Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 011539 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
1039 AM CDT Sat Oct 1 2016

1039 AM CDT

The going afternoon and evening forecast is in very good shape.
Satellite imagery indicates a pronounced area of jet-enhanced lift
across northeast Illinois on the west-northwest side of the low
center. This area of lift is forecast to weaken the next couple
hours and have already seen the concentrated area of moderate to
heavy rain weaken as well. Broad ascent will continue though
associated with the closed low, and do think scattered showers
and some pockets of drizzle will be seen into the afternoon. Going
forecast for temperatures and winds is on track.



307 AM CDT

Through Tonight...

The main focus for today is the timing of showers and
thunderstorms with the next disturbance expected to impact
portions of the area.

Water vapor imagery this morning shows the center circulation of
the upper low now centered over far southeastern Indiana. This
continues to allow small scale perturbations to rotate westward
along its northern periphery, namely over the lower Great Lakes.
One such disturbance is currently driving a complex of showers,
with some embedded thunderstorms over portions of northeastern
Indiana. Believe it or not, these storms will be headed for
portions of northeastern Illinois and northwestern Indiana this
morning as they shift westward within the deep broad cyclonic flow
over the region. Current timing of these showers and storms puts
them in and around the Chicago area by around 14Z (9am) this
morning. As a result, I have boosted POPs into the 70 to 80
percent range across the eastern CWA mid to late this morning.
Although it is not a good diurnal time of day for thunderstorms
this time of year, the northwestward influx of cold mid-level
temperatures (500MB temps -18 to -20C) associated with the
northward moving upper low, will support the presence of steep
enough lapse rates for charge separation over the area. Therefore,
I have also mentioned isolated thunderstorms in the forecast with
these showers this morning.

Additional showers and storms will remain a good bet (at least on
a scattered basis) over the eastern half of the area into the
early to mid afternoon hours as the main PV anomaly retrogrades
northwestward towards the area. Forecast guidance continues to
indicate that this will act to drive an inverted surface trough
northwestward towards southern portions of the Chicago area by
early afternoon. As a result, this will continue to act as a focus
for showers and some storms through mid afternoon, especially over
my southern and eastern CWA.

Any lingering showers or storms should come to an end over the
area by early this evening. However, there will remain a small
chance for a few showers later Saturday night as another area of
showers tries to drop southward over portions of northeastern



326 AM CDT

Sunday through Friday...

The upper low that has been hanging out around the region for the
past several days, and really outstayed its welcome, is finally
expected to gradually shift towards the eastern Great Lakes by
Sunday evening, then over New England on Monday. Attention will
then begin to focus on another upper level trough, which is
expected to dig over the western CONUS late Sunday and into
Monday. The good news is that it appears that we will be in for
some dry and warmer weather into midweek as the low-level flow
turns southerly across the central CONUS ahead of this western
CONUS trough. However, this system will shift gradually shift
eastward over the Plains, with surface low pressure likely to
shift over the Upper Midwest and into Ontario by mid to late week.
This still looks to drive a decent cold front eastward over the
area, sometime around Thursday. Showers and some storms will be
possible with the frontal passage. Cooler weather is likely in the
wake of this front for Friday into Saturday.



For the 12Z TAFs...

The current cluster of showers and isolated thunderstorms
extending through Northern Indiana into lower Michigan will
continue westward this morning. Based on the track of the surface
low the area of thunderstorms may reach GYY but then drift south
of the Chicagoland terminals. Therefore have not carried TS at
ORD/MDW at this point. But the main batch of showers in lower
Michigan will also make steady westward progress as a leading
shortwave moves through. The inverted trough axis will linger in
the area this afternoon as additional energy swings east around
the upper low. This will maintain at least scattered showers and
possibly even a storm closer to the lake through the afternoon

Feel the best bet for IFR conditions would be during the morning
when the first batch of rain showers arrive. Either way, expect
current MVFR to fill in more across the area with some mvfr vsbys
as well. With the rain IFR is possible with occasional 2-4sm
vsby. Current feeling for ORD/MDW that cigs will stay at 700-800
or higher. Low confidence exists for lower IFR conditions.

The low will approach southern Lake Michigan, and this will result
in lighter winds today, light and somewhat variable under the low
in NW Indy, and more of a NNE direction across northeast and north
central IL. Winds will slacken and eventually shift to a NNW area
wide tonight as the low moves overhead then drifts back
northeastward toward Lake Huron tonight into Sunday. Some lower
cigs again appear likely with vis reductions possible too.



306 AM CDT

An inverted low pressure trough will expand over the lake this
morning as the center of low pressure moves to the southern tip of
Lake Michigan. This will maintain an east-northeast wind today.
Waves at the Wilmette buoy have settled in the 3.5- 4 ft range or
so, and therefore will carry 3-5 occ 7 ft in the NSH and let the
small craft Advisory go with the wind field weakening. Expect
waves will hold in this range at least through the morning for the
IL nearshore. The low will eject to the northeast toward Lake
Huron tonight into Sunday which will finally allow high pressure
to begin moving east, and will also let winds shift to more NNW.
This will aid in decreasing the waves tonight into Sunday. The
high will build across the northern and western Great Lakes Sunday
night, and then it will shift to New England through the middle of
the week. The next low will head north of the lakes mid to late






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