Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221917

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
217 PM CDT Sat Oct 22 2016

1248 PM CDT

Through Sunday...

Skies continue to clear from west to east this afternoon as high
pressure shifts east.  High temperatures still appear on track
near seasonal norms in the upper 50s to low 60s. Patchy high
clouds will drift overhead tonight ahead of a shortwave that will
move east along the Canadian border. With high pressure departing,
low level southwest flow will continue tonight along with warm air
advection and support warmer lows tonight in the 40s.

12z NAM and GFS runs suggest that the cold front will hold off for
most of the day across northeast Illinois and northwest Indiana.
Another impressive thermal ridge ahead of the front coupled with
gusty southwest winds will allow all areas to reach the upper 60s
and many areas having a good shot at 70 (and higher south),
especially from the metro area southward. A late October 925 mb
temperature climatology has a median around 70 for ORD. Some late
day cooling is possible north where hitting those higher numbers
may not occur if the front speeds up a bit.




Sunday night through Saturday...

217 pm...Primary forecast concern this period is a storm system
to affect the area midweek.

A ridge of high pressure will build from the northern plains
Sunday night to the Great Lakes region Monday night. A cold front
will move across the area Sunday evening ahead of this high...
shifting winds northerly and bringing cooler temps to the area
after a warm Sunday.

Low pressure will develop over the central plains Tuesday and move
east to southern lower MI by Thursday morning. The track for this
system has been fairly consistent though minor timing differences
still come and go. Precip still appears to develop Tuesday
afternoon and Tuesday night ahead of this low across southern WI
though this has begun to slowly drift south into more of northern
IL. Pops have been adjusted up accordingly but for most of the
area...bulk of the precip should hold off until Wednesday as the
low approaches and then passes east of the area Wednesday night.
Timing differences noted above are for perhaps a slightly slower
movement which could maintain precip into Wednesday evening...
especially over the eastern cwa/northwest IN. Instability for
thunderstorms is limited but maintained slight chance for thunder
Wednesday afternoon. Depending on where Tuesday night precip sets
up...its possible some areas north of I-80 could see up to an inch
of rain...storm total from this system.

Another low moves across the upper midwest Friday/Saturday with
some timing differences between the gfs/ecmwf. This would turn
flow southerly with temps possibly warming into the 60s ahead of
the low. But a trailing cold front over the weekend or late in
the weekend could then bring another shot of colder air. cms


For the 18Z TAFs...

- Gusty SW winds Sunday shifting to W and NW through the afternoon

VFR deck in the 7-10kft range will move east through the
afternoon. We will maintain a SW wind tonight into Sunday as high
pressure moves east of the area. The gradient will tighten Sunday
morning ahead of low pressure that will drop southeast through the
upper Midwest and into southern WI Sunday afternoon. The low will
shift to eastern MI late Sunday afternoon with gusty winds
shifting more westerly and northwesterly late. There is a cold
front behind this low that most guidance keeps just north of the
terminals through at least mid Sunday afternoon. If this front
speeds the shift to northerly winds may occur a bit quicker, but
for now current guidance keeps the shift out of the 30 hr ORD TAF




217 pm...A ridge of high pressure will move east of the lake this
evening as low pressure over the western Dakotas moves east and
crosses Lake Michigan Sunday afternoon. As high pressure builds
across the northern plains Sunday night...the gradient will
tighten behind this front with northerly winds increasing up to 30
kts quickly Sunday evening. The high will settle over the lake by
Monday evening with northerly winds steadily diminishing Monday.

Another area of low pressure will develop over the central plains
Tuesday and move east Wednesday...crossing southern Lake Michigan
Wednesday night into early Thursday morning. Strong easterly winds
are expected to develop Tuesday night into Wednesday as this low
approaches. Gale force winds are possible...especially over
southern portions of the lake. Winds will shift northerly and then
northwesterly as they diminish...behind the low as it departs. cms





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