Area Forecast Discussion
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FXUS63 KLOT 170154

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

854 PM CDT

GOES differential water vapor RGB this evening nicely depicts
the well defined upper low over the central plains beginning to
shear out and grow increasingly deformed as it moves into an area
with more of a confluent flow. Short range numerical guidance is
in good agreement in weakening this system tonight as it moves
east, through strong forcing in advance of the feature is already
resulting in a blossoming arc of precipitation from IA east into
western IL then arcing south into downstate IL.

In advance of this arc of precip there resides a very dry air mass
with ACARs soundings this evening from MDW depicting a bone dry
atmosphere with the highest RH being 50% at around 500 mb.
Upstream, ILX/DVN soundings show the result of extensive virga and
top-down saturating with a much more moist profile, though dry low
levels linger.

The dry air mass will help keep a sharp cut-off to the northern
extent of the precip, which looks like it will reside in our CWA
overnight into early Saturday morning. For the most part, changed
very little with respect to precip timing, amounts, and location
tonight other than to perhaps slow the onset of the precip

The bigger question remains the precip type. Generally speaking,
ACARs soundings this evening have suggested that both the 12z and
18z GFS as well as the 18z NAM were verifying slightly too warm
(~1C) with the max temp aloft. While this is a small error in the
grand scheme of things, forecast soundings tonight have
temperatures tonight precariously close to the freezing line. This
is resulting in a fairly low confidence forecast in eventual
precip type. The RAP guidance thermal profiles have generally
been colder than going forecast and given the slight warm bias
noted in NAM/GFS forecasts as well as observed ACARS soundings
with wet-bulb temps well below zero, have trended the forecast
toward a slightly colder solution. The result is a bit more
snow/sleet northern extent of the precip with still a sizable area
of freezing rain from Ogle & Lee Counties east-southeast into Will
& Kankakee Counties and into NW IN from there. Given the magnitude
of the dry air present am concerned that sleet could be a bit more
prevalent (especially initially) in this swath, but confidence
isn`t such that I was comfortable to make any big shifts in

All in all, the going winter wx advisory looks on track and no
changes are planned this evening.

- Izzi


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 re-intensify. 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 00Z TAFs...

644 pm...Forecast concerns this period include a period of a
wintry mix early Saturday morning and timing of this precip.

Drier easterly flow is expected to weaken initial waves of precip
this evening but eventually...precipitation is expected to reach
the terminals overnight. Because of this dry air...start time may
need further adjustments as trends emerge this evening. There
will likely be a sharp cutoff to the precip area...near or just
north of a rfd/dpa/ord line. Thus confidence for precip at those
terminals is only medium...with better chances for mdw/gyy.
Precip type will likely vary due to location and intensity. Have
maintained light freezing rain and snow with this forecast but its
possible some areas may end up all light freezing rain while
areas on the northern edge of the precip are mainly light snow or
light sleet. Precip end time is also somewhat uncertain...with the
potential for light snow or light rain perhaps lingering at
mdw/gyy through mid Saturday morning.

Cigs will slowly lower to a mid deck later this evening and likely
into mvfr once precip develops. These mvfr cigs will start to push
south Saturday morning with clearing skies expected by afternoon.
East/northeast winds will continue through the period. As high
pressure approaches the area Saturday evening...winds are expected
to become light and variable after sunset. cms


332 PM CDT

Weakening low pressure will pass south of the lake tonight into
Saturday while high pressure remains draped across the lake. This
will result in northeast winds over the southern portion of the
lake and west to north then variable winds for the north half. A
cold front will drop south down the lake Sunday into Sunday night.
This will be followed by the next period of more significant
marine concern next Monday and Tuesday. A stronger high pressure
will set up over Ontario while fairly strong low pressure passes
near or south of the Ohio River. Brisk northeast winds will ensue,
with the track and strength of the low driving how strong the
winds get. Gales are possible Monday night, including for the
nearshore waters, though not yet indicated in the open waters
forecast. North-northeast winds will remain elevated but not
quite as strong on Wednesday, maintaining hazardous conditions for
small craft in the nearshore water.



IL...Winter Weather Advisory...ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
     ILZ039 until 8 AM Saturday.

IN...Winter Weather Advisory...INZ001-INZ002-INZ010-INZ011-INZ019
     until 8 AM Saturday.




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