effects. The best chance for a wintry mix remains in or
near the southern halves of Livingston, Ford, and Iroquois
counties. Light icy accumulations are possibly across those areas.
Axis of heaviest snowfall continues to run from eastern Iowa
towards central Indiana, including portions of the southwest half
of the CWA where snow totals in excess of 6 inches will be
possible. Expect a sharp gradient will occur along or just
northeast of a Rockford to Rensselaer line where confidence is
fairly low in snow totals. It`s quite plausible that portions of
northern Cook County into Lake County and eastern portions of
McHenry County see nothing. No changes planned to the going
headlines at this time.



228 PM CDT

Saturday night through Thursday...

Aside from the possibility of lingering light snowfall Saturday
evening in the southeastern portion of our forecast area,
increasing heights aloft and the southern periphery of a broad
surface ridge across the Great Lakes should allow dry conditions
to prevail for the remainder of the weekend into Monday afternoon.

Forecast highs on Sunday reflect the expectation for a band of
snow cover running from Lee/Ogle through Livingston and Benton,
keeping this area just about 3-4 degrees cooler than areas either
side of this corridor. This is actually being picked up by some of
the models both for Sunday and Monday, though given the degree of
late March insolation there is less confidence that the effects
will be as pronounced on day two of the melting. At the same time,
sky cover begins to increase again Monday afternoon ahead of the
next low pressure system, so this could slow the melting somewhat.

By Monday afternoon the surface and upper level ridging that had
been in place overhead begins to slide east as low pressure
develops across the Plains. Models are in decent agreement in
bringing the low and its associated frontal zone very slowly
across the area late Monday through early Wednesday. Snow will not
be a concern locally. Instead, warming mid-upper 50s temperatures
along with instability fueled by Gulf moisture could support some
convection, especially on Tuesday afternoon if present trends
hold. Will not include TSRA in the forecast just yet, but will
continue to monitor for the possibility.

In the dry northerly flow behind the front, Wednesday and
Thursday could be a few degrees cooler than earlier in the week,
but perhaps not too much below normal highs which will soon be in
the low 50s.



For the 00Z TAFs...

A ridge of high pressure will remain anchored over the lake and
centered north of the region. This will allow for a continued ENE
wind through the period, weakening some tonight, then picking back
up on Friday as a storm system organizes in lee of the Rockies.
Expect winds to get back up above 10 kt through the afternoon and
as the low will move into the central plains and increase the
pressure gradient.  More active weather in the form of light snow
arrives late Friday, likely after the peak evening traffic
period and largely remaining southwest of the terminals through
this TAF cycle. Expect more widespread snow overight Friday into
Saturday morning. Present expectations are for the heavy snow axis
to remain southwest of the terminals, but some minor shifts in the
storm could bring heavy snow farther north. At this time, an inch
or so of wet snow Friday night into Saturday morning is favored
for ORD/MDW, with temperatures remaining close to or above

Confidence is not super high on amounts given the
expected very tight gradient between heavy wet snow and much
lighter amount, but confidence on timing is medium to high.



228 PM CDT

High pressure across the lake will support relatively light and
somewhat variable winds through late Friday at which time a low
pressure system approaching from the Plains will start to increase
the gradient and turn winds easterly. The low moves well south of
the lake, through the Tennessee Valley, early in the weekend.
Meanwhile a ridge positioned from the western Great Lakes into
eastern Canada will help support ongoing easterly flow into early
next week.



IL...Winter Storm Watch...ILZ008-ILZ010-ILZ011-ILZ019-ILZ020-
     ILZ021...10 PM Friday to 4 PM Saturday.

     Winter Storm Watch...ILZ022-ILZ023-ILZ032-ILZ033-ILZ039...1 AM
     Saturday to 4 PM Saturday.

IN...Winter Storm Watch...INZ010-INZ011-INZ019...1 AM Saturday to 4
     PM Saturday.




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