Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 271932

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
232 PM CDT Mon Mar 27 2017

230 PM CDT

Through Tonight...

The immediate lakefront fog, at times appearing dense, continues
with a couple downtown Chicago cameras having shown dropping
visibility once again this early afternoon. According to City of
Chicago partners, the stratus remains just off the deck on Lake
Shore Drive. Will utilize SPS and Graphical NOWcast for messaging
of this dense fog into early evening. Dissipating time is
challenging to say. Dry air appears to be oozing its way down the
lake with improved marine webcams along Wisconsin and Michigan
shores, and would expect that trend to inch southward due to
increasing northerly wind speeds into the evening. Those stronger
north winds do not arrive in the immediate Illinois and Indiana
lake front areas until mid-evening, so expecting this to last at
least a few hours after dark.

The compact upper low over southeast Missouri this afternoon
continues to chug eastward. Weak but moist isentropic ascent and
transient mid-level frontogenesis is generating lift and some
showers over the CWA, mainly along and south of I-80 with a few
stragglers north. These showers should shift eastward early to
mid evening. Stratus will continue through the night with high
confidence. Cannot rule out some patchy drizzle at times beyond
the rain showers.

The north winds will steer in cooler air, with lows in the upper
30s forecast for many areas north of I-80, and lower 40s south.



230 PM CDT

Tuesday through Sunday...

Generally cloudy spring weather will prevail through the week
a slow moving mid week system bringing more rounds of rain. Flow
off the lake will be rather persistent keeping lake shore areas
chilly with inland pushes of the chillier lake air varying by day.

Tuesday will be precipitation free as tonight`s system departs to
the east. Weak high pressure ridging extending southward from
central Canada will keep north to northeast winds in place which
will keep lake areas in the lower 40s while inland portions of
the Chicago metro warm to the upper 40s/50 degrees. Areas beyond
the metro will see mid 50s or so. Cloud cover looks pretty
persistent through the day though some erosion/breaks may occur
later on the afternoon.

Another longwave trough, currently swinging across the southwest
U.S., will split with a southern stream upper trough cutting off
and drifting eastward into the central and southern Plains. This
will follow a similar pattern to the systems over the last several
days and remain centered south of the forecast area. This system
will spread waves of moisture/ascent and resultant rain northward
into the area. High clouds increase Wednesday but it is possible
some sun makes it through early. Most, if not all, areas remain
dry through Wednesday afternoon, though some sprinkles could occur
in the far southwest late in the day. Waves of rain arrive
Wednesday evening and continue through Thursday. The closed upper
low and attendant surface low take their time passing so
damp,cloudy and potentially showery weather continue into Friday
but coverage of precip will likely decrease over what will have
occurred Thursday. Could see some snow mix in with any rain
showers far north late Thursday night/early Friday morning. Temps
Wednesday will be similar to Tuesday. Thursday will see a larger
north- south spread in highs as a warm front nudges northward,
though it may not make it very far into the forecast area. Should
see upper 40s to lower 50s Friday but low cloud cover may persist
as will lake side cooling.

Saturday may turn out to be decent for most areas with high
pressure expected to move over the area. Light flow and likely
some sunshine should allow a lake breeze to make an inland push
allowing some areas of the Chicago/NW IN metro to lose at least a
few degrees in the afternoon. Still early for these details but
something to keep in mind. Guidance suggests a weak northern
stream wave passing Sunday bringing clouds and possibly a minimal
amount of rain. Another longwave trough will be moving onshore
over the western U.S. before splitting allowing another upper low
to move across the central/southern Plains and up the Ohio Valley
early next week, with a general repeat of recent systems favored.



For the 18Z TAFs...

The concerns with the Chicago area airports continue to be low
clouds and fog, as well as increasing northeast winds late this
afternoon into the evening.

Light northeast to east winds have started over the area and this
continues to keep stratus in place though a slight improvement has
been noted at most sites since 16Z. The exception is near the
lakefront which is upstream of ORD and MDW, so still lean toward
only a very gradual improvement in cloud bases. Confidence on
whether 500-700 ft vs 800-900 ft is quite low, but confidence in
at least IFR through the evening is medium-high. There has been
patchy fog, at times dense, near ORD and DPA in the past couple
hours, but that should be less of a threat this afternoon. MVFR
and occasional IFR visibility seems more likely.

Low pressure passing well to the south will spread
isolated/scattered showers up to I-80 and possibly a tad further
north. Have a VCSH at Chicago area airports but will monitor
trends and that could be pulled early in the afternoon.

While stratus is likely a good part of the day Tuesday, confidence
on specific height is low. Gradually drier air should be working
southward and would think that the cloud bases will gradually
lift. Also of note on Tuesday is that wind direction could back
to 350-360 during the morning, at least temporarily at ORD and
MDW. Confidence on this is low right now but have seen that before
in these northerly flow patterns with the low passing up the Ohio
River Valley, and some computer models are indicating that.



317 AM CDT

Low pressure over the central Great Lakes is departing to the
northeast this morning, while west to northwest winds are in
place. Expect a trend towards the northwest for much of the lake
this morning, and then more northerly throughout today. However,
speeds don`t appear to be too strong for much of today. Very moist
conditions still in place over much of the lake, with dense fog
likely being observed over the entire lake with the exception of
the Indiana nearshore. Have the dense fog advisory in place
through later this morning and do think this end time still seems
reasonable. Will need to monitor for fog to possibly linger longer
over portions of the lake today though, especially over the
southern end of the lake. High pressure to the northwest and low
pressure tracking through the region just south of the lake
tonight into Tuesday will allow northerly winds to increase.
Expect speeds of 15 to 25 KT with some gusts up to 30 KT over much
of the lake tonight, with these speeds and direction likely
continuing into Tuesday. These stronger winds and building waves
will likely provide conditions hazardous to small craft by late
tonight and especially into Tuesday. These stronger northerly
winds will persist through midweek.



LM...Dense Fog Advisory...nearshore waters
     until 10 PM Monday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...3 AM Tuesday to 9
     PM Tuesday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...3 AM Tuesday to 3
     AM Wednesday.

     Dense Fog Advisory...LMZ777-LMZ779 until 7 PM Monday.




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