Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 131101

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
601 AM CDT Sun Aug 13 2017

236 AM CDT

Through tonight...

Surface high pressure is centered overhead early this morning, and
this is resulting in nearly calm winds across the area. While
some patchy fog will be possible for a couple hours through around
daybreak, it appears that it will remain very patchy at best
based on current trends. Otherwise, with the light winds expected
today under the influence of high pressure, expect a lake breeze
to develop by late morning, and then shift inland across
northeastern IL and northwestern Indiana during the afternoon.
Afternoon high temperatures will again top out in the mid to
upper 70s. There is a good amount of higher level cloudiness to
our west and northwest early this morning, and it appears that
these clouds will shift over the area through the day, so expect
clouds to be on the increase the day, with partly sunny conditions
developing around midday.



245 AM CDT

Monday through Saturday...

Main forecast concerns this period are intermittent chances of
rain/thunderstorms beginning Monday evening through Wednesday...
with the best chances of rain for the week coming later Wednesday
afternoon and Wednesday evening. A secondary chance of
rain/thunderstorms comes later Thursday and Thursday evening with
a cold front.

Sprawling high pressure at the surface across the lower Great Lakes
Monday will give way to a sharp upper level shortwave undercutting
the central Canada ridging. Models trending more impressive with
this feature the past 24 hours with modest ascent now progged in
the warm sector ahead of an associated area of weakening low
pressure moving east from the plains. As this low weakens as it
moves up against sprawling Great Lakes high pressure Monday into
Tuesday... the lingering associated weak surface boundary may
continue to be a focusing mechanism for additional rain...albeit a
low coverage and generally light QPF scenario through Tuesday

Best coverage of rain/thunderstorms expected for the period
Wednesday afternoon and evening... shifting west to east across
the forecast area as an upper trough lifts out of the southwest.
This trough combines with return gulf moisture and a good warm
air advection pattern ahead of surface low pressure. Yet another
chance of rain/thunderstorms then comes again Thursday as a cold
front sweeps southeast across the region.

A modest warming trend in temps will be realized through mid week.
With a weak pressure gradient through Tuesday... afternoon lake
breezes Monday and Tuesday will keep lakefront regions somewhat
cooler then inland locations.

More tranquil conditions appear on tap for Friday and Saturday with
temps near seasonal averages.

Ed F


For the 12Z TAFs...

High pressure is currently centered overhead, and this will
continue to result in very light winds through the period. This
light wind regime should allow a lake breeze to develop and easily
push inland over the Chicago area terminals by early this
afternoon. Therefore, expect a period of easterly winds up to 8-10
KT this afternoon, before the winds abate again and turn
southerly tonight.

Overall VFR conditions are expected through the period. However,
some patchy fog at KGYY and KDPA may continue to reduce the
visibilities through 13z before burning off.



311 AM CDT

High pressure across southern Lake Michigan this morning will
prevail across the area into Monday.  Weakening low pressure moving
across the plains into northern Iowa and southern Minnesota will
dissipate Monday evening...allowing a weak pressure gradient and
hence light (and occasionally variable) winds to remain across Lake
Michigan through Tuesday night under lingering high pressure.
Southeasterly flow will then increase across the lake Wednesday as
high pressure finally exits to the east... and yet another area
of low pressure... this time better organized... moves east out of
the plains.

As this low pressure then moves to western Lake Superior Wednesday
night... and into Ontario Thursday... its associated cold front
will sweep southeast across Lake Michigan Thursday afternoon and

Ed F





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