Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201131

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
631 AM CDT Wed Mar 20 2019

249 AM CDT

Through Thursday...

No major changes to the forecast and still looking like a wet
Wednesday morning commute. Made tweaks to hourly PoP trends and
added hourly resolution through mid day tomorrow in line with
radar and CAM guidance trends. Extrapolating off the current
position of the area of rain over central IA up to about I-80
lends credence to the gradual northward shift of the rain that has
occurred with this system. It looks like most if not all
locations in the CWA in northern IL will receive measurable rain.

Fairly strong moisture transport in 925-850 mb layer from 25-35
kt LLJ will translate the 0.6"+ PWAT axis sampled by 00z TOP RAOB
eastward over our area, supporting steady to temporary moderate
rainfall rates. Main factor keeping rain amounts in check will be
the fast moving nature of the forcing from the short-wave and
isentropic ascent, with maximum amounts from near/south of I-88
down to around Kankakee River up to about 1/4 inch, with lighter
amounts north to around 0.1" in state line counties. Other item of
note is the rather breezy south-southwest winds that will
accompany the tail end of the morning rain and persist into the
early afternoon, especially southeast of I-55. Surface low will
open up but still looking at stout pressure falls mid morning
through early afternoon for gusts up to 30 mph in favored corridor
per forecast soundings.

Finally, tomorrow afternoon into the early evening, as another
stout short wave dives southeast with cold front crossing the
region with scattered showers looking like a good bet with that
front. Did not change PoPs for mid to late PM hours, but did
change showers to coverage wording given good confidence in them



220 AM CDT

Thursday night through Tuesday...

The main forecast concerns through the period continue to be the
increasing potential for a decent cold front to shift over the
area Friday, followed by brief warmup for the weekend, then the
possible arrival of another strong cold front Sunday night.

It appears that a weather system dropping southeastward over the
eastern Great Lakes will push a cold front down Lake Michigan and
across our area early Friday. With the passage of this front,
winds are likely to shift northeasterly off the lake setting up a
chilly afternoon (temps in the 30s) for areas around the lake.
While temperatures will be a bit warmer inland, temperatures here
will likely remain in the mid 40s.

Surface high pressure should quickly drop southward over the area
Friday night before shifting to our southeast during the day on
Saturday. After a cold start to the day (temps in the mid to upper
20s) temperatures should warm back to near 50 on Saturday. The
only exception to this will be areas along the lakeshore, where
it appears probable that a lake breeze will develop Saturday
afternoon and result in locally cooler conditions.

Temperatures should inch up a bit more for Sunday, but there will
be increasing cloud cover and chance for some rain showers in
association with an approaching weather disturbance. While there
continues to be uncertainties on the details of the forecast
Sunday night into Monday, there continues to be signs that another
strong cold front could drop southward across the area as strong
high pressure builds southward over the northern Plains. Not only
would this result in below normal temperatures for early next
week, but it could also result in a period of wintry precipitation
Sunday night into Monday. However, confidence on the wintry
precipitation is low at this time.



For the 12Z TAFs...

Expect a period of light rain this morning in association with a
weather disturbance pushing across the area. While initially CIGs
will remain VFR, expect some MVFR CIGs to develop over the area by
mid to late this morning and continue for a period into this
afternoon. The rain is expected to wane across the area by midday,
with much, of the afternoon remaining dry. However, another batch
of rain showers looks to move across the terminals early this
evening in association with an approaching cold front. While some
improvement to the CIGs are possible this afternoon and early
evening, expect them to trend back down into the MVFR range in
the wake of this front. Northwesterly winds will set up over the
area following this front, and continue into Thursday.






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