Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 140820

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
320 AM CDT Mon Aug 14 2017

319 AM CDT

Through tonight...

The main concerns during the period will be the potential for
a period of showers and thunderstorms over northern Illinois this
evening and into the overnight hours.

Currently, a line of showers and thunderstorms are shifting
eastward across north central Iowa. This activity is expected to
weaken over the next few hours as it moves into a progressively
more stable airmass. So it is unlikely that this activity would
poss much of risk for thunderstorms over north central IL this
morning. However, a stray remnant light shower cannot be totally
ruled early this morning. Otherwise, it appears most, if not all
of the day will be dry across the region.

Winds are expected to become south-southwesterly during the day
as a weak surface frontal trough of low pressure sets up over Iowa
and into southeastern Wisconsin this afternoon. This will allow a
warmer and more moist airmass to begin shifting back over the
area. As a result, expect temperatures today to warm a few degrees
over Sundays highs, which should put most areas into the low 80s
this afternoon. Slightly cooler conditions can be expected near
Lake Michigan in northeastern IL, however, as a lake breeze
develops this afternoon. It appears unlikely that this lake breeze
will move inland as far as yesterdays lake breeze, due to the
southwesterly winds, so any cooling would be confined to the
lakeshore areas.

New scattered thunderstorms are likely to develop in the vicinity
of the surface frontal trough to our west later this afternoon.
This activity should be aided by the approach of a decent
mid/upper disturbance currently over the Upper Midwest. These
thunderstorms then look to shift eastward across far northern IL
(north of I80) through the evening as the surface frontal trough
begins to approach the area. There is some uncertainty on the
exact timing of these storms, but it does appear that far northern
Illinois will have the best potential this evening. Overall this
does not look like a big severe weather producer, but given the
amount of downdraft CAPE, some of these storms could produce some
gusty winds.

The surface frontal trough is expected to gradually sag southward
into northern Illinois later tonight. Some showers and storms may
continue to fester along this boundary overnight, but overall
weakening instability and forcing for ascent should result in much
less storm coverage in Illinois overnight.

Some fog may also try to develop over portions of north central
Illinois late tonight as the surface flow weakens in the vicinity
of the surface frontal trough. This combined with ambient low-
level moisture could result in some fog into early Tuesday



226 PM CDT

Monday night through Sunday...

The surface ridge that has been parked overhead and providing
very mild and dry early-mid August weather will continue moving
east and allow return flow to begin in earnest Monday night.
Increasing low level moisture through the week will support
increased chances of showers and thunderstorms at least until late
Thursday when the passage of a cold front again dries things out
and allows ridging to develop for Friday and perhaps early

Models of course disagree on the timing and intensity of the
various upper shortwaves that would focus support for
precipitation. The best chance would appear to be on Wednesday
afternoon and evening when a more pronounced shortwave aloft
interacts with ample warm and moist advection at low levels. Hints
of a warm front in model guidance would further focus low level
moisture convergence early Wednesday afternoon, and the
approaching cold front would be favorable late Wednesday. For
Monday night into Tuesday night, and again Wednesday night into
Thursday, chances are lower given overall weaker forcing but
certainly worth continuing a mention of in the forecast. If the
cold front slows down then POPs may need to be raised for

It is not clear at this time range whether the entire upcoming
weekend would be dry, but the aforementioned ridging for Friday
and early Saturday would at least provide a start. It does appear
the longevity of the ridge would be less than what we are
presently experiencing.



For the 06Z TAFs...

Light and variable winds will turn south-southwesterly during the
day. However, the winds should be light enough to allow another
lake breeze to develop over portions of northeastern Illinois by
early afternoon. In spite of this, it appears that it will
struggle to move inland over cook county. As a result, it may
remain east of the terminal sites for much, if not all of the
day. Even if the lake breeze was to make it to ORD or MDW late in
the day, wind speeds in its wake would likely remain below 10 kt.
For these reasons, I have opted to keep the wind shift out of the
latest TAFs.

While the weather will remain quiet across the area through the
day, there will be a potential for a few showers and thunderstorms
over portions of northern IL and southern Wisconsin later this
evening and into the overnight hours as an upper level disturbance
approaches the area. However, due to the low probability for
thunder at the Chicago area terminals this far out, I have opted
to leave the mention out of all but KRFD.



226 PM CDT

Light flow over the lake in the near term will continue to allow
local thermal effects to dominate, with land breezes at night and
lake breezes in the afternoon. Stronger southerly flow develops by
midweek before a cold front crosses the lake late Wednesday or
early Thursday and veers winds westerly and perhaps all the way
to northerly for the second half of the week.






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