Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210709

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
209 AM CDT Tue Mar 21 2017


Through Tonight...

156 pm...Bulk of the showers and thunderstorms have moved east
and southeast of the cwa this afternoon but much of the short term
guidance suggests there will be festering showers through late
afternoon across parts of northwest Indiana and possibly into the
early evening hours. Confidence is fairly low with how widespread
this activity may become as well as how long it may persist.
Maintained low chance pops into the late afternoon but trends will
need to be monitored.

A cold front is slowly sagging south across the cwa this
afternoon shifting winds north/northeasterly and as these winds
increase during the mid/late afternoon...a more noticeable drop in
temps will be seen closer to the lake with lows tonight dropping
into the 30s. Cloud cover is struggling to clear across northeast
IL and southern Lake MI and while trends do support clearing
conditions this evening...confidence is also low. cms


304 PM CDT

Tuesday night through Monday...

Northwest flow still anticipated to be in place and although high
pressure will build southeast across the region during this time,
still am monitoring for the potential of some light rain Tuesday
afternoon. Narrow axis of forcing/precip still possible mainly in
the afternoon, and for areas south of I-80. Confidence still not
overly great, with guidance varying to the extent of any precip.
Most locations will likely stay dry but did leave low chance
wording for rain for the far southern CWA. This forcing quickly
shifts to the southeast by Tuesday evening, as high pressure
really begins to shift overhead Tuesday night into Wednesday. Dry
but cool conditions then expected by midweek. Pattern still
appearing to be active by the end of the work week with
anticipated large upper level system likely slowly moving east
towards the region. As this occurs, showers and even thunderstorms
will become more probable, especially by the weekend.



For the 06Z TAFs...

A surface high will continue to build east-southeastward into the
Upper Great Lakes by early Wednesday morning. This pressure
feature will continue to drive a northerly wind across the region
through the period. North-northwesterly winds will be favored
through the morning today. However, by early to mid afternoon
today the winds will shift back to the northeast with the passage
of a lake breeze enhanced cold front front. Wind speeds with the
passage of this front will likely remain in the 10 to 15 kt range
for several hours this afternoon. VFR conditions are expected
today. However, some lower CIGs are likely to shift southward over
the area this evening, though at this time it appears they may
remain at or above 3,000 feet.



207 AM CDT

Another cold front will shift southward over the lake today. In
its wake, a surge of much colder air is expected to shift
southward over the lake. This airmass will help set the stage for
a period of enhanced northerly flow up to 30 KT down the lake
later today and tonight. These stout northerly winds will also be
aided by a potent surface high, which will gradually build
southeastward from south central Canada into the Upper Great Lakes
today through Wednesday. With this period of stronger northerly
winds over the lake, expect high waves up to around 10 FT over the
southern shores of the lake. These hazardous conditions to small
craft will continue over the southern Lake Michigan near shore
waters through Wednesday afternoon.

Once this surface high shifts over the eastern lakes by early
Thursday morning, the winds over the lake will shift southward
for a period again during the day Thursday. However, another cold
front is likely to shift down the lake Friday night into early
Saturday. Following this frontal passage, another potent surface
high is likely to build over the Upper Great Lakes and southern
Canada as a storm system tracks eastward from the Plains to the
southern lakes region by Saturday night. These two pressure
patterns could result in a period of strong northeasterly winds
again on the lake for a period this weekend.



LM...Small Craft Advisory...LMZ743-LMZ744-LMZ745...4 PM Tuesday to 4
     PM Wednesday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...7 PM Tuesday to 4
     PM Wednesday.




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