Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181148

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
648 AM CDT Tue Oct 18 2016

342 AM CDT

Through tonight...

Summer-like overnight conditions continue early this morning,
with temperatures still in the low-mid 70`s at 3 am CDT. Deepening
surface low pressure just east of Lake Superior will continue
track off to the northeast this morning, with a trailing cold
front pushing southeast across the cwa this morning through early
this afternoon. As a result, parts of far northern/northwest IL
may see the warmest temperatures early this morning ahead of the
front, with model low-level thermal fields supporting recovery
back to around the 70 degree mark near the IL/WI border this
afternoon. Farther southeast, temps should warm to near 80 by
midday just ahead of the front. Forecast soundings maintain dry
moisture profiles across the area through the day for the most
part, which upstream 00z RAOBS from OAX, TOP support, though a
period of post-frontal stratus is likely per high-res model
soundings and observed post-frontal cloud deck across portions of
northern IA. Winds, which continue to gust 20-25 mph early will
continue to east as the gradient weakens with the approach of the
frontal pressure trough, shifting northwest with fropa. Gradient
appears to weaken enough later this afternoon that winds may turn
north-northeast along the Lake Michigan shore, producing an
onshore weak lake breeze especially from Chicago southeast into
northwest IN.

Weak surface high pressure ridge noses into the region tonight in
the wake of the cold front, which slows up to our south-southeast
across the Ohio Valley. Without a strong push of colder air, min
temps in the mid-40`s to mid-50`s are expected, with a nod to the
slightly cooler mos numbers especially north/northwest.



342 AM CDT

Wednesday through Monday...

Mid-week portion of the forecast features an amplification of the
upper level pattern, with a digging upper trough inducing a
surface low pressure wave which tracks along the stationary front
across the Ohio Valley late Wednesday through late Thursday.
Guidance has trended toward spreading rain farther north within
the elevated baroclinic zone with this system, particularly
across the southeast half or so of the LOT cwa from late Wednesday
afternoon into Thursday. Colder air spreads in aloft within the
upper trough axis Thursday, along with a secondary upper trough
which moves across the western Great Lakes region into Friday,
resulting in steepening low level lapse rates across the still-
mild Lake Michigan waters, and setting the stage for a period of
lake effect showers from early Thursday morning into the day
Friday initially for northeast IL and shifting into northwest IN
with time. GFS soundings over the southern end of the lake depict
a deep mixed layer with a few hundred J/kg of lake-induced SBCAPE
which could even support some thunder. Blustery north winds
developing during this time will pull colder air into the region,
with high temps only in the 50`s on Friday, which looks to be the
coolest day.

Models remain in decent agreement in developing a broad upper
ridge across the central and western CONUS by the end of the week,
helping to nudge the upper trough over our area east of the
region. Both the GFS and ECMWF continue to deepen the trough
however, closing it off across the eastern Lakes and New England
by Saturday morning. This may slow ending of lake effect over
northern Indiana and slow our temp rebound heading into the
weekend, though for now models do shift surface high pressure
across the area Saturday. Confidence lowers Sunday onward, as
models differ with the amplitude of a mid-level wave topping the
ridge to our west and dropping across the Lakes into early next
week. Have favored the weaker and slower ECMWF here, with a return
to average temps Monday-Tuesday.



For the 12Z TAFs...

No significant aviation concerns today, with much lighter winds
than on Monday. Primary focus is on a period of MVFR CIGs this
morning through mid day and then the potential for a lake breeze
boundary to get near MDW this afternoon. Southwest winds are still
gusting up to 20 to 25 kt ahead of a cold front that is into
northwest Illinois just prior to 12z. This front will shift winds
to the west with at least occasional gusts in the 15-20 kt range.
Winds will further veer to west-northwest around 10 kt this
afternoon. The pressure gradient will diminish enough to allow a
lake breeze to form and likely push inland into NE IL and NW IN,
including GYY. It is possible that the boundary and associated
wind shift could approach MDW by the mid to late afternoon, with
low confidence in it crossing the terminal. Have medium-high
confidence that ORD will maintain west-northwest winds through the
daylight hours. Winds will quickly diminish with sunset this
evening and stay light/VRB into Wednesday as high pressure moves

Regarding the MVFR CIGs, some patchy CIGs have developed ahead of
the cold front, followed by a more solid area of low-mid MVFR CIGs
immediately behind the front. Once these arrive, there should be a
3-4 hour period of MVFR CIGs at all terminals, with some potential
for them to hold on longer behind lake breeze at GYY. Confidence
is medium in CIG trends through mid day today, with high
confidence in VFR tonight and Wednesday.



429 AM CDT

The intense low pressure system responsible for the strong
south-southwest winds Monday through early this morning is
quickly lifting northeast from eastern Ontario into Quebec. The
trailing cold front is crossing northern Lake Michigan and will
sweep across the rest of the lake this morning. Winds will shift
to northwest and west and remain fairly brisk through much of the
afternoon on the north half, while diminishing fairly quickly on
the south half. High pressure will result in light and variable
winds on the southern portion of the lake on Wednesday, with
modest southwest winds to the north. The next cold front will
move down the lake Wednesday night, resulting in an extended
northerly wind episode Thursday into Friday. Speeds will be up to
30 kt on the south half. The cool air mass over still unseasonably
warm lake waters will be favorable for lake effect showers and
potentially waterspouts. Have not added waterspout mention to the
forecast yet, but may need to.

Allowed the Gale Warning for the near shore waters to expire,
though a few gale force gusts will occur prior to daybreak. Winds
will diminish quickly this morning after shifting west behind the
cold front, so the planned end time of 10 am CDT for the Small
Craft Advisory seems reasonable. Will likely need a Small Craft
Advisory Thursday-Friday for the northerly winds and building



LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 10 AM Tuesday.




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