Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201945

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

305 AM CDT

Through Tonight...

Concerns with the short term center on 1.) any redeveloping
isolated storms this afternoon and tonight, which if they can
will certainly have a heavy rain threat and possibly a severe
one, and 2.) near heat advisory criteria for the southern and
western parts of the forecast area this afternoon if unimpacted
by storms.

Early morning storms along the elevated instability gradient and
a pool of PWATs around 2 inches will slowly move southeast over
the northern and eastern CWA. With some cold pool assistance, this
should propagate through the area by 9-10 a.m. Heavy rain and
isolated pockets of 40+ mph winds will be the main concern.

How long the influence of the clouds and the rain-cooled air
persists is a challenge to say, as outflow should push south to
around I-80 or so. The remnant frontal zone will likely reside
slightly north of this and in general that broad corridor could
serve as a trigger for isolated afternoon storms if enough
convergence offsets the upper ridging. With low-level moisture so
high and mixed layer instability forecast to pool to around
2000-3500 J/kg by mid-afternoon, it may not take much for isolated
initiation. While the better westerlies will be north, enough
shear is forecast to support the isolated severe threat for hail
and wind, and obviously localized very heavy rainfall would be
seen under any storms.

South of the frontal zone (south of I-80 or so), afternoon
temperatures are presently forecast to climb into the 90s, based
on morning convective debris clouds fading, and outflow generally
remaining north. Mid 90s are attainable in this area and if they
look more likely by late morning -- given dew points in the mid to
upper 70s -- a Heat Advisory for heat index readings in excess of
105 will likely need to be issued. Confidence on this is just too
low with the convection at the moment, and the day shift should
have a clearer assessment by mid to late morning.

Later tonight, the quasi-stationary boundary will begin to lift
north as a warm front. Through the night, isolated storms along
and to the immediate north of the boundary are possible with
gradual isentropic ascent along it. Again a marginal severe
threat will exist, but not seeing any signs of a key short wave
disturbance to trigger anything widespread.



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.




229 am...Remaining stronger winds from outflow/thunderstorms late
Wednesday evening should diminish soon over the southern parts of
Lake Michigan. Confidence regarding wind speeds and directions is
somewhat low with additional thunderstorms expected near/over
southern Lake Michigan this morning. A weak frontal boundary is
expected to move south shifting winds northerly by late afternoon.
Prior to that and in the absence of any thunderstorms...its
possible the gradient will tighten enough for southwest winds into
the 15-25 kt range. Trends will need to be monitored through the
morning. Strong/erratic winds can be expected with any
thunderstorms that move over the lake.

A weak ridge of high pressure will move across the lakes region
tonight and then low pressure over the northern and central
plains will move east across the region Saturday night into
Sunday with a broad but weak area of high pressure settling over
the region early next week. cms


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




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