Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200729

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

1010 PM CDT

The convective line continues to very slight weaken as it heads
into northwest Indiana. Gusts of 45-55 mph were observed across
Will County including here at the WFO (53 mph). Base velocity
values on the KLOT radar do indicate some pockets near 60 mph,
however that has been as the storms have been moving away and the
radar vantage point is more elevated. The environment indicates a
little more MLCIN into far northwest Indiana, however given the
very moist low-levels, it likely will continue having 40+ mph
winds across Kankakee County (44 mph at IKK recently) and
northwest Indiana. Even further south toward Ford and Iroquois
Counties, outflow ahead of the storms looks to be producing 35-45
mph gusts.

Further north, underneath the MCV/broad bookend vortex, there have
been 40-50 mph non-thunderstorm gusts and sharp pressure rises in
far southern Wisconsin. While this MCV will drop south-southeast
into northern Illinois, it is uncertain on how far southeast these
winds will advance. The long-lived nature of this complex would
point toward at least some inroads of these winds into far
northern Illinois. It will be something to watch, especially in
areas that sustained wind damage from the earlier storms.



319 PM CDT

Through Thursday...

For the remainder of the afternoon into early evening, the main
concern will be isolated shra/tsra developing ahead of a lake breeze
pushing inland.  Latest vis satellite shows the lake shadow of clear
air moving inland with sct cu/tcu/shra/tsra developing in the
undisturbed warm/moist/unstable environment.  Without a significant
organizing trigger, other than the lake breeze boundary itself,
activity should remain isold/widely scattered.  There should be no
severe threat from any tstms that develop, there will be the
potential for locally heavy rainfall as pwats are around 2 inches
over the area.  This activity should largely be diurnal and
dissipate with sunset.  For the overnight hours, thunder concerns
will shift to a developing MCS moving through srn MN this afternoon.
The models have been all over the place with this system, both the
CAMs and the larger scale models.  The latest GFS seems to have the
better handle on this system and the short term wx/pop/qpf forecast
has trended closer to this solution than the NAM, especially, which
has been a more northern track outlier.  The general trend is for
the MCS to track along an instability corridor, through nern IA/swrn
WI and into nwrn IL by arnd midnight.  Timing is still a bit
uncertain and will depend on how the system develops upscale, but
latest thinking is that any thunder will hold off until arnd
midnight for the Rockford area and then spread southeast into the
Chicago metro area drug the early morning hours.  There has been
some suggestion in the guidance that some secondary development may
occur in a warm advection zone following the passage of the main
MCS.  This could allow some ts to linger into the morning hours
tomorrow.  There will be some severe threat with the main MCS, with
the primary threat being strong damaging wind.  Locally heavy
rainfall will also be likely, which could aggravate the ongoing
flooding issues on the Des Plaines and Fox Rivers.  Expect that the
system will be relatively progressive, so any heavy rainfall should
be transitory and not sit in one location.

For Thursday, increasing heat and humidity will become a concern in
addition to renewed thunderstorm chances in the afternoon.  The
trend for tomorrow and into the extended forecast period will be for
unsettles weather and periods of thunderstorms with periods of
locally heavy rainfall possible.  As for heat and humidity, expect
that temperatures across much of the area should reach into the low
90s with dewpoints in the low to middle 70s.  The highest temps and
dewpoints should be confined to locations south of the I-80
corridor.  Heat Index values may reach 105-107 F tomorrow south of I-
80 and a heat advisory may be required.  have trended a bit higher
than guidance on the temps and dewpoints, given the general synoptic
pattern, so will hold off on issuing a heat advisory at this time and
see how conditions evolve.  Thunderstorm chances should be low
tomorrow with mid level warm advection keeping some capping, but
will maintain a corridor of low chance to slight chance pops for
tsra along the main corridor of sfc instability.


306 PM CDT

Thursday night through Wednesday...

Biggest concern in the extended is convection that may lead to heavy
rain Friday and Friday night. Chances of storms continue through the
weekend before a dry period sets up for the first half of next week.

Expecting daytime convection to continue into Thursday evening/night
ahead of the cold front.  The cold front will stall over the far
southern portion of the forecast area.  Additional convection is
expected Friday. There will be two areas of focus: the stationary
front south of I-80 and an upper level disturbance shifting
northeast from Iowa into southern WI.  The GFS suggests PWAT values
will be 2-2.25 inches, which is well above climatology and near the
upper end of daily maxima for mid to late July.  Mid level flow is
around 40 MPH so while storms should be moving at a decent pace,
heavy downpours are likely.

The GFS appears to be capturing the current situation the best so
weighed the forecast in its direction. While thunderstorms are
possible across the forecast area Friday afternoon, think the best
chance will be south of a Rockford to Valparaiso line. However,
convection will spread to include all of the forecast area in the
evening, and an organized convective complex will likely shift
southeast through the region Friday night. This complex will lead to
additional heavy rain that parts of the area do not need.  Will
continue to highlight our thoughts about heavy rain in an ESF and
the hydrology section below.

Flow off the lake will likely keep temps along the lake in the 80s
while the rest of the region will be in the mid 80s to around 90.
Dewpoints in the 70s will lead to heat indices from 90-100 north
of I-80 to 100-106 south of I-80.

I have medium confidence in Saturday`s forecast as the convective
complex should still be overhead Saturday morning. Convection should
work over the atmosphere, and I am unsure how quickly we will be
able to recover Saturday afternoon. If we can recover, thinking the
best chance for additional storms will be along and east of I-55.
The hot, sticky conditions continue Saturday with highs in the mid
80s to the low 90s and heat indices in the mid 90s to up to 106

Cooler and drier air arrives Sunday with highs in the 80s. High
pressure slowly shifts east and reaches Lake Michigan Tuesday
afternoon. I have dry conditions in the forecast through at least
Tuesday night if not into Wednesday. Models are slowing the next
system mid week so Wednesday may be drier than currently forecast.




The Hydrologic Outlook (ESF) for along and north of I-88 has been
continued due to concerns of multiple rounds of storms through
early this weekend. Observational trends are supporting the
forecast in terms of a very moist atmosphere over the area from
now through Saturday, with pwats in excess of 2 inches. The
confidence on receiving a burst of heavy rain with a quick-moving
storm complex in far northern Illinois late tonight has increased,
but some uncertainty still resides in exact track of this system.
Where the center of it passes certainly will have potential for
1"-2" in around 1-2 hours.Typically, with such quick moving
storms, even if intense, the tendency is to produce widespread
averages more of 0.50"-1.00". If there were to be some slower
movement or backbulding, concerns would raise for renewed

Wet antecedent conditions particularly over the Des Plaines and
Fox River basins provide concerns through the rest of the week as
storm chances increase Thursday night and into Friday. Location
of this rainfall is even more uncertain, however there may be
higher concern of additional flooding given it is a regime that
could favor training storms and very efficient rain-producing



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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