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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KLOT 130953
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
453 AM CDT Sat Apr 13 2024

.KEY MESSAGES...

- Following summer-like warmth, a lake enhanced front will drive
  a significant temperature drop Sunday afternoon and evening
  along and near the southern Lake Michigan shore.

- Waves of showers and at least scattered thunderstorms are
  probable late Monday night through Tuesday night-early
  Wednesday. Some of the thunderstorms may be severe Tuesday
  afternoon through early Wednesday.

- Windy Tuesday afternoon through Wednesday with gusts to 40 mph
  and possibly stronger at times

- Unseasonable warmth continuing into Wednesday, except near
  the lake on Monday and along and near the IL shore on Tuesday,
  followed by a noteworthy cool-down late in the week

&&

.DISCUSSION...
-- Changed Discussion --
Issued at 330 AM CDT Sat Apr 13 2024

Through Sunday:

An absolutely great weather day is in store for the area today as
a surface ridge axis shifts overhead. The combination of plentiful
sunshine and the influx of a warmer airmass from the southwest
through the day will push temperatures up to around (or just
above) 70 degrees for most areas this afternoon. While it is also
expected to be pleasant along the Lake Michigan shore today, an
afternoon lake breeze may foster slightly cooler conditions right
along the shore. Fortunately, southwesterly winds farther inland
should remain just strong enough (10 to 15 mph) to hold the lake
breeze right along the lakeshore, thus limiting the inland extent
of the lake cooling footprint.

A southwesterly low-level jet will ramp up overhead tonight in
response to a quick eastward moving low pressure system across the
Upper Midwest into the Great Lakes. Continued warm air advection
with this low-level jet will support a rather mild night, as
overnight lows tonight only fall back into the mid to upper 50s.
These unseasonably warm temperatures continue Sunday, with inland
temperatures expected to to top out well into the 70s, if not
even into the low 80s in advance of an approaching cold front
dropping down the lake through the day. Onshore winds developing
in the wake of this front through the afternoon will then send
temperatures falling back into the 50s by early evening along the
southern Lake Michigan lakeshore.

KJB


Sunday Night through Friday:

High pressure will remain in control Sunday night through Monday,
with continued tranquil conditions, and unseasonable warmth once
again on Monday away from the Lake Michigan shore. Following a
cool Sunday night (compared to Saturday night), inland highs will
likely reach well into the 70s on Monday, while synoptic onshore
flow and lake breeze reinforcement keeps areas along the lake and
few to several miles inland in the mid 50s to lower 60s.

The main player in our weather Monday night through mid week will
be the ejection of a strong negatively tilted short-wave trough/upper
low and associated lee cyclogenesis. A warm front will sharpen
south of the local area on Monday night. Strengthening warm and
moist advection overnight across and north of the frontal zone
in response to lead positively tilted short-wave impulses and
modestly steepened mid-level lapse rates should result in an
uptick in elevated convection into early Wednesday. Progged
MUCAPE and effective bulk shear generally look sub-marginal for
any strong to severe threat with this initial activity.

Tuesday`s synoptic pattern and wind profiles continue to be
concerning big picture wise for the threat of severe
thunderstorms as early as Tuesday afternoon near the northward
advancing warm frontal zone. The key uncertainty in the whole
set-up is tied to the gradual slowing trend of the trough
ejection, resulting in a farther west surface low (near/west of
the MO River) until Tuesday night. Given the time of year
(cool Lake MI waters) and only modest pressure falls over our
area, the warm front could very well get hung up south of I-80
into if not through mid Tuesday afternoon, keeping IL shoreline
locales in the 50s until evening. Elevated convection north of
the front Tuesday morning could also serve to keep the warm
front reinforced south.

As the warm sector manages to advance north Tuesday afternoon
through Tuesday night with the surface low pressure drawing
closer, low-mid-level wind profiles become rather concerning for
rotating thunderstorms. That said, during the day Tuesday, with
the main 500 mb low well to the west, mid-level lapse rates are
forecast to remain fairly weak at 5.5-6.0C/km. With cloud cover
and weak lapse rates, even 70s temps and 60s dew points in the
warm sector may only yield limited destabilization. If robust
storms are able to develop, all hazards would be possible, with
the main threat zone conceptually I-80 and south until Tuesday
evening or Tuesday night.

Strengthening dynamics will result in robust low-level warm and
moist advection through Tuesday night in the warm sector with
southerly winds gusting up to 35-40+ mph, attempting to hold
nocturnal stabilization at bay as temps and dew points remain
propped up near to above 60F. If the severe threat is limited
during the day on Tuesday, especially with northward extent, a
rare for April nocturnal all-hazards severe threat is certainly
within the realm of plausibility Tuesday night per most recent
guidance trends.

The systems cold front will sweep across the area through
Wednesday morning as the ~990 mb surface low pressure tracks
toward northern Lake Michigan. Some showers will persist through
mid-day Wednesday and possibly into the afternoon in spots, with
a decreasing chance for additional thunderstorms. Given the
latest forecast timing of the cold front passage, the threat for
renewed severe weather will likely be to our east and southeast
on Wednesday. Westerly winds may potentially gust up to 40-45
mph behind the front Wednesday afternoon during strong pressure
rises and steepening low-level lapse rates. It will be one last
mild day before the noteworthy late week cool-down to solidly
below normal temps. In fact, we may be dealing with freeze
concerns Friday night in parts of the area, as the growing
season will be off to a fast start with all the warmth (and
additional rain) this week.

Castro
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&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
-- Changed Discussion --
Issued at 453 AM CDT Sat Apr 13 2024

Main Concern:

- LLWS tonight

VFR will again prevail through the period. Light westerly winds
will shift to the southwest in the early to mid afternoon
today. Expect some gustiness 15-20 kt. A strong low-level jet
(LLJ) will move overhead in the mid to late evening, resulting
in LLWS conditions. During the strongest LLJ, ORD and MDW will
likely gust to 20-25 kt or so from the south-southwest, while
other terminals only exhibit sporadic gustiness. A weak cold
front will shift across the area Sunday morning, resulting in
winds shifting to the west with gusts up to 15-20 kt.

Castro
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&&

.LOT WATCHES/WARNINGS/ADVISORIES...
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IL...None.
IN...None.
LM...Small Craft Advisory from 10 PM this evening to 7 AM CDT
     Sunday for the IL and IN nearshore waters.
-- End Changed Discussion --

&&

$$

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