Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 252001

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
301 PM CDT Sat Mar 25 2017

300 PM CDT

Through Tonight...

Expansive, stacked upper low center is sprawled across Missouri
this mid-afternoon with a plume of 1-1.25 inch precipitable waters
nosed ahead of this feature into our area. This moisture has
helped to fill back in the thicker clouds. There are some areas
of thinning and agitated cumuliform clouds across west central
Illinois, but in and immediately upstream of the CWA, any showers
are rooted aloft. The earlier slight concern of some storms moving
over the boundary and possibly becoming a bit feisty is even less
of a concern in our CWA given the thickened cloud cover impeding
any low-level instability.

In response to a small scale disturbance lifting northward near
St. Louis on satellite water vapor imagery, would expect showers
to continue to show an uptick across central Illinois and moving
into the CWA during the rest of the afternoon into early evening.
While the northward-oriented upper jet ahead of the low does
weaken some tonight, it still should support lift atop continued
moisture transport for showers to develop. Convective-allowing
model solutions support this, with clusters of showers and some
hints of isolated thunder moving over at times throughout the
night. Most unstable CAPE values of a few hundred J/kg look
probable, especially in the south late this afternoon into
tonight. Sometimes with closed upper lows at night too, it seems
like there can be a holding steady of isolated thunder when
otherwise it would dissipate.

The surface boundary extends from near Lacon to Kankakee and to
near Valparaiso. This should not move much further north this
evening. Temperatures may inch up a few degrees north of the
boundary, while will slowly fall south of the boundary, so all in
all not much of a change for tonight. The northeast flow north of
the boundary may drag some thicker fog into northeast Illinois,
including Chicago. Confidence on this is low though, and if the
boundary does inch a little further northward, it may increase the
potential for dense fog. Also apart from marine fog, there could
be some fog development areawide near and north of the boundary,
but with stratus already present that will likely be dominant over
dense fog.



252 PM CDT

Sunday through Saturday...

Main forecast concerns for the longer term forecast period will
continue to be timing of a series of systems bringing periodic
chances for pcpn to the region as well as temperature trends.

The latest guidance continues to indicate a series of southern
stream systems lifting out of the swrn CONUS and tracking across the
region.  The models remain relatively consistent on this trend,
though some differences in the track remain, with the GFS
trending a little south of the NAM/ECMWF, the general idea of the
associated sfc low lifting tracking from the middle Mississippi
Valley and across nrn IL/srn WI tomorrow remains consistent with
previous runs. As the low tracks slowly to the northeast, an
associated warm that is pushing nwd through IL/IN today will be
north of the CWA, the warmer air overspreading the region today
will remain in place through the day on Sunday, allowing
temperatures to rebound back into the lower 60s. Instability will
be on the increase in advance of the system and at least some
partial clearing along with forcing from the sfc low and mid-level
shortwave will keep some chance for thunderstorms over portions
of the area through the day tomorrow. Generally expect the greater
chances for TS to be near the greater forcing of the sfc low/mid-
level shortwave. For timing, expect that the most favorable window
of opportunity for TS will be through late morning or early
afternoon, with the system lifting to the northeast durg the
afternoon hours. There should be a grief lull in pcpn sunday
night as weak shortwave ridging crosses the region in advance of
the next system. The models are in a little better agreement on
the track and timing of the next system. With strong high pressure
building out of central Canada and into the nrn plains/upper
Mississippi Valley, the track of this system should be more to the
south, through srn IL/IN and the Ohio Valley. The most widespread
pcpn with this system Monday into Monday night should be over
portions of the area south of the I-80 corridor, but there should
still be some light rain over the nrn portions of the CWA as well.
Temperatures on Monday should still trend higher than normal,
with much of the area seeing temps in the upper 50s to lower 60s,
but winds on Monday should trend toward nely through the day,
which would keep the far nrn portions of the CWA and the lakefront
a bit cooler, as flow turns off of the cool Lake Michigan waters.
Lakefront locations should should see temperatures only in the
upper 40s to lower 50s. High pressure will then spread east across
the upper Mississippi Valley and the Great Lakes region through
midweek, keeping generally northerly winds in place through much
of next week. The temperature trend through next week should be
for warmer conditions inland and cooler near the lake. As high
dominates the region through midweek, conditions should generally
be dry. The next significant chance for pcpn will not come until
late next week as another srn stream system lifts out of the swrn


For the 18Z TAFs...

The concerns with the Chicago area TAFs are:

- Northeast winds through tonight with sporadic gusts this

- Isolated SHRA early this afternoon gradually upticking in
  coverage by late day into the evening, with potential for
  isolated TSRA.

- 400-700 ft cigs probable to continue through tonight, with
  potential overnight and Sunday morning to dip just below this,
  but confidence in that occurring and for how long is low.

- Potential for more showers on Sunday with winds veering to
  south, though low-medium confidence in when that shift occurs.

Low pressure across Missouri this afternoon will move to far
western Illinois by daybreak Sunday and to near/just north of
Chicago by mid-afternoon Sunday. The warm front with this remains
south of the TAF sites this afternoon, with northeast flow locking
in lower clouds. Webcams and observations indicate 400-600 ft
cigs from ORD and MDW back to the lake, so do not see these
changing much. Would think the better potential to dip a tad lower
is tonight, especially in-between any showers overnight. Cannot
rule out 200 ft given the synoptic scenario and bases already at
400 ft, but feel it is a low chance right now.

As for showers, isolated to scattered activity is expected to
develop this afternoon and pass over northeast Illinois, though
the highest coverage is not expected until late day into the
evening. Some of this may contain thunder, though the instability
is quite limited, so did not have confidence enough to include in
the TAFs. Confidence in shower coverage overnight into Sunday is
lower, though at least isolated should be present.

There is uncertainty on how far north the warm front will reach
tonight, which is key because south of this boundary conditions
are likely to be VFR. The front reaching GYY is a real
possibility, even by late afternoon. As for MDW and ORD, it may
get pretty close, but feel that winds should continue out of the
northeast/east and maintain the low clouds and occasionally MVFR
to IFR visibility.



300 PM CDT

Winds are trending to easterly today with speeds up to 30 KT over
much of the lake as low pressure over Missouri slowly moves
across the mid Mississippi Valley tonight. Hazardous conditions
for small craft will persist through tonight, mainly for the
Illinois nearshore waters in the persistent east flow. As the low
then tracks north through Illinois on Sunday and central Lake
Michigan Sunday night, anticipate that speeds will diminish to
the 10 to 20 KT range. High pressure is expected to build across
the upper Great Lakes region Tuesday through Thursday setting up
generally north to northeast winds over the lake with more
moderate wind speeds.


LM...Small Craft Advisory...IL nearshore waters until 5 PM Sunday.




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