Area Forecast Discussion
Issued by NWS Chicago, IL

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857
FXUS63 KLOT 211541
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1041 AM CDT Fri Jul 21 2017

.UPDATE...
1034 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
locations.

Rodriguez

&&

.SHORT TERM...
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
point.

Ratzer

&&

.LONG TERM...
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.

Eric

&&

.AVIATION...
For the 12Z TAFs...

Main aviation concerns continue to focus on timing periods of
greater thunderstorm potential across the terminals through early
Saturday.

A series of thunderstorm clusters have slipped past the terminals
to the south overnight, and will continue to do so early this
morning. These storms were occurring north of a stationary front
which stretches from the IA/MO border through central IL/IN. While
a few isolated showers have been noted as far north as KRFD...no
widespread SHRA/TSRA are expected at the terminals through early
afternoon. The front to our south will lift north as a warm front
this afternoon and this evening, with the potential for isolated
to scattered thunderstorm development later this afternoon near
the frontal boundary. A greater potential for storms appears to
come late tonight however, as strong to severe storms develop
northwest of the area and eventually propagate southeast into the
region after midnight. Much of the available guidance focuses this
activity across MN/WI...though many of the high-res models do
eventually sag activity into the terminals toward morning.

Winds are generally light/variable early this morning...but will
become southeast less than 10 kts later this morning...and more
south by tonight as the warm front lifts north. Low pressure
moving east across WI early Saturday will allow a shift to west-
southwest winds Saturday morning.

Ratzer

&&

.MARINE...
408 AM CDT

Light and somewhat variable flow early today will remain light but
generally evolve to south or southeasterly flow by the afternoon.
With such a weak gradient in place it is likely that thermal
effects will also support the development of lake breezes in many
locations. South to southeasterly flow continues Saturday ahead of
an approaching low pressure center and frontal zone. The frontal
passage late Saturday flips 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. The gradient then appears to relax again on
Tuesday through mid to late week.

Lenning

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022...4
     PM Friday to 9 AM Saturday.

IN...None.
LM...None.
&&

$$

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