Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 262325

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
625 PM CDT Sun Mar 26 2017

204 PM CDT


Around sunrise tomorrow the area will be sitting between two
closely spaced low pressure systems, one over Lake Huron and
another moving into southwest Missouri. The low to our northeast
would be the one that was responsible for the wet start to today,
but most of the activity with this system has already lifted
north and east of the area as of mid afternoon. The exception
would be the lingering activity in northwest Indiana and some
isolated showers over Illinois. This too would be expected to
taper off or lift farther northeast along with the departing low
this evening.

Under persistent overcast the area will begin to dry out a bit
overnight as winds turn northwest and dewpoints drop into the mid
40s, but this will be short lived before the upstream low reaches
the area tomorrow. There was some question about fog development
overnight with the relatively light winds, especially given the
recent rainfall, but the overall downward trend on low level
moisture makes this a tougher call. For now we will opt not to
mention fog except in the marine areas, but this will be
something to continue monitoring.



314 PM CDT

Monday through Sunday...

A primarily cloudy week lies ahead thanks to existing clouds and
a continued series of upper lows passing over the region. It
looks wet for at least for a large part of the area Monday
afternoon and then at some point again during the latter half of
the week. Northeast to east winds will prevail given the surface
low reflections passing to our south, which combined with the
clouds means more subtle temperature changes than we have seen.

Monday morning will start the week appropriately cloudy with
possibly some fog as noted in the above discussion. The next upper
low located across western Kansas this afternoon will track
eastward into the mid-Mississippi Valley by afternoon and up the
Ohio River Valley Monday night. This should remain fairly compact
but there is spread in solutions on how quickly this tracks
across the area. While this will bring rain into the CWA with high
confidence, due to its compact nature, the northward push of
moisture transport may be limited and thus the rain may cut off
somewhere in the northern CWA. Thunder potential is not great but
include a mention in the southeast given some elevated instability
and forcing for lift immediately ahead of the mid-level

With trying to account for clouds and the rain spreading in
during the afternoon, have undercut guidance in northern areas
and near the lake, but overall confidence on temperatures Monday
is somewhat low and may still be too mild by quite a bit including
across the south.

North winds behind the system will prevail on Tuesday, as will
likely quite a bit of low-level cloud cover. So again
temperatures may be too mild even with undercutting. Given the
long fetch of cool flow off the lake, shoreline areas including
downtown Chicago may top out only around 40. The brief window of
scattering cloud potential looks to be late in the day Tuesday or
Tuesday night, before mid/high clouds start moving in on

The next closed low in the wave train is expected to take a
similar path as its predecessor, however be more broad and closed
and thus slower. Given that closed nature, confidence is low on
the speed of this and the exact path. Have generally gone close to
a model blend. Confidence is almost highest in it being cloudy.
Temperatures look to be at or slightly below normal Thursday and
Friday before a gradual climb this weekend, but that again depends
on the end of the week system speed, remnant clouds, etc.



For the 00Z TAFs...

Main aviation forecast concern remains redevelopment of IFR/LIFR
cigs overnight and persisting into Monday morning.

Weak surface low pressure center was over far northeast IL between
UGN and ORD early this evening. Milder air in southwest winds
south of the low track had allowed cigs to improve into high
MVFR/VFR during the afternoon, though expect a gradual lowering
again from the west as winds diminish and become more west-
northwest through the evening. Extensive area IFR/LIFR exists just
upstream across far western IL/southern WI, and advection as well
as a gradual lowering with loss of daytime heating should result
in cigs lowering to IFR and eventually LIFR during the late
evening and overnight hours. Upper trough axis across the area
early in the evening may produce some spotty drizzle or very light
rain, though should be of little impact with drier air spreading
in aloft. Winds eventually veer to the northeast across the
terminals Monday, as a new area of low pressure moves into
downstate IL from the southern Plains. This will result in some
drying in the low levels, allowing cigs to rise back into IFR and
eventually MVFR during the day. Light rain/showers are expected to
spread into the region during the mid-afternoon hours, though
heavier precipitation is expected to remain generally south of the
terminals. This light precip may redevelop patchy IFR cigs during
the afternoon, but should then skirt southeast of the terminals
Monday evening. Northeast winds will increase a bit (12-15 kts)
Monday evening as well, while becoming more north- northeast.



327 PM CDT

Low pressure will pass northeastward over the central part of the
lake this evening. Webcam imagery from the Illinois and Wisconsin
shores has indicated undulations in visibility, with dense fog at
times, and given that low track believe that more dense is
probable. Some of this will likely linger into Monday morning. The
low path also means wind direction at times could be variable
across the central part of the lake as it turns from southeast
late today to southwest and then to northwest late tonight.

The next in a series of surface lows will pass up the Ohio River
Valley Monday evening. Behind this, northerly winds will sharpen
Monday evening through Tuesday morning. This will bring Small
Craft Advisory waves and possibly winds to the Illinois and
Indiana nearshores.

North, northeast, or east winds will prevail much of the week.
With the next low late in the week, these winds are presently
forecast to stiffen to 30 kt over the open water.



LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ777-LMZ779 until 9
     AM Monday.




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