Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180842

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
342 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 06Z TAFs...

The strong south-southwest winds and LLWS will continue through
early this morning, then winds will veer to west-southwest and
diminish some with the approach of a cold front. The cold front
will move through RFD shortly after daybreak and then the eastern
terminals by mid morning, shifting winds to westerly with gusts to
15 to 20 kt. Per obs behind the front in IA and NAM/WRF forecast
guidance, a period of MVFR CIGs will likely move in behind the
frontal passage, lasting for 4-5 hours. Winds will then shift to
northwest by the early afternoon, with CIGs scattering. As the
pressure gradient weakens this afternoon, a lake breeze may slip
just inland, and could shift winds to north or northeast at GYY.
Will monitor guidance trends for inclusion in next TAF. It appears
less likely that a wind shift would make it to ORD/MDW prior to
the lake breeze washing out, but it will be something to watch.



246 PM CDT

A warm front lifting through northern Lake Michigan has helped
create some instability across land portions of Wisconsin and Upper
Michigan for the afternoon. This instability will help for
thunderstorm development through the evening as low pressure draws
nearer to northern Lake Michigan. Thunderstorms will track eastward
across northern portions of the lake through the evening. In
addition to the thunderstorm threat, winds are expected to reach
close to gale force associated with a trailing cold front that will
cross the lake overnight. While there is a threat for some 35 knot
wind gusts, the short threat window precludes a gale warning at this
time. Southwest winds will turn more west-northwesterly behind the
front overnight with winds and weather subsiding for daytime
Tuesday. Waves over 4 feet will be possible until they wane in the
afternoon hours Tuesday. Benign marine conditions are expected for
the remainder of the week with winds generally northerly under 25kt.



     AM Tuesday.

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




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