Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201227 AAA

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

727 AM CDT

The final line of storms is moving through the area, with
occasional wind surges seen in velocity data or inferred in
reflectivity data with sharp front edge gradients and back edge
drying. Suspect gusts of around 50 mph are most probable in these
surges, most noteworthy at the time of this writing in Kendall and
LaSalle Counties.

Rainfall has been very efficient within this tropical-like air
mass and remains the primary concern. While the movement of these
storms has not been fairly quick, especially this back line, the
rates are heavy enough (1" in 30-60 minutes) to cause at least
localized flooding issues. This is especially true in urban areas.

This MCS should continue southeast and through the forecast area
by mid-morning, with some spotty storm development remaining
possible in its wake.



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 12Z TAFs...

646 am...Main forecast concern and challenge is thunderstorms.

Back edge of the thunderstorms for this morning extends from ugn
to sqi and should clear ord by 13z and mdw by 14z. There may be
some festering/lingering showers through midday but confidence is
low. Line of thunderstorms approaching will produce brief
torrential rain with vis possibly under 1sm.

Confidence for afternoon convection has decreased some in the
past 6 part because of how widespread and further
southwest this current activity has become. Models still show
scattered thunderstorm development mid/late afternoon across the
terminals but timing has slowed. More importantly...location of
new activity is uncertain and it could develop southwest of the
terminals. Given the lower confidence have opted to only extend
the tempo by an hour to account for possible later timing and
later shifts can adjust accordingly based on trends this
afternoon. Thunderstorm chances tonight will depend on afternoon
convection...which if it develops as expected would likely move
south/southwest with mainly dry conditions tonight. If afternoon
convection doesn/t materialize...the low level jet will likely
allow development overnight into Friday morning. And this may
still take place anyway...but confidence is too low to include
this potential in the tafs except at rfd.

Winds will be erratic for the next few hours. Outside of a strong
northwest wind with outflow/storms...there will likely be a period
of southeasterly winds which should slowly turn south/southwest
later this morning and more westerly by early afternoon. Higher
wind speeds/gusts from previous forecast are not expected to
materialize given the impact of these storms on the wind field. A
shift to the northeast is still expected mid/late this afternoon
but lower confidence on timing and this shift may also be impacted
by thunderstorms. Winds are expected to become light southeasterly
later this evening.

Final concern later tonight into Friday morning is fog which is
dependent on cloud cover and any thunderstorms. With light
winds...moist low levels and a dissipating boundary/front in the
area...its possible fog may develop in the usual areas and perhaps
become locally dense. Confidence too low to include with this
forecast. 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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