Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211320
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
820 AM CDT WED SEP 21 2016

.UPDATE...
810 AM CDT

Still monitoring a line of thunderstorms which is steadily pushing
southeast through northern Illinois at this time. Although this
line has remained sub-severe, gusty winds and small hail have
been observed. This will likely continue to be the case over the
next hour as it continues to drop southeast into areas in east
central Illinois and northwest Indiana. Do think the southwest
edge will be an area to monitor for more robust development, where
highest instability and dewpoint air resides. However, it should
stay sub-severe.

Monitoring upstream development across southern Wisconsin which
is also dropping southeast towards northern Illinois. Has a decent
probability of continuing southeast, but confidence is low with
the overall evolution and intensity of this development. Very
strong instability/steep mid level lapse rates overhead is a
little concerning though. Will provide an update on these next
thunderstorms soon.

Rodriguez

&&

.SHORT TERM...
333 AM CDT

Through tonight...

Primary challenge in the short term is with convective chances.

A well anticipated MCS has blossomed over MN and central WI early
this morning. Mean flow would suggest a mainly easterly movement
across central and southern WI, however, orientation of the
instability axis and forward propagating Corfidi vectors suggest
that a more southeasterly motion will occur. This would
potentially place northern IL in the line of fire later this
morning. SPC objective mesoanalysis data suggests that fairly
robust inhibition exists between us and this line, though given
the mesoscale organization that this system is exhibiting am
thinking that there is a pretty good shot that some of the showers
and storms will make it into far northern IL. Have beefed up pops
in anticipation of this occurring this morning, especially north
of I-88 and I-290, but fully expect additional revisions to pops
will be needed.

Questions then become how far south into the area does this
activity make it and how quickly cloudiness clears out in its
wake. Have introduced more of a temp gradient from north to south
for highs today, given potential for more clouds lingering north.
Suspect that this activity should dissipate by mid to late morning
at latest, possibly sooner, but could leave outflow boundary which
would have at least a chance of sparking additional isolated
storms this afternoon. Confidence is low, but did maintain some
low pops north into the afternoon due to this threat.

Tonight, it appears the focus for convection will be north of the
WI border and we should be dry. The activity tonight appears less
likely to propagate south into our area, so Thursday stands a
better chance at being dry and very warm CWA-wide.

Izzi

&&

.LONG TERM...
333 AM CDT

Thursday through Tuesday...

Thursday night into Friday will feature an amplifying trough over
Quebec into New England, which should force the quasi-stationary
front southward along with the focus for convection. Still
appears the majority of the showers/storms Thursday night will
stay north of the WI border, but there looks to be a better chance
of this activity spilling south of the border Thurs night than
tonight.

Front will increasingly become oriented NW to SE, probably
augmented some by the lake Friday. Lake Michigan remains
remarkably warm, in fact, the water temps Tues evening were the
warmest on record for so late in the season at the south mid-lake
buoy. The result will be only modest cooling behind the front and
probably not much forcing for precip given the drier easterly low
level flow and only weak/shallow frontal forcing in the presence
of large scale subsidence with the building upper ridge. Same
story on Saturday, with temps remaining above average and many
areas again climbing into the 80s, expect the immediate lakefront
where highs will hold in the 70s.

Upper level pattern this weekend into next week over NOAM is
forecast to remain very blocky with high amplitude ridging in our
area. Deep trough over the western CONUS will struggle to progress
east and rather be shunted more northward by the ridge, likely
resulting in a dry and seasonably warm weekend. Maintained the
chance pops from the blended model initialization for Sunday, but
suspect that Sunday will end up dry as well as medium range models
typically are too progressive with eastward progress of systems in
highly blocked flows like this one.

Temps could cool closer to climo early next week, but this is
contingent on the ridge actually getting shoved east and given the
run-to-run variability in the medium range guidance and the
tendency for these models to struggle in patterns such as this,
confidence is low in the forecast for early next week.

Izzi

&&

.CLIMATE...

Record highs and warm lows:

              Chicago            Rockford
           High      Low      High      Low
Wed 9/21  92(1970) 76(1931)  94(1920) 70(1895)
Thu 9/22  92(1956) 73(1895)  93(1937) 65(1959)
Fri 9/23  91(1937) 71(1891)  92(1937) 66(1920)

&&

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

Primary concern is the line of thunderstorms moving into
north central IL and how long the line will maintain it`s
integrity as it tracks to the southeast.

At 1120Z, TS are moving through the RFD terminal area and should
reach ORD/DPA by arnd 12Z. Expect a relatively short period of
steady TS, but will maintain the 2-hour TEMPO timing as there may
still be some isold/embedded TS within the trailing pcpn area.
Expect a quick windshift to nwly with gusts of 25-30 kt. Again,
the period of gustiness should be relatively brief, largely
associated with an outflow boundary just ahead of the leading edge
of the TS as well as the leading edge of the TS itself. Latest
radar imagery indicates a fairly extensive area of trailing pcpn,
the the general movement of the trailing pcpn is to the east while
the movement of the convective line is to the southeast, so, do
not expect an extended period of prevailing pcpn following the
passage of the TS line. While the swrn portion of the line is
showing signs of weakening, expect that it will hold together long
enough to bring TS to GYY as well.

For the remainder of the day, a warm front extending across the
upper midwest is expected to become a bit more well defined, invof
the IL/WI border. Old outflow boundaries invof the warm front
could initiate further activity durg the afternoon hours. Timing,
coverage and location is highly uncertain at this time, but the
environment will be supportive of additional tsra/shra. Given
increasing uncertainty and lack of any consistently from the
short term, high res guidance, will back off to a dry forecast for
the late afternoon hours.

The wind forecast through much of the period after 12Z remains of
very low confidence as the environment may become disrupted by
both tsra and any associated outflow boundaries.

&&

.MARINE...
315 AM CDT

High pressure is centered over the eastern Great Lakes with a
ridge axis extending southwest into the lower Mississippi Valley.
Low pressure is developing over the western plains with a trough
and associated warm front extending east across the upper midwest
to central lake michigan. The surface pressure pattern will
change little into Thursday night, with The exception of a large
area of high pressure spreading across southern Canada. The exact
location of the frontal boundary should meander a bit north or
south over the next few days, but in general, should remain across
the middle of the lake with winds having a sly-sely component. By
Thursday night and into Friday, the Canadian high will strengthen
and spread south across the upper Great Lakes, forcing the front
to push south as a cold front. The cold front will push south of
the Great Lakes Friday afternoon turning winds to nly-nely for a
short period before settling into a more extended period of
generally ely winds as the high remains over the ern Great Lakes
and a new low develops over the western plains.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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