152
FXUS63 KLOT 041937
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
237 PM CDT Tue Jun 4 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms develop mid to late
  afternoon, and increase in coverage this evening. A few
  stronger storms may produce with gusty winds and small hail,
  in addition to heavy rainfall and a localized flooding
  threat.

- Following isolated to scattered showers Wednesday evening,
  mainly dry and comfortable (seasonably cool) through Sunday.

- Breezy to windy on Thursday with gusts up to 30-40 mph.

&&

.DISCUSSION...
Issued at 236 PM CDT Tue Jun 4 2024

Through Wednesday:

Before the thunder activity concerns, there is a lake breeze
from Chicago northward, keeping a lid on temperatures near the
lake (70s) whereas most inland areas remain on the warm and
muggy side as temperatures, even with the clouds, march on into
the mid 80s.

A mesoscale convective vortex (MCV) continues to shift northeast
into eastern Iowa. Ahead of the MCV There is a fair amount of
cloud cover across northern Illinois and northwest Indiana,
which is limiting instability. There are some weak waves in the
south-southwest flow and we are starting to see some of the
radar echoes on KLOT reaching in the ground in the form of
mostly light shower activity. Expect this to drift northward,
with some erosion on its northern edge. Thunder coverage will
low with this leading activity until the MCV drifts a bit
farther NE, given that MLCAPE will remain a bit more limited
closer to 500 J/Kg. Behind the opaque mid and upper level cloud
shield continued moist advection with dewpoints in the mid to
upper 60s will lead to increased instability approaching 1000
J/Kg or so of MLCAPE closer to the MCV across central and
western Illinois where capping is weaker. Therefore expect an
uptick in shower and thunderstorm coverage west of I-39 and to
the southwest, but also there is a narrow corridor across
northwest Indiana. Some of this activity will migrate closer to
the Chicago area toward early evening.

MLCAPE will not be quick to erode this evening as the MCV
advances into northern Illinois. This would likely be a more
favored period for a bit more storm coverage. Gusty winds
would be more favored severe hazard given some elevated low
level lapse rates through early evening. PWATs will also
increase ahead of the MCV as low level flow gets enhanced, which
suggests pockets of heavy rain this evening. While the flow
aloft is initially somewhat weak, enhanced of the low-mid level
southwesterly flow would suggest the individual storm elements
will be moving fast enough to preclude any significant flash
flooding in spite of the high PWAT airmass. However, increased
low level jet/storm regeneration is of some concern for
localized flooding/ponding and is probably the hazard of most
concern this evening into the early overnight hours. The main
area of concern will be west and south of the core Chicago metro
area closer the low/slightly better instability.

A fairly sharp upper trough, currently across the northern plains
will steer a cold front through the area in the area in the
morning. Ahead of the trough showers should expand in a
scattered to numerous nature again overnight, with storm
coverage gradually decreasing overnight into early Wednesday.
With a deep moist profile, pockets of heavy rainfall may lead to
ponding on area roadways into the morning commute.

The cold front will usher in cooler and drier air in the
afternoon in the Chicago area and into northwest Indiana, and
the front will also bring the end to any lingering showers, at
least for a brief time in the afternoon.

KMD


Wednesday Night through Tuesday:

Following Wednesday morning`s cold frontal passage, a weaker,
secondary cold front is progged to move across the CWA Wednesday
evening and night. A push of low-mid level moisture ahead of
the boundary will result in some late day destabilization
amounting to as many as a few to several hundred joules of
MUCAPE over parts of our area. Models agree on a mid level
inversion keeping ELs topped out at around 500mb, which will
equate to around -10C. And with the vast majority of forcing for
ascent found below the freezing level, such a setup should
struggle to produce much lightning, but low- topped convective
showers certainly look attainable. Some models were thinking
that the moisture advection may be too meager to produce much of
anything, but the trend is in favor of a push of showers ahead
of the front. The more appreciable moisture, and in turn
instability, will be found across the northern and northwestern
CWA. It`s here that we`ll find the best odds at seeing precip as
well as an embedded thunderstorm, but again the thunder
potential overall is seemingly low.

All of the precip should be off to our east by Thursday morning.
A tight pressure gradient will set up across the region
between the departing storm system and an approaching high out
west. A 40-50 kt LLJ is also expected to move overhead, both of
which will bring windy conditions to the area on Thursday. The
LLJ diffuses some for Friday, but the surface pressure gradient
will remain in place bringing more breezy conditions on Friday.
The high pressure center will propagate across the lower Midwest
late Friday into Saturday which should keep precip chances
rather minimal heading into the weekend. A large upper low
expected to cycle over the eastern Great Lakes region late this
week and early next will reel in some cooler air aloft and help
keep conditions a tad cooler than normal this weekend with highs
progged in the lower and middle 70s.

Doom

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1149 AM CDT Tue Jun 4 2024

Key Aviation Impacts for the current TAF period:

- Scattered thunderstorms near the terminals in the late
  afternoon.

- Gusty southeast winds expected before the second round of
  storms

- A better chance (50 percent) for showers and storms at
  terminals this evening as showers continue overnight.

- Cigs are expected to lower overnight into Wednesday morning,
  low confidence in clearing time

VFR conditions prevail through this afternoon. Winds will be out
of the southeast and gusts are expected to strengthen through
the afternoon ahead of the showers. An isolated gust up to 30
knots cannot be ruled out completely, but better chances for
gusts to be 20 to 25 knots.

As a weak boundary moves northward in the late afternoon,
chances increase for showers and thunder to pass near the
terminals. Confidence in coverage for the first round remains
low. The chance for rain and thunder occurring at the terminals
is at about 25 percent or less before 00Z, but some thunder
will be around thus we will carry a VCTS mention. A southeast
lake breeze is expected to develop to help reinforce confidence
in southeasterly winds as the prevailing direction. However, as
the next round of storms move northward, it would not be
surprising if some occasional flipping to 190 happened between
00Z-02Z.

The next, more robust wave is expected to move over the
terminals after 00Z to bring the next round of showers and
thunderstorms over the region. Confidence increased enough to
convert the chances for thunder to a TEMPO group for TSRA at
area terminals, though timing may be adjusted in the next TAF
installment.

Showers will continue through the overnight as a cold front
moves over the area. Cigs are expected to lower tonight down to
at least low MVFR levels. There is a growing signal for IFR
conditions early Wednesday morning, but confidence was just low
enough to keep the SCT group in the TAFs for now. VFR conditions
are expected on Wednesday as conditions dry out and winds
become westerly with gusts around 20 knots.

DK

&&

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

&&

$$

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