Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 262017

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
317 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 18Z TAFs...

The midday radar shows a few isolated showers across northern
Illinois, with more widespread activity across northwest Indiana.
Trends generally should be on the dry side this afternoon, though
some brief showers certainly are possible. There is even a low
chance of an isolated thunderstorm, but confidence on timing and
location are far too low to warrant mention in the TAF at this
time. As for ceilings, despite some hint of clearing or at least
VFR skies, most locations remain solidly in MVFR and given the low
pressure system parked overhead, that is the favored trend
through the afternoon.

The remaining question is how winds will evolve. A fairly light
gradient has been allowing for some variability in direction so
far today, but the overall flow supports a southwesterly trend
going forward, with turning to northwest overnight and eventually
around to northeast tomorrow ahead of the next approaching

Rain and low conditions appear to be on the way again for tomorrow
afternoon. Confidence in timing and exact cig/vsby trends is not
high at this range.



405 AM CDT

Low pressure continues to slowly lift northeast through the mid
Mississippi valley this morning, with more of an easterly wind in
place. This low will move into northern Illinois today, and then
across the lake early this evening. This will provide highly
variable winds over much of the lake today into tonight, however,
do expect speeds to diminish today into tonight. The higher speeds
which provided conditions hazardous for small craft across the
Illinois nearshore have diminished. Higher waves are still in
place, and so have continued the Small Craft Advisory today. Will
monitor for a possible early end time though. Continue to monitor
fog over the south half of the lake, and at this time, it still
appears that the more dense fog is situated over the western
shores of the lake. It does look like this dense fog will continue
for much of today, and quite possibly expanding over much of the
southern half of the lake. Will monitor the possible need for
expansion of the dense fog advisory to remaining areas over the
south half of the lake.



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




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