Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240125 AAA

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

825 PM CDT

The going forecast of scattered/periods of showers tonight remains
on track. There was a slight diminishing in coverage since 7 pm
owing more to the passing of a shortwave than a true diurnal
downtick. While some diurnal dissipation should be the case
through the evening, variation in coverage is likely with
continued cold mid-level air and subtle short wave impulses ahead
of the main low center. The 00Z DVN and ILX soundings indicated
200-400 J/kg of MUCAPE and 25-40 kt of deep layer shear that will
continue to be tapped for showers with some organization and
spotty moderate to heavy rates. As an example, Pontiac Airport
had 1/2SM visibility around 7 pm with a shower. The potential for
funnel clouds likely has ended as surface winds are generally
light and variable with little for low-level convergence, and any
lightning should be few and far between.

Given the rain and low T/Td spreads, fog is a possibility. There
is quite a bit of existing cloud cover that would hint at low
stratus being more dominant than fog, but confidence is low.
Certainly at least shallow patchy fog is a good bet in some
locations, and would be likely and even dense in any areas that
have a thinning in cloud cover (presently not favored for most of
the CWA).



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.



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

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 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.



For the 00Z TAFs...

The main concerns are precipitation trends, winds and CIG/VSBY
trends. A slow moving upper level low pressure will continue to
bring waves of showers at least into Wednesday morning which will
be capable of reduced visibility. The more frequent or prevailing
rain/shower activity should on the large scale focus over the
eastern terminals, with isolated to at times scattered showers
near RFD. The wind field is quite light with a light pressure
gradient but as weak low pressure organizes south of area tonight,
tendency should be for gradually increasing northeast winds. By
mid-day Wednesday and likely through Wednesday evening, the
position of stronger low pressure off to the east will support
gusty north-northeast winds with gust speeds forecast of 18-20 kt
and potential for up to 25 kt at eastern terminals.

Regarding CIG trends, the most widespread low clouds are over
Wisconsin from MKE and to the north and northwest of there. Much
of the guidance, but not all, brings these low clouds southward
tonight with time and then lingers it into Wednesday. Am concerned
that this a bit overdone, so limited the duration of IFR in the
TAFs, leaning on probabilistic ensemble guidance for trend toward
improvement by the mid-late morning through the afternoon.
Confidence is low- medium with CIG trends overall. If the low
clouds do scatter as anticipated, some guidance hints at them
spreading back southwest Wednesday evening. The lighter wind and
moist regime overnight into Wednesday morning may support VSBY
reductions in BR outside of shower activity.



309 PM CDT

Low pressure dropping from Wisconsin into Indiana tonight and then
moving to Ohio Wednesday evening will support northeast winds
across the lake through Thursday night. Winds are lighter and more
variable Friday and Saturday under the influence of a ridge of
weak high pressure moving over the lake.






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