Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202017

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
317 PM CDT Thu Jul 20 2017

315 PM CDT

Through Friday...

Isolated light rain showers persist over northern IL, but the main
focus for convection will be along an advancing lake breeze and
south of I-80 near a weak boundary.  I have low to medium confidence
in how many storms will form along these boundaries and how long
storms will persist into this afternoon/evening. The more agitated
clouds are associated with the lake breeze pushing across Lake and
Cook counties in IL so thinking a few showers/storms are possible
over the next couple of hours.

The boundary further south is near the southern forecast area
boundary over McLean, Champaign, and Vermilion counties.  This
boundary is not as strong and satellite is not showing as much CU.
Therefore the general idea is isolated to widely scattered
showers/storms form along the lake breeze and near the southern
boundary through the early evening hours. Storms will then
concentrate along a Rochelle, IL to Rensselaer, IN line around
midnight.  Models have not been handling convection well today so I
have low confidence in the forecast this evening and tonight.

As far as the severity of the storms, my main concern is torrential
downpours as PWATs are around 2 inches.  The latest RAP analysis has
CAPE values over 3000 J/kg, but shear is lacking. The highest shear
values are 35-40 kt north of a Pontiac to Benton County Indiana
line. Therefore, all storms will produce torrential downpours that
may result in localized flooding, but only a few stronger storms may
produce large hail or damaging winds.

Outside of convection, dewpoints are very high in the mid 70s to
around 80 at some locations. Expecting fog to develop after midnight
especially where there isn`t nocturnal convection.  Dense fog is
possible, but didn`t have the confidence to put dense fog in the
forecast just yet.

Low confidence continues into Friday`s forecast. Models have very
different timing and paths for a decaying complex sliding southeast
across the forecast area. Kept precip chances in the chance range
due to my low confidence even though I do expect a complex to impact
the region.  Main impacts from this complex will be heavy rain and
gusty winds. Convective parameters look similar to today with CAPE
values over 3000 J/kg, but shear values of only 30 kt.

I made minimal changes to high temps Friday as I`m not sure how the
decaying system will impact temps.  If the system rolls through
earlier than currently forecast, we could be seeing 90s like today.
However, if it comes through later, temps may only reach the upper
80s before the storms cool the environment.  Either way, hot and
muggy conditions continue Friday.



243 PM CDT

Friday afternoon through Thursday...

Main forecast concerns/challenges are with continued rounds of
thunderstorms Friday evening through Saturday night, with severe
weather and heavy rainfall once again appearing likely throughout
this period. A Flash Flood Watch has been issued for portions of
north central and northeast Illinois Friday afternoon through
Saturday morning.

Friday Afternoon into Friday Evening...

Period begins active with guidance indicating progressive MCS to
likely move southeast through the region by either late Friday
afternoon or very early Friday evening. Given that there is no
real big change expected in the overall pattern, this appears to
be a highly probable scenario. Instability axis will still be
situated into northern Illinois and northwest Indiana, supporting
the idea that any upstream development will have a high chance to
move into northern Illinois in the late afternoon and early
evening hours. Still some uncertainty on exact track through
northern Illinois before shifting into northwest Indiana later in
the evening. A more focused impacted area may be realized during
the day Friday, but at this time, most locations along and north
of I-80 will have a high chance of observing this next complex of
storms. With this complex of storms, a damaging wind threat will
likely be the main hazard and given the likely progressive nature
of these storms, thinking any widespread flash flooding would be
low during this time. However, high PWATS around 2 inches and
strong instability in place will support torrential rainfall with
any storm. So can`t rule out minor flooding, especially as a quick
1-2 inches of rainfall will be possible.

Friday Night into Saturday morning...

This MCS will continue to shift to the east southeast through the
mid to late evening, still posing a damaging wind threat for
areas in northeast Illinois and northwest Indiana. At this time,
don`t see a large difference in the environment across these
locations, with these storms maintaining the same intensity and
damaging wind threat. As this complex of storms exits late in the
evening, should begin to already observe additional thunderstorm
development across areas from northeast Iowa and southwest
Wisconsin. Frontal zone position, additional upstream mid level
energy, and strengthening LLJ will likely support continued
thunderstorm develop along this axis throughout the night.
Instability/moisture axis and steering flow will once again
support any upstream development to move into areas in northern
Illinois and northwest Indiana with most locations observing these
additional storms. This situation becomes very concerning as
although there will remain a severe threat, the threat of flash
flooding increases during the overnight hours into early Saturday
morning. There is the potential for areas to receive a quick
additional amount of 2-3 inches given the possible training and
high likelihood of high rainfall rates/torrential rainfall. These
additional thunderstorms along with the area already observing
multiple rounds of thunderstorms with heavy rainfall and very wet
conditions in place, will provide a flash flood threat. Have
issued a Flash Flood Watch for portions of northern Illinois. Have
left out areas further to the south in Illinois and the counties
in northwest Indiana give some lower confidence in these
locations. Also, observed rainfall has not been quite as high
recently. Nonetheless, will continue to monitor possibly including
these areas with later forecasts.

Saturday and Saturday Night..

Confidence does lower with convective trends on Saturday and
Saturday night given some model variability with the synoptic
pattern. However, additional thunderstorm chances Saturday and
Saturday night still appear reasonable. Large scale forcing will
likely be in place along with a moist and unstable atmosphere,
supporting at least scattered development. Although the likely
pattern on Saturday does not appear favorable for widespread heavy
rainfall, the very moist conditions will still support heavy
downpours. With the possible pattern and the likely instability,
severe weather will also be a concern area wide.




315 AM CDT

The Hydrologic Outlook (ESF) for along and north of I-88 has been
continued due to concerns of additional rounds of storms through
early this weekend. Early morning storms are producing additional
rainfall up to or over an inch, primarily in north central
Illinois (Rockford/Byron areas). Lesser but still moderate rain is
expected over the Des Plains and Fox River basins. Just last
night`s rain brought a steadying of the hydrographs at several
locations, showing that it may not take too much to cause rises
given wet antecedent conditions. Concerns are particularly
heightened on Friday and Friday night, where the environment will
be even more favorable for very efficient rain-producing storms
(training storms, possible backbuilding, deep moisture, etc).
While there is uncertainty on location, in collaboration with WPC
and adjacent WFOs, the focus is presently most favored in southern
Wisconsin and far northern Illinois. WPC has gone ahead with a
moderate risk of excessive rainfall in that period.



For the 18Z TAFs...

Gusty southwest winds continue across the region. Expecting a lake
breeze to push onshore over the next few hours, with winds
becoming northeast at the terminals. The strong southwest winds
give me a little less confidence in northeast winds, but guidance
has been persistent and winds have turned northeast at Milwaukee
and Port Washington, WI.

Isolated to widely scattered showers and storms may form along the
boundary so have a VCTS for a couple of hours at the eastern
terminals. Confidence is low in how many storms form and exactly
where. Showers persist over northwest IL so kept a VCSH in at RFD
for the next couple of hours.

Any additional convection is expected to the west and south of the
terminals tonight. The other concern for tonight is if we do not
have any storms and skies are clear, fog is possible. Do not have
enough confidence to put fog in the TAFs right now though.

Winds veer to southeast by mid Friday morning. Another storm
complex is expected Friday, but have low confidence in timing and
track. Therefore, went with a PROB30 for storms.



315 PM CDT

Strong winds from a wake low this morning have dissipated. The baggy
pressure gradient has allowed a lake breeze to form. Despite gusty
southwest winds, the lake breeze is finally moving ashore over the
southwest end of the lake. Variable wind directions are expected
across the lake through tonight.

Winds become southerly tomorrow as high pressure shifts over the
eastern Great Lakes. One to two additional convective systems are
possible through Friday night. Any of these may produce a wake low
that briefly increases winds and waves to values higher than
currently forecast.

A weak low will pass over the lake Saturday night and winds
will become northerly behind it.  A second and possibly stronger
surge of north winds will push down the lake Sunday night into
Monday morning. North winds 15-25 kt are possible. High pressure
moves over the lake Tuesday.



IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     PM Friday to 9 AM Saturday.




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