Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170522

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1222 AM CDT Tue Apr 17 2018

226 PM CDT

Through Tonight...

A mid-level trough/circulation embedded within broad cyclonic
flow over the Great Lakes region currently tracking SE across the
CWA will continue to produce scattered to numerous snow showers
over the next couple hours. Otherwise, as the broader mid-level
low over western Ohio gradually shifts eastward through tonight,
snow showers will diminish in coverage and intensity.

A low stratus deck will remain across much of the area through
the night as plenty of low-level moisture lingers under a
strengthening subsidence inversion. There is a chance the low
clouds will scatter out across the far western CWA overnight based
off upstream trends along the MN/WI state line.

Min temps tonight are expected to range from the mid 20s NW to
near 30 across the far east. This puts the forecast low of 24F in
Rockford at the 4/17 record low set in 1983. If the clouds do
scatter out more than expected, it is possible temps could end up
a degree or two lower than forecast.



300 PM CDT

Tuesday through Monday...

The forecast concerns in the long term are centered on the next
closed low moving over the area on Wednesday into Wednesday night.
The main changes with the forecast for this time were to lower
temperatures in the north on Wednesday. There remains potential
for a brief period of morning icing in this area and the
likelihood of some Wednesday night snow on the backside.

For Tuesday, the ongoing cyclonic flow will gradually be easing,
but there will remain cold enough air around 850mb (around -9C)
to foster stratocumulus. This is especially true in the northeast
part of the forecast area including Chicago, while it likely will
be mainly sunny in the far western/southwestern forecast area.
Have trended more cloudy there with cooler temperatures. Winds
will be easing in the afternoon, and a more onshore wind is
expected to develop late in the day into early evening.

Attention then turns to the closed Pacific-origin low that will be
scooting eastward into western Iowa by early Wednesday morning.
As the wave becomes closed late Tuesday night into early
Wednesday, it will lift a moistening warm advection zone northeast
over the area. The moisture flux with this is slow at first but
saturation through ascent increases quickly by sunrise Wednesday
morning north of Interstate 80. The origin layer for lift and
precipitation looks to be below the ice layer, and also is plenty
warm so rain is expected to be the hydrometeor type. If
temperatures are below freezing (potential north of I-80) for a
few hours that may yield patchy icy conditions, especially given
the recent cooler weather and limited solar insolation. Also there
could be a little convection early in the morning given elevated
steep lapse rates. But confidence in precipitation occurring with
freezing surface temperatures is low-medium, and even if freezing
rain were to happen, temperatures will be moving upward. The
wildcard on whether it happens may be how fast temperatures drop
the previous (Tue) evening and then have to recover overnight as
winds increase and clouds thicken.

Going into the day Wednesday, the surface low around 999mb will
move east into the area by midday Wednesday. There has been a
slight southward trend the past several runs of guidance, and as
such the quickly-progressing surface warm sector will likely make
less intrusion into the region. Still have highs around 60 in the
south, but northern locations, especially remaining with an
onshore component may have trouble getting far into the 40s.
Wednesday late morning into afternoon may see a lull in
precipitation coverage with an increase into the evening as the
compact deformation area moves eastward. Have increased chances of
precipitation during that time, and the cold advection should
provide a changeover to snow, mainly north where surface
temperatures will already be cooler. Minor accumulation, mainly
on grass, looks once again possible.

Beyond the trend with the next upper low continues to be slower
and further south one, which may mean an extended period of
quieter weather over the area into the weekend. For those wanting
warmer temperatures, especially in Chicago, the low-level flow
looks to remain northeasterly keeping lake-modified air streaming
into the region.



For the 06Z TAFs...

The upper level low that brought persistent snow showers is
shifting east allowing high pressure to spread over the region.
Northwest winds will gust up to 20 kt early this morning, but VFR
cloud cover and dry conditions will continue. There is some
concern that high-end MVFR cigs may filter back over the
terminals early this morning, so I left them in the TAF, but I
have low confidence in MVFR cigs occurring. Cigs should lift to
VFR by mid morning.

Winds will shift to northeast as the high pressure gradient
weakens late this afternoon. East winds increase tonight to
greater than 10 kt under VFR cloud cover.



226 PM CDT

Low pressure over the eastern Great Lakes this afternoon will
drift eastward to southern Quebec through Tuesday night.
Meanwhile, a ridge extending from Manitoba to the Lower
Mississippi Valley will shift eastward, crossing Lake Michigan
Tuesday night. NW winds to 30 knots with a few gale force gusts
to 35 knots will continue into Tuesday morning ahead of the ridge.
Another low pressure over the central Great Plains Tuesday night
will cross far southern Lake Michigan Wednesday afternoon and the
Lower Great Lakes Wednesday night, bringing a period of easterly
winds to 25 knots on Wednesday and northerly winds to 30 knots
Wednesday night into Thursday. Winds generally under 20 knots are
then expected Thursday night through the weekend as a broad ridge
drifts across the Great Lakes region.



LM...Small Craft Advisory...IN nearshore waters until 3 PM Tuesday.

     Small Craft Advisory...IL nearshore waters until 10 AM Tuesday.




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