Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 280758
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
258 AM CDT Wed Sep 28 2016
257 AM CDT
Mostly cloudy skies will persist through the day and tonight, as the
upper level shortwave currently centered over Southern Lake Michigan
drifts south through tonight towards Central Indiana. The surface
feature will drift south as well, and continue to spread periodic
showers over Northern Illinois and Northwest Indiana. Some low-level
convergence areas are expected to pivot over the southern half of
the lake and much of the forecast area, which coupled with
considerable cold air aloft will lift parcels and quickly moisten to
produce periodic steadier showers. The warm lake water will help to
produce instability near the lake, and could be enough to produce
some convection through tonight. Based on latest hi-resolution
guidance, expect the coverage for any precipitation today and this
evening to remain scattered.
With respect to highs today, current indications are that the thick
cloud cover will hold temps in the upper 50s across much of the
area. The exception will be locations adjacent to Lake Michigan
where the warmer marine environment will bleed inland and modify
temps into the low 60s. As the shortwave begins to weaken and drift
southeast away from the area, the precip shield will also shift
east/southeast. Thick cloud cover is expected to linger tonight,
which will help slow temps from falling below the low/mid 50s
257 AM CDT
Thursday through Tuesday...
Thur through Fri: Cutoff upper level shortwave continues to be
progged to drift southeast towards the Tenn valley Thur aftn/eve and
deepen. Meanwhile an expanssive surface ridge over Ontario will
spread southwest through the Central Plains, helping to sharpen the
backedge of the departing surface wave over the eastern portions of
the forecast area. Expect clouds to thin considerably along the
western CWA by late Thur, but this may only be short-lived as a
moist channel is poised to feed west from the Atlantic along the
northern fringes of the surface wave over Southern Ohio late
Thur/early Fri. Guidance continues to prog precip chances returning
from east to west Thur aftn, but more focused on the Fri period. But
with the surface ridge sharpening up the western edge, it is
possible the far western CWFA could remain dry Thur ngt into much of
Guidance continues to indicate the 500mb vort max will be
retrograding across the Ohio Valley to the Great Lakes later in the
day Fri, which will help to push the precip shield west across
Northern Illinois. Temps will continue to be unseasonably cool, with
highs mainly in the 60s Thur to near 70, then as the upper level low
drifts northwest Fri highs could struggle to move beyond the upper
Sat through Tue: Across much of Manitoba/Ontario/Quebec is a 500mb
ridge that continues to block the northward progression of the
cutoff low over the Great Lakes; however, ensemble members continue
to indicate that over the weekend this block will weaken and will
allow the shortwave to lift northeast and depart the Great Lakes. By
the second half of the weekend a trough is poised to slide south
across the Western CONUS, which will help to increase mid-lvl
heights/ridging over the Central Plains/Great Lakes Mon/Tue of next
week. This should help to return temps to near seasonal conditions.
As for precip chances, the best chance doesn`t arrive until closer
to the middle of next week.
For the 06Z TAFs...
1233 am...Forecast concerns include wind directions...cigs and
potential for showers.
Northerly winds early this morning may turn light northwest or
even variable for a time overnight before the gradient begins to
tighten toward dawn when wind directions will become northerly.
With time winds should be north/northeast mid/late morning and
then likely remain north/northeast for the rest of the period.
However...its possible winds could stay north or turn back to
north/northwest at times...so some variability in wind directions
is possible. Speeds will increase to 10-15kts later this morning
and then continue to increase this afternoon with gusts into the
mid 20kt range possible...but confidence regarding how strong wind
gusts will become is low.
Light showers from southeast WI back toward the Quad Cities will
rotate across the terminals overnight into the morning hours...but
this precip should remain light and possible if it remains more
scattered...some areas could stay dry. Opted to continue mention
of light showers in this forecast later this morning for now.
Additional more convective showers will possible this afternoon
into early this evening but coverage is uncertain and for now just
Cigs will be problematic as they are currently vfr and some
guidance is now suggesting nothing more than high mvfr for the
Chicago terminals. Cigs to the north are generally mvfr and with
shower activity not expected to be too heavy...removed ifr
mention but trends will need to be monitored...especially once
winds strengthen and come in from the lake. Eventually...any lower
cigs should lift back to vfr by late afternoon/early evening. cms
224 am...An area of low pressure over northern Lake Michigan will
move south and merge with a second area of low pressure over
central Illinois. This combined low will slowly move southeast to
the Ohio Valley tonight into Thursday. A strong ridge of high
pressure will build across the upper midwest and Ontario tonight
and Thursday and this will tighten the gradient across the lake...
especially southern Lake Michigan tonight when gales are possible.
The low is expected to move back west Friday and Saturday...
centered over Indiana. This will maintain a strong gradient across
the lake with northeast winds to 30 kt expected for a prolonged
period...likely through Friday night. The gradient will slowly
weaken as the low drifts northeast across the eastern lakes
Saturday night into Sunday.
This low pressure combined with colder air aloft will allow the
potential for waterspouts to develop through Thursday morning...
ending from north to south this afternoon through Thursday
LM...Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 7 PM Wednesday.
LMZ779...7 PM Wednesday TO 3 AM Thursday.
Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...1 PM
Wednesday TO 7 PM Wednesday.
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