Area Forecast Discussion
Issued by NWS Chicago, IL

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341
FXUS63 KLOT 200720
AFDLOT

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

.UPDATE...
930 PM CDT

No major changes to the forecast message of a wet Wednesday
morning commute, the day of the vernal equinox at 4:58pm CDT. Made
tweaks to hourly PoP trends and added hourly resolution through
mid day tomorrow in line with radar and CAM guidance trends.
Extroplating 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
now looks like most if not all locations in the CWA in northern IL
will receive measurable rain.

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, at least widely scattered showers appear probable. Did not
change PoPs for mid to late PM hours, but did change showers to
coverage wording given good confidence in them occurring.

Castro

&&

.SHORT TERM...
1232 PM CDT

Through Wednesday...

Seasonal conditions remain in place afternoon with plentiful
sunshine, some scattered cumulus, and readings in the upper 40s
to near 50. Water vapor imagery depicts an increasing cloud
shield out ahead of the upper low currently across the central
Plains. Warm advection ahead of the compact upper low will drive
an area of rain across the area late tonight into Wednesday.

Morning RAOBs indicate an axis of PWAT values in excess of 0.6",
not a direct gulf of Mexico feed but an axis of increased
moisture. Southwest flow ahead of the low will induce isentropic
ascent and eventually lead to lower level saturation. For tonight
the ascent is more modest, but will increase overnight into more
so Wednesday morning. Ensemble plumes suggest most areas will see
under 0.25" of QPF, but the lift and moisture are supportive of
higher totals mainly along and south of I-80 and into northwest
Indiana.

The stronger shortwave energy will pass to our south, but broad
height falls and the main trough axis will still be shifting
through Wednesday afternoon, thus cloudy conditions will persist
with some light rain/showers still possible. Given the extensive
cloud cover, expect highs to fall shy of 50 in most areas.

KMD

&&

.LONG TERM...
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.

KJB

&&

.AVIATION...
For the 06Z TAFs...

Rain will be developing over the area towards daybreak as our next
weather disturbance shifts over the area. Expect a period of light
to moderate rain during the early morning, which may produce a
short period of IFR visibilities. Low MVFR CIGs will also develop
over the area for the morning. While CIGs may improve some in the
afternoon, it appears that another batch of rain showers could
move across the terminals late in the afternoon and early evening
as a cold front moves over the area. CIGs may then trend back down
into the MVFR range later tonight with winds shifting to the northwest
in the wake of the front.

KJB

&&

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

$$

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