Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200818 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
318 AM CDT Thu Jul 20 2017

318 AM CDT

Convective clusters, possibly evolving into a loosely-defined MCS
by daybreak, will continue to inch southeastward into northern
Illinois early this morning. There has been evidence of small cold
pools assisting bulging clusters southward, where wind gusts of
around 40-45 mph have been observed in southwest Wisconsin. The
biggest concern will be flooding from heavy rainfall, as several
sites have had 0.75"+ in an hour in southwest Wisconsin, including
Mineral Point, WI which recently had 1.29" in an hour. With wet
antecedent conditions, especially north of I-88, the threshold for
flash flooding is lowered. In addition, this will be moving over
parts of the CWA, including some of the Chicago metro during the
morning rush hour, potentially causing bigger issues if that

Overall confidence is low-medium on if this will bring a flash
flooding threat, as the instability may not have advanced too far
into our area to keep big time rain rates up, at least into
northeast Illinois. There is a marginal severe weather threat,
especially in north central Illinois where the instability axis
resides. Will likely be headed for SPSs to mention winds of 40+
mph, cannot rule out some brief further intensification of any



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.



305 AM CDT

Friday through Sunday...

Focus then shifts to what appears to be an active Friday and
Friday night, as multiple more pronounced short wave disturbances
are forecast to reside in the westerlies. The frontal zone will
continue to be the primary focus mechanism for storms, and
confidence is increasing that this should reside in the northern
CWA by midday Friday. Uncertainty is still high on whether that
keeps the heaviest rain just north of the area or actually coming
across it, as waves of convection would likely modulate things a
little southward more so into our area. Concerns center on heavy
rain with storm movement parallel to the boundary and low speeds
of forecast backbuilding vectors. In addition, model QPF bulls
eyes -- while not perfectly placed -- do correlate to the signal
we are seeing. Have gone ahead and mentioned heavy rain in the
forecast for north of I-80 Friday night. In addition, stronger
shear will support organized storms at times in the region, with
deeper cores capable of hail and any bowing segments bringing a
wind threat. Supercell structures would seem plausible Friday
evening along this frontal zone given degree of 850-500mb
crossover and hodographs, however it is just challenging to say
where specific convective threats will shape up given how much
each round of convection affects conditions for the next.

Like a broken record, temperatures will again depend on convective
influences. The southern half of the area runs the potential of
reaching Heat Advisory criteria again. Dew points should reside
in the mid-upper 70s areawide giving a very muggy feel.

Saturday will be more of the same, though the forcing mechanism
may be more so the approaching cold front, as low pressure is
expected to be moving eastward over or just north of the area.
Strong, heavy-rain producing storms will again be possible, though
the emphasis especially by Saturday night, may begin to shift more
so into the southern CWA. Dew points ahead of the boundary may
reach their peak on Saturday, so again will have to watch for
heavy advisory. Offshore flow looks to possibly be the strongest
this day, offering downtown Chicago the warmest day in the
stretch, if not impacted by convection.

By Sunday, the front is forecast to move through, with northwest
winds gradually changing air masses to start next week.


Monday through Thursday...

A large area of high pressure will be building across the Great
Lakes region Monday and is expected to slowly build south and
east through the middle of next week. This is now slower then
previous forecasts with dry weather expected through at least
Wednesday morning. The Gfs continues to show a cold front moving
south across the region Wednesday night into Thursday. The Ecmwf
has slowed this front and the Canadian is generally dry through
the end of the week keeping the high over the southern lakes
region. So confidence regarding precip chances during the middle
of next week is low...but maintained chance pops for now. This
pattern would suggest generally seasonal temps with lower
humidity...along with lake breezes. Though some increase in
dewpoints is possible as flow eventually turns more southerly on
the back side of the high. cms


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 06Z TAFs...

1236 am...Main forecast concern is the potential for two rounds
of thunderstorms through this evening.

Focus now shifts to thunderstorms developing/occurring across
western WI. Short term/hi res guidance is in good agreement with
this activity moving southeast into northern IL around daybreak.
There remains some differences as to where the strongest storms
may track...which could be west of the Chicago area terminals but
given the current trends...have maintained tempo mention with
minor tweaks to timing. Expect to be able to refine this more
with the 09z updates. There will likely be an outflow boundary
with this activity shifting winds northerly with or just before
the storms. Duration of this next round of thunderstorms may be a
bit longer than the activity Wednesday evening and as a result
there may be a period of ifr cigs/vis. These thunderstorms are
expected to weaken as they move south of the terminals by mid

Second potential round of thunderstorms would be mid/late this
afternoon and while did include tempo mention...confidence on both
timing and coverage is only medium. A weak frontal boundary will
be moving into northern IL and this should allow for at least
scattered thunderstorms. At the same time winds will be shifting
northeast off the lake...likely aiding additional development...
which will slowly move to the south southwest away from the
terminals by late afternoon. Beyond this time period...confidence
for additional thunderstorms is too low to mention. However...
there is some possibility of thunderstorms developing early Friday
morning especially across northwest IL.

Winds will likely settle to light southeasterly early this morning
and turn more south/southwest toward sunrise. There may be a brief
increase in southeast wind speeds through the next hour in the
wake of the departing complex. A wind shift with the first round
of convection to northerly is possible which may create chaotic
wind directions thru mid morning. Forecast soundings suggest a
period of strong westerly or southwesterly winds mid/late morning.
Confidence is low due to uncertainty of departing thunderstorms
but added this to the tafs. Winds will likely remain west/
northwest until second round of thunderstorms and the associated
outflow/wind shift.

There is also the potential for fog late tonight into Friday
morning. confidence is too low to mention but with multiple rounds
of rain and light winds...some fog is possible. cms



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





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