000
FXUS63 KLOT 132343
AFDLOT

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

.KEY MESSAGES...

- Unseasonably warm temperatures (upper 70s to around 80!) are
  expected Sunday before an enhanced lake breeze steadily drops
  temperatures near the shoreline into the evening.

- 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 continues through Tuesday, except near
  the lake on Monday and along/near the Illinois shore on
  Tuesday, followed by a noteworthy cool-down late in the week.

&&

.DISCUSSION...
Issued at 411 PM CDT Sat Apr 13 2024

Through Sunday Night:

Temperatures will top off into the upper 60s to low 70s today with a
sturdy warm air advection pattern in place. The eastward propogating
low level ridge will hold the southwest winds through Sunday
morning. An overnight 50-60 knot low level jet will aid in keeping
the southwesterly warm air flux strong with mild lows in the mid to
upper 50s. In spite of the strong winds just above the surface,
model soundings suggest that as the boundary layer decouples
suggesting that gusts will be less frequent. There still exists the
potential for turbulent mixing to result in sporadic gustiness
through the near-surface stable layer up to 25 mph especially within
the warmer urban core of the metro where it may remain weakly mixed
at times. An increase in mid-level moisture will additionally arrive
overnight as a weak mid-level wave moves through the region embedded
within the broader upper ridge. No precipitation is expected as the
low levels remain too dry.

The continuation of the warm air advection pattern Sunday will send
temperatures up into the upper 70s to lower 80s, which is around 10-
15 degrees above average! Sunday will not only look to become the
warmest day so far for the year, but will likely be the warmest day
through the rest of the upcoming week. A weak cold front will move
through Sunday morning turning winds to the north-northwest which
will stall some of the warming across far northern Illinois. A
reinforced lake breeze will then push onshore into the afternoon
resulting in a steady cooldown in the afternoon into the evening
near the lakeshore. The overnight temperatures will cool down toward
the upper 40s to low 50s.

Stump/Petr


Monday through Tuesday night:

After a relatively cool start to the day on Monday, temperatures
across most of the area should rebound back into the 70s amidst
mostly clear skies on the backside of a departing surface high.
Locales near the Lake Michigan shore will be the main exception to
this as an onshore flow component to the winds will keep
temperatures here several degrees cooler than farther inland. In
fact, it`s quite possible that there could be a 20+ degree gradient
across the Chicago metro on Monday with the southern and western
suburbs currently forecasted to reach the mid 70s while locations
along the Illinois lakeshore may struggle to break the mid 50s!

Monday night into Tuesday, a deep upper-level trough will eject out
of the southwestern CONUS towards the Upper Midwest and acquire a
negative tilt, spurring a fairly robust episode of lee cyclogenesis
to our west in the process. There had been a noted slowing trend in
this process and the subsequent arrival time of the resultant low
pressure system and its associated fronts over the past several
ensemble and deterministic model runs, but the last couple of
guidance suites have remained relatively steady state with respect
to timing, bringing the system`s warm front into our area Tuesday
morning and its cold front on Wednesday morning. EPS/GEFS ensemble
members have also started to converge on a more precise track for
the center of the surface low, now favoring a track from central
Iowa through central/southern Wisconsin during the Tuesday night
through Wednesday afternoon time frame.

In the big picture, confidence at this point is fairly high that
there will be periods of showers and storms while find we find
ourselves near and between the system`s fronts late Monday night
through Wednesday. This should initially come in the form of
elevated, sub-severe convection on Tuesday morning as warm air
advection/isentropic ascent funnels air parcels over the surface
warm front into an environment characterized by steepened mid-level
lapse rates but relatively marginal MUCAPE and effective shear. It
is possible, if not likely, though, that this convection could
stall the northward progress of the warm front for some time which,
given the southeasterly surface flow north of the warm front, would
result in relatively cool lake-influenced temperatures hanging
around in far northeastern Illinois for much, if not all, of the
daytime. This would also have implications for when and how windy
it will get during the daytime on Tuesday with southerly gusts to
35-40+ mph expected to be present south of the warm front.

Tuesday afternoon, warm sector convection is likely to fire to our
southwest, closer to where the stronger forcing for ascent will be.
This convection will likely become organized quickly given the shear
profiles in place and could feature both supercellular and/or linear
characteristics as it marches northeastward towards and eventually
into our forecast area during the late afternoon or evening. By
this time, instability will probably be waning a bit, but could
still be sufficient for the maintenance of severe thunderstorms
capable of generating all modes of severe weather well into the
night, especially with the continued favorable shear and forcing.
Confidence in this is still medium at best though, and we`ll
still have to see what kind of take some of the shorter range,
higher resolution forecast guidance has on all this before we can
start to nail down the more salient, finer scale details of the
severe weather threat with higher confidence.

Ogorek


Wednesday through Saturday:

The system`s cold front will eventually sweep across the area on
Wednesday as the occluded, weakening surface low pressure tracks
toward central Lake Michigan. Some showers should persist through
mid-day Wednesday and possibly into the afternoon in spots, with a
lower chance for additional thunderstorms. Given the latest forecast
timing of the cold frontal passage, the threat for renewed severe
weather will likely be to our east and southeast on Wednesday, but
if the system slows any further, then it`s conceivable that such a
threat could materialize in our southeastern CWA. Westerly winds
could potentially gust up to 40-45 mph behind the front Wednesday
afternoon during strong pressure rises and steepening low-level
lapse rates. A noteworthy cool-down to solidly below normal
temperatures is then in store for the latter half of the week
into the weekend. In fact, we may be dealing with frost/freeze
concerns Friday night and/or Saturday 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.

Ogorek/Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 628 PM CDT Sat Apr 13 2024

Aviation Forecast Concerns:

- LLWS late this evening and overnight, with 45-55 kt low level
  jet. Sporadic surface gusts

- Wind shift to northeast expected for Chicago metro sites
  Sunday afternoon/early evening.

Surface low pressure was centered over eastern Dakotas/Minnesota
border early this evening, with warm, breezy south-southwest
winds across the Midwest including our terminals. Sunset will
likely bring a quick drop in gusts early this evening, though
a 45-55 kt low level jet developing across the area mid-late
evening will result in low level wind shear conditions
developing before midnight. We`ll also likely see at least
sporadic gusts redevelop especially at our usual metro sites
(KORD/KMDW) as the LLJ ramps up. These conditions will persist
through the night, though the low level jet magnitude will
likely ease a bit toward sunrise before moving off to the east
of the area.

The surface low will pass north of the area overnight, but will
trail a cold front across the area Sunday morning, which will
result in a shift to northwest winds at the terminals by late
morning. As mixing develops after sunrise, we may see brief
bump in wind gusts (around 25 kts) as we start to tap into the
bottom of the departing low level jet, though winds should
gradually ease by midday. As winds gradually turn more north-
northwest during the afternoon, a lake breeze is expected to
push inland off Lake Michigan. KGYY will see this by early
afternoon, with the lake breeze expected to reach KMDW during
the afternoon and KORD later still. There remains a several hour
spread in the timing at KORD in model guidance at this
distance, ranging from 20Z-24Z for the wind shift. Opted to
maintain our current 23Z from previous forecast for now, as
confidence is fairly low with the exact timing, but refine with
future issuances.

Otherwise, VFR expected through the period.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
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.

&&

$$

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