Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 141754

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1254 PM 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



330 AM CDT

Tuesday through Sunday...

The main forecast concern is shower and thunderstorm potential,
particularly Wednesday through Wednesday night. The short-wave
tonight will have a weak surface low reflection moving across the
central and eastern Great Lakes, which will drag a weak cold front
southward on Tuesday. Mid and upper forcing will be weak in the
presence of height rises. The progress of the frontal boundary
likely slow south of I-80, with some moisture pooling, weak
convergence and perhaps a weak wave possibly enough to kick off
isolated to widely scattered showers and storms. It appears the
greater chance for this to occur would be south of I-80 closer to
the frontal boundary, so PoPs will likely need refinement/tightening
up should these trends continue in the guidance today. Severe
weather is not expected given light flow aloft resulting in weak
deep layer shear. Progged thermal profiles support highs in the
lower-mid 80s, except upper 70s in far northeast Illinois and
along the lake shore.

A trough over the high Plains will cause surface cyclogenesis by
Tuesday evening over eastern CO/western KS vicinity. Warm
advection over the stalled frontal zone could yield showers and
elevated thunderstorms overnight into early Wednesday, but it`s
possible more appreciable activity holds off until during
Wednesday morning (perhaps in the form of a decaying MCS). There
is plenty of uncertainty in the evolution, track, timing and
strength of the mid week system in the latest guidance, along
with mesoscale convective details, thus confidence is low in
trends with this forecast.

A warmer and more humid air mass will be drawn northward as the
previously stalled frontal boundary lifts northward as a warm
front. Stronger forcing from the trough will result in a higher
likelihood of showers and storms by late Wednesday into the
evening, but probable to be well ahead of the cold front.
Magnitude and any risk for strong to severe storms will be driven
by how much instability can be realized, pending cloud cover
concerns, as well as diurnal timing possibly being somewhat

While the exact track of the surface low is highly uncertain, cold
frontal passage does appear it will occur during Thursday, with
the higher shower and thunderstorm chances possibly focused
along/southeast of I-55. Strong wind fields for mid August and
resulting deep layer shear could also yield a risk for strong to
severe storms with the cold front if sufficient instability can be
realized, particularly if fro-pa can be slowed a bit. Friday
should be mainly dry and slightly cooler behind the front, with
low shower and thunder chances indicated for now over the weekend,
though there is plenty of uncertainty/low confidence based off
large differences in the guidance.



For the 18Z TAFs...

South-southwest winds continue this afternoon, and the winds
should be strong enough to keep the lake breeze east of ORD and
MDW. Scattered thunderstorms are still forecast to form this
evening near RFD and then shift east through the mid to late
evening. I have medium-high confidence in showers and storms
forming near RFD, but I have medium-low confidence in timing and
coverage. As such, I kept TSRA as is in the TAFs.

Light winds tonight will probably result in fog mainly at RFD. A
cold front sinks south early Tuesday morning and winds become
north to northeast. Low cigs are also expected with the front. IFR
cigs are most likely at RFD, while other sites will likely see
MVFR cigs. Clouds scatter out in the mid to late morning, and
thunderstorm activity Tuesday should be well south of the



347 AM CDT

Fairly light and generally southerly winds can be expected over
the lake today. A weak cold front will drop southward tonight,
turning winds northeast and then north, earliest on the northern
portion of the lake. Weak low pressure moving into the central
lakes will usher the front south across the remainder of the lake
Tuesday morning. The northerly winds on Tuesday will likely top
out at up to 20 kt.

A stronger system will likely take shape by mid week, with
strengthening south and southeast winds ahead of it, particularly
on Wednesday night/early Thursday. There is plenty of uncertainty
with the track and strength of the surface low into Friday, which
will impact wind directions and speeds Thursday. In the wake of
the system Thursday night into Friday morning, brisk northwest
/north-northwest flow could certainly be stronger than the 15-25
kt currently indicated in open lakes forecast with a favorable
surface low track. Speeds should diminish fairly quickly later
Friday as high pressure spreads in from the west. Periods of
thunderstorms are also likely with this system.






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