300
FXUS63 KLOT 182009
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 PM CDT Fri Apr 18 2025

.KEY MESSAGES...

- Scattered showers/storms likely late this afternoon and
  evening, some possibly severe with large damaging hail.

- Much cooler Saturday with shower chances lingering near/south
  of Hwy 24, especially early.

- Periods of showers and possibly embedded thunderstorms on
  Sunday becoming more widespread Sunday night. There remains a
  conditional threat for strong to severe storms Sunday
  afternoon and evening mainly for the southern portions of our
  area.

&&

.DISCUSSION...
Issued at 309 PM CDT Fri Apr 18 2025

Through Tonight:

Extensive cloud cover persists across the area this afternoon
within the warm sector of a surface low centered near Dubuque, IA.
In spite of the cloud cover, breezy south winds continue to aid
in the advection of warmer temperatures. In fact, as of this
writing temperatures have warmed into the mid to upper 70s, save
areas along the Lake County IL Lake Michigan shore. Cooler
temperatures (in the 60s) have persisted here due to the
continued onshore flow following this mornings outflow boundary.
Expect these breezy and warm conditions to persist into this
evening as a cold front gradually shifts eastward into the area.

The primary weather concern continues to center around the
threat for strong to severe thunderstorms, particularly this
evening (roughly 6 PM through 11PM) as the cold front shifts
eastward across the area. A persistent and exceptionally strong
EML and associated capping inversion continues to be sampled in
recent AMDAR soundings out of MDW and RFD. This is expected to
curtail near surface based storm development, at least for the
next few hours mid to late this afternoon. In spite of this,
elevated storms, based above the EML inversion, will continue to
fester across far northern sections of IL for the next few
hours as a weak impulse tracks northeast into southern WI. Any
severe threat with these initial storms should be confined to
some instances of marginally severe hail near and north of the
WI state line.

As we head into early this evening (after 6 PM), the surface cold
front, which as of this writing resides west of the area across
eastern IA, will slide eastward across northern IL. It is near
this boundary in which additional (potentially near surface
based) storms look to develop. As mentioned in previous
discussions, questions remain with the exact timing and overall
coverage of storms near the frontal boundary this evening. Much of
this continued uncertainty revolves around how effectively the
EML capping inversion will erode into this evening, especially
considering that most of the forced ascent and cooling looks to
come as a result of frontal scale forcing.

Nevertheless, any storms that do develop near the front this
evening will do so in an environment quite favorable for severe
storms. The combination of strong deep layer shear and very
steep mid-level lapse rates would be supportive of some
supercells capable of producing instances of very large hail
(1.5"+). Large hail looks to be the primary severe threat this
evening with these storms, though cannot rule out the potential
for some damaging wind gusts and possible a brief tornado,
particularly with any storms that are able to become near
surface based. The highest threat area for these severe storms
will largely be along and south of a line from near LaSalle-Peru
to Chicago, with overall lower potential the farther northwest
of this line you are. Expect the threat of storms to end north
of I-80 later this evening. However, storm activity is likely to
linger overnight well south of I-80, though the severe
potential is expected to gradually ease here after midnight.

KJB


Saturday through Friday:

Some showers from decaying overnight convection moving across
our southern CWA should linger through the 7-10am CDT timeframe
on Saturday across our far southern CWA, primarily south of
US-24 (60-70% PoPs). The rest of the day and through Saturday
night/early Sunday, additional impulses interacting with the
low-level baroclinic zone focused to our south may bring
occasional (primarily light intensity) showers at times to
portions of central Illinois and south of the Kankakee River in
Indiana (20-50% PoPs/highest south of US-24). Otherwise, it will
be notably cooler on Saturday with cloudy skies and perhaps
some sunny breaks in the afternoon. Highs will only reach the
mid 50s to around 60F inland and likely be limited to the upper
40s along the immediate lakeshore. This will be followed by lows
in the lower to mid 40s on Saturday night.

On Sunday, the negatively tilted upper trough ejecting from the
southern Plains and associated surface cyclogenesis will result in
a surface low tracking to the mid MS Valley late Sunday night to
central Lake Michigan early Monday afternoon. This is expected to
result in a breezy (easterly gusts to 30 mph), cool, and showery
at times Easter Sunday, unfortunately. It doesn`t look to be an
all-day washout, however, with a few rounds of forcing interacting
with the warm/moist advection over-topping the warm frontal zone.

The forecast surface low path and timing represents a further
slowing trend vs. a few model cycles ago. This is important with
respect to the northward progression of a warm front lifting north
and the potential for a conditional surface based severe threat
late day Sunday into the evening. Given the system`s slowing
trend, it appears the warm front will be hung up just south of our
southern CWA, with perhaps a chance for isolated thunderstorms
south of the Kankakee River Sunday afternoon.

Sunday night is when the most widespread showers and isolated to
scattered embedded thunderstorms are anticipated, along with gusty
southeast winds. Rainfall totals may end up near or just upwards
of an inch near and west of I-39 in northwest to north central
Illinois, with amounts tapering gradually off to the east. Can`t
rule out some ponding on roads with any heavier convective rain
rates Sunday night, though the overall flooding threat appears low.
The surface low will track just to our northwest late Sunday
night and then to our north on Monday, with a few lingering
showers possible until late morning/midday in the cold advection
regime behind the cold front. The afternoon will feature breezy
west- southwest winds gusting to 35 mph, and some upside potential
remains with this.

Castro

For Tuesday onward, the upper-level pattern is forecast to
become more zonal (west-east oriented) and persistently milder.
Both deterministic and ensemble guidance continues to indicate
that subtle shortwaves may pivot through the pattern and result
in additional periods of showers and perhaps thunderstorms (first
window would be late Tuesday-Tuesday evening). Given the
uncertainty in timing and coverage of such activity, see no reason
to alter the periodic 20-50% POPs offered by the NBM through the
end of next week, but suspect that many dry hours will occur as
well. On a more positive note, continued mid-level warm advection
(850mb temps forecast in the 10-12C range) is expected to keep
temperatures in the above normal category for mid-April with highs
in the upper 60s to 70s and overnight lows in the 40s to around
50F. The exception however, will be for those near the lake where
the potential for daily lake breezes may keep readings notably
cooler.

Yack/Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1256 PM CDT Fri Apr 18 2025

The main weather concerns for the 18Z TAF period include:

* Scattered showers and thunderstorms this evening with hail and
  associated gusty winds possible.

* MVFR cigs likely this evening through Saturday morning.

A system of showers and thunderstorms will move into the area
from the west later today. Scattered showers could get going
as early as the late afternoon with a few embedded thunderstorms
possible, although chances are greatest after around 02Z. A few
storms this evening could be strong to severe and feature
damaging hail and locally strong winds. Storms should move away
later in the evening with scattered showers lingering into the
early overnight.

MVFR cigs are expected to park over the TAF sites later this
evening, although periods of MVFR may be possible earlier in the
evening with the rain. MVFR should then hang overhead through
Saturday morning before lifting to VFR for the afternoon. There
is a signal for a possible few hour period of IFR cigs overnight
tonight. Confidence was too low for a mention in the TAF for
now, except at RFD where a TEMPO for IFR was introduced for
03-07Z.

SW winds will gust into the 25 to 30 kt range this afternoon
with a couple of gusts over 30 kt possible. They`ll gradually
ease during the evening and overnight while veering to NW.
Confidence in wind trends on Saturday is on the lower side. A
signal exists for a few hour period of NNE winds Saturday
morning (mainly 09-12Z timeframe) before flopping back to NNW
for the better part of the morning, although persistent NNW is
slightly favored. Additionally, we`re expecting a quick shift to
easterly at the Chicago sites, but timing is uncertain. Best
guess at this point is mid-late afternoon, but we could see this
shift as early as around noon.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 9 PM CDT this evening for Winthrop
     Harbor to Wilmette Harbor IL.

     Gale Warning until 9 PM CDT this evening for Wilmette Harbor
     IL to Michigan City IN.

&&

$$

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