Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161737

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1237 PM CDT Fri Mar 16 2018

330 AM CDT

Through Saturday...

A challenging precipitation type forecast for later tonight. A
period of freezing rain is expected and as noted in recent AFDs,
the uncertainty continues on the northeastward extent and what
impacts to roads will be across the area. In the zone that does
receive freezing rain, elevated surfaces/objects are likely to see
some ice accumulation, and given a few hour duration the roads
may see patchy ice as well. A collaborated Winter Weather
Advisory has been issued for the most confident area of freezing
rain for several hours. The impact time looks to be centered on
overnight, but could start in some areas late this evening and
extend in some locations to just after sunrise Saturday.

Satellite imagery early this morning shows the well-developed
closed upper low across the Kansas/Nebraska border region. Warm
advection-driven echoes on regional radar over Iowa and eastern
Missouri have shown a struggling trend to advance northeast and
that should be the theme through today. Nearly full sun this
morning will become more filtered as high clouds start to spread
in this afternoon, but not before allowing the drier low-level
air mass to warm into the lower to mid 40s in most areas. The
heart of Chicago with the onshore flow looks to stay shy of 40.

The upper low will start to feel the effects of the strong upper
ridging currently in place over the Great Lakes this evening,
which should lead to 1.) a gradual system weakening into the
overnight and 2.) a sharp northeast gradient in precipitation.
However, during the mid-evening, the leading axis of
precipitation could be modest and even convective as it spreads
into the southwest CWA, assisted by differential instability
wrapping northeastward ahead of the low. Precipitation type should
be rain at first in the southwest CWA, and cannot rule out some
thunder. As the system slows and gradually weakens late this
evening into overnight, an axis of deeper moisture convergence
should temporarily set up draped east-southeast over the
area...most favored on the suite of guidance where the current
Winter Weather Advisory is. Given the northeast dry low-level
flow, a fairly sharp gradient on the northeast side should be

The warm layer aloft in this system is not particularly warm, with
the maximum wet-bulb aloft mainly below 1.5C in the area of
interest. However, the depth of the isothermal layer is several
thousand feet, and sufficient for melting of hydrometeors using
the Bourgouin method. This combined with the wet-bulb cooling in
the inherent drier air mass presents the rare mid-March freezing
rain threat. Some precipitation will go toward wet-bulb cooling
and thus not the ice, and it is unlikely all QPF will be ice
overnight, so ice amounts in may locations that see freezing rain
may only be a few hundredths, and if so roads will likely be
alright. However, at this time the QPF forecast is for an axis of
0.10-0.15 to fall as ice during that precipitation type window for
freezing rain, most of that favored 11 pm - 4 am on high-
resolution guidance. Some light sleet or snow mix in is possible,
especially on the northeastern edge of the precipitation.

Air temperatures at the surface early this morning are running
well below freezing and northeast flow will only inch down 925mb
temperatures over the next 24 hours, so after dark tonight would
think central and northern areas would not have a problem dropping
to or below freezing, though it may not be more than a couple
degrees below. Again this time of year boundary layer temperatures
can be quite challenging to get much below freezing in a
precipitation event. As for the pavement temperatures, they were
noticeably warm yesterday after a sunny day, but dropped into the
30s into the evening (32 presently at Pontiac on an IDOT sensor).
So confidence on impacts, especially road impacts, is low at this

Precipitation should be weakening in the pre-dawn hours on
Saturday into the morning as the low dampens and slides east-
southeast. Anything falling Saturday morning would likely be
liquid due to lack of ice aloft, but should be quite light.
Depending on cloud trends, temperatures on Saturday look to warm
into the 40s for most non-lake adjacent areas.



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-ILZ019-
     ILZ020-ILZ021-ILZ022-ILZ023...11 PM Friday to 8 AM Saturday.

IN...Winter Weather Advisory...INZ010-INZ011...11 PM Friday to 8 AM




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