Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240759

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
259 AM CDT SUN JUL 24 2016

347 PM CDT

Through Sunday...

A Severe Thunderstorm Watch has been issued for northern Illinois,
generally along and north of I-80, through 10 pm this evening. No
changes made to the heat headline or any issuance for Sunday due
to uncertainty and residual effects of nighttime convection.

Convective cluster in northeast Iowa has raised concerns as cloud
tops have rapidly cooled and is further showing signs of
organization. Mesoscale convective vortex (MCV) across central
Iowa is likely the driving feature and has become more clear this
afternoon on satellite with mid-level speed max supporting
effective shear of 35 to 50 kt. Also there is support for a cold
pool to develop and grow. The effective surface front and
instability gradient, affected by outflow from storms to the
north, is draped across far northeast Iowa and far near/just south
of the WI/IL state border. This likely would be the path that
organized storms would travel eastward, though propagation would
indicate a slight south of eastward movement too.

There is high uncertainty on just how organized these storms will
become as well as how far south this evening. There is capping in
place the further south into the area and the storms are
immediately moving into somewhat worked over air due east of
them. But given the degree of instability (MLCAPES of 3500 to 5000
J/kg) and moisture (PWATS around 2 inches) over a large part of
the DVN CWA and ours, at least some convective cluster(s) are
likely to make it into our area. The best timing estimate of these
reaching the Rockford area appears to be around 7 pm and the
Chicago area 830 to 11 pm. Again this is assuming the storms have
some form or organization.

The primary hazards with these evening storms would be pockets of
severe winds due to high downdraft CAPES and precipitation
loading, especially any more organized activity. Near the outflow
boundary there may be a localized tornado threat due to backed
low-level wind flow, and the boundary layer is quite moist for
maintaining low-level instability into the evening. Storm motion
could be fairly slow especially anything that develops ahead of
the main cluster(s) and would be of greatest concern for localized

There is now naturally less confidence on late night/daybreak
Sunday storms but maintaining likely in northern areas where
vort max is pegged to move eastward across Wisconsin and the low-
level jet points over our area. The less organized evening storms
are the more likelihood of overnight storms and that they could
bring a marginal severe weather risk.

For Sunday, high dew points in the mid to upper 70s are expected.
The thermal ridge immediately ahead of the cold front will support
mid 90s for highs if we can clear and have full heating. There is
high uncertainty of this so that is why no heat advisory at this
time. One may be needed for a large portion of the CWA, including
Chicago, if the going forecast continues. The front itself will
probably spark scattered storms although how much coverage will
likely be impacted by how storms behave tonight.



213 PM CDT

Sunday night through Saturday...

Sunday evening through Monday...The upper trough currently moving
east across southern Saskatchewan will swing across
western/central Ontario Sunday night and Monday. The associated
cool front will be dropping southward across the southern half or
so of the local area Sunday evening. Will continue with chance
pops Sunday evening/night across the south to southeast half of
the area as the front will be slow to progress and there is a
decent chance of an embedded shortwave moving through the main
upper flow. Some activity may linger Monday morning across the far
southern counties but surface high pressure will be quick to build
in bringing drier air with it. 850 mb temps cool to 16-17C Monday
with abundant sunshine expected once any lingering clouds clear
south. This supports highs in the mid to upper 80s but onshore
flow will develop near Lake Michigan keeping areas within a few
miles of the lake in the lower 80s. Lows Monday night will fall
into the mid 60s with upper 60s/around 70 in the Chicago Metro.
High pressure will be overhead Tuesday and shift to the east
Tuesday night. High temps will be similar to Monday with
continued cooling near Lake Michigan.

Wednesday through Saturday...The generally zonal upper flow
of Monday and Tuesday will buckle a bit mid week with medium
range guidance suggesting a more pronounced larger scale trough
developing into and over the weekend. Until then...the surface
pattern is fairly ill-defined through Wednesday but the upper
flow patter supports the passage of embedded shortwaves. This will
probably lead to transient shower/storm chances Wednesday and
Thursday. A more coherent surface low may move across Thursday
with a stronger upper wave noted in guidance but this has not
been the most consistent feature. Guidance is in decent agreement
on a well defined upper trough crossing the area sometime Friday
or Saturday which will bring a more organized chance for storms
but the timing is not clear at this range. Low/mid level temps do
not fluctuate much through late week. Cloud cover may be rather
persistent from Wednesday onward with any convection also working
to hold temps down some as well. Will continue with highs in the
lower to mid 80s...but may end up being warmer if convection is
kept to a minimum.



For the 06Z TAFs...

1255 am...Showers with embedded thunder will slowly drift
southeast over the coming hours, with the bulk of the
precipitation falling out of a mid level cloud deck. While
confidence is medium, feel that the showers and storms over
Wisconsin will drift more east of south and will move over Lake
Michigan. We have some instability available which may allow some
showers to fester through the night, but there is some capping in
wake of the evening storms and therefore feel that a dry overnight
forecast would be the rule, or maybe some VCSH. Some guidance does
attempt to shift some of the activity southeast across the
terminals this morning, but confidence in this scenario is low.

Additional thunderstorm development is expected this afternoon
into Sunday evening ahead of an approaching cold front. Timing has
differed from various models but trend now appears later and
maintained vicinity mention for now.

Winds should shift back from the current southeasterly to
southerly overnight with gusty south/southwest winds expected
today, shifting to northwest later tonight.



252 AM CDT

Southwest winds will increase today to around 25 kt ahead of low
pressure that will move east through northern Ontario today. A
cold front behind this low will move south across the lake tonight
with modest northwest winds behind it. High pressure will take
control through mid week with fairly benign conditions across Lake
Michigan. Thunderstorm chances exist today and then return later
in the week.



IL...Heat Advisory...ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-
     ILZ020-ILZ021-ILZ022-ILZ023-ILZ032-ILZ033-ILZ039...11 AM
     Sunday TO 7 PM Sunday.

IN...Heat Advisory...INZ001-INZ002-INZ010-INZ011-INZ019...11 AM
     Sunday TO 7 PM Sunday.




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