934
FXUS63 KLOT 190207
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
907 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

&&

.UPDATE...
Issued at 859 PM CDT Sat May 18 2024

This evening, we find a weak cold front draped across eastern
IA and up through central WI propagating eastward toward
northern IL. Clusters of strong thunderstorms can be found ahead
of the front across central and northern WI where there is
notably better forcing for ascent, both along the face of the
front and from features aloft. What little activity we had
brewing out to our west a bit earlier this evening has fizzled
away meaning areas upstream are rain-free for the time being.
While most high res guidance has dry conditions prevailing along
the front, if not producing just a few sprinkles, the RAP and
HRRR have consistently been resolving a little bit of
shower/storm redevelopment along the line within the next few
hours just prior to reaching the CWA. Some low level deformation
appears to be the primary driver behind this redevelopment as
it looks to stretch the low level vort lobe and provide us with
a little uptick in pre-frontal forcing over the next few hours.
With dry low level air in place, LFCs across northern IL are
pushing 10,000+ ft AGL, which equates to around 700 mb, with
more and more MUCIN building up as the evening progresses. And
with such shallow moisture to work with beneath awfully dry mid
level air, it`s going to be tough to overcome these dampers and
produce any precip, even with the added forcing. If anything
does manage to go up, chances are seemingly greatest across far
north central IL late this evening with the environment becoming
even less favorable to support precip as the front moves into
the Chicago metro later tonight. The current forecast brings
slight chances across our northern CWA with the front, and that
seemed appropriate still. That being said, the short term
forecast remains very much on track.

Doom

&&

.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 /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 614 PM CDT Sat May 18 2024

Key Aviation Messages:

- Slight (<20%) chance of a shower or thunderstorm late tonight
  with a cold front moving through the area.

- South-southwest winds will shift north-northwest behind the
  front pre-dawn Sunday, then northeast around/after sunrise.

Surface low pressure was over far western Ontario early this
evening, with a cold front trailing through western WI into
eastern IA. South-southwest winds (with a few 15-20 kt gusts
prior to sunset) will prevail ahead of the cold front into the
overnight hours, before shifting north-northwest behind the
front which should move through KRFD 06-07Z and KORD/KMDW
09-10Z. Isolated to widely-scattered showers and thunderstorms
were occurring ahead of the front currently, but are expected to
diminish in coverage with the loss of diurnal heating after
sunset. High-res CAM guidance suggests most of these would
dissipate before reaching the Chicago terminals later tonight.
KRFD has a little better chance, but probability is too low for
TAF mention.

Winds will shift northeasterly early Sunday morning, and will
remain from that direction through the day, before trending more
easterly Sunday evening. VFR conditions should prevail through
the period.

Ratzer

&&

.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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