Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 130129
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
829 PM CDT Sat Aug 12 2017

.UPDATE...
828 PM CDT

Evening Update...

Forecast appears to be generally in good shape this evening, with
the only tweaks made to lower overnight min temps a bit in some
areas, especially the Fox/Rock river valley regions where a few
upper 40`s are possible again tonight.

00Z surface analysis depicts a broad area of high pressure
stretching from the Upper Mississippi Valley to the western Lakes.
The center of the high will move across the forecast area
overnight and early Sunday, as subsidence persists in the wake of
the mid-level trough exiting the region to the east, and
downstream of another short wave propagating through the Northern
Plains. Mainly clear skies and light/calm winds in the vicinity of
the surface high center should allow for strong radiational
cooling overnight, which should again allow for temps dipping to
the lower to middle 50`s across most areas outside of the Chicago
urban core, and a few upper 40`s most likely in our typically
cooler spots similar to this morning. These calm/clear conditions
will also set up the potential for some patchy shallow fog, again
mainly away from warmer urban locations.

Otherwise, no other significant changes made to the going
forecast. Surface winds turn more east-southeasterly tomorrow as
the high drifts off to the east of the area. Dry and pleasant
conditions will continue with a little moderation in temps. It
does appear that high cloud cover will increase from the
west/northwest during the afternoon as the Northern Plains short
wave moves into the Upper Mississippi Valley, but pretty good
weather to finish up the weekend.

Ratzer

&&

.SHORT TERM...
232 PM CDT

Through Sunday...

A broad upper level trough continues to swing across the Great
Lakes this afternoon. Water vapor also shows a closed upper low
moving eastward across North Dakota which will take its` time
passing north of the area later Sunday into Monday. A shortwave
ridge is out ahead of this circulation and will pass overhead
tonight. Surface high pressure stretching from Minnesota to
Missouri will move in tonight and crest the area early Sunday.
Late afternoon clouds will dissipate into early evening and the
north to northeast breeze will diminish. Light and variable or
even calm winds will prevail overnight and radiational cooling
will ensue supporting some patchy/shallow fog development, mainly
in rural and less urban areas. Light winds will be in place Sunday
and favor a east to northeast direction early then trend
southeasterly toward evening. Expect to see less stratocu
development but high clouds will likely increase through the day
ahead of the advancing upper low to the north. Temps should warm
to levels several degrees higher than today given more sunshine
and some modest mid level warming.

MDB

&&

.LONG TERM...
251 PM CDT

Sunday night through Saturday...

An extended period of dry conditions is expected through much of
the long term period. High pressure will be parked over the Great
Lakes and upper Midwest into early next week. The weak pressure
gradient persisting across the region will allow the development
of daily lake breezes. While temperatures should trend back closer
to near seasonal normal levels, low to middle 80s through the long
term forecast period, lake breeze development will keep the
lakefront cooler than inland locations through Wednesday. The
longer range models are trending a little faster in tracking a
series of weak shortwaves passing across the middle and upper
Mississippi Valley Tuesday and Wednesday, bringing the next chance
for any pcpn to the region. The longer range guidance is also
trending stronger with a shortwave lifting out of the central
Rockies Tuesday night or early Wednesday, inducing a deepening
surface low. While the GFS and ECMWF differ in the strength of the
low, they generally agree on the track and timing, with the main
sfc low lifting to western Lake Superior Wednesday night. The low
should continue to lift northeast while dragging a cold front
across the region Thursday. Scattered showers and thunderstorms
may develop in advance of the front, but the greatest chance for
pcpn will be Wednesday into Wednesday night as the front pushes
across the area.

&&

.AVIATION...
For the 00Z TAFs...

650 pm...North/northeast winds generally under 10kts early this
evening will become light and variable overnight as weak high
pressure moves across the area. Winds will become light easterly
Sunday morning and turn southeasterly Sunday afternoon...though
its possible a lake breeze may keep winds more easterly at ord/mdw
and northeast at gyy. Patchy fog is possible overnight into Sunday
morning though confidence is low and fog is most likely in the
usual/rural locations. Continued with tempo mention at rfd/dpa but
trends will need to be monitored later tonight. cms

&&

.MARINE...
251 PM CDT

Low pressure over southern Quebec this afternoon will continue to
lift northeast as high pressure spreads across the upper
Mississippi Valley. This will maintain brisk northerly flow
through this evening. The pressure gradient will gradually
relax from the west allowing winds to diminish overnight tonight.
Small craft advisory conditions will persist over the Indiana
nearshore waters east of Burns Harbor through this evening.

The high pressure will spread into the Great Lakes region tonight
allowing winds to diminish overnight. High pressure ridging will
remain over the the Great Lakes region into the middle of next
week. allowing for relatively light and somewhat variable winds
while the ridge of high pressure dominates the area. South flow
will increase Wednesday ahead of low pressure moving east out of
the northern Plains.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Beach Hazards Statement...INZ001-INZ002 until 10 PM Saturday.

LM...Small Craft Advisory...IN nearshore waters until 10 PM Saturday.

&&

$$

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