Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161951

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
251 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".



250 PM CDT

Saturday night through Friday...

The main forecast concern/uncertainty in the long term portion of
the forecast regards handling of a storm system that may pass
near or south of the area early in the work week. Sunday will be a
pleasant day under surface high pressure after a chilly start to
the day. Dry boundary layer conditions and full or nearly full
sunshine will enable temperatures to warm into the lower to mid
50s. The only item of note is the passage of a lake breeze for
shoreline areas and a bit inland during the afternoon, which will
result in sharp cooling behind the wind shift, to the upper 30s to
around 40 near the lake by the late afternoon.

Regarding the early week storm system, a northeastern North
American/north Atlantic blocking pattern, known as a negative
North Atlantic Oscillation (-NAO), is forecast to reintensify. A
short-wave embedded in the deep trough along the Pacific Coast
will eject over the Intermountain West over the weekend, becoming
a vigorous closed mid-upper low with attendant surface low over
the southern High Plains on Sunday night. The track this system
takes and how it interacts with the blocking pattern is something
that the guidance has been struggling with over the past few days.

The 12z GFS and several GEFS members were more bullish on a
slightly farther north low pressure track and northern extent of
precipitation shield versus the ECMWF and Canadian operational
models and most ensemble members. This is still a period to watch
for some precip potential, which at this point would have a
better chance for areas well south of I-80 (where chance PoPs are
currently indicated), though at 78+ hours out, can`t rule out
further shifts. If precipitation does extend north into the area,
thermal profiles would likely support a change to wet snow on
Monday night into early Tuesday. Outside of the uncertain precip
potential, this period will be marked by northeast winds gusting
to 25-35 mph at times, strongest lake side.

After this possible Monday-Tuesday system, expect a few quiet and
cool days, with current trends indicating a decent chance for
another storm system to affect the region starting some time next
Friday. The other main story next week is once again the
persistent and at times strong onshore flow (especially Monday-
Tuesday) which will result in chilly conditions lakeside, with
most of the week in the 30s/possibly around 40 for these areas.
Inland areas will not escape the cooler than average temperatures
for the latter part of March, aided by the -NAO.



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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