Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211140

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
540 AM CST Wed Feb 21 2018

414 AM CST

Last minute update based on radar trends was to cancel remaining
Illinois counties in winter weather watch, as precip moving east
quickly this morning. Advisory remains in effect until 15Z/9 am
CST from the IL/IN state line east for mix of freezing rain/sleet.



351 AM CST

Today through Tonight...

Main short term concerns focus on winter/flood headlines early
this morning, with precipitation ending over the eastern portion
of the forecast area over the next few hours.

Surface cold front moved through the eastern portion of the area
last evening, allowing temperatures to gradually fall below
freezing from the northwest. Freezing line was approximately from
Champaign to the south side of Chicago as of 3 am. Moist southwest
flow aloft continued to overrun the low-level cold air, which had
resulted in rain changing to a mix of freezing rain and sleet
overnight, though there has been a fairly narrow axis where
lingering precipitation and freezing surface temperatures have
overlapped, and thankfully air temps in that zone have generally
been between 30-32 degrees. After temps in the 50`s and 60`s over
the past two days, this has generally resulted in icing mainly on
elevated surfaces, while ground/pavement have generally remained
wet. Have allowed western portion of winter weather advisory to
expire at 3 am as scheduled, and will keep an eye on western edge
of precipitation this morning for potentially trimming the western
periphery of the remainder of the winter weather advisory over
the eastern parts of the cwa. IL portion currently goes through 6
am CST, with northwest IN until 9 am CST. While most roads are
likely in good shape, it only takes a little glazing to cause
problems, and have no plans to cancel remainder of headline until
precip ends.

Also under headline management, cancelled the remainder of the
flood advisory this morning, as it is unnecessary with an areal
flood warning in effect for the remaining portion. Precipitation
is expected to continue to transition out of the area during the
morning hours, and should clear our Indiana counties by mid
morning. Model guidance depicts strong drying in the mid and lower
levels during the day, though forecast soundings and time-height
sections indicate considerable high clouds will linger across the
area. Some thinning of the cirrus will likely allow for partly
sunny conditions west/north, while clouds hang on more in the
southeast. Temps will be a good 20-30 degrees lower than
yesterday, with highs closer to average in the low to mid 30`s.

The cold front settles south of the Ohio River by tonight, though
the elevated baroclinic zone will linger across MO, IL and IN
into the weekend beneath persistent southwesterly upper flow. Low
level flow in the 925-850mb layer veers tonight across this
baroclinic zone in response to a mid-level short wave emerging
from the southwestern states, along with a more minor amplitude
disturbance tracking north of the Ohio Valley. Some guidance,
particularly the NAM and GEM, develop some light precipitation
across parts of central IL/IN, including the far southeastern tier
of WFO LOT counties, after midnight tonight. NAM forecast
soundings down around CMI/LAF indicate marginal thermal profile
for snow, with a deep isothermal layer near 0C potentially
supporting a little freezing rain or a mix of snow/freezing rain.
The GFS is a little warmer aloft, supporting the freezing rain
idea, though not as well saturated in low levels (and doesn`t
produce measurable precip). With this in mind, have maintained
some low chance pops for mixed precip after midnight across the
far southern tier of counties. Could be some minor ice accums in
that area, which will have to be monitored as new guidance comes
in later today.



351 AM CST

Thursday through Tuesday...

Active weather pattern looks to persist through the weekend, as
a series of systems move through southwest flow aloft, and
interact with the frontal zone which lingers just to our south
during the period.

The weak disturbance which is expected to bring some light
precipitation to the region late tonight will exit the area
Thursday morning, with light freezing rain/sleet or snow across
the south changing to rain as temps warm diurnally in the
morning. Another more substantial mid-level short wave is progged
to move across the area later Thursday night into early Friday,
with a greater threat of precipitation across the cwa. As will be
the trend for the next few days, proximity to the frontal zone
will maintain relatively warm temps aloft, making precipitation
type forecast a little complicated. With surface temps below
freezing along/north of I-80 Thursday night, this again may
support some light freezing rain across the northern half of the
cwa, which would continue into Friday morning before surface temps
edge above freezing and allow precip to turn to just rain before
the wave exits and precip ends midday.

Another more amplified wave is progged to affect the region over
the weekend, with precipitation developing as early as Friday
night as warm advection begins aloft. Once again, warm air aloft
and temps at or below freezing Friday night would potentially
produce freezing rain initially, changing to rain Saturday morning
as surface temps warm. Medium range guidance differs a bit with
the track of the surface low with this system Saturday, though
both deeper/farther north GFS and weaker ECMWF would likely
maintain temps warm enough for rain into Saturday night, before
precip ends with the cold frontal passage.

Other than nighttime temps at/below freezing, daytime temps are
expected to reach the mid/upper 30`s Thursday, and the 40`s
through the weekend.



For the 12Z TAFs...

Overall quiet weather day on tap as rainfall that impacted the
region the past couple days finally moves east of the terminals.
Some lingering MVFR ceilings will be in place this morning though
should scatter out at some point early to mid morning. VFR
conditions will likely prevail through the remainder of the
period. Northwest winds this morning will veer to the northeast
early this afternoon and thereafter should maintain an easterly
component through remainder of the TAF period.



417 PM CST

Areas of dense fog over the southern portion of the lake should
clear this evening. Brisk northwest winds up to 30 kt behind cold
front passage this evening will diminish on Wednesday as strong
high pressure overspreads the lake. Speeds will be fastest to
diminish on the northern portion of the lake. On the south part
of the lake, winds will shift to northerly and will be slower to
subside. Hazardous conditions for small craft are expected in the
Indiana nearshore from the pre dawn hours of Wednesday through
Wednesday evening. Depending on how strong winds are when they
shift northerly, it`s possible that a Small Craft Advisory may be
needed for the Illinois shore as well. The next period of
heightened concern for strong winds over the lake will be
primarily over the weekend, particularly Sunday, as a strong low
pressure system may affect the region.



IN...Winter Weather Advisory...INZ001-INZ002-INZ010-INZ011 until 9 AM

LM...Small Craft Advisory...IN nearshore waters until 9 PM Wednesday.




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