Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201620
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1120 AM CDT FRI MAY 20 2016

.UPDATE...
1119 AM CDT

No significant changes to going forecast. Pretty impressive
looking mid/upper low on w/v imagery this morning tracking toward
The area. Normally in late May, this would be a cause for concern
but 12z raobs and forecast soundings depict quite the dry layer
around 700 mb with dewpoints colder than -40F in addition to low
dewpoints and lack of instability. Main sensible weather impact is
the mid level overcast that will cap highs around 70/a few low 70s
inland today. Onshore flow will keep lakeshore in 50s to low 60s.

Have not made changes to low (~20%) POPs into southeast CWA late
this afternoon and evening, but having some doubts about whether
anything other than a few very light sprinkles or even just virga
will be able to occur. It`s possible that the CWA gets split with
isolated showers into west central IL under upper low this
afternoon and better coverage of showers moving into the Ohio
Valley, with dry conditions in between. Late morning regional
radar does seem to be hinting at this and to note, 12z 12km NAM
does keep the whole area dry through this evening. Will monitor
trends and update as needed this afternoon. Even if it does rain
in east central IL and NW Indiana south of Kankakee River, it will
be light and spotty and cause no impacts.

RC

&&

.SHORT TERM...

Today through Saturday night...

300 am...A weak area of low pressure will move northeast across
the Ohio Valley today and tonight with the back edge of its
showers moving across eastern IL and northwest Indiana. Initially
any precip will be falling into a fairly dry easterly flow but by
late this afternoon into this evening...there is some potential
for a few showers across the far southeast cwa and introduced low
chance pops. For the rest of the area...increasing high/mid clouds
from this system will make for mostly cloudy or cloudy skies by
midday with the clouds exiting later tonight. Temps into the upper
40s in some locations this morning but with no precip today...
guidance highs back in the upper 60s/near 70 seem ok but cooler
near the lake.

High pressure will build south across MN/IA/MO on Saturday and a
few models squeeze out some light qpf Saturday afternoon mainly
north of the cwa as a weak trough passes across the northern
lakes. Stayed dry with this feature but will need to be monitored
with later forecasts. North/northeasterly winds will keep the lake
shore areas cooler as the rest of the area should see highs well
into the 70s. cms

&&

.LONG TERM...

Sunday through Thursday...

300 am...High pressure will move across the area on Sunday with
mostly sunny skies expected. The light winds will allow a lake
breeze to move inland during the afternoon keeping the lakeshore
areas cooler. But highs inland should easily reach mid/upper 70s
and bumped highs up to around 80 across the western cwa.

The high will slowly move across the eastern Great Lakes and Ohio
Valley Monday into Tuesday and its speed will influence how fast
precip chances return Monday night into Tuesday as a trough across
the plains tries to move east. Monday looks dry and if it remains
mostly sunny...highs likely in the lower 80s. A more southerly and
a bit stronger wind may keep lake cooling right along the IL
shore.

Both the GFS/ECMWF have trended slightly slower and drier for
Monday night and Tuesday with perhaps some decaying activity
around sunrise Tuesday. Maintained chance pops during this time
period but there will likely be several dry hours. And if any
precip is indeed only in the morning...Tuesday could end up being
another day with highs in the lower 80s and possibly enough of a
southwest component for no lake cooling at the shore. Dewpoints
also expected to rise into the 60s on Tuesday making humidity
levels noticeably higher.

Uncertainty increases Tuesday night through the end of the week
as there will likely be at least one system moving across the
midwest. Difficult to pin down timing from this distance with
chance pops for the remainder of the period. Highs likely to be
well into the 70s given the dewpoints but dependent on cloud cover
and precip coverage/timing. Precipitable water values increase
into the 1.5 range Wednesday and Thursday...so locally heavy rain
will be possible. cms

&&

.AVIATION...
FOR THE 12Z TAFS...

Mainly quiet weather-wise for the area airports today and
tonight. Increasing east to northeast winds should reach 10-12 kt
at ORD and MDW this afternoon and early evening. Confidence on
this is fairly high especially between 20Z and 01Z...with less
confidence on the exact hourly start and end times of these 10 kt
plus winds. Clouds today will be mid-level VFR altostratus. While
possible a few light showers make it as far north as a Peoria to
Champaign line this afternoon...any activity is expected to be
light with low cloud tops and remain south of the TRACON area.

The winds by daybreak Saturday morning may back to around due
north but speeds will remain light.

MTF

&&

.MARINE...
125 AM CDT

Surface high pressure will have the majority of influence over the
lake through the weekend meaning little of impact weather.
Developing weak low pressure across the Ohio River Valley late
today into tonight will slightly sharpen the pressure gradient
over the far southern part of the lake bringing north to
northeast 15-20 kt gusts into the Illinois and Indiana nearshore
areas. Also of minor note this weekend...a weak cold front will
drop down the lake Saturday night...re-enforcing a little more
northerly component into Sunday morning.

Next week the prevailing surface pressure pattern is forecast to
be low pressure across the Rockies and Plains states while high
pressure across the eastern U.S...a pattern equaling southerly
flow over the lake. Through at least Thursday there does not
appear to be any periods favoring above 25 kt winds.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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