Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 232031

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
331 PM CDT Wed Oct 23 2019

247 PM CDT

Through Thursday...

Aside from a few isolated sprinkles across portions of the area
over the past few hours, most areas have remained dry under partly
sunny skies. In spite of this, it still does appear that we could
experience a short period of some showers this evening, primarily
across far northern IL (north of I-88). This will be in response
to the approach of the mid-level disturbance, and an associated
weak surface low now over southwestern IA. As this disturbance
shift eastward across northern IL this evening it appears likely
that there will be some showers, especially north of the track of
the surface low over far northern IL and southern WI. Any light
rain should abate by late evening, then a cold front will drop
southward over the area overnight. Winds will turn north-
northwesterly in the wake of the front, and continue on Thursday.

Temperatures on Thursday will be a few degrees cooler than todays
readings. Expect highs generally in the low to mid 50s south, and
upper 40s to around 50 north.



330 PM CDT

Thursday Night through Wednesday...

There are no significant forecast concerns from an impact
perspective through the weekend, though there are a few items of
interest. At the end of the period, a stronger storm system could
impact the region, though there is plenty of uncertainty. In
addition, toward or just after day 7, confidence is increasingly
high in a period of unseasonably cold winter-like temperatures.

On Thursday night, with the elevated baroclinic zone draped over
central Illinois and Indiana, there could be some light f-gen
driven banded precip. NAM has been most aggressive in bringing
this into portions of the CWA, but guidance consensus would favor
south of the southeast CWA, though did hang onto some slight
chance PoPs for areas south of US 24 in central Illinois.
Otherwise, still expecting a good deal of lingering post-frontal
cloud cover, stratus south and mid-high clouds north. Despite
light winds from high pressure over the region, this should
prevent better radiational cooling and precluding colder
overnight lows. The rest of Friday will be quiet and seasonably
cool with increasing sun into the afternoon.

On Friday night, while surface high pressure will be gradually
pulling eastward, favorable position of ridge axis over our area
and clear skies will yield good radiational cooling conditions and
lows in the lower to mid 30s outside of Chicago and upper 30s in
the city, with potential for upper 20s in favored cold spots.

Turning ahead to Saturday, global operational models have come
into much better agreement on a southern stream wave and
associated surface low tracking across the western Ohio Valley and
then the central and eastern Great Lakes Saturday through
Saturday night. With this said, the trend on models that have this
system is a bit east of previous runs. Thus, PoPs for rain are
still at most in the high chance range, highest I-55 and southeast
Saturday evening. Given very dry air mass the system will be
lifting into, a sunny start and dry conditions appear to be a
decent bet north of I-80 until mid afternoon. Precip potential for
north central IL/northwest IL portions of the CWA will depend on
the exact track of the low pressure area, and once again trend on
12z ECMWF and Canadian model was east of previous runs. At this
time, Saturday- Saturday night system does not appear to be a
significant rain producer. High temps Saturday will be slightly
below normal in the mid to upper 50s. Sunday will be the nicest
day of the weekend, with seasonably mild temps.

Looking into next work week, medium to long range operational and
ensemble guidance is in excellent agreement in a large scale
pattern change that will at least eventually favor unseasonably
cold conditions for the eastern half of the US. Over the weekend,
anomalous mid and upper level ridging will amplify off the west
coast and up to Alaska. This is known as the negative phase of the
East Pacific Oscillation and is often associated with our coldest
air masses during the cool seasons.

The big question is how this pattern change progresses. There
will be a plunge of winter-like cold southward and downstream
troughing in response to the AK ridging. However, if the cold
trough first dumps into the west, temporary height rises east
could bring storm track over or or west of our area. This is what
the typically reliable ECMWF and its ensembles are favoring, while
the GFS suite is quicker to bring cold into our area. A ECMWF-
like scenario could favor a strong fall system with a period of
mild readings ahead of it Tuesday into early Wednesday and shower
(even some thunder) potential, followed by unseasonably cold air
pouring in on the eve of Halloween. Given this discrepancy in the
guidance, temperature and precip trends later next Tuesday into
Wednesday are of lower confidence and will take some time to iron
out. But we can say that while it is over 7 days out, Halloween
itself is likely to be a cold one. See the CPC 6-10 Day Outlook
for more.



For the 18Z TAFs...

Overall, there are no significant weather concerns for the
terminals. There will, however, be a a wind shift this evening as
a cold front drops southward over the area. It appears that prior
to its arrival that the winds could become light out of an
easterly direction as early as 23z at the eastern terminals, but
the more persistent northerly winds should hold off until mid to
late evening. Some light rain showers or sprinkles will also be
possible at times this afternoon and early evening over northern
IL. This should not cause any major issues though as it should
remain light and within VFR conditions.

Some MVFR CIGs look to develop following the cold frontal passage
later this evening and overnight. Currently it appears CIGs could
fall down to around, or just below 2,000 feet agl for a period
into early Thursday morning. Thereafter, VFR conditions should
return for the day Thursday.






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