000
FXUS63 KLOT 130521
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1221 AM CDT Sat Apr 13 2024

.KEY MESSAGES...

- Following summer-like warmth, a lake enhanced front will bring
  an abrupt temperature drop to northeast Illinois and
  northwest Indiana Sunday afternoon-evening.

- Waves of showers and a good chance (30-40%+) for thunderstorms
  Monday night through Tuesday night, some of which may be
  severe, particularly Tuesday afternoon and evening.

- Warm and windy on Tuesday with southerly winds potentially
  gusting to around 40-45 mph.

&&

.UPDATE...
Issued at 844 PM CDT Fri Apr 12 2024

Isolated showers continue to diminish in areal coverage and
intensity across northwest Indiana this evening, and will come
to an end shortly. Otherwise, clearing skies and diminishing
winds are the trends this evening, with quiet and milder weather
expected into Saturday.

Deep upper level trough/closed low was evident in vapor imagery
across the eastern Lakes and the Northeast this evening, with a
series of sheared mid-level vorts propagating southward within
the western periphery across the western Great Lakes. This
sheared vort axis has shifted east of the area since late this
afternoon, allowing subsidence and warming mid-level
temperatures and the loss of diurnal surface heating resulting
in the loss of instability across our eastern cwa. Showers and
earlier isolated thunderstorms have thus waned, and continue to
decrease in coverage and intensity as they shift southeast
across northwest and central Indiana. The few isolated showers
lingering currently over Newton/Jasper and eastern Benton
counties should move out within the hour, allowing clear skies
to prevail across the cwa tonight.

Strong northwest surface winds have also diminished markedly
since late afternoon, with the gradual weakening of both the
surface pressure and isallobaric gradients associated with the
deep low over southwest Quebec pulling away as well as the loss
of diurnal instability. Winds will continue to diminish
overnight and eventually back more westerly across our Illinois
counties toward morning as weak surface high pressure ridging
builds in from the west. Winds will shift southwest on Saturday,
allowing low level warm advection and abundant sunshine to warm
temperatures into the upper 60s and lower 70s by afternoon.
Winds may be just light enough to allow a slight southeasterly
lake breeze to develop along the immediate Illinois Lake
Michigan shore in the afternoon, which may keep locations right
along the lake just a little cooler.

Going forecast has all of the above in good shape, and other
than some minor adjustments to early evening pops and short-term
temp trends, no significant changes are needed.

Ratzer

&&

.DISCUSSION...
Issued at 335 PM CDT Fri Apr 12 2024

Through Saturday Night:

Widely scattered showers have developed across portions of the
area this afternoon, namely north of a Rockford to Joliet to
Fowler, IN line. These have managed to grow a bit taller than
anticipated which has resulted in greater coverage than
previously anticipated. In fact, some have managed to grow tall
enough to support electrification, with lightning strikes
observed in portions of Jasper County, as well as north of the
state line in southeast Wisconsin. As was the case yesterday,
there is a narrow convergence axis generally along/near the I-65
corridor that could support funnel cloud development yet again
though the better potential exists just to the southeast of the
forecast area where the RAP non-supercell tornado parameter
(NST) is up to 2. With any of the showers gusty winds up to 45
mph and pea sized hail/graupel will be possible along with a few
lightning strikes. Winds and shower coverage diminish with
sunset with clear skies overnight.

Saturday looks pleasant weather-wise with light west winds to
start the day and temperatures warming into the upper 60s to
lower 70s, warmest southwest. Winds will turn southwesterly
during the afternoon and gradually increase as warm air advects
into the region between the developing Northern Plains surface
low and high pressure across the Gulf Coast states. A low-
amplitude wave will traverse within the upper ridge which could
bring the potential for a few showers toward daybreak Sunday but
for now have kept the overnight hours dry.

Petr


Sunday through Friday:

A relatively mild start to the day on Sunday will ensure that most
of the area will be in the 70s come the late morning. Going into the
afternoon, some locations, particularly in the southwestern half of
our forecast area, should see temperatures climb to 80F for the
first time this year. At around the same, however, our northeastern
locales will likely start to see temperatures tumble as a lake-
enhanced front start to press inland, and eventually, between
nocturnal cooling and cold air advection behind the front,
temperatures should bottom out in the 40s overnight. There also
continues to be a fairly low (~15-20%) chance that a few showers
sprout along the frontal boundary in the afternoon (mainly in
northwest Indiana), but capping, dry air, and limited forcing
support aloft will likely be too much for any convergence-driven
attempts at convection along the boundary to overcome. However,
if these limiting factors are able to be overcome, the presence
of instability aloft associated with an EML over the region will
allow for any established convection to be capable of producing
lightning.

Mostly clear skies and modest warm air advection off of
southeasterly winds should allow for temperatures to recover back
into the 70s during the daytime on Monday across most of the area.
Locales near the Lake Michigan shore, particularly those in
Illinois, 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.

Monday night into Tuesday, a deep upper-level trough will eject out
of the southwestern CONUS towards the Upper Midwest, spurring a
fairly robust episode of lee cyclogenesis to our west in the
process, and bringing a much better chance for showers and storms to
our area. Showers and storms could be seen as early as Monday night
as warm air advection/isentropic ascent start to ramp up well in
advance of the developing low pressure system, but the main time
frame of interest to watch for potential severe weather in our area
will be Tuesday afternoon into Tuesday night as that is when we will
find ourselves in the system`s warm sector, which should be
characterized by surface temperatures in the 70s, dew points in the
upper 50s to mid 60s, and breezy southerly winds (with gusts
potentially approaching and exceeding 40 mph at times if rain and
cloud cover don`t end up being too much of a hindrance).

It`s still a little too early to dive into the specifics of this
potential severe weather threat as, still being four days out,
shifts and changes in forecast guidance are bound to happen and
alter the overall forecast expectations for any given event. In
fact, over the past several ensemble and deterministic model
runs, there has been a clear trend towards a slower and not-as-
deep low pressure system, which generally would be less favorable
for the overall chances and potential magnitude of the severe
weather threat in our area. That being said, even with a slower
and weaker surface low, the track of the surface low remaining
to our northwest should still draw at least a modest amount of
instability into northern Illinois and northwest Indiana which,
amidst seasonably strong deep-layer shear, would still present
the opportunity for storms to be severe as they track through the
area. The latest CIPS Analog guidance and CSU`s Machine Learning
probabilities corroborate this thinking, so the Storm Prediction
Center`s Day 5 15% contour that encompasses the entirety of our
forecast area continues to look appropriate at this time, but
we will continue to monitor forecast trends.

Showers and perhaps a few storms may linger around into Wednesday
before another system moves through the region during the latter
half of the week. This late week system could bring another round of
showers and storms to the area, though overall confidence in this is
low at this time. Much higher confidence exists in cooler
temperatures returning to the area going into the upcoming weekend.

Ogorek

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1221 AM CDT Sat Apr 13 2024

Main Concern:

- LLWS tonight

VFR will prevail through the period. West-northwest winds
overnight through mid day will shift to the southwest in the
early to mid afternoon. Expect some gustiness (to ~20 kt),
especially at RFD. A strong low-level jet (LLJ) will move over
the area 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.

Castro

&&

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

&&

$$

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