Severe Thunderstorm Warning 35, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
255 PM CDT Mon Jun 18 2018

.SHORT TERM...
238 PM CDT

Through Tuesday...

Main concerns for the short term forecast period will be
temperature/dewpoint/heat index values this afternoon and then
thunderstorms and associated impacts this evening.

As of 2:00 PM CDT, temperatures have reached the lower to middle 90s
with dewpoints in the 70s.  With temperatures possibly rising
another degree or two this afternoon, heat index reading should top
out close to 105F for much of the CWA.  Thus, the Excessive Heat
Warning will remain in effect for cook County and the Heat Advisory
will remain in effect for the remainder of the CWA until early this
evening.

The next concern will be timing of a cold front pushing south
through wisconsin and Lake Michigan and timing of thunderstorms
associated with the boundary.  Scattered thunderstorms have already
begun to develop south of I-80 in a zone of strong instability, but
weak forcing along a modest convergence boundary/trough axis.  These
showers/storms have the potential to produce lightning, brief heavy
downpours or a strong wind gust, but with very quick ewd
progression, these storms do not pose a severe threat.

Of greater concern will be thunderstorms associated with the frontal
boundary pushing south through Wisconsin and Lake Michigan.  While
winds shear is rather weak, sfc instability will be be quite strong,
with SB Cape up to 4000 J/kg across northern IL ahead of the front.
Latest radar trends and high res model guidance suggest that the
bulk of the convective activity will be along and post-frontal.  Obs
and guidance also indicate high levels of moisture pooling invof the
front with pwats in excess of 2 inches.  Heavy downpours will be
likely with these storms.  The biggest question at this point is
whether there will be much in the way of echo training with the
storms.  850-700mb flow is just south of westerly, so if the
southward progression of the front is slow enough, there could be
some echo training of storms producing heavy rainfall.  Thus, there
is a flood or flash flood threat durg the period or max convective
activity late this afternoon into early evening.  Isolated strong
wind gusts will also be a possibility with some storms.  As the
front pushes south, temperatures will drop off rapidly, particularly
along the nern IL lakefront.  It is quite possible that temperatures
along the lake front could drop from the low 90s to arnd 70F in a
very short amount of time.  While the greatest chance for
thunderstorms will be with the cold front as it pushes south, there
will be a lingering chance for some showers or isolated
thunderstorms overnight.  Latest guidance suggests that the front
could stall out over the southern portions of the CWA, so temps
there may only drop into the low 70s.  Locations north of the front
should drop into the lower to middle 60s.

&&

.LONG TERM...
254 PM CDT

Tuesday night through Monday...

Overview...Rainy and stormy conditions continue through Wednesday.
Most of the area will be dry Thursday, but the far southern portion
of the forecast may receive additional precipitation Thursday.  The
next low brings more rain with it Thursday night through Saturday
morning. Temperatures will be much cooler especially along the lake.

The stalled front remains over the southern portion of the forecast
area, and showers and storms are still expected across much of the
area Tuesday evening and night.  The front shifts south into central
IL Wednesday, and precip shifts south through the day with it.  The
boundary may stay just north enough to produce showers and storms
south of a Pontiac, IL to Rensselaer, IN line Wednesday night into
Thursday. My confidence is low though in precip chances Wednesday
night.

The next low reaches the region Thursday afternoon/evening bringing
more widespread rain and storms. Rain comes to an end Saturday
morning as the low reaches Michigan.  Guidance differs on how strong
a couple of upper level waves will be early next week, so I kept
precip chances in the chance or lower range. Monday looks dry.

Temperatures will be cooler throughout the extended especially along
the lake. Prolonged onshore flow through Friday will lead to high
temps in the upper 60s to around 70 along the lake. Outlying areas
will be in the 70s to 80s. Widespread 80s and higher dewpoints
return early next week away from the lake.

JEE

&&

.CLIMATE...

Here are the standing records for Chicago and Rockford:

                   Chicago      Rockford

Mon Jun 18  High  98 (1954)    99 (1994)
            Low   77 (1921)    74 (1905)

&&

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

Forecast concerns remain thunderstorm timing and cold front/wind
shift timing late this afternoon/early this evening.

Southwest winds 10-15kts with gusts to 20kts will continue through
this afternoon while gradually veering to more westerly.

There is a small chance for an isolated shower or thunderstorm
this afternoon, but much of the activity is expected to remain to
the south of the terminals.

A cold front is still expected to sag south across the terminals
by early this evening with winds shifting to the northeasterly.
This cold front may be pushed south faster by thunderstorms across
southern WI today, faster than guidance is currently showing
should thunderstorm activity and outflows become more widespread.
Have pushed up timing slightly, by an hour in difference to
slightly faster timing from the latest guidance and the ongoing
coverage of TS over srn WI ongoing at 18Z, which is a little
farther south than previously anticipated.

The greatest coverage for thunderstorms should be post-surface
frontal aligning more with the 850mb boundary in the stronger zone
of isentropic lift north of the surface boundary. Thunderstorm
coverage should on a dissipating trend by late evening with some
lingering showers or perhaps an isolated thunderstorm overnight.
Any stronger storms that develop will likely produce torrential
rainfall with ifr/lifr cigs/vis. Erratic/gusty winds may also
occur from outflow boundaries.

Guidance has been trending cigs into mvfr overnight and down to
ifr levels Tuesday morning in the cooler air moving inland from
Lake Michigan.

&&

.MARINE...
254 PM CDT

A cold front is moving down the lake, and winds are north 10-20 kt
behind the front. This front will reach the south end of the lake
this evening, and it will become stationary across the southern
Great Lakes and Ohio Valley through the end of the week. Winds will
generally be east to northeast through the end of the week. Periods
of thunderstorms will be possible through tonight...as they drift
south with the front. Higher wind speeds are possible from outflow
boundaries and storms.

The next low will reach the lake late this week, but will quickly
reach southern Quebec early Sunday morning.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Excessive Heat Warning...ILZ014 until 7 PM Monday.

     Heat Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-ILZ032-
     ILZ033-ILZ039 until 7 PM Monday.

IN...Heat Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 until 7 PM
     Monday.

LM...None.
&&

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
605 PM CDT MON JUN 18 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0600 PM     HEAVY RAIN       GURNEE                  42.38N 87.94W
06/18/2018  M1.18 INCH       LAKE               IL   TRAINED SPOTTER

            SINCE 455 PM

&&

IZZI