Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231942
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
242 PM CDT Tue May 23 2017

.SHORT TERM...
240 PM CDT

Through Wednesday...

Deep low pressure will continue to sink south from the corn belt
to the lower Mississippi valley Thursday. The first in a series of
waves ahead of the low will shift northeast through the flying
area this afternoon. Expect an increase in shower coverage and
intensity over the coming hours with the enhanced lift from this
wave, with some moderate showers due to a narrow moisture plume
precipitable water values in excess of 1 inch There is a small
pocket of instability across north Central Illinois into adjacent
Wisconsin where the best near term chances for embedded
thunderstorms exists. With the upper low shifting farther east,
cold temperatures aloft will allow some of this weak instability
to shift over northeast Illinois and northwest Indiana. This
combined with the vort max still warrants a low chance thunder
mention. RAP forecast of non-supercell tornado threat are still
low but low level cape does exist in very localized pockets near
the cold frontal boundary where low level vorticity is enhanced,
and with some limited clearing there is still a limited severe
weather concern for gusty winds as the vort max encounters the
front this afternoon.

As the low sinks south tonight into Wednesday, Expect continued
waves of showers as additional energy spreads northward ahead of
the low, with coverage likely decreasing though possibly becoming
more narrowly focused on eastern half of the area. There could be
a modest rainfall axis that will setup tomorrow, with the I-55
eastward corridor still favored, but still challenging to pinpoint
at this time. A low thunder chance exists across east Central IL
and northwest IN as well. Elsewhere just expect occasional showers
and cool cloudy conditions.

KMD

&&

.LONG TERM...
240 PM CDT

Wednesday night through Tuesday...

Looking ahead through the extended part of the forecast, there are
not many periods without a mention of rain chances.  We transition
away from one upper low late this week but into another this
weekend, with only weak ridging in between.  However, despite the
frequent chances for rainfall, it does not appear that each day will
be a washout. In general the showers should be to some extent
diurnally driven under a cyclonic upper level circulation, which
means at least the early part of most days could be favorable for
outdoor activity.

The most likely period for a break in activity appears to be
Thursday afternoon into the early part of Friday as the first system
moves out and cooler, drier air moves in with northerly surface
flow.

Warmer and more humid air returns for Friday and Saturday.  The
biggest uncertainty for this forecast period comes late Friday and
into the weekend, with the deterministic GFS and ECMWF in poor
agreement with the timing of an upper wave and surface low pressure
system moving through the southern Great Lakes. Presently the much
faster ECM...as well as the GEM...brings this system through late
Friday and leaves behind just some light activity for Saturday.  The
GFS is about a day slower and shows widespread rainfall Saturday
afternoon and evening. Both have a stationary front extending east
from a larger center of low pressure across Oklahoma early Friday.
They both also depict a wave forming along the front as the one that
pushes through the local area. Given the proximity of the boundary
and the uncertainty with the movement of the surface wave, will
continue to carry increasing precip chances late Friday, especially
south, and again Saturday afternoon and evening for a larger portion
of the area.

Models are in somewhat better agreement in depicting another break
in the weather early Sunday, then a large upper low becoming parked
over the Great Lakes from late in the weekend through at least
midweek. This would support ongoing widespread chances of rainfall,
especially during periods of afternoon heating.

Lenning

&&

.AVIATION...
For the 18Z TAFs...

Concerns:
-Expanding showers embedded thunder this afternoon
-Brief IFR vsby in moderate rain or t-storms
-MVFR cigs lowering to IFR or possibly LIFR
-Continued showers and lower cigs into Wednesday
-Winds becoming erratic this afternoon and settling northeast

Deep low pressure will continue to sink south from the corn belt
to the lower Mississippi valley Thursday. The first in a series of
waves ahead of the low will shift northeast through the flying
area this afternoon. Expect and increase in shower coverage, with
MVFR vsby, possibly briefly IFR. There is a small pocket of
instability across north Central Illinois into adjacent Wisconsin
where the best near term chances for embedded thunderstorms
exists. With the upper low shifting farther east, cold
temperatures aloft will allow some of this weak instability to
shift over the terminals this afternoon. This combined with the
vort max still warrants the VCTS mention, through coverage may not
be that large. Largely VFR cigs are expected but occasional MVFR
is possible.

Expect continued waves of showers through much of the period as
additional energy spreads northward ahead of the low. Precipitable
water values in excess of 1 inch suggest occasionally moderate
rain with reduced visibilities. The other concern is with cooler
air spilling southward as the surface low will shift to south
central Indiana, model guidance paints IFR cigs, possibly even
LIFR cigs or vsbys late tonight as light winds from the northeast
and moist conditions linger. Confidence is not high on the extent
or coverage, but it is a favorable setup for low cigs overnight
into early Wednesday, possibly alleviated by periods of rain.

KMD

&&

.MARINE...
428 AM CDT

While low pressure is situated just to the west of the lake this
morning, associated boundary is bisecting the lake. Northerly
winds are occurring over the north half while southerly winds are
in place over the south half due to this boundary. Expect these
winds to continue for much of the day, until winds turn northerly
over the entire lake tonight. Monitoring some patchy fog across
the north part of the lake and at this time, it looks to be mainly
land fog. However, did add in fog mention in the forecast and will
continue to monitor trends this morning. Will also need to monitor
all areas of the lake later today into this evening for
additional fog development. Expect the northerly winds tonight to
increase, with waves increasing tonight into Wednesday. These
higher waves and winds will then likely remain through Thursday,
with hazardous conditions for small craft returning Wednesday
through Thursday.

Rodriguez

&&

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

$$

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