361
FXUS63 KLOT 200530
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1230 AM CDT Mon May 20 2024

.KEY MESSAGES...

- Period of showers and thunderstorms Monday afternoon, some
  possibly severe with locally heavy rainfall.

- Summer-like warmth expected through Tuesday night, interrupted
  only by showers/storms Monday and Tuesday.

- Breezy to windy on Monday and Tuesday, with southwesterly
  gusts up to 30 mph on Monday, and southerly to south-
  southwest gusts up to 30-40 mph Tuesday afternoon and evening.

- Scattered (mainly non-severe) thunderstorms possible Tuesday
  morning and then after a break, thunderstorm chances return
  Tuesday evening/night, including some threat for severe
  weather, primarily for the western CWA (near/west of I-39).

&&

.UPDATE...
Issued at 844 PM CDT Sun May 19 2024

Minor adjustments to the going forecast made this evening,
primarily to hold onto chance thunderstorm pops longer in our
south this evening. Also made adjustments to temps/sky cover
based on obs trends for lake-cooling and convective outflow/cold
pool footprint.

Surface stationary front near the LOT/ILX cwa borders became
the focus for scattered strong thunderstorms this afternoon and
early evening. These storms produced locally heavy rainfall of
2-3 inches across parts of Benton county, as well as several
30-40 mph wind reports from personal wx stations One report of
damage was received from Fowler IN, where wind damage to roof
shingles occurred with one of the stronger storms. Convective
outflow has muddied the frontal location, though it still
appears to be along/just south of our southern cwa border this
evening. A few weaker isolated elevated thunderstorms may fester
along/north of the boundary across our southern tier of
counties through late evening, as southerly low- level return
flow begins to develop atop the frontal zone and convective cold
pool, along the southern periphery of a mid-level short-wave
tracking northeast from IA into WI.

Farther to the west, a more vigorous mid-level short wave was
evident in evening RAOB/water vapor imagery across the central
Plains. This wave (likely convectively enhanced by strong MCSs
over KS/southwest IA) is forecast to propagate northeast into
the mid- Mississippi Valley through Monday morning. This will
further increase south-southwest low-level return flow into the
region late tonight into Monday, lifting the stationary front
back north as a warm front across the WFO LOT forecast area, and
increasing the potential for warm-advection forced showers and
thunderstorms to develop into our western counties and the I-39
corridor toward sunrise.

Have increased chance pops slightly and held onto them through
this evening across our south for ongoing festering isolated
storms there. Also lowered temps/dew points where lake-cooling
footprint has been more pronounced, and where convective cold
pool lingers from late afternoon storms in the south as well as
increased cloud cover there. Could see needing to increase pops
tomorrow morning a little more quickly farther east into the
Chicago metro, but will allow mid-shift to monitor evolution of
convection west of the Mississippi.

Ratzer

&&

.DISCUSSION...
Issued at 315 PM CDT Sun May 19 2024

Through Monday Night:

A cooler and much drier airmass continues to work in across
northern IL on east-northeasterly winds along the southern
periphery of a surface high centered over southern WI. However,
the higher theta-e airmass, which has been destabilizing all
morning, is not that far away. As if this writing, it resides
in closer proximity to a west-to-east oriented surface frontal
boundary near or just south of US-24 across central parts of IL
and IN. Regional satellite imagery has been showing some building
CU near this boundary over the past couple hours, with some of the
better CU development currently located across southern Iroquois
east-northeastward into Benton county, IN. This is the general
area we will be watching for isolated thunderstorm development
over the next couple hours as any remaining weak capping continues
to quickly erode.

While weak flow through the troposphere will curtail any threat
for organized storms into this evening, a high amount of DCAPE
(in excess of 1,000 j/kg) and very steep low-level lapse rates
will support the potential for some pulse type storms capable of
producing isolated instances of gusty winds up to 45 mph. The weak
steering flow will also result in nearly stationary to very slow
moving storms (at 10 mph or less), so some very localized
instances of heavy rainfall could materialize. These isolated
storms should diurnally weaken after sunset this evening.

The only other area of showers and storms we will have to keep an
eye on into this evening is the area of weakening convection
currently moving across IA. Fortunately, this activity is expected
to continue weakening with eastward extent evening as it moves
into a drier and more hostel environment across northern IL.
We should thus end up with a relatively quiet night for the area.

Our weather will begin to turn active on Monday as the frontal
boundary to our south lifts northward and opens up the gates for
the moist and unstable airmass to our south to return northward.
Meanwhile, a notable mid-level impulse is expected to become
convectively enhanced tonight by a severe complex of storms over
KS. This feature is expected to be a key player with our threat
of thunderstorms Monday afternoon. It appears that some of the
decaying remnants and outflow from the Plains complex will be
shifting eastward across eastern IA by mid to late Monday
morning. Assuming that lingering cloud debris and outflow from
this dyeing MCS does not hinder the northward return of the high
theta-e airmass to the south, renewed thunderstorm development is
likely (70%+ chance) as the airmass over northern IL
destabilizes through the morning. Thunderstorms could develop over
northern IL as early 1 or 2 in the afternoon.

Deep layer flow, and the associated deep layer shear, will
increase modestly for a period as the main impulse shifts over
southern WI into the afternoon. Deep layer shear is not expected
to become overly impressive. However, with the prospects for up
to 30 to 35 kt deep layer bulk shear, it certainly seems plausible
that some organized multi-cell clusters and possibly a few
supercells will materialize. Instances of damaging winds and hail
would be the main threat from these storms Monday afternoon. Some
locally heavy rainfall will also be possible with these storms as
PWATs increase to around 1.5". While significant widespread
flooding is not likely, some minor instances of flooding could
result. This would be particularly true over the Chicago urban
areas, along with any areas that experience a period of east-
northeastward training of storms. The threat for these storms
will wane into Monday evening following the eastward departure of
the main mid- level impulse.

KJB

Tuesday through Sunday:

Tuesday & Tuesday night:

Strong to severe convection is once again expected to develop over
the central Plains Monday evening and probably congeal into an MCS
that will track toward the mid-Mississippi Valley. Some weakening
remnants of this MCS will bring a chance of widely scattered
showers and embedded non-severe thunderstorms to mainly areas near
and north of I-80, where our forecast has 30-40% PoPs. A warm
front is progged to be surging rapidly northward Tuesday morning
with better support for convection quickly shifting well north of
our CWA by mid day. The GFS finally trended toward the general
consensus of the remainder of the guidance (ECMWF, NAM, GEM), that
would support a windy (southerly gusts up to 30-40 mph) and
unseasonably warm (almost hot) Tuesday afternoon, with minimal
rain chances in our area after any AM lingering AM convection
dissipates.

Explosive severe thunderstorm development is likely over central
or western Iowa into northwest/north central Missouri Tuesday
afternoon. This convection likely wouldn`t reach our western CWA
until after sunset Tuesday, if not later. By the time it arrives,
boundary layer will be nocturnally stabilizing with growing
MLCINH and weakening MLCAPE. The most likely scenario would be
storms being in a weakening phase as they arrive Tuesday evening.
Depending on the extent of mesoscale organization that takes
place, still certainly can`t rule out a threat for severe
convection, primarily in our western CWA through mid evening.

Current thinking is similar to our previous forecast, that areas
near/west of I-39 in line with SPC`s day three level 3 (enhanced
risk) are most favored for the evening severe threat. If there is
a severe threat it would likely be in the form of damaging winds
and perhaps a QLCS tornado threat, but again, this will depend
heavily on strong mesoscale organization of convection that allows
it to overcome growing and increasingly large MLCINH over our
area Tuesday night. System dynamics and strong southerly low-level
flow and warm/moist advection plus a well organized MCS may be
enough to bring the severe threat farther east across the level 2
(slight risk) area. Though this appears less likely at this time
with the current thinking on timing, faster eastward propagation
of the expected MCS may keep the severe threat going deeper into
the evening and early overnight farther east.

Progged 925mb temps continue to support high temperatures making a
run at the lower 90s. Given uncertainties on potential debris
cloudiness, didn`t make any meaningful changes to the NBM`s mid-
upper 80s Tuesday, but quite plausible that some areas breaking
the 90 degree mark. If temps do overperform, that would increase
the chances of boundary layer moisture mixing out, potentially
lessening instability/increasing inhibition a bit more Tues night.

Wednesday & Wednesday night:

The GFS and its ensembles (and the typically unreliable outer
ranges of the NAM) continue to be on their own Wednesday with
respect to our weather, in particular the precip chances. A closer
look at the GFS and NAM reveals that it is forecasting Tuesday
night`s cold front to be more anafrontal in nature with the front
at 850mb still displaced a couple hundred miles northwest of the
sfc cold front. The GFS/NAM then seem to break out convection
rooted between 825-875mb near the elevated front. This GFS/NAM
solution is an outlier versus the ECMWF and GEM (and an
overwhelming majority of their respective ensemble members), which
continue to strongly support a cleaner frontal passage with
breezy, seasonably warm (70s to around 80F), and dry conditions
Wednesday. If the front slows a bit, portions of the southeastern
CWA (primarily southeast of the Kankakee River Valley) could have
a chance for a few isolated showers and thunderstorms to develop
in the afternoon.

Thursday and beyond:

Thursday looks to be sunny and seasonably warm with comfortable
humidity as the baroclinic zone moves well south of the area.
Next northern stream shortwave should result in increasing
chances of showers and storms again, perhaps as early as late
Friday, but more probable Friday night into Saturday. Timing,
track, intensity of this system is still subject to change this
far out, but varying chance range PoPs in the official forecast
appear reasonable. It will be at least a few days until we can
hone in on specifics for the all-important Memorial Day Weekend
forecast regarding effects on the many planned outdoor activities.
Temperatures don`t appear they will be cooler than normal, but not
as warm as what we experienced this weekend (unfortunately, for
pool and beachgoers).

- Izzi/Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1230 AM CDT Mon May 20 2024

Key Aviation Impacts for the current TAF period:

- Scattered and possibly strong thunderstorms expected today,
  arriving in the late morning for RFD and around or just after
  noon for the Chicago terminals

Dry conditions and light winds out of the east, becoming
southeasterly ahead of the next storm system arrives. There may
be the occasional spotty shower that develops in the morning out
ahead of the main system; however, consensus suggests that the
early showers will leave RFD and head toward southeastern
Wisconsin, so it was kept out of the TAFs for now. As for the
main system, winds will increase out of the southeast ahead of
the storms with gusts around 25 knots. Widely scattered storms
are still likely with the only major change to the forecast
being increasing the arrival time to around 15Z at KRFD and as
early as 18Z at KORD. Lightning, strong westerly wind gusts, and
potentially some hail are the main impacts expected with these
storms. Additionally, MVFR cigs and lower vis can be expected
during the heaviest rainfall as they pass over terminals.

Latest guidance suggests that the main system should move east
of Chicago after 22Z. There could be some lingering light
showers behind the line, but with lower confidence in projecting
exact location of that, it was kept out of the TAF. Winds are
expected to be out of the southeast and below 10 knots behind
the rain, but they may be variable in the immediate wake of the
storms. Cloud cover is expected to remain over the region, but
cigs should be at VFR levels.

DK

&&

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

&&

$$

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