Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KLOT 200842

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
342 AM CDT Thu Oct 20 2016

342 AM CDT

Today through Friday night...

Short term forecast concerns focus on lingering rain across the
southeastern part of the forecast area today, along with cooler
temperatures and breezy north winds. Lake effect showers are
expected to develop tonight and persist through Friday for
northwest Indiana, while gradually decreasing cloudiness and
diminishing winds across the Illinois portion of the forecast area
will support areas of frost late tonight and again late Friday

Early morning surface analysis depicts weak surface low pressure
across Arkansas and the Bootheel of Missouri, along a nearly
stationary frontal boundary which extends through the Ohio Valley.
The low is progged to deepen a bit today as it continues to track
along the boundary into far western Pennsylvania by this evening,
in response to a fairly vigorous mid-level short wave propagating
across the region. Deep forcing for ascent, including low level
frontogenetic forcing within the elevated baroclinic zone north
of the surface front, persistent height falls with the advancing
short wave trough, and upper divergence in the right entrance
region to the upper level jet streak, will continue to produce
light to moderate rain across parts of downstate IL and much of
Indiana through the day. For the WFO LOT cwa, rainfall is expected
to remain generally southeast of a roughly Pontiac to Chicago
line, with the steadiest rain and greatest rainfall amounts across
the southeastern tier of cwa counties. Rain will diminish and move
east/southeast late in the day, as the initial mid-level trough
axis passes across the area. A secondary upper trough will dig
southeast from the Northern Plains to the western Great Lakes
through tonight, providing a push of colder air into the region
tonight and Friday. This colder air will lead to increasing lake-
induced instability over Lake Michigan tonight, which combined
with a long northerly fetch of moderately strong winds is expected
to lead to development of lake effect rain showers which will
affect mainly northwest Indiana overnight and Friday. Model
forecast soundings continue to depict fairly impressive
thermodynamic profiles over the south end of the lake, with lake
surface to 850 mb delta-T values approaching 18C by early Friday,
and several hundred J/kg of lake-induced SBCAPE. Given this
environment, isolated thunder and isolated waterspouts are
possible, with the greatest convergence focused just east of Gary
across northern Lake and Porter counties through Friday morning.
The low level flow gradually backs more northwesterly Friday
afternoon, which should shift the convergence axis and the best
chances for lake effect showers east of the cwa during the

Farther west, at least partial clearing is expected later today
and tonight along with gradually diminishing winds as weak surface
high pressure ridging approaches the Mississippi Valley from the
west. Combined with drier air, (sub-freezing surface dew points),
this will likely allow for overnight lows dipping to the lower and
middle 30`s across north central Illinois and the development of
areas of frost by sunrise Friday. Considered frost advisory
headlines, though the prospect of continued patchy cloud cover
within secondary upper trough and light but persistent north-
northwest wind late tonight, think it best to defer to day shift
which will have a better look at regional features this afternoon
to make that decision. The surface ridge axis moves across the
area Friday night, which will again present the potential for
frost/freeze conditions by sunrise Saturday with perhaps better
low level conditions, despite slightly warmer temperatures
developing aloft.



342 AM CDT

Saturday through Wednesday...

Dry weather and moderation back to average or better than average
temperatures are expected this weekend into early next week.
Little potential for organized rainfall is evident until closer to

Guidance is in good general agreement in building a broad upper
ridge across much of the central CONUS into the weekend, though
there is evidence of a short wave flattening the flow from the
Northern Plains to the Great Lakes on Sunday. This initially
allows weak surface high pressure ridge to push across the
forecast area Saturday, before a weak surface wave moves across
the region Sunday. With stronger high pressure firmly blocking the
return of Gulf moisture however, the only significant affect on
our sensible weather conditions is likely to be a slight warm up
into the 65-70 degree range Sunday before the weak cold front
moves through. The upper ridge looks to quickly build back into
the Plains and Mississippi Valley early next week however,
preventing any real intrusion of colder air. GFS and ECMWF then
both depict a short wave trough pushing through the ridge Tuesday
night into Wednesday which would bring a slightly more developed
surface low into the area by the end of the period. In addition to
initially maintaining slightly above normal temps, it would also
present the next real threat of organized precip as the low and
trailing cold front push across the area Wednesday-Thursday.



For the 06Z TAFs...

Concerns today include low CIGs, the duration of these CIGs and
gusty northerly winds. A cold front will move south early this
morning, reaching the terminals shortly after daybreak.
Accompanying the front will be stronger northerly winds and low
CIGs. There may be a small lag until the onset of the strongest
winds after frontal passage, with speeds of 10-15 kt and gusts of
15-20 kt. GYY will likely increase to 15-20 kt sustained with
gusts around 25 kt. The direction in the morning should vary
between 350-010, and then could shift to more NNW to N in the
afternoon at ORD/MDW/DPA/RFD (330-360). Winds will then diminish
tonight, except at GYY.

Satellite and METAR obs show the lower clouds behind the front,
so have high confidence in them occurring. Main uncertainty is
whether there will be a brief period of IFR. In this TAF, added a
scattered IFR mention and will monitor trends. Confidence in IFR
occurring is low, with high confidence in MVFR. The rain should
stay southeast of the terminals today, but closest to GYY.
Parameters will become favorable for lake effect clouds and
showers tonight, though GYY would likely be only terminal to be



232 AM CDT

A cold front is moving down the lake this morning, with brisk
northerly winds behind it, and these will continue through Friday
night. Gusts/speeds will be up to 25 kt on the north half and up
to 30 kt today through this evening on the south half and near
shore waters, followed by up to 25 kt for the entire lake Friday
night before easing late. Much colder air moving over the still
mild lake waters will result in sufficient instability for the
potential of waterspouts and thunderstorms tonight through Friday

West and southwest winds will return Saturday into Saturday
evening, with continued uncertainty in wind direction Saturday
night into Sunday, especially on the north half. A weak low
pressure area will then cross the lake Sunday afternoon or
evening, setting the stage for another period of brisk northerly
winds into Monday, though likely of shorter duration. Strong high
pressure will spread into the region on Monday afternoon, causing
winds to diminish quickly.

A Small Craft Advisory is in effect this morning through Friday
morning for the Illinois near shore waters and mid this morning
through Friday afternoon for the Indiana waters. The advisory may
need to be extended for a portion of the Indiana near shore into
Friday night to account for waves slowly subsiding.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...7 AM Thursday TO
     10 AM Friday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...10 AM Thursday TO
     4 PM Friday.




WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.