486
FXUS63 KLOT 182019
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
319 PM CDT Sat May 18 2024

.KEY MESSAGES...

- Small chance (~20%) for some isolated/widely scattered
  showers/storms later this evening as a weak cold front moves
  into the area.

- A few additional isolated showers and storms possible Sunday
  afternoon and early evening, mainly south of the Illinois and
  Kankakee River Valleys.

- Summer-like warmth expected through Tuesday night,
  interrupted only by lake cooling Sunday; then during any
  showers/storms Monday and Tuesday.

- Scattered thunderstorms likely Monday afternoon and evening
  across portions of northern Illinois, a few of which could be
  strong, and capable of causing heavy downpours and ponding

- Scattered (mainly non-severe) thunderstorms possible Tuesday
  morning and then after a break, widespread thunderstorms in
  the evening, including a severe threat for parts of the area

&&

.DISCUSSION...
Issued at 315 PM CDT Sat May 18 2024

Through Sunday Night:

Very warm and somewhat more humid weather has set up across the
area this afternoon in advance of an approaching cold front
across the Upper Midwest. While there have recently been a few
attempts at thunderstorm development across northwestern IN where
weaker capping exists, no more than some isolated sprinkles or
light showers are expected east of I-57 this afternoon. Elsewhere,
partly cloudy and warm conditions will persist the remainder of
the afternoon as inland temperatures top out in the mid 80s.
Slightly cooler conditions are expected near the IL Lake Michigan
shores due to an onshore southeasterly wind component.

Diurnal heating and destabilization in advance of the approaching
cold front to our west-northwest is expected to foster scattered
thunderstorm development across parts WI and northeastern IA later
this afternoon. While this activity should persist into this
evening as the cold front begins to approach northwestern IL, a
gradual weakening trend is expected through the evening as the
boundary layer stabilizes. Nevertheless, I opted to add some
slight chance POPs (20%) along the front across northern IL later
this evening. Areas in and around the Rockford area stand the best
chance to have a few of these weakening showers and storms in the
10pm to midnight timeframe.

The cold front is expected to stall out over my southern counties
in central IL and IN on Sunday. North of the boundary, a drier
airmass will filter into the area on east-northeasterly winds.
Accordingly, a less humid day is expected for much of interior
sections of northern IL (especially north of I-80). Temperatures
will be the coolest near the lake, where readings will hold in the
upper 60s to around 70. Farther inland, temperatures will top out
in the low to mid 80s.

Much of the area will be precipitation free on Sunday due to the
drier easterly winds. However, a moist and unstable airmass will
remain in place to the south near the stalled frontal boundary
across central IL and IN. Therefore, as capping weakens in the
afternoon, some isolated showers and thunderstorms cannot be ruled
out, especially south of the Kankakee and Illinois River Valleys.
While weak flow through the troposphere will curtail any threat
for organized storms, any showers or storms that do manage
to develop Sunday afternoon will be very slow moving at 10 mph or
less. This could thus promote some very localized instances of
heavy rainfall.

These isolated storms should diurnally weaken after sunset Sunday
evening, thus setting up a relatively quiet night for the area. We
will have to keep an eye on convective trends to our west late
Sunday night, however. Numerous thunderstorms are expected to
develop and convectively enhance a mid-level disturbance across
the Plains Sunday afternoon and evening. The decaying remnants of
these thunderstorms are expected to shift east-northeastward
across the Corn Belt Sunday night, and it is possible that some of
them make it into northern IL into early Monday morning. We thus
continue to carry some low end chances (~20%) west of I-39 late
Sunday night to account for this possibility.

KJB

Monday through Saturday:

On Monday, a lead short-wave and its associated surface low
lifting into the upper MS Valley will drag a warm front north of
the area. While the strongest forcing will pass north of our area,
the southern flank of the short-wave will provide some weak height
falls. With strong heating bringing temps up into the mid-upper
80s and dew points into the 60s amidst 6.5-7.0 C/km mid-level
lapse rates, looking at up to 1500 J/kg of MLCAPE. Areas
near/northwest of I-55 or I-57 should see enough erosion of CIN
(capping) that combined with the modest large scale forcing,
expecting scattered thunderstorms to develop toward mid day and
especially in the afternoon over the western or northwestern half
or third of the CWA and then track northeast.

Marginal (less than 30 kt) of deep layer shear and the modest
mid-level lapse rates appear to be limiting factors for a more
appreciable severe threat, though fairly steep low level lapse
rates and seasonably high PWATs may contribute to wet downbursts
with 50 to perhaps 60 mph gusts across the northwest CWA, in line
with the SPC day 3 level 1 (marginal) severe risk. Our official
forecast holds onto likely PoPs north of I-80 in the evening and
overnight, though with weak flow aloft, diurnal loss of
destabilization, and minimal large scale forcing, the highest
thunderstorm coverage may in reality be confined to only an hour
or two after sunset.

Looking into Tuesday, global guidance has been consistent in
bringing a lead short-wave northeastward across the mid-MS Valley
during the morning hours. There may be enough of a MUCAPE
reservoir for scattered showers and thunderstorms during this
time, focused near/northwest of I-55, where we have 30-50% PoPs in
our forecast. Morning timing and marginal deep layer shear suggest
primarily non-severe thunderstorms, with maybe a chance of an
isolated marginally severe hail or wind producer. Similar to our
previous discussion, Tuesday afternoon continues to look like a
gap in the convection that should support temps at least in the
mid to upper 80s, and possibly tagging 90F if clouds really scour
out. Southerly winds will gust up to 30-35 mph.

The powerhouse cyclone for late May that has exhibited an overall
slowing trend vs. a few days ago will have the surface low
northwest of Des Moines early Tuesday evening (around 00z).
Initial supercells initiating west of the MS River may tend to
congeal into a squall line or bowing segments into the evening,
with a coherent but possibly weakening severe MCS tracking
eastward across the area during the evening. Given the slightly
later and less favorable timing, areas of the CWA within the 30%
probs area in the SPC Day 4 outlook correspondingly appear to have
the highest severe chance. With this said, strong dynamics and
strengthening southerly low-level flow may offset diurnal
stabilization enough to keep the severe (mainly wind) threat going
farther east, overall fitting the 15% area on the outlook.

Thunderstorm coverage should wind down overnight Tuesday night
and then expecting the cold front to clear the area by Wednesday
morning. Barring a slightly slower trend in the cold frontal
passage, Wednesday will be a breezy, cooler (highs in low-mid
70s), and dry day for most of the area. In this scenario
(slightly slower cold front), the best chance for any showers
and a thunderstorm will be southeast of I-55, where chance PoPs
were maintained. Dry and pleasant weather Thursday, with highs
in the mid to upper 70s, may very well persist through the day
on Friday, before shower and thunderstorm chances possibly
return at the end of the period. An early glance at Memorial Day
Weekend points toward seasonable temps and some potential for
showers and thunderstorms at times, but that`s about all that
can be said this far out.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1216 PM CDT Sat May 18 2024

VFR conditions are expected through the period, with only some
scattered to occasionally broken CIGs around 5,000 feet
expected this afternoon. Winds will remain light (up to around
10 kt) from the the south-southwest through this evening. A weak
cold front will then move across the terminal airspace tonight
causing winds to shift north-northwesterly during the predawn
hours of Sunday. Winds are then expected to settle into an east-
northeasterly direction during the day Sunday as a surface high
builds in over southern WI.

A small chance (10-20% chance) exists for a couple of isolated
showers or storms tonight as a cold front moves into the area.
The best chance for these storms will be west of the area today.
Currently it is anticipated that these storms will weaken as
they shift eastward into northern IL and southern WI through
the evening. The possibility even exists that the storms
diminish altogether before the front reaches the Chicago area
terminals overnight. For this reason, I have left a formal
mention out of the TAFs at this time.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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