Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181513

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1013 AM CDT Wed Apr 18 2018

1012 AM CDT

Overall forecast still looks on track across the area. Main change
this morning was to remove thunder mention from the eastern 2/3 of
the CWA. Radar presentation over NW IL is rather convective in
nature, but lightning has diminished to just a few isolated strikes
as of 10am. Though steep mid to upper-level lapse rates exist per
12Z DVN and ILX RAOBs, forcing is occurring below a high EML around
600hPA. Therefore, most of the meager elevated instability is not
being realized.



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.



351 AM CDT

Thursday through Tuesday...

Significant weather not expected at this time.



For the 12Z TAFs...

MVFR cloud cover is spreading westward across the terminals, and east
winds will begin gusting to 20 kt this morning. Low pressure
currently over Missouri will pass over Central IL today.

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 many 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. I have medium confidence in when rain
will turn to snow at the terminals. 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 around midnight, northwest
wind gusts 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 until 1 AM

     Winter Weather Advisory...ILZ005-ILZ006 until 1 AM Thursday.

LM...Small Craft Advisory...Winthrop Harbor to Gary IN until 7 PM

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




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