771
FXUS63 KLOT 191149
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
649 AM CDT Sat Apr 19 2025

.KEY MESSAGES...

- Showers ending this morning south of the Kankakee River,
  otherwise dry and much cooler today and tonight.

- Shower chances return Sunday, mainly in the morning, but
  remaining seasonably cool.

- Rising temperatures expected Sunday night with a period of
  showers and perhaps some gusty thunderstorms overnight.

&&

.DISCUSSION...
Issued at 315 AM CDT Sat Apr 19 2025

Through Sunday Night:

We`re watching a couple of boundaries over or approaching the
CWA early this morning, including the initial synoptic cold
front moving east out of our CWA as of 07z. A secondary E-W
oriented cold front along with its lake enhanced counterpart
are crossing south into IL now. The secondary and lake enhanced
fronts will result in wind shift off the lake and an abrupt
temperature drop prior to sunrise downwind of Lake Michigan.

While the primary synoptic front will continue to move eastward
in concert with its parent sfc low over Lower Michigan and
associated northern stream shortwave trough, the south flanks of
this front will lay out east-west across down state IL and IN
as it becomes aligned with the west-southwesterly flow aloft
associated with the sub-tropical jet (STJ). Small perturbations
embedded within the STJ will continue to force a large area of
showers and thunderstorms northeastward along and north of the
boundary. Rain should impact areas mainly south of the Kankakee
River early this morning with this shield of rain gradually
slipping south of our CWA today as the frontal boundary slowly
sags southward and a drier air mass encompasses the rest of our
CWA.

Attention will then turn to the vigorous closed mid-upper level
low over the Four Corners. This system is progged to ride the
STJ eastward into the southern High Plains tonight, before
lifting negatively tilted northward into the Midwest by Sunday
evening. As this system ejects eastward, the frontal boundary
that will have stalled out downstate will begin lifting
northward Sunday into Sunday night in response to a deepening
sfc low tracking from OK north into IA by late Sunday evening.

North of this boundary today and tonight, look for dry
(after the aforementioned showers exit our southern counties
this morning) and much cooler conditions. As the warm air
advection ramps up in advance of the approaching system, a band
of showers will likely lift northward across our CWA Sunday
morning, with coverage of rainfall likely greatest over our
western CWA. While there could be some showers around at times
Sunday afternoon, it currently looks like many dry hours are
possible as lead impulse lifts north of the area taking the
strongest WAA driven showers with it.

Sunday will be rather chilly during the day, particularly north
of I-80 where easterly winds off the still cold lake will make
for rather raw feeling temps. However, Sunday evening the warm
front should surge north across the area and lead to rising
temperatures Sunday night. In fact, temperatures should rise
into the 60s over most of the area Sunday night, especially
south of I-80.

Another round of showers and probably some thunderstorms will
occur Sunday night in advance of the approaching cold front.
Within the warm sector Sunday night, some weak destabilization
of the boundary layer is possible, which could slow the
weakening of an extensive upstream potentially severe QLCS as it
moves across our CWA overnight. The most likely scenario is that
it will be in a decaying state and sub-severe by the time it
reaches our CWA, however given the very strong wind fields just
off the deck (>50kt down to about 1500ft AGL), it wouldn`t take
very much instability for some wind threat to sneak into our
CWA. This looks to be a low probability of occurrence at this
point, but something worth monitoring this weekend.

- Izzi

Monday through Friday:

The upper trough and its associated surface low that is
expected to bring us some rain on Sunday/Sunday night (see short
term discussion for details) will be lifting across WI and
northern MI on Monday. While showers are expected to be ongoing
Monday morning (perhaps even an isolated storm along the cold
front in far NW IN), they will gradually taper from southwest to
northeast by Monday afternoon. However, the tight pressure
gradient (6-hr pressure rises on the order of 9-10 mb) and cold
advection in the wake of the low will generate gusty west-
northwest winds through Monday evening. In fact, gusts on Monday
could very well peak in the 35-40 mph range with higher gusts
up to 45 mph possible depending on the degree of mixing. The
cold advection will also cap temperatures in the mid-50s to
around 60 Monday afternoon before readings cool into the upper
30s and lower 40s Monday night.

Heading into Tuesday, guidance remains in good agreement that a
more zonal (west-east oriented) upper level pattern will
establish across the CONUS through the end of next week. While
these patterns often offer periods of tranquil weather, there is
a notable signal for shortwaves to traverse the pattern and
bring periodic chances for showers and perhaps thunderstorms.
The first of these periods is forecast to occur late Tuesday
into Wednesday as a shortwave over the northern Plains forces a
cold front through northern IL and northwest IN. Depending on
when exactly the front moves through the region, it seems
plausible that sufficient instability should be in place to
generate thunderstorms, but the chances for any severe weather
look low. Another period of wet weather is forecast toward the
end of next week (Thursday through Saturday timeframe) as a more
notable shortwave lifts into the area. However, there is much
more variability in the timing and evolution of the late week
system making this part of the forecast lower confidence. That
said, have maintained the 40-60% POPs from the NBM for now but
suspect more refinement will be made over the coming days.

Regardless of the rain/storm chances, temperatures through much
of next week look to be above normal for mid-April. Therefore,
expect high temperatures in the upper 60s and 70s with overnight
lows in the 40s and 50s. Though, the typical daily lake breezes
will keep readings cooler along the lake shore most days.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 649 AM CDT Sat Apr 19 2025

- IFR with OCNL LIFR CIGS expected near the lake through mid
  morning before CIGS likely improve to MVFR by late morning and
  likely VFR this afternoon
- Northeast winds at ORD/MDW will likely become fairly
  light/variable or westerly late morning through mid afternoon
  before another potential shift to east.

IFR and OCNL LIFR CIGS near the lake likely to persist through
mid morning before gradually improving to MVFR, then probably
VFR this afternoon. Medium-high confidence in the general trend
in CIGS today, but lower confidence on the precise timing.

The extent of cloud cover this afternoon, particularly lower
cloud cover, could play a role in the ultimate wind direction
and speed this afternoon. Influence of the lake enhanced front
that moved through before dawn is likely to wane this morning
with winds likely becoming fairly light and variable midday.
Guidance that is more aggressive in clearing out skies this
afternoon would support a period of west to northwest winds
closer to 10kt, prior to a lake breeze shifting winds back to
easterly late this afternoon. An alternative scenario which is
also possible is fairly light and variable winds persisting
through the afternoon into the evening with no appreciable wind
shift to westerly or lake breeze this afternoon if lower clouds
are more prevalent this afternoon. Overall, confidence is low,
but kept trends from previous TAF showing the first scenario
with a period of westerly followed by a lake breeze.

A stiffer easterly wind will develop at all terminals late
tonight and in particular during the 24-30 hour period of the
ORD and MDW TAFs. There`ll probably be showers in the area
toward the end of the 30 hour TAF for ORD/MDW as well, though
probably remaining VFR during any rain Sunday morning.

- Izzi

&&

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

&&

$$

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