Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211755

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1255 PM CDT Fri Jul 21 2017

.UPDATE...1030 AM CDT...

Working on updates to the pop/qpf forecast for early this evening
through the overnight hours, and will be significantly increasing
pops to account for high likelihood of upstream MCS development
this afternoon to move east southeast into northern Illinois by
early/mid evening. Concerns are increasing for a damaging wind
threat across northern Illinois, and then likely northwest Indiana
later in the evening. This initial complex of storms will also be
capable of torrential rainfall. Given potential setup late this
evening and overnight, additional heavy rainfall and flooding
remains a concern especially across the Flash Flood Watch



402 AM CDT

Today through Tonight...

Main forecast concern is with timing rounds of thunderstorms
across the forecast area, and with the potential for severe
weather and additional heavy rainfall especially later tonight.

Early morning surface analysis places a nearly stationary frontal
boundary from roughly the IA/MO border eastward across central IL
and IN. Southwesterly low level jet around 25 kts has driven a
couple of small, slightly elevated MCS`s along the cool side of
the boundary early this morning, primarily affecting the southern
half of the cwa. Expect these thunderstorms to continue to track
to the east-southeast across locations mainly south of a Dixon-
Joliet and Rensselaer line early this morning. While locally heavy
rainfall has occurred with some of these storms (2.00-2.50 inches
per 88D estimation), the storms have been fairly progressive and
this area has seen some of the least rainfall in the cwa over the
past few days, thus no significant precip/flood problems are
anticipated there this morning.

Attention then turns back to our northwest with a mid-level short
wave tracking across the Plains, and associated surface low
pressure which is progged to develop into the Upper Mississippi
Valley by this evening. As this occurs, southeast and eventually
more south surface winds will allow the stationary front to our
south to lift north as a warm front, with 00Z model runs trending
a bit farther north with the boundary into far southern WI by
later tonight. Very warm, moist air mass (temps in 90`s, dew
points in 70`s to near 80) will support strong destabilization
this afternoon once morning MCS debris exits, setting the stage
for scattered thunderstorm development along the northward
shifting warm front. Primary focus for organized strong storms is
expected to by across MN/western WI by mid-late afternoon, with
eventual upscale growth into an MCS expected tonight.

As previously noted, many of the synoptic scale models have
trended farther north with the warm front and also with the
heavier QPF axis with strong storms into tonight. Some of the
high-res guidance continues to bring storms and heavy rainfall
potential across far northern IL overnight, and this appears
reasonable given the effect of lingering clouds/boundaries from
convection early in the day, and low level instability which is
expected to be across the region into tonight. Thus have shifted
qpf axis a bit northward (in agreement with WPC and surrounding
offices), but still have 1-1.50 inch coverage across northern tier
of IL counties. Expect timing of greatest heavy rain threat to be
after midnight, as MCS slowly sags southeast and low level jet
veers more westerly resulting in slower propagation and the
potential for training as Corfidi vectors indicate.

Have made no changes to flash flood watch at this point, though
if models continue to trend in similar fashion could see removal
of southern tier of FFA counties allowed. SPC has enhanced day 1
severe risk just upstream of IL cwa (actually gets into far NW),
with wind- producing bowing segments possible as late day/tonight
MCS moves into the area.

With warm front lifting north-northeastward across the area
today, very warm and humid conditions are expected. If not for
convective cloud debris this morning and residual boundaries/cool
pool from morning storms, would expect to see a few mid-90`s
potentially in the far south/west counties. Have generally kept a
bit of a conservative approach given these limiting factors, as
well as a light southeast wind off the lake farther northeast.
Current forecast touches 105 degree heat index in a couple spots
briefly in warmest part of afternoon, though short duration and
uncertain spatial coverage does not warrant heat headline at this



402 AM CDT

Saturday through Thursday...

Saturday afternoon heat and humidity will again push apparent
temperatures toward or over heat advisory criteria with the support
of southwest flow ahead of an approaching frontal zone.  Model QPF
fields and MSLP trends reflect disagreement in the timing and low
level forcing related to the passage of the front, perhaps due in
part to the potential stabilizing effects from widespread convective
activity expected tonight and in the predawn and morning hours on
Saturday. Still feel that, at the least, isolated to scattered
activity is possible areawide for the Saturday afternoon period,
but toward evening and overnight the focus shifts mostly south of
the Chicago metro area.

A considerable pattern change is then in store, with Sunday looking
like a transition day toward drier weather with cooler temperatures
and lower dewpoints as an upper trough digs over the area and a
broad surface ridge approaches the Great Lakes from the northwest.
Sunday appears to be just a bit cooler and drier than Saturday,
but the change will be more evident on Monday and Tuesday when
some locations do not escape the 70s. By mid to late week the
ridge likely will move far enough east to allow more typical
warmth and moisture to start streaming back into the area, but at
this range we do not appear positioned for a late-week repeat of
the recent very warm heat and high humidity.



For the 18Z TAFs...

Primary concerns center around the timing of thunderstorms at the
terminals tonight.

A few showers may continue eastward this afternoon, but at this
point we do generally feel these will remain in a tempered state
with some capping in pace.

Model spread is still high enough to retain tempered confidence,
likely medium as this point, regarding specific timing on thunder
impacts at the terminals, but confidence has increased regarding an
earlier arrival time for the leading edge of storms this evening and
for additional development overnight.

Expect a leading storm complex to move into area this evening, and
these storms will likely pose a concern for gusty northwest winds.
Concerns then shift to what appears to be a a favorable setup for
backbulding storms during the overnight hours. While this is
expected to happen, whether this is over the terminals or a bit
south is still in the air, but our feeling overnight is that there
will likely be some very heavy rain producing storms. The
boundary does appear to linger over our terminals on a NW to SE
axis from RFD through Chicago, so the best course of action is
continue the thunder mention into the overnight hours across the
terminals. Confidence on how quickly storms shift south of the
terminals is also low, and tweaks may be needed for this
additional development.

Confidence is pretty high that after 12z things will shift south of
the terminals and the impacts will dwindle. At this point confidence
on wind direction diminishes, so future adjustments may be
needed. Additional development is possible tomorrow afternoon,
but the bulk of guidance is drier than previous forecast will
likely hold dry.



1251 PM CDT

Strong to severe thunderstorms are expected over the southern
third of the lake this evening, with gusts in excess of 30 kt
certainly plausible, along with heavy rain.

Otherwise, Thermal circulations exist this afternoon across
nearshore areas with a broad southeast wind regime in place. A low
pressure trough will cross the lake tonight with the main low
center not expected to cross the lake until later Saturday which
will maintain the east wind component until the low moves south of
the lake Saturday night.

Once the associated low shifts east, the flow around to the north
where it will remain and gradually increase into early next week, to
the point that small craft advisories may be needed along the
southern shores on Monday with a full northerly fetch wind. The
gradient then appears to relax again on Tuesday through mid to late



IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     PM Friday to 9 AM Saturday.




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