998
FXUS63 KLOT 192335
AFDLOT

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

.KEY MESSAGES...

- Expect showers at times, with breezy and cool conditions on
  Sunday.

- Rising temperatures are expected Sunday night with another
  round of showers and embedded thunderstorms. A few gusty
  thunderstorms may occur overnight.

- Monday will be windy with gusts up to 35-45 mph from the west-
  southwest. Expect well above normal temperatures inland the
  rest of next work week after a cool Monday.

&&

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

Through Tonight:

Low clouds, and even a period of drizzle, have persisted across
parts of eastern IL into northwestern IN through the morning
following the passage of a lake enhanced cold front early this
morning. However, a west-northwesterly wind shift, and the
associated influx of some slightly drier low-level air early this
afternoon, has allowed for a gradual improving trend across
interior parts of northern IL (generally west-northwest of the
I-55 corridor as of this writing. I suspect this improving trend
will continue eastward across much of the Chicago metro area
during the mid-late afternoon hours. With this improving trend,
expect temperatures to warm a few degrees (into the mid 50s across
eastern IL and northwestern IL and into the low 60s across
interior sections of northern IL) mid to late this afternoon
before sunset.

For tonight, we are not expecting much in the way of rain, aside
from the low (15%) chance for a few showers far south overnight.
However, attention will be focused on the robust mid/upper level
trough/low that will spawn low pressure across the southern
Plains tonight. This weather system is expected to impact our
sensible weather during the day on Sunday. For more on this,
reference the discussion below.

KJB

Sunday through Sunday Night:

The vigorous closed mid-upper level low over the desert
southwest will track eastward into the southern High Plains
tonight, before lifting negatively tilted northward into the
Midwest by Sunday evening. As this system ejects east-
northeastward, the frontal boundary that will have stalled out
downstate will begin lifting northward Sunday into Sunday night
in response to a deepening surface low tracking from Oklahoma
north into Iowa by late Sunday evening.

As warm air advection ramps up in advance of the approaching
system, a band of showers will likely lift northward across our
CWA Sunday morning into the early afternoon. Coverage of
rainfall will likely be greatest over our IL counties, perhaps
focused inland of the lake. While there should be some scattered
showers around at times in the mid to late afternoon hours as
well, it currently looks like many dry hours are possible
(especially with southward extent) as the lead impulse lifts
north of the area taking the strongest WAA driven showers with it.

Sunday will be fairly chilly by late April standards for most
of the area, especially when factoring in easterly winds gusting
to 25-30 mph, overcast skies, and rain at times. Forecast highs
are mainly in the lower 50s north of I-80 (only mid to upper
40s along the Illinois shore) and in the mid 50s to lower 60s
south (warmest south of US-24 towards evening). Temperatures
will steadily rise as the warm front lifts north of the area
Sunday night, to 60F or so all the way up to or just north 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 (best chance of this far southwest CWA. This continues to
look like it has a low probability of occurrence at this point,
but certainly something worth monitoring.

Castro/Izzi

Monday through Saturday:

The upper trough and its associated surface low that will
bring us some rain and thunderstorms on Sunday/Sunday night
will be lifting across WI and northern MI on Monday. Isolated to
scattered showers will continue until mid morning while lifting
north and then largely come to an end by the late morning.
More notably, the tight pressure gradient (6-hr pressure rises
on the order of 9-10 mb) and cold advection in the wake of the
low pressure will generate gusty west-southwest to west winds
through early Monday evening.

In fact, gusts on Monday will likely peak in the 35-45 mph
range, with higher gusts up to 50 mph or so possible depending
on the degree of mixing. The ECMWF and its ensemble (EPS) have
been the most bullish with Monday`s winds in the last several
model cycles. Given the run to run consistency thus far and
better support from the NAM and GFS Bufkit soundings, adjusted
gusts up to around the NBM 90th percentile. The ECMWF/EPS
scenario would entail a decent chance that we`d need a wind
advisory, with 90th percentile gusts from the EPS in the ~50-55
mph range. The cold advection and only slowly diminishing cloud
cover will also cap temperatures in the mid-50s to around 60F
Monday afternoon before readings cool into the mid 30s to 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 dry conditions, there has
been a persistent signal for shortwaves to traverse the pattern
and bring periodic chances for showers and perhaps
thunderstorms.

The first window where showers and some storms may occur is
forecast late Tuesday and Tuesday night and into Wednesday as a
shortwave over the northern Plains forces a cold front through
northern IL and northwest IN. Guidance variability precludes
higher confidence in this period. Thus expect fairly substantial
refinements to be made in the PoP forecast. Sufficient
instability may be in place to generate thunderstorms, but the
chances for any severe weather look low.

The next potential period of unsettled, at times showery
weather is forecast toward the end of next week (later Thursday
into Saturday morning timeframe) as a more notable shortwave
lifts into the area. However, as is common at this lead time,
there is plenty of variability in the timing and evolution of
the late week system making this part of the forecast lower
confidence. That said, generally maintained the 30-60% POPs from
the NBM for now. It does look like expansive high pressure
building in north of the Great Lakes should result in dry and
fairly pleasant/comfortable conditions the rest of Saturday and
through Sunday.

Regardless of the periodic rain chances, temperatures through
much of next week look to be above normal for late April.
Therefore, expect high temperatures inland of Lake Michigan well
into the 60s and 70s with overnight lows in the 40s and 50s.
Wednesday and Thursday could even make a run at 80F in parts of
the area! The typical daily lake breezes will keep readings
notably cooler (50s) along the lakeshore most days.

Castro/Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 635 PM CDT Sat Apr 19 2025

Aviation forecast concerns for the 00Z TAFs:

- Winds turn easterly by mid-evening or so and remain east
  through Sunday while becoming breeze with gusts near 25 kt in
  the afternoon/evening.

- Area of -SHRA lifts across terminals mid-late Sunday morning,
  with a lull likely for Chicago area sites late afternoon/early
  evening before rain returns. Some thunder potential later
  Sunday evening, though too low for TAF inclusion at this
  distance. Patchy MVFR cigs possible in SHRA, most likely
  later Sunday evening.

Surface wind field has been messy across the Chicago metro
terminals this afternoon/early evening, as a backdoor cold front
stalled west of ORD/MDW before moving back to the east of both
fields in the last 1-2 hours. This will maintain modest west
winds early this evening, before winds again turn east-
northeasterly with the help of a late lake breeze. Most high-res
guidance (which have not handled wind details well today) bring
east winds back into ORD/MDW by 01Z-02Z. Winds should then
remain easterly for the duration of the forecast, becoming
breezy with gusts 20-25 kts during the day/evening Sunday and
shifting a little more southeast Sunday evening.

The next weather disturbance approaches the area Sunday, with an
area of showers expected to spread north across the terminals by
late morning. Showers will persist through mid-late afternoon
before decreasing in coverage for a time into Sunday evening at
least for the Chicago terminals. RFD will likely see greater
shower coverage persist. Patchy MVFR conditions are possible
within these showers, especially at KRFD. Another more vigorous
push of showers, and possibly a few embedded TSRA are then
expected to return late. MVFR/patchy IFR conditions are more
likely with this round for the Chicago terminals. Thunder
coverage is of low confidence at this distance and have not
included in the TAFs at this time, though could potentially see
a need for a short window late Sunday evening with further
forecasts.

Ratzer

&&

.MARINE...
Issued at 326 PM CDT Sat Apr 19 2025

Have issued a Gale Watch for the Illinois and northwest Indiana
nearshore waters from Monday morning through early Monday
evening.

Low pressure at or just under 1000 mb will track from
Iowa Sunday night to northern Lower Michigan Monday afternoon
and across Lake Huron Monday evening. Breezy east-southeast
winds Sunday night may result in hazardous waves for small craft
along the Illinois nearshore. Then in the wake of the system on
Monday, winds will quickly shift to southwest and then west
while rapidly increasing due to a tight pressure gradient. While
the top end magnitude of the speeds/gusts varies amongst the
guidance for Monday, in general felt the signal was strong
enough to issue a gale watch given expected Monday morning
onset. For now, the forecast will indicate gale speeds/gusts to
around 35 kt but the more bullish guidance suggests gales up to
40 kt or even a bit higher are possible. Increasing stability
offshore will likely limit the gales to the first 2-3 miles of
the nearshore. Westerly winds will steadily ease Monday evening.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Watch from Monday morning through Monday evening for the
     IL and IN nearshore waters.

&&

$$

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