Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180847

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
347 AM CDT Wed Apr 18 2018

346 AM CDT

Through tonight...

Main forecast concerns are with the expected wintry weather of
light freezing rain, sleet, and snow this morning through late
this evening. There also remains a chance of thunder this morning
through midday across much of the CWA. Only changes to the current
Winter Weather Advisory were to delay the start time a few hours
for McHenry and Lake IL counties.

In the near term, dry conditions in place at this time as the
upper level system and deep surface low are situated across the
central Plains. These dry conditions will likely remain early this
morning and possibly into mid morning, as most guidance in fair
agreement with the slower trend to the start time of precip this
morning. This makes sense given the closed nature of this system,
along with any advection processes leading to precip development
are focused right near the system. This strong WAA ahead of this
system will help to continue to warm the column overhead this
morning though, with mainly liquid expected with the arrival of
this precip along and west of I-39 by early/mid morning. Some
small concerns for a small window of light freezing rain along and
north of I-88 this morning, however, this threat has lessened
further. Despite temps in the upper 20s, WAA should help temps
rise by the time this precip arrives, limiting any icing threat.
Given the small chance, have still made mention of this brief
window in the forecast.

Previous concerns with the extent of any ice present aloft which
could support sleet remain, and have continued this trend in the
forecast. Although sleet may not be as widespread and long lived
in anyone location, still think most locations along/north of I-88
will see at least a brief window of sleet this morning as this
precip continues to spread over much of the CWA. This will likely
be more tied to convective trends with the arrival of strong
forcing and steep mid level lapse rates. So have continued mention
of sleet today, mainly for the morning though. An abundance of
thunder is still noted right near this system across northwest IA.
Although the extent of the instability may weaken with its
eastward movement, do think the forcing and steep lapse rates will
continue to support the possibility for thunder later this
morning through midday. Intensity of these storms should be
limited by the overall lack of instability, however, once again,
they may act to support more sleet in some locations.

By midday/early afternoon, thunder chances lessen with a window
of rain or drizzle likely for much of the cwa. By mid to late
afternoon, the system will begin to shift to the east with CAA
supporting a cooling column. This will coincide with the arrival
of deeper moisture, with a transition over to snow expected from
the northern CWA southeast into the early evening. All snow
expected for much of the CWA by this evening, with a period of
light to moderate snow likely. Locations north of I-88 have higher
chances for more accumulating snow, with areas in the Winter
Weather Advisory likely seeing the highest of 1-3 inches. Snow
totals lessen to around an inch or less further to the south. Snow
expected to exit to the southeast fairly quickly after the
midnight time frame.

Only changes to the Winter Weather Advisory were to adjust the
start time for McHenry and Lake IL counties. Given the possibility
of a window of light freezing rain, sleet, and then accumulating
snow later today into this evening, the Advisory still appears to
be on track. Although there may be a window around midday when
frozen precip chances will slightly lower, think this will be a
small window. The transition over to snow later today/evening for
locations outside of the Advisory should not create any impacts
which would warrant an advisory at this time. However, will need
to monitor this period though, as further cooling into the evening
could support more accumulating snow on area roadways.



304 PM CDT

Wednesday night through Tuesday...

Main concern will be the ongoing wintry system. As the surface low
continues to shift toward the eastern Great Lakes Wednesday,
evening, enough cold air will funnel in from the northwest to
transition most areas to snow/rain and eventually all snow. While
the higher QPF is expected during the midday hours Wednesday,
cyclonic flow associated with the upper low and a healthy
deformation field should allow precipitation to continue or even
pick back up a bit for most areas through the evening. Northern
areas near the Wisconsin border, and areas closer to the lake may
still pick up another inch or so of snow in the evening, with
some wet accumulations mainly on grass farther south. Higher
pressure will funnel in after midnight which will quickly cutoff
the deeper moisture, with some snow possible overnight downwind of
Lake Michigan.

We will remain in cool northwest flow on Thursday, and while not
the bitter cold airmass we have had earlier this week, we will
still have highs only in the 40s for most locations, coolest

Upper ridging will develop downstream of a cutoff upper low across
the southwest for the weekend, and this will allow expansive
surface high pressure to take up residence across the Great Lakes
Friday and through the weekend. The good news is dry weather will
be place into early next week. With the high centered over or
east of the lake, northeast winds will maintain our seasonally
cool airmass, with highs only slowly creeping up, not nearing
seasonal norms until Monday. It will be much cooler at the lake
in this pattern. Saturday will also feature more clouds than other
days as the western upper low will shift through the lower
Mississippi valley, being held south by a mini rex block (ridge
due north of the low), so this will keep most of the precip out of
our area.Warmer southwest flow will bring another seasonal day
Tuesday ahead of cold front which would signal our next chance
precip mid week.



For the 06Z TAFs...

Cloud cover increases this morning as low pressure currently over
Kansas approaches the region. East winds will begin gusting to
arnd 20 kts and slowly back to northwest through the early

Precip will spread from west to east from mid morning through the
early afternoon, and the previous forecast looks on track so I
did not make any adjustments. The main precip type should be rain,
but if any thunderstorms form, precip could be a mix of rain,
sleet, or even small hail. The thunder threat subsides in the
early to mid aftn, but gusty east winds and drizzle will continue.
IFR cigs and vsby are expected.

Rain/drizzle will become snow in the late aftn/early eve as cold
air moves in behind the low. The snow should at least produce IFR
vsby, psbly LIFR vsby, but I don`t have enough confidence to put a
TEMPO with LIFR vsby in just yet. Winds back to NNW with the cold
air. Precip ends, northwest winds diminish, and VFR conditions
are expected early Wednesday morning as the low pulls away.



207 AM CDT

I did not make any changes to the going headlines.

Low pressure currently over Kansas will track across Central IL
today, and then reach New England Thursday. High pressure will then
build over the Plains, and north winds will increase to 30 kt across
the lake. A few gales are possible, but guidance has backed off on
wind speeds, so I do not think a gale watch or warning is needed.
The high will reside over the Great Lakes Friday and Saturday.



IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ008...10 AM Wednesday
     to 1 AM Thursday.

     Winter Weather Advisory...ILZ005-ILZ006...1 PM Wednesday to 1 AM

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...1 PM
     Wednesday to 7 PM Thursday.

     Small Craft Advisory...LMZ744-LMZ745...4 PM Wednesday to 1 AM




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