Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241139

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

315 AM CDT

Through Thursday...

Unseasonably strong upper low over Iowa is moving steadily south
this morning with GOES-16 low (7.34 micron) and mid (6.95 micron)
water valor imagery depicting the upper low nicely and also of
note is a strong shortwave over Arkansas. As the IA upper low
moves south today, the strong shortwave trough over AR is forecast
to pivot around the southeast and then eastern flanks of the upper
low. GOES-16 imagery confirms there is a healthy supply of
moisture with this feature which should spread north today across
Indiana as the shortwave lifts north and surface low pressure
deepens in response. A healthy TROWAL is likely to develop with
the northwestern extent of this precipitation probably making it
into at least our southeast third of our CWA later this
afternoon, with a pretty sharp cut off to the northwest.

Other than this precip, anticipate the slug of rain over NE IL
currently to decrease in coverage and intensity as the upper low
drops southward. A good deal of the CWA will spend most of the day
with little or no precipitation, with some isolated instability
showers probably developing western CWA this afternoon to the west
of the shadow of more stable air spreading inland from Lake
Michigan. Any spotty diurnal showers west will quickly fade at
sunset with guidance now in pretty good agreement in shifting the
precip with the secondary s/w east of our CWA during the early
evening, with our CWA likely rain-free by midnight, if not sooner.

Some partial clearing is expected tonight, but guidance is
currently suggests winds should stay up enough to prevent fog from
becoming a big problem tonight.

Temps will remain cool for this time of year today, especially
near the lake where a brisk wind blowing down the full fetch of
the lake will probably hold temps mostly in the low to mid 50s
near the shore, while inland areas likely climb into the low to
mid 60s.

Thursday, temps will moderate and we should see at least partly
cloudy skies. Gradient will favor NNW winds, which will
undoubtedly allow a healthy lake breeze to form in the afternoon
and penetrate inland. Behind the lake breeze, temps along the
shore should drop into the low to mid 50s, while inland areas
moderate into the mid 60s to low 70s (warmest west).

- Izzi


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 12Z TAFs...

Latest radar imagery showing solid area of light showers finally
exiting to the northeast. This trend will continue and although
some isolated light showers may linger for a few hours at the
start of the period, do think the majority of today will be dry.
RFD may see a slightly longer period of isolated light showers
this morning. Then, after a dry period this afternoon, it does
look like light showers may briefly return this evening. However,
this should not be any long duration rain, with conditions once
again drying out through tonight. VFR ceilings have been observed
for most of the night, likely due to the rain overhead. As this
rain is exiting, do think there is still a higher likelihood of
IFR ceilings this morning. This trend is occurring at this time,
with GYY and MDW now observing IFR conditions. So have kept IFR in
this morning, but may have not kept these lower ceilings in long
enough with IFR possibly staying in place through midday. A slow
improvement is then expected soon there after, into the afternoon.
Ceilings remain tonight, but confidence is lower with how low
they will get. Have maintained similar forecast, but will need to
monitor for the possibility of lower ceilings tonight into
Thursday morning. Northeast winds will be in place for much of
today, with speeds steadily increasing today.



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.



LM...Dense Fog Advisory...LMZ740-LMZ777-LMZ779 until 9 AM Wednesday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...1 PM Wednesday to
     10 AM Thursday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...4 PM Wednesday to
     4 PM Thursday.




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