Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181843

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

140 PM CDT

Through Wednesday...

A cold front continues to push across the region this afternoon,
and stretches from roughly Gary through Pontiac at 18Z. Out
ahead of the front, temperatures in the upper 70s with dewpoints
in the mid 60s will contribute to close to 1000 J/kg MLCAPE by mid
afternoon, so cannot rule out a shower or isolated thunderstorm
developing but better chances will be off to our east where there
is very modest upper support associated a sheared shortwave
pushing across the Great Lakes. Behind the front, winds are
turning northwest and easing while skies are clearing out. Latest
runs of the RAP and HRRR continue to suggest a lake breeze will
form and push inland across NW Indiana and a little into Cook
County mid to late this afternoon which will hasten temperatures
dropping back through the 60s for those areas. Otherwise, expect
quiet weather this evening and overnight as high pressure builds
towards the mid/upper Mississippi Valley.

High pressure will continue to dominate most of the day Wednesday
providing light northeast flow and primarily dry conditions.
Despite the cold frontal passage, we will continue to experience
above normal temperatures Wednesday with mid to upper 60s north of
I-80 and low 70s along and south of I-80. An upper level
disturbance currently digging across the Inter-mountain West is
progged to lift across the central plains Wednesday. Weak mid
level height falls will begin to overspread the local area by mid
to late in the afternoon with increasing mid and high clouds.
Models do generate some QPF up through roughly the I-80 corridor
by 00Z tomorrow evening, though soundings show dry air persisting
below 700mb. Could be some high-based light showers or sprinkles
moving up from the south into the local CWA, but better chances
for showers will be closer to the baroclinic zone stretching from
downstate Illinois into southern/central Indiana.



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 18Z TAFs...

A cold front is pushing across the region this afternoon with
winds turning NW and becoming less breezy behind. MVFR cigs have
mostly dropped southeast of the terminals this afternoon and
expect VFR to prevail through the remainder of the period. As
winds continue to diminish this evening, a lake breeze is expected
to form and push inland across GYY and likely MDW where winds will
shift to NNE or NE respectively. Winds will become light and
variable after sunset and overnight, then expect a light N or NE
wind during the day 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






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