Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KLOT 270557

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1257 AM CDT Mon Mar 27 2017

735 PM CDT

The upper level trough is moving through overhead which has likely
caused the blossoming of light rain showers and drizzle currently
on radar. Decided to cover the increase of precip with ocnl
showers and areas of drizzle as not expecting any site to get
measurable precip. Expecting showers to end from southwest to
northeast as the trough continues northeast. Dry conditions are
still expected after midnight and through at least Monday morning
before more rain arrives.

The other change to the forecast was adding patchy fog through the
night. I am not expecting dense fog due to overcast skies. No
other sig changes were made to the going forecast.



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 06Z TAFs...

Main forecast concerns are with persistent IFR/LIFR ceilings
through much of the forecast period, fog and reduced vis this
morning, and then a wind shift to the northeast later this

Ceilings steadily lowering once again at this time, with IFR
ceilings quickly falling to LIFR. The only site which has not
quite observed this trend is GYY, but anticipate this trend to
impact this site as well over the next 1-2 hours. These lower
ceilings then expected to be in place for much of today. Guidance
does vary to the extent of any improvement later today into
tonight, and have maintained forecast improvement to MVFR.
However, will monitor for the potential for IFR ceilings once
again later tonight. Fog will likely lower through early this
morning with RFD and DPA likely getting into the more dense fog,
with vis below 1 mile. This may impact ORD and MDW, and will need
to continue to monitor for this potential. Westerly winds will
gradually turn to the northwest this morning, and then to the
northeast by mid morning. Speeds should initially stay below 10
KT, but do anticipate speeds at or above 10 KT by early this



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.




WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.