Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161935

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
235 PM CDT Fri Mar 16 2018

233 PM CDT

Through Saturday...

A compact surface low and mid-level trough across north-central
Kansas will track eastward tonight while shearing apart between
the southern and northern streams. Though dynamics are becoming
less favorable as the system approaches the CWA, sufficient low-
level moisture transport and warm-air advection should maintain
modest ascent across most of the CWA overnight. Precip should
begin across the SW CWA around 03Z, then expand NE into the
Chicago Metro around 06-07Z. Precip should then gradually diminish
across the entire area from 10-14Z. Cloud cover will then
gradually diminish from north to south through the day Saturday as
upper ridging and surface high pressure edge into the area from
the north.

Surface dew point depressions of around 20F have been prevalent
upstream across lower MI this afternoon. The continuous dry E/ENE
feed in the low-levels should maintain surface wet-bulb temps below
freezing through the night. However, it will also result in a sharp
northerly cutoff in precip. Based on latest guidance, have continued
to sharpen the northern edge of the precip, with QPF reaching to a
rough line from Beloit to Woodstock to Evanston.

Low-level temp profiles supports nearly all precip to melt during
descent. Across the northern fringes of the precip generally along
and north of I-90, a sufficiently deep layer of sub-freezing wet-
bulb temps exists for a mix of freezing rain and sleet/snow. Precip
type then gradually transitions to all freezing rain across central
portions of the CWA, to a mix of rain and freezing rain across
southern portions of the CWA.

While most of the icing is expected on elevated surfaces, ground
temps near freezing in some locations should result in icing on non-
elevated surfaces. With ice accumulations generally in the 0.1" to
0.2" range for most areas, have expanded the Winter Weather Advisory
to all of the CWA except the WI bordering counties. With that said,
icing conditions for Kane, Dupage and Cook counties should remain
confined to the southern portions of counties. As for sleet/snow
accumulations across the northern portions of the precip shield,
current expectation is for less than 0.5".



345 AM CDT

Saturday Night through Thursday...

Sunday looks to be a nice day inland with lower to mid 50s favored
if indeed sunny as forecast, with high pressure ridging over the
area. A lake breeze will drop lake adjacent area temperatures
noticeably in the afternoon.

A similar synoptic system to the one in the short term is
expected to develop across the central Plains Sunday night into
Monday. A split flow jet pattern makes this challenging to say how
this system will track eastward. The 00Z Global guidance and the
GFS ensemble favor this track south of our area Monday Night into
Tuesday at present, but will be something to watch.

The northeast flow reinforced with this will keep temperatures
slightly below normal Tuesday-Wednesday, and continue lake
cooling through mid and even possibly late week. Cannot rule out
some lake effect showers Tuesday Night-Wednesday.



For the 18Z TAFs...

VFR conditions should prevail for all terminals through this
evening, with a few lingering 2-3kft stratocumulus into the mid
afternoon for the eastern terminals. East to northeast winds will
remain near 10 knots through the period.

A shield of precip with MVFR ceilings will spread northward
across all sites late this evening through the overnight. Precip
will primarily be liquid, with some snow mixing in for the
northern terminals. With temps hovering near freezing, -FZRA will
be the primary precip type overnight, especially on elevated
surfaces. Also, a sharp precip gradient will exist across far NE
IL, but confidence in the northern extent of the precip has
inceased enough to include ORD in prevailing wx for a brief window

After precip ends around 12Z Saturday, MFVR ceilings will
gradually give way to SCT VFR clouds through the morning at all



247 AM CDT

I kept the Small Craft Advisory as is as waves should quickly
subside and no longer pose a threat to small craft by mid-morning.

North winds to 30 kt will weaken this morning as high pressure
spreads over the lake. Wind direction will vary across the lake this
afternoon with west winds over the northern end of the lake and
northeast to east winds over the southern end of the lake. The
high dissipates tonight while low pressure over Kansas weakens as
it reaches the Mid Mississippi Valley. The low continues to weaken
as it passes over the Ohio Valley Saturday. North winds set up
Saturday as the low passes to the south. A weak low passes over
the northern Great Lakes Saturday night/Sunday followed by high
pressure early next week. The next low forms over the Plains
Sunday night, and the low passes over the Ohio Valley Tuesday.



IL...Winter Weather Advisory...ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
     ILZ039...11 PM Friday to 8 AM Saturday.

IN...Winter Weather Advisory...INZ001-INZ002-INZ010-INZ011-
     INZ019...11 PM Friday to 8 AM Saturday.




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