Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241917

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
217 PM CDT Wed May 24 2017

215 PM CDT

Through Thursday...

Sharp upper level trough with pinwheeling lows will continue to
impact the region in the near term bringing overcast skies
areawide and scattered showers. Water vapor imagery this
afternoon reveals two upper lows embedded within a broader
longwave trough centered over the Mississippi Valley. A western
low is digging south across Missouri into Arkansas while another
low is lifting north across the Ohio Valley. The western low
resulted in some scattered showers over the western CWA earlier
today but is otherwise exiting the area with mid level height
rises building across the mid and upper Mississippi Valley. More
widespread precipitation is in place with the eastern low and some
of the showery activity with this low will impact areas mainly
east of the I-55 corridor through the remainder of the afternoon
and evening. There is some very modest instability over
northwestern Indiana, 100-200 J/kg MLCAPE which may result in an
isolated embedded thunderstorm or two, but overall lightning
potential is low for the CWA.

The western upper low is progged to dig into the deep south late
this evening and overnight which will help kick the eastern low
farther east out of the CWA. This will result in diminishing
precip chances through the late evening and overnight hours.
Lingering low level cyclonic flow through mid afternoon Thursday
will likely keep much of the CWA locked under cloud cover, but
ridging building into our west mid to late in the afternoon should
result in some clearing west and perhaps a few peeks of sunshine
east. Temperatures are expected to be several degrees warmer
tomorrow, with highs in the upper 60s to around 70 inland, but
only in the 50s and low 60s along the lake front.



315 AM CDT

Friday through Tuesday...

Medium range forecast gets a little tricky with generally lower
confidence in the details. In the wake of the departing upper low,
we should transition to largely a zonal flow over much of the
CONUS late in the week into the first part of the weekend. Due to
the zonal flow, shortwave troughs moving across the country will
be low amplitude and because of this models are struggling with
the timing of shortwaves and subsequent rain chances late in the
week into the weekend.

Unfortunately, blending model solutions together results in
precip chances in the forecast almost every period from Friday
through Monday because the widely varying model solutions. This is
likely misleading as the majority of the weekend will be dry. Due
to the lower than average confidence in patterns like this, made
very few changes to blended model initialization grids.

GFS 7 day running mean skill scores have been very low recently,
so leaning on the ECMWF and its 51 member ensembles suggests that
the baroclinic zone will likely set up pretty far to the south of
our CWA Friday into Saturday, with precip threat Friday, Friday
night, through early Saturday looking very low over most of our
CWA. Best chances (which still appear low) would be our southern
counties Friday night.

By Saturday night through Monday, look for another unseasonably
strong large deep tropospheric closed low to dig southeast into
the Great Lakes region. A fair number of EPS members and the
operational ECMWF support a shortwave trough zipping east along
the southern flank of this digging long wave trough. This
shortwave could be close enough to bring some threat of showers
and thunderstorms north into our CWA Sat night, mainly southern
CWA and looks to be one of the better chances of seeing
widespread-ish rain this weekend.

Sunday and Monday look mainly dry, though depending on the
positioning the coldest air aloft, couldn`t rule out some isolated
to widely scattered instability showers and perhaps a storm
during mid afternoon through early evening hours. At this point,
neither Sun nor Mon look to be a washout and could very well still
end up being mostly dry.

Temps should be closer to normal this weekend with lake cooling
likely Saturday, but looking less likely Sunday and Monday with
westerly surface winds most likely to bring the 70 degree warmth
right up to the lakefront.

- Izzi


For the 18Z TAFs...

Low pressure over the Ohio Valley this afternoon will meander over
Indiana and Ohio over the next 24 hours with a modest northerly
breeze and primarily MVFR conditions in place across northern
Illinois and northwest Indiana. Ceilings will pose the primary
forecast challenge this afternoon. Ceilings are expected to hold
steady or improve slightly this afternoon, though erring on the
side of persistence in the meantime as confidence in the details
is low. There is some lingering patchy IFR this morning (at RFD
and GYY), though anticipate that should lift to low MVFR over the
next couple hours. There is potential that ceilings could lift to
VFR for several hours mid to late this afternoon into the evening,
but confidence is too low to include in the TAF at this time.
Showers are expected to increase in coverage across northern
Indiana this afternoon and eventually into southwest Michigan late
afternoon into the evening. The best coverage of showers is
expected to stay east of the terminals, but cannot rule out some
of the showers rotating west across the terminals this evening.
Overnight, expect ceilings to lower back to low MVFR with IFR
again a possibility.



416 AM CDT

Despite low pressure situated just to the south of the lake early
this morning, gradient over the lake has been supportive of
relatively lighter winds for the most part. In this pattern, dense
fog has developed mainly over the south half of the lake. Have
issued a dense fog advisory for the south half of the open waters
and for a portion of the IL nearshore waters. Have this advisory
going through mid morning today, but will need to monitor trends
to possibly extend this longer into the day. This low does
strengthen today, and expect northerly winds to increase
throughout the day, with 15 to 25 KT winds in place. Do think a
period of to 30 KT winds are possible this afternoon as well.
These elevated winds and waves look to persist tonight into
Thursday, until this low begins to exit to the east. Have issued a
Small Craft Advisory for these elevated winds and waves, that go
into Thursday. The IL side only goes through Thursday morning
while the IN side goes through the afternoon due to lingering
higher waves.



IL...Beach Hazards Statement...ILZ006-ILZ014 until 7 PM Thursday.

IN...Beach Hazards Statement...INZ001-INZ002 until 7 PM Thursday.

LM...Small Craft Advisory...IL nearshore waters until 10 AM Thursday.

     Small Craft Advisory...IN nearshore waters until 4 PM Thursday.




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